Riverflow reconstruction from precipitation data

In the United Kingdom few river catchments have overflow records longer than 35 years. The wealth of precipitation data in the United Kingdom enables long records of riverflow to be produced with a catchment model calibrated over the recent period. For most parts of England and Wales it is shown that monthly sequences of riverflow that account for 90–95 per cent of the variance of the measured riverflow can be produced back to 1860. Reconstructions are made on 10 catchments (size range 400–4500 km2) in both upland and lowland Britain. Reliable reconstructions are possible using as few as 4 rainguage records. The reconstructions provide not only a valuable database for the study of the effect of climatic change on riverflow, but also allow stationarity, homoscedasticy and changes in the frequency of extreme events to be examined. It is shown that on the 10 catchments extreme droughts with a duration of 18 months are three times more likely to have occurred between 1925 and 1979 than between 1870 and 1924. The significance of this and other findings are discussed in the context of water resource analysis. The reconstructed riverflows, apart from their obvious use for water resource analysis, also provide a valuable climatological database for the possible further extension of riverflow using proxy climate indicators such as tree rings.