TRANSPIRATION AND EVAPOTRANSPIRATION V1ITh ALEPPO PINE (Pn'ius HALEPENSIS MILL.) SEEDLINGS UNDER VARYING SOIL MOISTURE AND SOLAR RADIATION LEVELS Howard G. Halverson A Thesis Submitted to the Faculty of the DEPARThENT OF WATERSHED MANAGEt"ItENT In Partial F\1fil1rnent of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 1962 STAT4ENT BY AUTHOR This thesis has been sulmdtted in partial fuLfillment of requirements for an advanced degree at The University of Arizona and is deposited in The University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGD: Howard G. Halverson APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: (2 A. .MCCB Head Department of Watershed Management 11 ACKGW1LEDG4ENTS The author wishes to express his appreciation to Dr. A. L. McCnb for his assistance in setting up the study and editing this paper. Professor P. B. Rowe also has the author's gratitude for his assistance in this study. Appreciation is rendered to Mr. L. P. HmUton and the personnel of The Plant Materials Center of the University of Arizona for generously providing a location and necessary equipuent for this study. Mr. H. L. Silvey also has the author's thanks for his assis- tance in collecting soil and constructing the bench area. The author would like to express his appreciation to Dr. H. Tucker for his able assistance in the statistical analysis. The Institute of Atmospheric Physics must be mentioned for their cooperation in providing solar radiation data. 111 TABLE OF CONTENTS Page INTRODUCTION 1 REVIEW OF LITERkTURE 2 U U U RESEARCH METHODS Soil Moisture Depletion Study Soils Climate Pot Treatment Analysis 12 13 15 Wilting Point Study 15 16 Analysis Soil Moisture and Solar Radiation Study 16 16 17 17 19 Soils Climate Pot Treatment Analysis 20 RESULTS 20 Soil Moisture Depletion Study Transpiration Evapotranspiration Evaporation Comparison of Transpiration, Evapotranspiration and Evaporation 20 22 24 26 Wilting Point Study 33 Soil Moisture and Solar Radiation Study 34 43 DISCUSSION Water Loss Under Different Soil Moisture and Solar Radiation Levels 43 Wilting Point 4-4 iv Table of Contents (Continued) Page SUMMARY 46 BIBLIOGRAPHY 4$ APPENDIX 51 V LIST OF TABLES Table 1 Title Mean water losses by transpiration for different levels of incident solar radiation 2 Mean water losses by evapotranspiration for different levels of incident solar radiation Page 20 22 Mean water losses by evaporation for different levels of incident solar radiation 24 Soil moisture percentages at permanent wilting of tree seedlings under two sources of water loss and four levels of solar radiation 33 5 Analysis of variance of wilting point data 34 6 Average daily rates of water loss in grams by transpiration under different levels of solar radiation and soil moisture 35 Average daily rates of water loss in grams by evapotranspiration under different levels of solar radiation and soil moisture 37 Average daily rates of water loss in grains by evaporation under different levels of solar radiation and soil moisture 39 Analysis of variance of unweighted means of moisture loss data from soil moisture and solar radiation study 41 3 4 7 9 APPENDIX 1 Soil moisture retention at different atmospheric pressures for soil used in moisture depletion study 2. 52 Particle size distribution of soil used in soil moisture depletion study 52 Data on temperature, relative humidity, and solar radiaation from May 15 to August 22, 1961 54 4 Soil moisture retention at different atmospheric pressures in soil moisture and solar radiation study 57 5 Particle size distribution of soil used in soil moisture and solar radiation study 57 3 vi LIST OF FIGURES Figure 1 2 3 4 5 6 7 9 10 U 1 2 Title Page Mean daily water losses in grams by transpiration for different levels of soil moisture and solar radiation 21 Mean daily water losses in grains by evapotranspiration for different levels of soil moisture and solar radiation 23 Mean daily water losses in grains by evaporation for different levels of soil moisture and solar radiation 25 Mean daily water losses in grains by transpiration, evapotranspiration, and evaporation at different levels of soil moisture under 100 percent solar radiation 2g Mean daily water losses in grams by transpiration, evapotranspiration, and evaporation at different levels of soil moisture under 70 percent solar radiation 29 Mean daily water losses in grams by transpiration, evapotranspiration, and evaporation at different levels of soil moisture under 49 percent solar radiation 30 Mean daily water losses in grams by transpiration, evapotranspiration, and evaporation at different levels of soil moisture under 6 percent solar radiation 31 Mean daily water losses by transpiration, evapotranspiration and evaporation during a soil moisture depletion study 32 Mean daily water losses in grains by transpiration under four different levels of soil moisture and solar radiation 36 Mean daily water losses in grams by evapotranspiration under four different levels of soil moisture and solar radiation 3 Mean daily water losses in grams by evaporation under four different levels of soil moisture and solar radiation 40 APPENDIX Diagram of design of soil moisture depletion study showing the position of each pot by code number 52 Diagram of design of soil moisture and solar radiation study showing the position of each pot by code number vii 57 INTRODUCTION There have been few basic studies concerning the rates of transpiration, evapotranspiration, and evaporation as related to soil moisture and solar radiation. Such studies are essential to a more complete understanding of the hydrologic cycle and of its modification by land and vegetation management practices. In arid regions water is a limiting factor in almost every sphere of human activity. Much interest is centered in obtaining more water for a variety of human uses and purposes. The magnitude of the yields of econcmtLc watershed plants is often determined by the amounts of water available to these plants. At the same time the amounts of water occurring as streuflow is dependent in part upon the kinds and density of the vegetation occurring on the watershed. To understand these reciprocal relationships and to make intefligent watershed management decisions,the processes relating to water losses must be studied in both field and laboratory. The basic purpose of this study was to provide a qualitative index of the relations among water loss, solar radiation, and soil moisture. The study used Aleppo pine as an indicator due to its rapid establishment when transplanted under the climatic conditions at Tucson, Arizona. The study was a fixed model so quantitative applications of results to field conditions or to other populations are not justifiable. This study is part of a larger study designed to study plant-water re- lations in the pine zones of Arizona. 1 REVIEW OF LITERATURE Knowledge of the processes utilizing soil moisture is important in watershed managnent. There is a large amount of reference material available concerning transpiration and evaporation. In view of this fact only the more pertinent material is presented here as a background. The question of whether soil water between field capacity and permanent wilting point is equafly available to plants for growth 1as been disputed for nearly 30 years (Stanhifl, 1957). Veihmeyer and Hendrick- son (1955) proposed the view that water is equafly available to plants at any point between field capacity and the permanent wilting point. Their work was done using Aleppo pine sealed in a suspended tank equipped with an automatic weighing device. Graphical results indicated that there was no difference between the rates of transpiration immediate- ly after irrigation and when the soil mass was near the wilting point. Similar results were obtained in subsequent experiments with other plants. Slatyor (1957) stated that the permanent wilting point was not a soil constant but a plant osmotic characteristic. He further pointed out that the evidence of Veihmeyer and Hendrickson was based on lysimeter studies where results may be altered by uneven root distribution. There- fore, the measurement of plant responses and the interpretation of these data may be in error. Lane and McComb (l94) using several species of plants reported 2 3 that a decrease in transpiration with increasing soil moisture tension existed. They also reported that the soil moisture percentage at the permanent