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Estimating Deep Drainage Between Irrigations¹

ABSTRACT

This paper treats the situation, common in irrigation, in which the water table is deep enough to be ignored and applied water moves downward by drainage and is taken up by plants. When plants are transpiring, soil water content is reduced by both deep drainage and plant extraction. Drainage rates from a wetted depth of soil are related to the total water contained in that depth when there is no plant use. This same relation can be used to estimate drainage following an irrigation when the soil is cropped. However, in a series of irrigations, there is a delay between the end of irrigation and the beginning of drainage, and drainage is overestimated. If reduced irrigation efficiencies can be tolerated, overirrigation can lengthen time between irrigations, especially if evapotranspiration rates are low.

¹ Contribution from the SWCRD, ARS, USDA, in cooperation with the College of Agriculture, Washington State Univ., Pullman. Scientific Paper no. 3652.

² Research Soil Scientist, and Soil Scientist, respectively, USDA, Prosser, Washington 99350.

Received for publication July 1, 1971. Accepted for publication August 17, 1971.