Irrigation Management -- Water Requirements and Water Balance

This paper seeks to discuss irrigation water requirement estimates in the light of current practice, important developments during the 1970's, significant research and future research and applications of that research. Each of these are elaborated in more detail in the text of this paper. A major addition to the science and art of estimating irrigation water requirements has been to replace the often ambiguous "potential evapotranspiration" with "reference crop evapotranspiration". In the past decade a series of experiments relating irrigation water applications to crop yield now permit a much better economic analysis of the use of water for irrigation. The estimation of monthly irrigation water requirements was facilitated, particularly for varying climatic conditions with the United Nations publication "Crop Water Requirements" by Doorenbos and Pruitt (1977). Estimation of daily water requirements for purposes of irrigation scheduling has been refined by the development of an albedo model and a wind function for the Penman method, that is variable throughout the season, Wright (1981). Several western states are experiencing lawsuits or other legal deliberations involving seasonal irrigation water requirements because of conflicts between groups of water users or water right transfers from agriculture to industry or municipal use. Irrigation scheduling continues to be refined from the standpoints of predicting ET, verifying yield conditions and other factors like production and peak pumping power reduction. Future research probably will include emphasis on breeding crops that require less water, refinements on the relationships between yields and water consumption, refinements in methods of estimating irrigation water requirements, and the development of irrigation schemes that minimize water and energy requirements. For other methods and more detail the reader is referred to sources such as Doorenbos and Pruitt (1977), Jensen (1974), Barman. et al. (1981)