wilting point varied between species. However, these differ- ences may have been due to differences in root penetration of the soil mass together with differences in extent of suberization, rate of respiration, and protoplasmic differences in the water absorbing area of the roots of different species. Stanhill (1957) summarized the results of 0 experiments relating growth to soil moisture and showed that in 65 cases the greatest yields were associated with the wetter soil moisture regime. Only in a carrot seed crop was the greatest yield associated with a dry regime. The 14 experiments that showed no response to soil moisture were older experi- ments in which fruit growth was the yield criteria. He attributed these results to the ability of the flowering parts to compete with vegetative parts for water during periods of moisture stress. Kramer and Kozlowski (1960) suggest that much of the confusion relating to water availability and transpiration in relation to soil moisture has resulted from failing to measure the internal water balance of the plant and to quantitatively estimate the factors controlling availability of soil water to the plant and the rate of water loss from the plant. The work of Richards and Wadleigh (1952) showed that availability of soil moisture to plants depended on the shape pf the moisture tension curve. Gingrich and Russell (1957) found that the growth of corn roots at equal moisture stress was greater when the stress was developed 4 osmotically than when it was developed as soil moisture stress. They attributeI the lesser growth under soil moisture stress to a rapid change in water transmission rates in their soil as soil moisture stress was increased. The effect was most pronounced in the 1- to 3-atmospheres range. Slatyer (1957) has attempted to show some of the relationships among transpiration, soil moisture, soil moisture stress, diffusion pressure deficit, osmotic pressure, and relative turgidity. He concluded that since transpiration is primarily a passive plant process it need not cease but may be reduced as soil moisture stress increases due to stomatal closure and reduced rates of water movement in unsaturated soils. Transpiration may continue after the death of a plant, limited by the energy available for evaporation, the resistance to water movement into, through, and out of the plant, and by the rate of flow of soil water into the roots. Kramer (1937) reported that water absorption lagged behind transpiration during the day but exceeded it at night. He found no differ- ences in the lag between woody, herbaceous, and succulent species. In all cases transpiration changes preceded absorption changes so he concluded water intake was largely governed by water loss. Bierhuizen (195) showed that transpiration and evapotranspiration decreased dth increasing soil moisture tensions under conditions of high evaporative potentials. inconclusive. Under low evaporative conditions the results were He suggests that some of Veihmeyer's and Hendrickson's experiments may have been conducted under conditions of low potential 5 evaporation. Closs (1958) states that two processes may be involved in determining transpiration. tial water absorption. They are: potential transpiration and poten- The most important one in any given situation depended upon where along the soil moisture tension curve the plant was located. Bierhuizen (1958) suggested that the transpiration rate is greater at higher light intensities, increases with available soil moisture and may become constant at high moisture levels. Recently, Kuiper and Bierhuizen (1958) studied the influence of several envirorm'iental factors on transpiration of cut leaves in potoineters and of entire plants grown in soil. They found that Fick's diffusion law can be applied to transpiration of cut leaves in potorneters. They also presented a formula to estimate the transpiration of plants in soil. Kuiper and Bierhuizen found the effect of soil moisture levels on transpiration was great under conditions of high potential transpiration and small under low potential transpiration conditions. They sug- gest that soil structure and oxygen content of the soil may become limiting factors in soil moisture absorption under certain conditions. Bierhuizen (195$) states that some of Veihmeyer's early work showing soil moisture to be equally available was done on coarse- to medium-textured soils. On clay soils deep-rooted species may show wilting responses to moisture deficits before the soil moisture reaches the permanent wilting point. Kuiper and Bierhuizen (1958) found a linear relationship between light intensity and transpiration. At low light intensities (up to 6 17.5 x lO ergs/cni.2/sec.) 6 percent of the light effect was due to stomatal changes and decreased diffusion resistance to water. Less than 14 percent was due to a temperature change and the accompanying increased vapor pressure deficit. At high light intensities the linear relationship was thought due mainly to an increase in leaf temperature and vapor pressure deficit. They stated that in experiments with plants in soil, the soil moisture tension affects the total diffusion resistance of leaves independent of light intensity. The effect of soil moisture on diffusion resistance may be due partly to incipient drying and partly to an osmotic stoinatal reaction. Abd el Rahman, Kuiper, and Bierhuizen (1959) found a linear relationship exLsted between transpiration and incident light intensity with the exception of the lowest light intensity under controlled temperature and relative humidity conditions. They attributed the deviation of transpiration at the low light intensity to morphological differences in the plants grown at these light intensities. Tests on 25 sample trees showed that light and soil moisture were the most prominent factors affecting transpiration (Rothacker, 1949). A decreased transpiration rate occurred with increased soil dryness. Plants in sealed containers placed on a tower at the crown levels of a forest stand and exposed to full solar radiation lost twice as much moisture as similar plants at the ground level in the same length of time. Hendrickson (1942) stated that quantitative results from container experiments such as those Rothacker reports cannot be applied to watershed areas. 7 Wiegand and Taylor (1961) stated that evapotranspiration, the suni of transpiration plus evaporation, must be analyzed on the basis of certain plant factors. Two such factors are the adaptations of land plants for mad.muni photosynthetic advantage and developuent of xylem which provides a supply of water to the mesophyll cells of the leaf. The effectiveness of stomata in control of transpiration is much debated. Stomata do occur at the only point at which protection against transpiration would be effective, namely at the plant-air interface. These authors also discuss three stages or periods in the drying of porous solids by evaporation. During the constant rate period evaporation is 1 mi ted by external conditions. rate periods of drying can be separated. The two distinct falling The first is controlled by the rate of surface evaporation and the second is controlled by the rate of internal liquid diffusion. Rowe (194) found that evaporation losses from the upper 12-inch depth of soil of annually burned plots in California chaparral were great-- er than evapotranspiration losses from undisturbed plots. He also stated that bared soil surfaces showed more rapid drying between storms than undisturbed plots. Zahner (l95a) found that a hardwood imderstory increased the rate of soil moisture depletion in pine stands A chemical treatment of the understory was more efficient than a controlled burn in preventing moisture loss. By the end of the growing season the soil in both treated and untreated areas had reached the wilting point and moisture loss had almost ceased. meyer (1955). These results agree with those of Hendrickson and VeihThese authors stated that the greatest water loss occurred 8 during the growing season. Water loss decreased rapidly at the end of the growing season. Netz and Douglass (1959) working with a 66-inch depth of Piedmont soil found that evapotranspiration losses from a pine plantation were greater than evaporation losses from a barren area. These authors found that evaporation from the surface 5-inches of soil was greater than evapotranspiration during a 40-day drying period. that