Precipitation is water in the form of rain or snow falling to the ground. Some water returns to the atmosphere by evaporation from the leaves of plants and from the ground, and by transpiration by plants. Except in the most arid environments, precipitation normally exceeds evapotranspiration. The surplus water eventually makes its way through the drainage system—although it may be stored first on the surface, in depressions and ponds, or in the soil as soil moisture and groundwater. Overland flow, which occurs when not all the precipitation can infiltrate the soil, moves quickly to streams and rivers. Infiltrated water moves more slowly—as throughflow and interflow in partially saturated soils, and as groundwater flow in saturated soils. Basin channel run-off is the combined result of quickflow (overland flow plus interflow) and baseflow (groundwater flow).
