Abstract: Based on the consideration of regional mean runoff as the difference between regional mean rainfall and infiltrations of surface soil layer over an arbitrary mesoscale region, a parameterization scheme for regional mean runoff over heterogeneous land surface under climatic rainfall forcing is presented. Surface soil infiltration and its spatial distribution for an arbitrary region can be accurately estimated with the fitted spatial probability density function (PDF) from the regional rainfall observations, and the mathematical description of soil water infiltration processes and its experimental expressions, and based on those theoretical and experimental expressions, a statistical dynamic estimation scheme for regional mean runoff was developed. In other words, regional mean runoff may be considered as the difference values between mean rainfall and infiltrations over a given heterogeneous land surface region, and the mean infiltration for the region may be calculated from two parts of infiltrations for saturation and non-saturation areas. In the physical process for water cycle of land surface, the soil water infiltrations occur where there is certain water resource over a given underlying surface. When the atmospheric precipitation falls on the ground surface, a portion of rain waters is first trapped by surface plant cover and adsorbed by surface soil, and then the residual portion of rain waters may form the surface runoff. This suggests that the key for estimating soil surface runoff is to estimate the storage of soil waters. A parameterization scheme is developed by deriving the water infiltration expression from the soil water flux equation, and by deriving the heterogeneous soil water content and the regional mean runoff expression over heterogeneous land surface under climatic rainfall forcing from the PDFs of describing spatial distributions of soil water and rainfall, respectively. The reliability and feasibility of the parameter scheme expression for regional mean runoff are confirmed by comparing the estimated results of the Yangtze River delta mean runoff in 1996 by both the parameter scheme and mosaic method.