A tightly bound soil–water scheme within an atmosphere–land–surface model

Abstract

Summary The concept of tightly bound water, in which a reservoir of soil water is bound tightly within small soil pores but is still available for evapotranspiration, is parameterised for the first time within the land surface scheme of a fully-coupled regional-scale atmosphere-land surface model. The Weather Research and Forecasting (WRF) regional climate model and the NOAH land surface scheme are selected and a case study is performed on the Olifants River Basin in the Limpopo region of South Africa. Accurate knowledge of water availability in this water-stressed region is of great importance for adaptation and future water policy. Results of a simulation forced by ERA40 re-analysis show that the standard land surface scheme is unable to reproduce the observed runoff despite rainfall and atmospheric conditions similar to observed. This version of the model over-estimates mean annual runoff by 120%. The tightly bound water scheme shows a significant improvement, reducing the bias to 22%. The inclusion of the tightly bound water scheme has little effect on the basin average annual rainfall despite increasing annual evapotranspiration. The tightly bound water physics dampens the response of runoff to precipitation and provides additional de-coupling between precipitation and runoff, increasing the variability in this relationship. Simulations with the WRF model forced with both 1980s and 2040s CCSM3 data show that the tightly bound water scheme significantly reduces runoff in different climates and projects a greater relative future decrease in runoff, from 4% to 10% for the same precipitation decrease of 2.5%. The scheme also affects the projected changes in spatially averaged 100-year return precipitation and runoff with significance at the 0.9 confidence level.