A relation describing the change in the amount of water stored within a defined volume owing to transfers of water across the boundary of the volume. The general water balance equation is W = P E Q , where P is precipitation, E is evapotranspiration, Q is runoff and W is the change in storage. The water balance is the basis of the hydrological method of determination of glacier mass balance. Typically in glaciological research the defined volume is a drainage basin tributary to a discharge measurement station near to the glacier margin, and not all of the basin is glacierized. Transfers of water by precipitation and evapotranspiration will include transfers not passing through the boundary of the glacier, and stores of water will include lakes, seasonal snowpatches, soil and aquifers as well as the glacier. Changes in each of the non-glacial stores must be accounted for to isolate the glacier mass balance. It can be convenient in glacier hydrology to distinguish between meltwater runoff from glacier iceand firn and runoff from the basin-wide snowpack, the latter including the snow on the glacier. Denoting these stores by I and N respectively, and assuming that changes in other stores are negligible, I = N Ei qin = Pn En Qn.