Empirical Methods of Estimating or Predicting Evapotranspiration Using Radiation

Abstract

PREDICTIONS of evapotranspiration\nare basic parameters for the engineer\nor agronomist involved in planning\nand developing water resources.\nEstimates of evapotranspiration are also\nused extensively in assessing the irrigation\nwater-management efficiency of existing\nprojects, future project drainage\nrequirements, and the magnitude of\ndeep percolation losses under existing\nmanagement practices. Water delivered\nto farms and fields must provide for\nevapotranspiration and unavoidable percolation\nbeyond the root zone due to\nunsaturated flow caused by gravity. The\nfirst is dependent on meteorological conditions\nand the vegetative characteristics\nof the crop when water is not limiting.\nThe second is dependent on management\nsuch as the amount of excess\nwater applied by rainfall or irrigation,\nor the moisture level maintained, and\nit is not directly dependent on meteorological\nconditions. Actually the relative\namount of deep percolation between\nirrigations under common practices\nis more apt to be inversely rather\nthan directly related to evapotranspiration.\nMeasurement of irrigation water\ndelivered to an area and surface runoff,\ncoupled with reliable estimates of evapotranspiration,\nprovide the most practical\nestimates of deep percolation under\nexisting management systems. Full\nutilization of water resources will require\nmore reliable estimates of evapotranspiration\nin the future, and these\nestimates must not include deep percolation.