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Effective Available Water and Its Relation to Evapotranspiration Rate, Depth of Wetting, and Soil Texture¹

ABSTRACT

The effect of evapotranspiration (Et) rate on effective available water (EAW) was evaluated for three soils ranging in texture from sand to silt loam, at evapotranspiration rates of about 0.4 to 1.3 cm per day, and at two depths of wetting. In all cases, as evapotranspiration rate increased, drainage following irrigation decreased and as a result EAW increased. The increase in EAW due to Et rate was relatively greater in the sand than in the finer textured soils. If only midsummer Et rates are considered, EAW is estimated quite well from a usual field capacity test, provided soil depth and soil texture are considered in determining the sampling time for characterizing field capacity. If one considers usable water referenced to some safe depletion level rather than available water referenced to the 15-bar percentage, the effect of Et is more important. EAW is influenced differently by depth of wetting in different textured soils.

¹ Contribution from the Western Region, ARS, USDA, in cooperation with the College of Agriculture, Washington State Univ., Pullman. Scientific paper no. 4013.

² Soil Scientists, USDA, Professer, Washington.

Received for publication April 3, 1973. Accepted for publication May 31, 1973.