Estimate of the size of future water resources in an aquifer, catchment area, or geographical region, which involves an evaluation of all the sources of supply or recharge in comparison with all known discharges or extractions.; A summation of inputs, outputs, and net changes to a particular water resource system over a fixed period.; (1) (Hydrology) An accounting of the inflows to, the outflows from, and the storage changes of water in a hydrologic unit or system. Also see Water Balance. (2) (Conservation and Planning) The calculated amount of water a household should use based on the type and number of fixtures, landscape requirements, and size of family.