In order to plan the development of a source of water or to plan for meeting a demand, accurate data is required concerning the volumes of water in the system and the flow rates, including the replenishment rate. The variability of flow, particularly river flow, is important from the point of view of risk of drought or flood and sizing of storage reservoirs. Rivers therefore need to be gauged over a period of time in order to understand the flow variations, and the longer the record, the better. More particularly, accurate and sufficient flow measurements are needed. The locality of the flow measurements is also important from the point of view of accuracy, but more particularly of relevance to the proposed development of the river. The same applies to other sources, e.g. groundwater. Extensive data is needed to assess the extent and yield of aquifers.