moisture loss is related to soil depth. They concluded Under vegetation it is a reflection of root concentration and from bare soils it is a reflection of the characteristics of evaporation. Koshi (1959) found that no differences in soil moisture retained throughout the growing season existed in the top 12-inches of soil under an undisturbed oak stand, a thinned oak stand, and a cleared area in Texas. In the 12-- to 24-inch horizon the moisture losses on the cleared plots were distinctly less than for the other two treatments. Failure to calculate interception losses when estimating soil moisture depletion by soil sampling under field conditions may alter the results of an experiment (Lull and Axley, 1958). Interception losses may amount to as much as 10 percent of the total rainfall. This may account for part of the differences in water loss between forested and bared plots. Zahner (1955) found that the rate of water loss between oak and pine stands was little different for corresponding depths. The upper 3-feet of soil lost water twice as fast as the layers below three feet. Gardner and Firan (l95) stated that there are two ma.xiinum 9 soil moisture evaporation rates: one is the potential rate as determined by external conditions, and the other is determined by the rate at which water can be transmitted in the soil. Water will be limited by the lesser of' the two and may vezynearly equal the lesser. 1easurements of transpiration of intact five-stamen tamarisk (Tamarix pentandra Pall.) plants by using an infrared gas analyzer (Decker and Wetzel, 1957) showed that transpiration increased linearly with increasing light intensity up to 600 foot-candles. was reached between 600 and 3,000 foot-candles. Light saturation Arizona cypress (Cupres- sus arizonica Greene) gave results essentially the same as those obtained with tamarisk. These authors also found that transpiration decreased with increasing humidity and increased with increasing temperature. In a study of diurnal variations of transpiration (Parker, 1957) the evening drop of transpiration seemed clearly related to decreases in light intensity. Occasional periods of cloudiness during the day had no apparent effect in depressing transpiration rates. Janes (1954) noted that even though leaves may absorb some water from a saturated atmosphere, this water did not replace soil moisture in promoting growth. He also pointed out that leaves under a partial mois- ture stress may have reduced rates of photosynthesis. Peters and Russell (1959) reported that the relative water losses from field corn by transpiration and evaporation from 1954 through 1957 resulted in the conclusion that transpiration accounted for only 30 to 50 percent of the total water loss. The remainder was attributed to evaporation. Harrold et al. (1959) using plastic covered lysimeters reported 10 that evaporation accounted for over half of the total water loss from field corn. Holt and Van Doren (1960) used plastic covered plots to estimate the water losses from field corn. 50 percent of the total water loss. Evaporation accounted for 40 to The low water loss on bare plots indicated a low rate of evaporation when the soil surface was dry. Recent gains have been made in research regarding the relative magnitudes of evaporation, evapotranspiration, and evaporation. However, more research is needed before the effects of vegetation manipulation and land management practices can be fufly evaluated. RESEARCH METHODS Soil Moisture Depletion Study The first part of this study of the relations among soil moisture, solar radiation, and water loss involved deriving a soil moisture depletion curve. The study was designed to investigate the rates of water loss by transpiration, evapotranspiration, and evaporation over a range of soil moisture and solar radiation levels. The results may help explain the magnitudes of the processes involved in water loss. Seedlings of Aleppo pine (Pinus halepensis Mill.), two years old, grown in plastic pots were used for part of these determinations. Than- spiration was studied by using pots that contained a tree seedling with the soil protected from evaporation; evapotranspiration, by pots that con- tained a tree seedling with the soil exposed; and evaporation, by pots that contained soil only. Soils. Four groups of 15 pots were filled with a forest soil from the Bear Wallow area of the Santa Catalina Mountains of Arizona. The soil used in this experiment was collected under a mixed stand of ponderosa pine (Pinus ponderosa Laws.) and douglas fir (Pseudotsuga taxifolia (Poir) ]3ritt.). to remove rocks. It was screened through a quarter-inch mesh The soil had a field capacity of 26 percent soil mois- ture using 1/3-atmosphere of tension as a simulator, and a wilting point of nine percent soil moisture using 15-atmospheres of tension. 11 Other 12 soil moisture tests were run at 3.4 and 5-atmospheres of tension (Table 1, Appendix). The mechanical composition of the screened soil was 48 percent sand, 47 percent silt, and 5 percent clay (Table 2, Appendix). The oven-dry weight of the soil was determined for each pot and proved to be in the range of 4000 to 5000 grams. Climate. A continuous record of relative humidity and tempera- ture was obtained during the experimental period by placing a hygrothermograph in the experimental area. Day temperatures ranged from 96° F. to over 110° F. and night temperatures from 56° F. to 840 F. Although some of the day temperatures were high for plant growth, they did not extend over long periods of time (Table 3, Appendix). During the experimental period the daily maximum temperature increased from a high of 100° F. during the last two weeks of May to a daily maximum of over 110° F. during the period from June 13 until June 27, 1961. Another period of maximum daily temperatures over 110° F. pre- vailed from July 7 until July 22, 1961. After July 27, the daily maxi- mum temperature dropped to approximately 100° F. for the remainder of the experimental period. Daily minimum temperatures averaging 60° F. were recorded from May 15 until June 8, 1961. The minimum daily temperature increased to an average of 800 F. within 10 days and remained at this level until the end of the experiment on August 22, 1961. Relative humidity during the experimental period ranged from 5 percent to 100 percent. A minimum daily relative humidity of less than 15 percent prevailed from May 15 until June 9, 1961. The highest 13 minimum relative hwniciity was 42 percent recorded on August 15, 1961. Madmum relative humidity never exceeded 65 percent between May 15 and June 17, 1961. After June 17 madnium daily relative humidity increased steadily until July 28, 1961. After July 28 relative humidity values above 85 percent were recorded almost every day until August 22, 1961. High relative humidity values were obtained during summer convec- tional storms but lasted for only a short time. A record of total solar radiation in gram-calories/cxn.2/day was obtained from the Institue of Atmospheric Physics, The University of Arizona (Table 3, Appendix). Pot Treatment. In this part of the study treatments were repli- cated five times; each replication consisting of the following three treatments. To study evapotranspiration, seedlings of Aleppo pine were planted in five randomly selected pots out of each group of 15. To study transpiration five other pots were treated by planting a seedling in the pot and then covering the soil surface with 4 mu neutral polyethylene film and sealing it to the tree trunk with grafting wax to minimize evaporation. The remaining five pots in each group of 15 were left as a bare soil check on evaporation. For convenience, the pots were numbered in such an order that the first five pots out of each group of 15 held a tree with the soil covered with polyethylene, the second five pots held a tree seedling, and the remaining five pots contained soil only. Each pot in the experiment was 10-inches in diameter and allowed 78.54 square inches of surface area when filled with soil. 10-inches deep and filled with 7 to 9 inches of soil. Each pot was The pot was equip- ped with a bottom liner of one-inch thickness fiberglass to minimize 14 water loss through the bottom of the pot and two plastic tubes leading into the soil to allow uniform watering throughout the soil depth. The two tubes were placed 3-inches and 6-inches below the surface of the soil, respectively. The