The Impact of Root Weighting on the Response of Transpiration to Moisture Stress in Land Surface Schemes

Land surface schemes (LSSs) for large-scale climate models use a variety of different methods to represent the influence of soil moisture on transpiration. One area in which they differ is in the treatment of vertical soil moisture distribution. While some schemes use root weighting to integrate moisture stress throughout the root zone, others use only the bulk root-zone soil moisture. The sensitivity of transpiration to surface root fraction is examined in a simple off-line soil moisture model and a complex Deardorff-type LSS. The seasonal cycle of transpiration is found to be quite sensitive to surface root fraction, with transpiration becoming progressively more susceptible to moisture stress as the surface root fraction is increased. Varying the surface (10 cm) root fraction between 10% and 90% produces local transpiration difference of up to 80 W m−2 and relative annual transpiration differences of up to 28%. The sensitivity of root-weighted methods to surface root fraction is found to be partly dependent on other aspects of the parameterization. Thus, differences between the treatment of vertical root distribution in LSSs have a considerable impact on simulated transpiration. These results have implications for LSSs and for the large-scale climate models in which they are employed.