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 th
e 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.
A period of one year, synchronized with the natural progression of the hydrological seasons by specifying the calendar date of its first day. Generally in glaciology the hydrological year is found to
be convenient because it begins near the start of the accumulation season and ends near the end of the ablation season. For example the appropriate dates are 1 October to 30 September in the mid-latitudes of the Northern Hemisphere. The concept of the hydrological year is most useful where the accumulation and ablation seasons are well differentiated, as on mid-latitude glaciers and most high-latitude glaciers, but it is less well suited to those regions in which there are more than two hydrological seasons, as in the tropics, or in which most of the accumulation occurs in the same season as most of the ablation, as in monsoon climates (see summer-accumulation type.).