RIVERFLOW RECONSTRUCTIONS FOR 15 CATCHMENTS OVER ENGLAND AND WALES AND AN ASSESSMENT OF HYDROLOGIC DROUGHT SINCE 1865

Recent years have seen increased risks of water supply problems becaus e of reduced rainfall in some seasons. Over England and Wales most riv erflow records begin during the 1950s and 1960s. The short record leng th, therefore, makes assessment of the rarity of some low flow sequenc es difficult. A means of extending riverflow records is to reconstruct them using a simple catchment model and the plentiful monthly rainfal l totals available for England and Wales. Riverflow data on a monthly basis have been reconstructed for 15 catchments for the period since t he 1860s. The reconstructions are produced with a regression-based cat chment model that relates lagged values of rainfall to runoff. The sta tistical nature of the model requires that any reconstructions be asse ssed over an independent dar:a period. All perform almost as well as d uring the calibration periods. Using the reconstructions, recent hydro logic droughts are put in to a longer term context. On none of the cat chments is the most extreme 6 months (April-September) average riverfl ow found to occur during the 1986-1995 period. On a number of catchmen ts, the extreme year is 1976 or 1984. Flows were low during 1989-1992 and 1995, with the former four year period being unusual, because the years occurred in succession. On all 15 catchments, however, an earlie r sequence occurring between 1865 and 1980 is more severe. (C) 1998 Ro yal Meteorological Society.