Surface water, together with ground water and atmospheric moisture may be viewed as a relatively isolated hydrologic system (Fig. 1.1.6) within which water circulates. Across the surface-atmosphere interface, water is transferred to the surface from the atmosphere chiefly in the form of precipitation and transferred to the atmosphere from the surface by evapotranspiration (evaporation plus transpiration from plant surfaces). Across the surface-ground interface water moves by process of recharge and discharge of underground aquifers.
Evapotranspiration, recharge and discharge are determined by determined by factors that vary significantly over relatively small areas. Precipitation (characterized by seasonal averages) is relatively constant over significantly larger areas. Thus, from a global perspective, it is appropriate to focus on precipitation leaving other processes to be considered when the focus narrows specific localities.
