The proportion of the incident solar energy used in the evapotranspiration process is a measure of the water requirement of plants. It has been found that in Hawaii approximately 60 per cent of the insolation is used in evapotranspiration during a 12-mo period of alfalfa production. Monthly water-requirement estimates were made from the empirical equation 
where W = water requirement in inches per month, and S = solar radiation in calories per square centimeter per month.
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1 Climatologist and Director, respectively.