Surface-water hydrology is a field that encompasses all surface waters of the globe (overland flows, rivers, lakes, wetlands, estuaries, oceans, etc.). This is a subset of the hydrologic cycle that does not include atmospheric, and ground waters. Surface-water hydrology relates the dynamics of flow in surface-water systems (rivers, canals, streams, lakes, ponds, wetlands, marshes, arroyos, oceans, etc.). Ground-water supplies are obtained from aquifers, which are subsurface units of rock and unconsolidated sediments capable of yielding water in usable quantities to wells and springs. The hydrologic characteristics of aquifers and natural chemistry of ground water determine the availability and suitability of ground-water resources for specific uses. Ground water is the part of precipitation that enters the ground and percolates downward through unconsolidated materials and openings in bedrock until it reaches the water table. The water table is the surface below which all openings in the rock or unconsolidated materials are filled with water. Water entering this zone of saturation is called recharge. Ground water, in response to gravity, moves from areas of recharge to areas of discharge. In a general way, the configuration of the water table approximates the overlying topography. In valleys and depressions where the land surface intersects the water table, water is discharged from the ground-water system to become part of the surface-water system. The interaction between ground water and surface water can moderate seasonal water-level fluctuations in both systems. During dry periods base flow, or ground-water discharge to streams, can help maintain minimum stream flows. Conversely, during flood stages surface water can recharge the ground-water system by vertical recharge on the watercovered flood plain and bank storage through streambed sediments. The net effect of ground-water recharge is a reduction in flood peaks and replenishment of available ground-water supplies. Ground and Surface Water Hydrology covers fundamentals of subsurface flow and transport, emphasizing the role of groundwater in the hydrologic cycle, the relation of groundwater flow to geologic structure, and the management of contaminated groundwater.