buried end of each tube consisted of a circular section with several holes punched along this length to allow uniform lateral watering throughout the soil mass. The weight of each pot with equiinent, oven-dry soil, and tree was determined to the nearest gram and soil moisture changes were determined by weighing. Tree seedlings were planted in the plastic pots on February 12, 1961 and allowed an establishment period under field capacity soil inoisture conditions until May On May 15, 1961 15, 1961. the pots were moved to The Plant Materials Center of the University of Arizona. They were randomly placed by groups on a prepared bench two and one-half feet above the ground with a roof of 4 mu neutral polyethylene four feet above the bench surface to prevent normal summer precipitation from entering the pots. The sides of the bench were not enclosed so normal air temperatures prevailed throughout the measurement period. To prevent solar radiation from striking the sides of the pots, thus increasing soil temperatures, a 6-inch border was constructed around the edge of the bench and the interval between the pots on the bench was filled with excelsior. At the start of the measurement period each randomly selected group of 15 pots was placed under one of four solar radiation levels. Solar radiation levels were 100 percent, 70 percent, 49 percent, or 6 percent solar radiation. For convenience, the pots were numbered in groups 15 of 15. Pot numbers 61 to 75 were placed under six percent solar radiation, 76 to 90 were placed under 49 percent solar radiation, 91 to 105 were placed under 70. percent solar radiation, and 106 to 120 were left under full incident solar radiation. For a diagram of the design see Figure 1, Appendix. The soil in each pot was watered to 26 percent which corresponded to field capacity as determined by 1/3-atmosphere of tension. Daily water losses were determined by weighing each pot to the nearest gram from the time the soil was brought to field capacity until the seedlings were permanently wilted. Analysis. This required about 30 days. Daily water losses were not related to leaf area but were left on a per pot basis due to the difficulty of determining the actual internal leaf area involved in transpiration and to aflow a direct comparison of transpiration, evapotranspiration, and evaporation (Decker, 1955). Wilting Point Study t'hen the plants were permanently wilted at the termination of the soil moisture depletion study the moisture percentages in each pot were determined from the total pot weight. The wilting behavior of the Aleppo pine seedlings was noted on the experimental plants by a change in hue and a dessicated and curled appearance of the juvenile needles and from the daily water loss values. These served as guides to determine when permanent wilting had occurred. The purpose of this work was to determine the soil moisture percentage when the tree seedlings reached permanent wilting over a range 16 of incident solar radiation levels. The results may help explain failures in tree reproduction under restricted light conditions. Analysis. Evaluation of this part of the experiment was supplenien- ted by using analysis of variance. Soil Moisture and Solar Radiation Study The second major section of this study of the relations among soil moisture, solar radiation, and water loss involved establishing daily water loss averages under different pot treatments, soil moisture, and solar radiation combinations. The results may add some information on the processes involved in soil moisture loss. Similar two-year-old seedlings of Aleppo pine (Pinus halepensis Mill.) were used for this work. Transpiration studies were conducted by planting a tree in a plastic pot and covering the soil surface with polyethylene. Evapotranspiration was studied by using a tree seedling with an exposed 8oil surface contained in a plastic pot. Plastic pots containing soil only were used to study evaporation. Soil. Four groups of 15 plastic pots were filled with a forest soil from the Bear Wallow area of the Santa Catalina Mountains of Arizona.. The soil used in this part of the study was collected under a mixed stand of ponderosa pine (Pinus ponderosa Laws.) and douglas fir tadio1ia (Foir) ]3ritt.). mesh to remove rocks. (Pseudotsuga It was also screened through a quarter-inch Moisture tension analysis of this soil at 1/3- atmosphere revealed a "field capacity" moisture content of 22 percent. A similar test at 15-atmospheres of tension showed the permanent wilting point to be at nine percent soil moisture. Other soil moisture 17 values were determined at Appendix). 3.4- and 5-atmospheres of tension (Table 4, Iechanical analysis of the screened soil showed 50 percent sand, 46 percent silt, and 4 percent clay (Table 5, Appendix). The oven- dry weight of the soil in each pot ranged from 4000 to 5000 grams. Climate. This section of the experiment was run concurrently. with the other sections so climatic conditions were the same (Table 3, Appendix). Treatment. Each of three treatments in this part of the study was replicated five times. Evapotranspiration was studied by planting seedlings of JLleppo pine in five randomly selected pots out of each group of 15. Five other pots received a treatment to study transpiration con- sisting of planting a tree seedling in the pot and covering the soil surface with a 4 mil neutral polyethylene film and sealing the polyethy- lene to the seedling stem with grafting wax to minimize water loss by evaporation. The remaining five pots of each group were left as bare soil surfaces to study evaporation. The same pot numbering scheme was again adopted as was previously employed. The first numbered five pots of each group of 15 contained a tree seedling with the soil covered, the second numbered five pots held a tree seedling, and the remaining numbered pots held soil only. The pots used in this experiment contained fiberglass bottan liners and watering tubes identical with those described earlier. Tree seedlings were transplanted to the plastic pots on February 2 and February 3, 1961 and allowed an establishment period under field capacity soil moisture conditions until May 15, 1961. On May 15, 1961 these post were also transferred to The Plant Materials Center and placed 18 on the bench that was previously described. At the start of the measurement period each group of 15 pots was brought to a selected soil moisture level (22 percent, 18 percent, 14 percent, or 10 percent) as indicated by pot weight and placed under full solar radiation conditions in a completely randomized design. 1 to 15 were watered to 22 percent soil moisture, 16 to to 18 percent soil moisture, moisture, and 46 Pot ninnbers 30 were watered 31 to 45 were watered to 14 percent soil to 60 were watered to 10 percent soil moisture. sign of the experiment is shown in Figure 2, Appendix. combination was continued for 16 days. The de- This measurement Pots were weighed to the nearest grain daily and were rewet to the selected soil moisture level each day by adding water to the pots as described hereafter. After each pot was weighed and the proper amount of water measured into a graduated cylinder the water was added to the pot by thirds. One third was placed into the tube leading 6-inches below the soil surface, one third was placed into the tube leading 3-inches below the surface and the remainder was added evenly to the surface of the soil. This method was used to insure that the soil mass in each pot was watered equally throughout the soil depth. After the full solar radiation measurements had been completed various screens were erected over groups of pots to allow various incident solar radiation (70 percent, 49 percent, and 6 percent of the total) levels and measurements including watering were continued in the same manner and for the same length of time as previously described. At the end of the experiment measurements had been recorded for all soil moisture and solar radiation combinations with the exception of the groups of pots having 19 10 percent soil moisture and an incident solar radiation level of 70 percent and 49 percent of total solar radiation. The experimental seedlings did not survive at this low soil moisture level. Tree growth during the period was not measured but appeared to be negligible. Analysis. Analysis of this section of the experiment was supple- mented by using analysis of variance of unweighted means. RESULTS Soil Moisture Depletion Study Transpiration. To obtain a measure of the rate at thich tran- spiration was depleting soil moisture each pot was weighed daily. correction was made for the green weight of the pine seedling. A The average weights of soil moisture lost for five replicates during the first eight days of the wilting period, during the remaining II to 24 days before the tree seedlings were pernianentr wiJted, and during the entire period are presented in Table 1. Moisture losses in relation to soil moisture are displayed graphically in Figure 1. 