Some detailed water balance studies of research catchments

Abstract

Aspects of the water balance during the drought period 1975–76 are reported for the Institute of Hydrology’s experimental catchments in Cumbria, East Anglia, the Thames Valley and upland Wales. Summer (April–September) and winter (October–March) totals of precipitation, streamflow and potential evaporation during the drought were compared with mean values for seasons preceding it; where soil moisture was measured by neutron probe, losses from actual evaporation were also compared. Yield dropped proportionately less in relation to rainfall where catchments contained appreciable storage, such as the Cam catchment in East Anglia with its chalk–glacial drift aquifer, or the Wye and Severn catchments in upland Wales which contain storage areas of peat underlain with glacial drift. The stream draining the Oxford clay of the Ray catchment in the Thames valley, on the other hand, dried up entirely in the second summer of the drought. The paper suggests that the comparison of water yields from the Wye and Severn catchments, which are under hill pasture and coniferous forest respectively, gives results which have considerable bearing on the future management of water resources from upland areas when the aim of management is to maintain supplies of water even during periods of drought as extreme as the years 1975–76. The effect on reservoir opera­tion of neglecting to allow for change in land use is illustrated by a hypothetical example using an artificial 30 year streamflow sequence containing a drought year with very long return period.