A simple model for estimating the sensitivity of runoff to long-term changes in precipitation without a change in vegetation

Forecasts of changes in precipitation (P) and potential evaporation (PE) can be applied to hydrologic models calibrated on existing conditions to obtain predictions of changes in runoff. This study describes an alternative approach, which uses a simple soil-moisture accounting model with a small number of independent and physically based parameters to explore the sensitivity of runoff to climate change for three simplified climates. The climate types chosen initially are those for which a piecewise analytical solution can be obtained so that computer programmes involving numerical solutions can be verified before being applied to field data. Sensitivity factors are calculated for the various cases and their relationships with climatic conditions and soil conditions are explored. Breakpoint Values were determined for each type of climate studied. These correspond to situations in which the soil becomes momentarily saturated once during the seasonal cycle but does not remain saturated for any finite duration. For humidity ratios greater than the breakpoint, the sensitivity of runoff to precipitation increases abruptly. For the climates studied, the sensitivity factor approaches the value of the soil parameter c as the humidity index approaches zero. The other climates studied exhibit the same sensitivity at this limit. A particular feature of the model is that analytical solutions can be determined in many cases to check and confirm the results of the numerical simulations. (C) 1999 Elsevier Science Ltd. All rights reserved.