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

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%.