1Jhen solar radia- tion treatnents are listed according to the amounts of water lost during the first eight days, from the largest to the smallest, they show the following order: 100 percent, 70 percent, 49 percent and 6 percent. The results showed that transpiration decreased as soil moisture decreased under all levels of incident solar radiation. However, after the first eight days or when the soil moisture had reached approdmate- ly 22 percent the decline was fairly constant regardless of the solar radiation level. Table 1. Mean water losses by transpiration for different levels of incident solar radiation Solar Radiatio cal./cm. / percent day 100 70 49 6 775 542 30 47 First grams 240 22 132 109 20 days Remaining Period grams days 145 195 194 11 11 24 24 24 Total grams 35 423 326 290 50 100 % SOLAR RADIATION . 0__ 70 % 'I 49 0/ - _O 6% 40 (I) S 4 Q I 30 >- 4 0 7 Lii / a- U) U) 0 / 4/ -J /',+ /-/.-' + w I.4 - _+_ 12 24 20 16 SOIL MOISTURE Figure 1. -+ + 0 0 a 28 PERCENT Mean daily water losses in grams by transpiration for different levels of soil moisture and solar radiation. 21 22 Evapotranspjratjon. The same procedures were used in measuring evapotranspiration as described for transpiration. The average weights of soil moisture lost during the first eight days of the wilting period, during the remaining 17 to 20 days before the seedlings were permanently wilted, and during the entire period are presented in Thble 2. lJhen the solar radiation treatments are listed accord- ing to the amounts of water used during the first eight days, frcn greatest to smallest, they fall in the order 100 percent, 49 percent, 70 percent and 6 percent. After the first eight days of the wilting period the soil moisture losses were approdniate1y the same regardless of incident solar radiation. Therefore, after the soil moisture had dropped to 19 percent, solar radiation did not affect water loss. Moisture losses in relation to soil moisture percentage are displayed in Figure 2. The curves did not follow the same pattern as did the tabulated water loss values. This effect was due to high water losses during the first two days under 70 percent solar radiation. Water loss decreased rapidly during the next The curves showed that water loss decreased both with decreas- six days. ing soil moisture and decreasing solar radiation during a period when no water was added. Table 2.Mean water losses by evapotranspiration for different levels of incident solar radiation. Solar Radiation cal./cm.2/ day perceift 100 70 49 3go 6 47 775 542 First grains 448 333 346 274 days Remaining Period grams days 225 229 234 264 18 17 17 20 Total grams 673 562 580 538 100 % SOLAR RADIATION ' 70% I 00- I, 49 0/ / 6 ------ " " + + 80U) 4 a: 0' --- o 7 - -+ 0 0 12 SOIL Figure 2. 20 16 MOISTURE 24 28 PERCENT Mean daily water losses in grams by evapotranspiration for different levels of soil moisture and solar radiation. 23 24 Evaporation. With the exception of correcting water loss values for the weight of a tree seedling the same procedures were used as described for transpiration. The average weights of soil moisture lost during the first eight days of the moisture depletion period, the remaining 24 days, and for the entire period are presented in Table 3. Water loss under different solar radiation treatments,when listed from the largest to the smallest over the first eight days follows an order of 70 percent, 100 percent, 49 percent, and 6 percent incident solar radiation. followed for total water loss. The same order is The differences are probably not large enough to be significant. Moisture loss in relation to soil moisture percentage is shown graphically in Figure 3. Moisture losses by evaporation decreased with decreasing soil moisture but were not related to incident solar radiation. The curves and data show the rate at which soil moisture was depleted by evaporation during the period when no water was added. Table 3.Mean water losses by evaporation for different levels of solar radiation. Solar Radiatio cal./cm. / percent day First 8 days grains Remaining Period grams days Total grams 100 775 426 239 24 665 70 542 463 254 24 717 49 380 382 206 24 588 6 47 377 194 24 571 100 0/, SOLAR RADIATION . 70 0/, o 160- 49 % rj 01 F, + + / ; 'I 80- 0 (I) Cl) 0 -J 0 U 40 F- 4 0 ' D_ B-+'j. - - q. - O 0 I2 SOIL Figure 3. 20 16 24 28 MOISTURE - PERCENT Mean daily water losses in grams by evaporation for different levels of soil moisture and solar radiation. 25 26 Comparison of Transpiration, Evapotranspjratjon, and Evaporation. In aU solar radiation treatments evaporation exceeded either transpiration or evapotranspiration at high soil moisture percentages. Evapora- tion remained the greatest cause of water loss until the soil moisture dropped to l percent where evapotranspiration became the greatest. Al- though evaporation losses were the greatest at all levels of soil moisture under 70 percent solar radiation this was probably not significant due to experimental error. l The high evaporation losses at soil moistures above percent were probably due to direct absorption of radiation and heat and to the high temperatures and low relative hiunidities under which the experiment was conducted. The temperature effects could be more pro- nounced when small pots containing only a small volume of soil were used such as in this experiment. In all cases evaporative losses decreased sharply with decreases in available soil moisture indicating that the surface soil moisture was governing water loss (Wiegand and Taylor, 1961). Evapotranspiration losses were intermediate between evaporation and transpiration losses at soil moisture percentages above l percent. This may have been clue to a change in the microenvironment at the soil surface in the pot brought about by the addition of a tree seedling. At soil moisture percentages below l percent evapotranspiration became the greatest method of water loss except at the 70 percent solar radiation level. Evapotranspiration did not decrease as sharply as evaporation in- dicating that moisture removal from the soil by transpiration became more important as evaporation was restricted by a dry surface layer. 27 Transpirationa]. water loss was the lowest of the three and in all cases did decrease as soil moisture decreased. Low transpirational water losses under low levels of incident solar radiation may have been due partly to depletion of food supplies within the plant and eventual starvation (Abd el Rahnian, Kuiper and Bierhuizen, 1960) but mainly to less available energy. Transpiration, evapotranspiration and evaporation under different levels of incident solar radiation are compared in Figures 4, 5, 6, and 7. A comparison of transpiration, evapotranspiration and evaporation averaged over all incident solar radiation levels is presented in Figure . In this case evaporation was the greatest source of water loss at high soil moisture levels but assumes a position intermediate between transpiration and evapotranspiration after the eighth day of measurement. The eighth day of measurement corresponds to about l percent soil moisture. The effect of solar radiation appeared to be greatest on transpiration and least on evaporation. This may have been caused by direct absorption of solar energy by tree seedlings which were highly sensitive to this form of energy. Evaporation may have been controlled by air temperature to such an extent that the effects of solar radiation were obscured. Evapotranspiration would show both effects. 160EVAPORATION TRANSPIRATION EVAPOTRANSPIRATION ---- - 4 a w 80 a- U) Cl) 0 -J ,, 40- / /0/ / / / / / / / w I4 0 12 20 16 SOIL 24 28 MOISTURE - PERCENT Figure 4. Mean daily water losses in grams by transpiration, evapotranspiration, and evaporation at different levels of soil moisture under 100 percent solar radiation. 