Monthly evapo‐transpiration losses from natural drainage‐basin

With limited restrictions the hydrologic cycle in a given area may be expressed essentially as follows: P = (R + E + ΔFm) in which P represents the precipitation during a given period, R that portion which has reached or will reach the stream‐channel either through surface or subsurface paths, E that part which is evaporated from land and water surfaces and transpired by vegetation during the same period, and ΔFm the change in field‐moisture content during the period. In general both E and Fm are unknown. When the period of time considered in the above expression is long, ΔFm may be neglected and then the evapo‐transpiration losses equal the difference between P (rainfall) and R (runoff) [see 1 of “References” at end of paper]; on the other hand, when the period of time considered is short and considerable rain has fallen then E may be neglected and ΔFm equals the difference between P and R represents water stored as an increment to field‐moisture to be disposed of by evapo‐transpiration during subsequent rainless periods [2].