Investigating the sensitivity of a land surface scheme's simulation of soil wetness and evaporation to spatial and temporal leaf area index variability within the global soil wetness project

A suite of sensitivity experiments focusing on leaf area index (LAI) was co nducted within the GSWP framework using one land surface scheme. First, the LAI was changed from the original 1 degrees x 1 degrees resolution provide d by GSWP so that it was determined by vegetation type. This was then varie d by plus and minus one standard deviation. Additional experiments explored sensitivity to the global use of a single LAI value, to aggregation of LAI to coarser resolution, and to changes in the seasonal variation in LAI. Re sults showed a sensitivity to LAI such that changes in total evaporation of 30 W m(-2) (July average) were common. Changes in soil wetness occurred wh ich commonly exceeded 5 %, and sometimes exceeded 10 %, of the soil moistur e at saturation. These changes in soil wetness are large enough to suggest that LAI needs to be provided with care within GSWP, and that any soil wetn ess product requires LAI to be provided with reasonable accuracy (perhaps w ithin +/- 1 and certainly within +/- 2). Any problems in the LAI data (part icularly in the magnitude rather than the precise seasonal variation) will lead to errors in the simulation of a soil wetness product.