The background and development of a theoretical method to solve the water balance equation for land areas is discussed. A forcing function is considered that is essentially determined by the product of absorbed solar energy multiplied by monthly precipitation; the response function is soil moisture in its month-to-month variations. A very simple parameterization is provided by one nondimensional surface characteristic named the “evaporivity” (which measures the fraction of absorbed insolation utilized during the month in the vaporization of concurrent precipitation) and a characteristic lag-time interval of the order of 2 to 3 mo to express “delayed” evapotranspiration and runoff. The solution is obtained by a closed integration of the water balance equation (rather than employment of regression or correlation methods) and yields a coherent set of data on monthly evapotranspiration, runoff, levels of exchangeable soil moisture, and storage changes. For verification, area averages for the central ... Abstract The background and development of a theoretical method to solve the water balance equation for land areas is discussed. A forcing function is considered that is essentially determined by the product of absorbed solar energy multiplied by monthly precipitation; the response function is soil moisture in its month-to-month variations. A very simple parameterization is provided by one nondimensional surface characteristic named the “evaporivity” (which measures the fraction of absorbed insolation utilized during the month in the vaporization of concurrent precipitation) and a characteristic lag-time interval of the order of 2 to 3 mo to express “delayed” evapotranspiration and runoff. The solution is obtained by a closed integration of the water balance equation (rather than employment of regression or correlation methods) and yields a coherent set of data on monthly evapotranspiration, runoff, levels of exchangeable soil moisture, and storage changes. For verification, area averages for the central ...