2 60EVAPORATION TRANSPIRATION E VA POTR ANS PIR AT ION C,, 4 20 >- 4 a o U / / 80- /0 a- / U) C,, 0 -J U 40- 0,' 0/ oo___ 4 0-9-- 9-o___ - 0. -S 0 0 0 2 12 SOIL Figure 5. 20 16 24 28 MOISTURE - PERCENT Mean daily water losses in grains by transpiration, and evaporation at different levels of soil moisture under 70 percent solar radiation. 29 EVA POR ATION TRANSPIRATION EVAPOTRANSPIRAT ION c 80 p a(I) U) 0 -J 40w o__ o-cr 0 12 20 16 SOIL o 24 28 MOISTURE - PERCENT Figure 6. Mean daily water losses in grains by transpiration, evapotranspiration, and evaporation at different levels of soil moisture under 49 percent solar radiation. 30 160- EVAPORATION TRANSPIRATION - - - EVA POT RAN S P1 RAT I ON C,) $20C, >- 4 80w 0. U) U) 0 -J 0 40- - -0 w I- 4 0 0 - .----.- -.0 $2 20 $6 24 -, 28 SOIL MOISTURE - PERCENT Figure 7. Mean daily water losses in grams by transpiration, evapotranspiration, and evaporation at different levels of soil moisture uxxler 6 percent solar radiation. 31 160- EVAPORATION TRANSPIRATION EVAPOTRANSPIRATION eO3Cl) U) 0 -J c 40- LU I4 0 0 I I 8 16 TIME Figure . 24 32 DAYS Mean daily water losses by transpiration, evapotranspiration, and evaporation during a soil moisture depletion study. 32 33 Wilting Point Study The percentages of soil moisture at permanent wilting for the indicator plants used in the moisture depletion study are presented in Table 4. Overall statistical analysis of these data (Table 5) showed that differences, significant at the five percent level, existed between transpiration and evapotranspiration with evapotranspiration reducing the soil moisture percentage to a lower level in all cases. Solar radiation levels were also significant. Soil moisture was reduced to a lower level by evapotranspiration than by transpiration alone. These data are mean values for five ex- periniental plants each having the same treatment. With the exception of the 49 percent solar radiation treatment with polyethylene covered soil all means decrease, aligned in an order of increasing solar radiation. Comparisons among individual wilting percentages showed that in both transpiration and evapotranspiration the 6 percent and 49 percent incident solar radiation levels gave permanent wilting points significantly higher than the wilting points under greater levels of solar radiation. This may have been due to restricted root growth, water absorption, and photosynthate production. Table 4. Soil moisture percentages at permanent wilting of tree seedunder two sources of water loss and four levels of solar radiation. lings Source 100 Solar Radiation (Percent) 70 6 49 Total Average Transpiration 15.78 16.15 20.39 19.25 71.57 17.89 Evapotranspiration U.96 12.69 14.81 15.31 54.77 13.69 34 Table 5. Analysis of variance of wilting point data. Source Degrees of Freedom Sum of Mean Squares Square F Pot treatment 1 173.98 173.98 35.72* Solar radiation 3 106.68 36.23 7.44* Interaction 3 12 81 4.24 32 155.91 4.87 39 451.38 Error Total * 0.87 Significant at the five percent level. Soil Moisture and Solar Radiation Study Measurements were made of the rates at which soil moisture was depleted by transpiration, evapotranspiration, and evaporation under four different levels of solar radiation and four soil moisture percentages. Each value reported is an average of from 29 to 75 indisrLdual observations. Soil moisture exerted a definite influence on daily transpiration- al losses but the role of solar radiation was obscure although greatest loss appeared to occur at full solar radiation. As soil moisture became limiting transpiration decreased in the same manner as in the moisture depletion study. The decrease was present under all levels of incident solar radiation. The actual incident solar radiation probably did not greatly influence the average daily water loss due to the high temperatures and low relative humidities of the environment under which the determinations were made. Data for mean daily water losses by transpiration under different 35 levels of soil moisture and solar radiation are presented in Table 6 and 9. Figure 6. Table Average daily rates of water loss in grams by transpiration under different levels of solar radiation and soil moisture. Solar Radiation Soil Moisture (Percent) 22 18 10 14 (Percent) 100 70 49 6 Total Average 52 37 40 41 34 27 36 24 12 18 19 5 13 6 170 121 30 62 16 22 42 6* 5* Total 103 88 100 84 Average 26 22 25 21 6 * Measurements not taken, values added for computation assuming that actual observations would be intermediate to other observed values at 10 percent moisture. Evapotranspirational water losses foflowed the same pattern as transpirational losses. Soil moisture levels exerted the greatest in- fluence on daily water loss. The results showed a decreasing rate of water loss as soil moisture was depleted under constant solar radiation. This was the same result pattern as was obtained in the soil moisture depletion study. A solar radiation effect was present between fufl solar radiation and the reduced levels of solar radiation at all soil moisture levels. The 100 percent solar radiation level allowed a much greater rate of daily water loss than did the lower levels of solar radiation. This may have been partiJ]y due to the experimental procedure of taking all the observations under 100 percent solar radiation during the same period of time. The observations of evapotranspiration under 100% SOLAR RADIATION 50- 70% 49 % 6% 40U) 2 4 30- 20 0 0 5 10 SOIL 1igure 9. 15 20 MOISTURE - PERCENT Mean daily water losses in grams by transpiration under four different levels of soil moisture and solar radiation. 36 37 the other three levels of solar radiation were taken over the next two months. Thus a variation in climatic conditions over the measurement period may have altered the results. At low solar radiation levels the effect may have been obscured by high temperatures and low relative humidities. Daily evapotranspiration losses under different levels of soil moisture and solar radiation are presented in Table 7 and Figure 10. Table 7. Average daily rates of water loss in grams by evapotranspiration under different levels of solar radiation and soil moisture. Solar Radiation (Percent) 100 70 49 6 Total Average Soil Moisture (Percent) 22 18 14 10 209 139 139 343 177 83 26 93 87 73 39 30 36 23* 20* 630 158 430 108 188 47 86 22 17 Total Average 495 294 276 269 124 74 69 67 * Measurements not taken, values added for computation assuming that actual observations would be intermediate to other observed values at 10 percent moisture. Daily water losses by evaporation were related to soil moisture and solar radiation as in the soil moisture depletion study. Evaporative losses were definitely influenced by decreasing soil moisture but followed no definite pattern with regard to incident solar radiation. soil moisture became limiting evaporation was retarded. As A dry soil sur- face was not present in this study because water losses were replaced daily by watering. The reduction in evaporative loss was due to an 100 0/, SOLAR RADIATION 70% 200 - 49% 6% 160p / 120- 80 U) U) 0 -J c LU 40- I- 0 0 5 10 SOIL 15 20 MOISTURE - PERCENT Figure 10. Mean daily water losses in grams by evapotranspiration under four different levels of soil moisture and solar radiation. 3 39 increased resistance to evaporation as soil moisture decreased (Table 8 and Figure II). Table 8. Average daily rates of water loss in grains by evaporation under different levels of solar radiation and soil moisture. Solar Radiation Soil Moisture (Percent) (Percent) 22 100 70 49 18 14 10 99 6 68 54 51 22 21 34 4 388 218 161 20 97 55 40 5 175 45 62 106 6 Total Average 30 5* 5* Total Average 334 131 157 165 84 33 39 41 * Measurements not taken, values added for computation assuming that actual observations would be intermediate to other observed values at 10 percent soil moisture. Analysis of variance of unweighted means of all soil moisture loss data are presented in Table 9. The pot within treatment sum of squares was divided by the harmonic mean to obtain a testing term. The F test showed that the interaction was non-significant but that pot treatments and soil moisture-solar radiation levels were significant at the five percent level. A compariaon of the data showed that water losses by transpiration, evapotranspiration, and evaporation were significantly different. The order exhibited by the water losses, from largest to smallest, was evapotranspiration, evaporation, and transpiration. In this section the magnitudes of water loss by evapotranspiration and evaporation were reversed from those found in the moisture depletion study ; evapotranspiration was the greater in this case. A comparison too o, SOLAR RADIATION 70Db 49% 6% (,)160 4 I >- 1' H 120 4 C) o80 Ui C/) C/) / 0 J / 40- 0 0 -ft ;/..7 I I I 5 10 15 SOIL MOISTURE 20 PERCENT Figure 11. Mean daily water losses in grams by evaporation under four different levels of soil moisture and solar radiation. 