in Chapter10, droughts were considered as rainfall deficiencies; these have immediate effects on vegetation growth and evaporation losses. When there are subsequent deficiencies in river flows, it is the bulk users of water, namely industry and the large urban concentrations of domestic consumers, that begin to suffer. Water Authorities and Water Companies whose duty it is to maintain public supplies of water are always concerned for their resources during periods of low flows and by the increase in demand that dry spells generally stimulate. A thorough knowledge of the minimum flows in those river providing water supplies is therefore essential.