Pan Evaporation Measuremnts as a Criterion for Predicting the Irrigation Requirements for Various Crops

Measurement of evaporated water from various devices has assisted in determination of the evaporation losses from large storage reservoirs. Evaporation is a large consumer of open-reservoir stored water which is used for irrigation, municipalities, manufacturing and numerous other purposes. The use of evaporation pans for measuring the quantity of water evaporated produces better results for estimating the resource extent of our stored water. Precautionary steps can then be taken toward the possible occurrence of predicted water shortages. This is essentially the principle involved in determining the consumptive use of moisture by irrigated crops. Consumptive use is commonly referred to as the sum of the volumes of water used by the vegetative growth in transpiration and building of plant tissue and that evaporated from adjacent soil. It is necessary to be able to predict the storage supply in the soil and apply additional water when it can be stored efficiently. This additional moisture should stop any approaching drought period for the particular crop involved. Various methods have been used to determine the consumed moisture by crops under irrigation. Harry F. Blaney and Wayne D. Criddle have developed a method by which they used climatological data along with measured consumptive use of irrigated crops to determine coefficients for many irrigated areas of Western United States. Once their coefficients are determined and tested satisfactorily it is a relatively simple process to predict with a reasonable degree of accuracy the moisture-use pattern of a particular crop if the necessary climatological data are available for the area. Numerous other approaches toward the prediction of the moisture use habits of crops have been made by other investigators. In 1951 and 1952 at Logan, Utah, Vaughn E. Hanson experimented with the correlation of evaporation from a Weather Bureau Class A Pan and consumptive use of four crops. Work on this problem was then initiated as a regional project with the Irrigation Engineers under Vaughn E. Hansen’s supervision doing some work at each of several irrigation research stations in the Midwest and the West. Discussion of this work brought about the possibility that some new method to measure evaporation, which would be both practical and applicable to the individual irrigation farm, could be designed. The letter that follows was written by Vaughn E. Hansen in May of 1953 to Leonard J. Erie, Irrigation Engineer, Brookings, South Dakota; Stephen Mech, Irrication Engineer, Prosser, Washington; Walter Meyer, Irrigation Engineer, Garden City, Kansas; P. Earl Ross, Irrigation Engineer, Weslaco, Texas; Hayden Rouse, Irrigation Engineer, Gunnison, Colorado; and Bryon Tomlinson, Irrigation Engineer, Riverton, Wyoming