The Thornthwaite moisture index is useful as an indicator of the supply of water in an area relative to the demand under prevailing climatic conditions. This study examines the effects of long-term changes in climate (temperature and precipitation) on the Thornthwaite moisture index in the Delaware River basin. Temperature and precipitation estimates for doubled-CO2 conditions derived from three general circulation models (GCMs) are used to study the response of the moisture index for steady-state doubled-CO2 conditions and for gradual changes from present to doubled-CO2 conditions. Results of the study indicate that temperature and precipitation under doubled-CO2 conditions will cause the Thornthwaite moisture index to decrease, implying significantly drier conditions in the Delaware River basin than currently exist. The amount of decrease depends, however, on the GCM climatic-change scenario used. The results also indicate that future changes in the moisture index will be partly masked by natural year-to-year variability in temperature and precipitation. ?? 1992 Kluwer Academic Publishers.