The role of evaporation in the hydrologic cycle

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Abstract
Hydrologists have recognized a serious need for means of measuring water‐losses resulting from evaporation and transpiration from drainage‐areas, from land‐surfaces as well as from free‐water surfaces. The fact that only a portion of the water from a given rainstorm can be accounted for as surface‐runoff and base‐flow is due to the fact that a part is required to replace the field‐moisture which has been removed from the soil by evaporation and transpiration since the previous rain.In the unit‐graph method of computing hydrographs from precipitation‐data the principal difficulty is due to uncertainty as to the magnitude of the runoff‐coefficient. In the channel‐storage method the chief difficulty is related to lack of knowledge as to the rate at which field moisture‐deficiency develops. In both cases, if data on losses from the watershed by evaporation and transpiration were available, they could be subtracted from the precipitation‐values and remainders obtained which would permit the determination of the actual amount of water available for runoff.

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