LONG-TERM VARIABILITY IN THE WATER BALANCES OF THE PLYNLIMON CATCHMENTS

Water balance data have been collected since 1969 from the Institute of Hydrology's long-term experiment on Plynlimon, Mid-Wales, with the primary aim of quantifying the differences in evaporation between the 67% forested Severn and the grassland Wye catchments. Evaporation differences between the two catchments, and hence between the two land uses, have been established with increasing precision as the experiment has proceeded; however, the absolute hydrological responses to natural and man-induced fluctuations in climate take longer to quantify. Analysis of trends in time series data from the catchments indicates increased rainfall and flow over the study period, particularly during the summer, with the difference between them, actual evaporation, declining markedly in both main and sub-catchments. A partial explanation for the reduction implicates a decline in atmospheric demand for moisture, caused by increased cloud interception of incoming radiation, colder air temperatures which lead to a lowering of the vapour capacity of the atmosphere and, most markedly, a decline of 30% in specific humidity deficits above the crop. Other potential physiological controls are discussed, including changes in atmospheric carbon dioxide and tree root damage by acid rain.