Hydrological Systems

Freshwater movement within Earth systems is affected and governed by water sources, evaporation, transpiration, surface and subsurface runoff, and groundwater recharge and flow. Subsurface runoff typically dominates in temperate environments, while overland flow processes dominate in dry catchments. These transport processes initiate in headwaters of drainage basins and the propagation of water downstream is regulated by stream networks, landforms, soil properties, and anthropogenic activities. Soil wetness is an important control on hydrologically active portions of catchments, with wet conditions supporting more widespread contributing areas. Lakes are fed primarily by streamflow, but in some cases direct precipitation can be a major input. Outflows from lakes represent the dominant water loss, but evaporation from lakes can be significant in dry areas. While climate change is affecting atmospheric conditions, in most areas it is difficult to disentangle the effects of land management changes on hydrological processes from those due to climate change.