40 41 of the mean water losses under the soil moisture-solar radiation treatments showed that only two significant groups of water loss values existed. The water losses attributed to transpiration, evapotranspiration, and evaporation under all incident solar radiation levels at 22 percent soil moisture and under 100 percent and 49 percent incident solar radiation at 18 percent soil moisture were significantly greater than the rest of High soil moisture and incident solar radiation will increase the means. water loss. Comparisons of individual means were not performed since the interaction was not significant. Table 9. Analysis of variance of unweighted means of moisture loss data frcn soil moisture and solar radiation study. Degrees of Freedom Source Sum of Squares Mean Square F 2 29,149 14,575 7.36* Soil moisture-Solar radiation treatment 13 70,140 5,375 2.72* Interaction 26 22,024 847 151 1,291,857 Pot treatment Pot within treatment 0.42 1,979# * Significant at the five percent level. # Pot within treatment sum of squares was divided by the harmonic mean of 4.322 to obtain a testing term. A comparison of the results of this experiment with the results from the soil moisture depletion study showed one difference. In this study evapotraflsPJJ'at10fll water loss exceeded the water loss by evapora- tion alone. This reversal of results was attributed to the experimental 42 procedure of watering each pot to a selected soil moisture level each day. The constant wetting of the soil surface allowed some water to remain in a more available position for water loss by evapotranspiration as evaporation could take place directly from the soil surface as the tree seedlings removed water from the deeper portions of the soil. Under the conditions imposed by the depletion study a dry soil surface was allowed to restrict evaporative losses and may have depressed transpiration by drying out the soil around the seedling roots in the top portion of the soil. rational water losses were the lowest in both studies. Transpi- DISCUSSION Water Loss Under Different Soil Moisture and Solar Radiation Levels The results of the soil moisture depletion study and the soil moisture-solar radiation study indicate that soil moisture exerts a definite influence on water loss but that the influence of solar radiation is obscure. Loss by transpiration decreased with decreasing soil moisture over the entire depletion curve. This would indicate that the rates of transpiration are influenced by the availability of soil moisture to the plant roots. Evapotranspiration and evaporation were also related to soil moisture. While soil moisture was abundant and soil moisture tension was increasing slowly, the rate of water loss was strongly influenced by moisture levels. At lower moisture levels water losses did not decrease as rapidly but a downward trend was still present. The failure of solar radiation to be clearly a major factor in controlling the rate of water loss may be due to the uncontrolled temperature and relative humidity conditions in the experiment. A linear relationship has been observed by some workers between water loss and incident light intensity (Kuiper and Bierhuizen, l95). How- ever, these workers used controlled temperature and relative huii dity conditions. In this experiment the combination of high soil moisture and high solar radiation levels did produce significantly higher mean 43 daily water losses. A comparison of the rates of water loss by transpiration, evapotranspiration, and evaporation revealed that transpiration depleted soil moisture most slowly. In all cases it was significantly slower than evapotranspiration or evaporation. The apparent reversal of the rates of evaporation and evapotranspiration in the two studies dealing with the average daily water loss has been attributed to the experimental procedure. It is probably safe to assume that evaporation from a bare soil surface would be greater than evapotranspiration from a similar soil with cover when both soils are at or above field capacity in the surface layer (Rowe, l94). Direct evaporation from a bare soil surface would not require an expenditure of energy to move water through the plant. Under field conditions in arid areas where the soil surface is rapidly dried evapotranspiration would soon assume a leading role in soil moisture depletion. This would be especially true on deep soils during extended periods without precipitation. Wilting Point The results of the wilting point study showed that water loss by evapotranspiration was significantly greater than water loss by transpiration alone. The differences were probably due to evaporation from the upper layers of the soil decreasing the average soil moisture percentage in the pot below the wilting point of the seedlings. The results also indicate that statistically significant differenôes in wilting percentages may be obtained under different levels of solar radiation. This effect may have been influenced by several outside 45 factors. One is that plants under low solar radiation and high temper- atures may not produce enough photosynthate to sustain the plant, root growth may be restricted, and water absorption lowered. The eventual depletion of food supplies within the plant may hasten permanent wilting. However, with only one exception, higher incident solar radiation levels did induce permanent wilting at lower soil moisture percentages than did low solar radiation levels. These differences are reasonable even though errors may have been made in judging permanent wilting. The one exception occurs in the transpiration experiment where trees under six percent solar radiation wilted at a lower soil moisture value than those under 49 percent. It is possible to conclude from this study that tree seedlings grown under high incident solar radiation levels will survive under lower soil moisture conditions than tree seedlings grown under restricted solar radiation. Moisture tolerances within a species do vary according to the amount of solar radiation received. silviculture. This has important implications in SU14MARY A study was made of the rates of water loss by transpiration, evapotranspiration, arid evaporation from soil in 10-inch plastic pots. Transpiration and evapotranspiration were studied using Aleppo pine (Pinus halepensjs Mill.) in sealed and non-sealed pots. The effects of different levels of soil moisture and solar radiation were measured. In a soil moisture depletion study the water loss from the tree seedlings wider covered and exposed soil conditions was related to incident solar radiation and soil moisture. Moisture loss decreased both with decreasing soil moisture and decreasing solar radiation. Evaporation from a bare soil surface was not clearly related to incident solar radiation. Evaporation was related to soil moisture and decreased with decreasing soil moisture. when the soil was at or near field capacity evaporation was the greatest source of water loss. As soil moisture became less available evapotranspiration assumed the leading role in rate of water loss. The soil moisture at the permanent wilting points of the tree seedlings were significantly higher under covered soil conditions than under exposed soil conditions. Also, statistically significant differ- ences existed supporting the conclusion that tree seedlings which received greater incident solar radiation wilted at lower soil moisture percentages. 