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During the past decade increasing attention has been given to various engineering type equations designed to estimate, from available climatic data, the amount of water used by plants. Some of the basic assumptions involved in these equations are examined in the ligh to frecent advances in knowledge of the physical processes at the earth‐atmosphere interface. It is found that the majority of equations involving the concept of potential evapotranspiration or consumptive use tend to assume a homogeneous soil moisture regime infinite in horizontal extent. Therefore, it has been difficult to apply these equations to irrigation practice, which obviously must create isolated areas of above normal moisture. Consequently, the problem is approached from the strictly physical point of view, and it is shown that a solution can be obtained, provided that sufficient knowledge of soil interactions is available.

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