CIRCULAR DECEMBER 146 1963 PRECISION IRRIGATION with SOLAR ENERGY Agricultural Experiment Station A UB U RN UN IVE RS IT Y E. V. Smith, Director Auburn, Alabama CONTENTS Page SOLAR ENERGY AND SOIL MOISTURE RELATION OF EVAPOTRANSPIRATION AND SOLAR ENERGY GROWTH AND QUALITY OF CROPS INTERVAL BETWEEN IRRIGATIONS COMPLETELY AUTOMATIC IRRIGATION 4 4 5 6 8 8 9 10 FACTORS TO CONSIDER FOR PRACTICAL USE SUMMARY LITERATURE CITED FIRST PRINTING 2M, DECEMBER 1968 PRECISION IRRIGATION with SOLAR ENERGY TOK FURUTA, W. C. MARTIN, JR., and FRED PERRY Department of Horticulture IMELY APPLICATION of water is important to crop growth and production. However, applications made too frequently are as harmful as subjecting crops to drought. Experience will help determine when a crop needs water. Research on irrigation of various farm crops has resulted in the development of many methods for determining proper time and way of irrigating. Irrigation guides include direct measurement of soil moisture by inserting or placing various devices in the soil, formulas based on climatological data (2, 14, 15, 19, 21, 22), change in leaf color (1), water evaporation from porous surfaces (6, 11), various types of irrigation gauges (4, 18), and measurements of stomatal opening by infiltration of liquids (8). The term "evapotranspiration" is used to describe depletion of soil moisture. This is the summation of direct evaporation from the soil surface, plant utilization, and transpiration. Irrigation research in greenhouses (13, 16) has emphasized means of uniformly distributing water. From this has developed many usable and reliable systems, such as perforated plastic pipe or tubing, special nozzles, and various systems of using individual tubes for potted plants. Considerably less attention has been given the problem of determining when water should be applied, using some measurable standard rather than methods depending on the experience of the grower. Tensiometers are usable instruments for greenhouse T conditions (7, 10, 13, 16). Problems of proper placement, maintaining proper operations, and limited use with potted plants indicate the desirability of other measurements to determine irrigation needs of crops under greenhouse conditions. SOLAR ENERGY AND SOIL MOISTURE The amount of solar energy influences the amount of evapotranspiration. Formulas to estimate evapotranspiration include some measure of light. Direct measures of solar energy have been highly correlated with evapotranspiration (9, 12). In these studies expensive instruments measuring incoming or net radiation have been used. Because of practical limitations in using such expensive instruments, indirect measurements of solar energy by use of black and white Livingston atmometers have been suggested. Tukey, Fluck, and Marsh (20) developed a simple, inexpensive portable instrument that measures incoming radiation. Measurements of solar radiation with this light integrating totalizer was highly correlated with measurements made with the Epply pyrheliormeter. RELATION OF EVAPOTRANSPIRATION AND SOLAR ENERGY In experiments at the Auburn University Agricultural Experiment Station, the amount of evapotranspiration was highly correlated to the amount of incident solar energy as determined with an illumination totalizer. Highly significant correlations were found between plants grown in pots and those in benches, Table 1. Significant correlations were also found between evapotranspiration and evaporation of water from Livingston atmometers. TABLE 1. CORRELATION COEFFICIENTS OF EVAPOTRANSPIRATION AND MEASURES OF SOME ENVIRONMENTAL FACTORS Correlation coefficients Factors compared Potted plants Plants 1st period Evapotranspiration to: 2nd period + .98 + .98 in benches Solar energy Evaporation from white atmometer Difference in evaporation of black and white atmometer Solar energy + evaporation from a white atmometer [4] + .98 + .98 + .98 + .93 + .98 + .93 +4.96 +-.98 Evaporation of water from white atmometers is influenced by humidity. Light as well