46 47 A second study related transpiration, evapotranspiration, and evaporation to four fixed levels of soil moisture and solar radiation. Statistically significant differences exLsted which supported the conelusion that greater soil moisture allowed greater rates of water loss. The effects of incident solar radiation were obscure but an effect may have been present between full solar radiation and reduced levels of solar radiation. BIBLI GRAPHY Bierhuizen, J. F. 1958. Some observations on the relation between transpiration and soil moisture. Netherlands Jour. Agric. Sc!. 6(2):94-98. Reprinted in Tech. Bull. No. 4 (1959). Institute for Land and Water Management Research, Wageningen, The Netherlands. Closs, R. L. 1958. Transpiration from plants with a limited water supply. Climatology and Microclimatology Canberra Symposium. UNESCO. Pp. 168-171. Decker, J. P. 1955. The uncommon denominator in photosynthesis as related to tolerance. For. Sci. 1:88-89. Decker, J. P. and B. F. Wetzel 1957. A method for measuring transpiration of intact plants under controfled light, humidity and temperature. For. Sc!. 3: 350-354. Gardner, W. R. and N. Fireman 1958. Laboratory studies of evaporation from soil columns in the presence of a water table. Soil Sc!. 85:2i-249. Gingrich, J. R. and N. B. Russefl 1957. A comparison of effects of soil moisture tension and osmotic stress on root growth. Soil Sc!. 84:185-194. Hendrickson, A. H. 1942. Determinations of the losses of moisture by evaporation from soils in a watershed area. Trans. Amer. Geophys. Union 23: 471-477. Hendrickson, A. H. and F. J. Veihmeyer 1955. Daily use of water az depth of rooting of almond trees. Proc. Amer. Soc. Hor. Sc!. 65:133-138. Harrold, L. J., D. B. Peters, L. B. Dreibelbis and J. L. NcGuinness 1959. Transpiration evaluation of corn grown on a plastic covered lysimeter. Soil Sc!. Soc. Amer. Proc. 23:174-178. Holt, R. F. and C. A. Van Doren 1960. Water utilization by field corn in western Minnesota. Agron. Jour. 53:43-45. 48 49 Janes, B. E. 1954. Absorption and loss of water by tomato leaves in a saturated atmosphere. Soil Sci. 7:l9-197. Koshi, P. T. 1959. Soil moisture trends under varying densitites of oak overstory. Southern For. Expt. Sta. 0cc. Paper 167. New Orleans, La. Kramer, P. J. 1937. The relation between rate of transpiration and rate of absorption of water in plants. .Ainer. Jour. Dot. 24:10-15. Kramer, P. J. and T. T. Kozlowski 1960. Physiology of trees. 642 pp. New York, Toronto, London. McGraw-Hill Book Company, Inc. Ku.iper, P. J. C. and J. F. Bierhuizen 195g. The effect of some environmental factors on the transpiration of plants under controlled conditions. Mededelingen Van De Landbouwhogeschool Te Wageningen, Nederland Reprinted in Tech. Bull. No. 4 (1959). Institute for Land and Water Management Research, Wageningen, The Netherlands. 5(ll):l-16. Lane, R. B. and A. L. McComb 194g. Wilting and soil moisture depletion by tree seedlings and grass. Jour. For. 46:344-349. Lull, H. W. and J. H. .Axley Forest soil-moisture relations in the coastal plain sands of For. Sd. 4:2-19. southern New Jersey. l95. Metz, L. J. and J. E. Douglass 1959. Soil moisture depletion under several Piedmont cover types. Tech. Bull. 1207. USDA. Washington, D. C. Parker, J. 1957. The cut-leaf method and estimations of diurnal trends in transpiration from different heights and sides of an oak and a pine. Dot. Gaz. 119 :93-101. Peters, P. B. and M. B. Russell 1959. Relative water losses by evaporation and transpiration in field corn. Soil Sci. Soc. Amer. Proc. 23:170-173. Raliman, A. A. Abd el, P. J. C. Kuiper and J. F. Bierhuizen 1959. Prelirn.inary observations on the effect of light intensity and photoperiod on transpiration and growth of young tomato plants under controlled conditions. Mededelingen Van De Landbouwhogeschool Te Wageningen, Nederland 59(11):1-]2; Reprinted in Tech. Bull. No. 16 (1960). Institute for Land and Water Management Research, Wageningen, The Netherlands. 50 Richards, L. A. and C. H. Wadleigh 1952. Water and plant growth. (In Soil PhysLca1 Conditions and Plant Growth. Pp. 73-251. Academic Press, Inc., New York). Rothacker, J. S. 1949. Sumnary of the exploratory phase of the Clear Creek transpiration study. TVA Div. For. Relations. Norris, Tenn. Rowe, P. B. 1948. Influence of woodland chaparral on water and soil in central California. Calif. Dept. of Natural Resources, Div. of Forestry. 70 pp. Slatyer, R. 0. 1957. The influence of progressive increases in total soil moisture stress on transpiration, growth and internal water relations of plants. Australia Jour. Biol. Sci. 10:320-336. Stanhill, G. 1957. The effect of differences in soil moisture status on plant growth; a review and evaluation. Soil Sci. 84:205-214. Veihmeyer, F. J. and A. H. Hendrickson 1955. Does transpiration decrease as soil moisture decreases? Trans. Amer. Geophys. Union 36:425-L49. Wiegand, C. L. and S. A. Taylor 1961. Evaporative drying of porous media. Agric. Expt. Sta. Spec. Report 15. Utah State University. Zahner, R. 1955. Soil water depletion by pine and hardwood stands during a dry season. For. Sci. 1:258-264. Zahner, R. 1958. Hardwood understory depletes soil water in pine stands. For. Sci. 4:178-184. A P P E N D. I X 51 52 Table 1. Soil moisture retention at different atmospheric pressures for soil used in moisture depletion study. Tension atmospheres Table 2. Soil Moisture percent 15 00 9.30 5.00 13.35 3.40 14. 8 0.33 26.31 Particle size distribution of soil used in soil moisture depletion study. Particle size Cuposition sand 4 silt 47 clay 5 percent Total 100 74 75 66 6i 67 70 72 71 Figure 1. 65 62 69 98 99 104 105 94 93 91 103 101 102 100 96 92 97 95 81 77 83 89 90 78 79 87 88 85 76 80 82 84 86 106 111 110 109 119 107 117 108 120 112 113 116 115 118 114 Diagram of design of soil moisture depletion study shodng the position of each pot by code number. 64 63 73 6 54 Table 3. Data on temperature, relative humidity, and solar radiation from May 15 to August 22, 1961. Temperature Date 1961 May 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Max. Mm. 102 105 106 109 109 102 96 104 106 106 102 108 109 106 102 110 106 57 62 57 59 60 56 57 57 58 107 109 100 104 10]. 110 56 56 60 64 61 60 63 57 59 61 Relative Humidity (percent) Mm. Max. 32 48 46 40 44 49 52 45 42 48 46 46 43 45 6 5 8 9 7 7 9 15 12 9 9 12 12 12 11 15 50 46 13 40 II 44 47 45 10 15 14 15 13 10 12 13 16 18 20 15 12 14 25 23 21 20 18 22 14 Solar Raiation cal./cm./day 802 768 723 813 770 825 678 808 812 815 720 681 813 761 625 812 804 June 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 no 110 110 no 109 106 no no 110 no no 108 nO no no 57 62 57 62 61 65 67 68 69 69 70 70 81 80 72 78 80 81 80 56 42 40 46 48 47 47 49 49 39 46 49 74 63 59 48 68 821 828 815 810 807 712 816 819 798 774 782 714 777 788 781 747 626 754 772 771 756 Continued 55 Continued Table 3. Temperature Date Solar Rad2iation Mm. 22 23 24 25 26 27 28 29 110 110 110 110 109 110 105 106 108 70 58 84 83 81 77 82 81 77 77 48 22 18 56 54 20 20 63 22 51 61 75 76 21 29 31 20 751 754 754 637 760 744 727 683 732 1 109 106 102 104 108 107 110 110 110 110 110 110 109 110 110 109 110 110 110 110 110 110 110 107 107 109 106 100 79 76 67 64 26 27 26 14 15 15 20 22 22 17 17 21 26 21 22 26 25 24 20 22 23 33 24 27 20 16 22 26 33 33 28 679 481 578 780 772 745 746 775 775 747 740 755 730 753 742 726 751 741 741 726 715 449 719 641 748 667 636 570 498 570 690 30 July (percent) Max. 1961 June Relative Hunddity °F. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 97 99 102 71 73 83 77 80 80 82 85 85 82 78 80 79 81 80 78 80 75 72 75 75 76 78 74 76 76 80 76 Max. 97 93 54 38 53 65 6 81 58 68 51 50 72 60 84 69 69 70 76 99 95 92 96 95 88 97 74 80 87 92 Mm. cal./cm. /day Continued 56 Continued Table 3. Temperature Date °F. 1961 Max. Mm. 106 102 109 96 106 109 110 110 105 104 107 110 102 101 96 98 103 105 110 106 109 75 74 75 72 77 78 78 76 79 76 76 76 80 74 74 Relative Humidity (percent) Max. Mm. Solar Radiation cal./crn. /day August 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 73 74 76 76 84 80 82 83 100 100 75 85 73 90 100 94 70 99 70 100 100 97 100 100 100 49 80 20 26 19 25 19 15 18 20 22 27 23 20 22 24 42 24 21 24 20 20 22 544 576 605 519 729 694 706 706 690 480 571 665 573 573 573 573 621 589 651 672 672 57 Table 4. Table 5. Soil moisture retention at different atmospheric pressures in soil moisture and solar radiation study. Tension atmospheres Soil 1Ioistue percent 15.00 9.20 5.00 11.47 3.40 13.69 0.33 22 00 Particle size distribution of soil used in soil moisture and solar radiation study. Particle size Composition percent sand 50 silt 46 clay 4 Total 5g 20 30 39 2 9 6 26 19 34 32 37 43 44 47 23 56 42 25 49 40 22 10 5 24 15 27 4 33 12 51 52 13 53 54 57 46 7 31 41 17 U 14 35 55 45 59 60 29 36 21 58 38 16 18 50 3 1 Figure 2. 2 4a Diagram of design of soil moisture and solar radiation study showing the position of each pot by code number.
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