A method of determining the mass balance indirectly by solving the water balance for the change in storage W in a drainage basin W = P E Q , with P the precipitation, E the evapotranspiration and Q the discharge, each of these quantities being a total over a stated span of time. In practical work the hydrological method can be applied only to an entire drainage basin. It does not provide any information on the spatial distribution or gradients. The quantity W will include changes in storage in lakes, seasonal snowpatches, soil and aquifers as well as in the glacier. Each of these changes must be accounted for to isolate the mass balance of the glacierized part of the catchment, but the changes in storage other than in the glacier and the snow cover are often assumed to be negligible over Annual periods.