as humidity influence evaporation from a black atmometer. Results of experiments reported indicate the possibility of using either solar energy measurements, or the difference in evaporation between a black and white atmometer as a means of determining evapotranspiration from greenhouse soils and to determine the need for irrigation. The combined use of both measures did not increase precision. There was a problem of keeping the atmometers clean while in use. Dust and other particles tend to cling to the atmometers. In addition, algae growth occurred on its surface, much as it does on the surface of clay pots. Weekly washings were necessary to keep the surface clean. Because of the problem of keeping the atmometers clean and the possibility of completely automating watering operation, the use of illumination totalizers is preferred as a means of determining irrigation frequency. Instrumentation is simple and inexpensive. GROWTH AND QUALITY OF CROPS A number of crops was grown for studying the practical use of solar energy measurements to. determine irrigation needs. Crops for cut flowers included pompom and standard chrysanthemum, snapdragon, and aster. Crops for potting included chrysanthemum, rose, hydrangea, azalea, and Easter lily. Methods based on experience and tensiometers where applicable were used as comparisons. The results of these studies, Tables 2 and 3, show that applying water on the basis of solar energy measurements was practical. The quality of the products-cut flowers or potted plants was not TABLE 2. INFLUENCE OF LIGHT ACCUMULATION AT THREE LEVELS BETWEEN IRRIGATIONS ON GROWTH AND QUALITY OF POT PLANTS Crop Easter Lily (1 plant per 6-inch pot) Chrysanthemum (1 plant per 4-inch pot) Measure of growth and quality Flowers, av. no. Plant height, in. Length of flower, in. Flowers, av. no. ...........Plant height, in. Fresh weight of tops, g-.-..... [5] Check 4.4 22.7 6.0 3.0 Light accumulation 1X 2X 4X 4.7 22.7 6.0 3.4 7.0 15.5 4.0 18.9 5.5 3.0 6.6 11.6 4.2 15.8 4.2 2.5 5.5 6.5 6.6 14.0 TABLE 3. INFLUENCE OF IRRIGATION ON BASIS OF LIGHT ON GROWTH AND QUALITY OF STANDARD CHRYSANTHEMUM FLOWERS Item Method of determining irrgiation need Cheek . Solar energyv 32.9 8.2 21.8 36.6 Stem length-Average, in.-------------------9.8 Coef. of variation, pt.-------------------Weight of shoot-Average, oz.-3.2------3.2------33.2 19.6 Coef. of variation, pet.-----------------4.8 Salable flowers per sq. ft.------------------------7.0 Grade distribution' Extra fancy, pet.------60.0 Fancy, pet.-------------------------21.5 -No. 1, pet. ---------------------------2.0 Design, pet.--------------------------9.5 Culls, pet. ------------------------1Standards developed by Gaylord and Hoxie (7). 4.4 7.0 55.0 26.5 2.0 9.5 reduced and in some cases the quality was superior. Solar energy measurements provided a definite standard easily understood. INTERVAL BETWEEN IRRIGATIONS Experiments conducted at Auburn were also to determine the amount of light accumulation desirable between irrigations. The soil held 0.42 in. of available water at field capacity for each 6 inch layer. At the 2X interval, Table 4, weight or stem length was not influenced. However, flower size was reduced and consequently a of flowers was of lower grades. Noticeable wilting larger of foliage of large plants occurred when the 2X interval was used. -number 4. Plant size or stage energy as an indicator 'of water needs. Soon after potting chrysanthemum plants, the frequency of irrigation for the :2X light treatment was approximately the same TABLE INFLUENCE OF LIGHT ACCUMULATION BETWEEN QUALITY OF CHRYSANTHEMUMS IRRIGATIONS ON -of growth was, important in utilizing solar Light Distribution by grades' Av. Av. wt. Av. flower stem per aecumula- Extra Fancy No. tion between 1 Design length stem diameter irrigations fancy In. Oz. In. Pct. Pct. Pct. Pct. -35.9 2.4 0.6 34.5 11.7 37.5 1X Shasta -2.3 35.6 9.9 0.6 45.0 33.3 2X 5.0 3.0 0.8 30.6 35.0 50.0 0.8 1X Giant Betsy 4.6 2.9 30.3 69.1 0.0 24.1 0.0 2X Ross ' Standards developed by Gaylord and Hoxie (7). Variety [6] TABLE 5. NUMBER OF IRRIGATIONS FOR POTTED CHRYSANTHEMUMS WITH VARYING AMOUNTS OF LIGHT ACCUMULATION BETWEEN IRRIGATIONS Irrigation applications Date Check Light accumulation No. Jun e 21-30 .