Effect of sub-grid-scale variability of soil moisture and precipitation intensity on surface runoff and streamflow

Land surface scheme simulations, which differ only in their parameterizatio n of surface runoff, are performed for the Amazon and Mississippi River bas ins at similar to1.88 degrees x 1.90 degrees resolution. In the first simul ation, soil moisture and precipitation intensity are assumed to be uniforml y distributed. In the second simulation, precipitation intensity is assumed to be exponentially distributed, and soil moisture subgrid variability is expressed in terms of the variable water-holding capacity of the grid cell. The effect of subgrid variability of precipitation intensity and soil mois ture is taken into account to model surface runoff more realistically. Nati onal Center for Environmental Prediction (NCEP) reanalysis data with its pr ecipitation estimates adjusted to long-term values are used to drive the la nd surface scheme, and flow routing is performed to obtain streamflow at th ree locations in each river basin. Comparisons with observations indicate t hat the inclusion of subgrid variability results in slightly improved strea mflow simulations in terms of reduced root-mean-square errors and better co rrelation coefficients. Sub-grid-scale variability also results in signific ant changes in the magnitude, time, and frequency of surface runoff generat ion, partitioning of total runoff into surface runoff and drainage and slig htly drier soil moisture conditions.