ABSTRACT
Precipitation represents the vital input of water to the surface hydrological system. To most people three types of precipitation come immediately to mind: rain, snow and hail. Some of the pollutants are deposited directly from the atmosphere as dry deposition and some are absorbed into the precipitation process and reach the ground as rain or wet deposition. As evaporation rates are high once the sun rises, such small quantities of moisture are soon returned to the atmosphere, so the contribution of dew to the local water budget is likely to be negligible. The direct methods utilize microwave techniques that have been available since the 1970s but have become available for precipitation estimation only recently as the physics of the interactions became better understood. Evapotranspiration is governed mainly by atmospheric conditions. Evapotranspiration is related to the gradient of vapour pressure between the surface and the air.