---------- ---------- -------------------July 1-10 ------------------------------- July 11-20 ---- ---- -------- ---- ---- ------------ ---July 21-31 - ----------------------------------- -- -- -- -- -- -- -Au g. 1-10 -- -- ----- ----- ----4 1X No. 9 5 6 10 7 7 8 10 5 2X No. 5 4 4 6 3 as the control plants, Table 5. However, after 4 weeks of growth, frequency of irrigation for the 1X treatment was more nearly that of the check. This development was expected as the consumptive use of water increases with plant growth. Type of pot and soil mixture also influenced the results. As irrigation frequency was reduced, that is more light between irrigations, the amount of plant growth decreased because of insufficient amounts of water. This decrease was less severe in a sand-peat-clay soil mixture than a sand-peat mixture. The sandpeat-clay mixture held more available water than the sand-peat mixture. Use of plastic pots also resulted in larger plants. The amount of evapotranspiration would be less, resulting in more moisture for plant growth in plastic pots that are not porous. plastic pots, soil mixture did not influence the results when water was applied frequently. But with clay pots, there was a definite increase in plant size with the sand-peat-clay mixture. With COMPLETELY AUTOMATIC IRRIGATION Systems designed for automatic surface irrigation have been proposed using tensiometers (17) or moisture blocks (5). To study TABLE 6. INFLUENCE OF POT TYPE AND SOIL MIXTURE ON GROWTH OF CHRYSANTHEMUMS IRRIGATED WITH VARYING AMOUNTS OF LIGHT BETWEEN APPLICATIONS Type of pot Soil mixture lX1 g 11.5 14.7 16.3 16.8 Fresh Weight of tops4X g Clay--------------- Sand-peat----------Sand-peat-clay------Plastic-------------- Sand-peat----------Sand-peat-clay------1Light 5.7 7.1 7.3 9.9 g 3.0 3.1 3.3 5.7 accumulation between irrigations. [7] complete automation, a series of tests were conducted using chrysanthemums as test plants. Watering at periodic intervals such as daily or every third day often resulted in excellent growth of plants. However, it was noted that soil often was too wet or became too dry, depending on weather and size of plant. While this system might be usable, efficient water use correlated to actual plant needs was not obtainable. Soil moisture tensiometers were modified to become switches (17) that would turn on the water. It was difficult to keep the system working properly. Variable results because of difficulty in locating tensiometers in a representative area or improper functioning were experienced. Automatically irrigating by means of measuring accumulated solar energy proved effective. Production and quality were excellent, Table 7. TABLE 7. CHRYSANTHEMUMS GROWN WITH COMPLETELY AUTOMATED IRRIGATION, USING LIGHT ENERGY CONTROLS Method to Distribution by grade' fancy Fancy No. 1 Design Stem length Av. of. Av. determine need Coefvar. pertra Pct. 25.6 22.0 t. ef . flowers spet. stem Check Solar energy automatic control 4.0 Pct. 8.5 Pct. 53.5 46.0 Pct. 26.5 45.0 Pct. 2.5 2.5 In. 38.3 36.1 Pct. 7.7 11.6 Oz. 3.1 2.7 No. 4.7 5.0 SStandards by Gaylord and Hoxie (7). FACTORS TO CONSIDER FOR PRACTICAL USE These experiments have shown the possibility of determining need for irrigation by measurements of solar energy. Semi-automatic and completely automatic operation are possible. A reliable, measurable standard is used and water application is timed to actual plant use. Moisture loss from soils to air is a result of evaporation from the soil surface or transpiration. The relative importance of each depend in part on the amount of soil surface covered by vegetation. When the surface is completely covered with vegetation, most of the loss comes from transpiration. [8] TABLE 8. AMOUNT OF AVAILABLE WATER IN THREE SOIL MIXTURES Soil Mixture Available water 1 peat, 1 perlite ____________________________0.60 gal. per cu. ft. of mixture ___________________________________ per cu. ft. of mixture 0.23 gal. 1 peat, 1 sand 0.28 gal. per cu. ft. of mixture 1 clay loam, 1 peat, 1 sand Total moisture loss by evaporation and transpiration is evapotranspiration. This moisture must be replenished at the proper time for economic production of crops. Applying water too often results in poor plant growth, waste of water, and higher production costs. Delayed application results in lowering of quality and economic loss. Major moisture loss from plants by transpiration occurs through stomates. Light is the controlling factor or key in determining the opening of the stomates. If moisture deficits do not occur, the stomates will remain open as long as plants are in light. To properly employ this criteria of water need, the grower must consider the following factors: (1) soil mixture, (2) soil volume, (3) type of container, and (4) type of plant. Soil mixture and volume must be considered because they regulate the amount of available water. Normally sandy soils are irrigated more frequently than clay soils, and a shallow soil is irrigated more frequently than a deep soil. For typical soil mixtures, the amounts of available water for typical soil mixtures are given in Table 8. Soil moisture is not depleted in the same amount by plants grown under the same conditions. Size of plant as well as plant species will influence the amount of water utilized (3). SUMMARY Results of experiments by the Auburn University Agricultural Experiment Station, Auburn, Alabama, have shown that evapotranspiration is highly correlated to solar energy. Additionally, solar energy measurements, utilizing a simple, inexpensive illumination totalizer has been shown to be a reliable guide for determining need for irrigation. Semi-automatic and completely automatic applications of irrigation has been possible. [91 LITERATURE CITED (1) BILBRO, J. K., JR., CLYMA, W., NEWMAN, S. J., JONSEN, M. E. SLETTEN, W. H. AND Preliminary Evaluation of the Leaf-Color-Change Method as an Indicator of the Optimum time to Irrigate Cotton on the Hardlands of the High Plains. Texas. Agr. Expt. Sta. Prog. Rept. 2159:1-7. 1960. (2) BLANEY, H. F. Irrigation Requirement of Crops. Agr. Engin. 32:66. 1961. (3) BLOODWORTH, M. E. Some Principles and Practices in the Irrigation of Texas Soils. Texas Agr. Expt. Sta. Bul. 937:1-55. 1959. (4) BOUWER, H. Integrating Rainfall-Evaporation Recorder. Agr. Engin. 40:278-279. 1959. (5) BouYoucos, C. J. A New Electric Automatic Irrigation System. Agron. Jour. 44:448-451. 1952. (6) COMIN, D. AND WILSON, J. D. The Use of an Evaporation Index in Timing the Irrigation of Mulch Crops. Ohio Agr. Expt. Sta. Bimonthly Bul. 26(209):63-69. 1941. (7) GAYLORD, F. C. AND HOXIE, C. E. Market Grades for Ornamentals, Chrysanthemums and Carnations. Mimeo HO 79+3:1-17. 1960. Purdue Univ. Agr. Expt. Sta. (8) HALEVY, A. H. The Influence of Progressive Increase in Soil Moisture Tension on Growth and Water Balance of Gladiolus Leaves and the Development of Physiological Indicators for Irrigation. Proc. Amer. Soc. Hort. Sci. 76:620-630. 1960. (9) HALKIAS, N. A., VEIHMEYER, F. J., AND HENDRICKSON, A. H. Determin- ing Water Needs for Crops from Climatic Data. 207-233. 1955. Bul. 180:1-4. 1961. Hilgardia 24(9): (10) HOLLEY, W. D. Watering Carnations. Col. 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Nature 176:1276. 1955. (19) (20) THORNTHWAITE, C. W. 1948. An Approach Toward a Rational Classi- fication of Climate. Geog. Rev. 38:55-94. C. TUKEY, L. D., FLUCK, AND MARSH, R. An Illumination Totalizer for Integrating Light from Either Natural or Artificial Sources. Proc. Amer. Soc. Hort. Sci. 75:804-810. 1960. (21) TURc, L. Le bilan d'eau des sols. Relations entre les Precipitations, l'evaporation et l'ecoulement. Annales Agron. 5:491. 1954. (22) . Le bilan d'eau des sols. Relations entre les Precipitations, l'evaporation et l'ecoulment. Annales Agron. 6:5. 1955. [ 11 ]