METHOD TO DELINEATE BLOCKS IN BTOPMC MODEL FOR LARGE SCALE WATERSHEDS

Abstract

BTOPMC (Block wise TOPMODEL with Muskingum-Cunge flow routing method) is an extension of TOPMODEL. The BTOPMC uses the block average saturation deficit instead of basin average value that is proposed in the TOPMODEL to calculate local saturation deficit. In this paper a methodology has been proposed to determine the optimum number of blocks when applying to large scale basins. The model was applied to simulate hydrological processes in the Mekong River basin upstream of Pakse with catchment area of 277, 000 square kilometers. Four parameters namely; lateral transmissivity under saturated conditions T_o, decay factor m, maximum root zone storage S_rmax, and Manning's coefficient n, were assigned depending to land use. The Manning's coefficient along the main stream and tributaries were assigned as a function of slope and the best Manning's coefficient value at the most downstream location. Daily observed discharges at Luang Prabang gauging station were used as an upstream boundary condition to the model. The watershed was divided into several blocks in order to understand the block size effect on hydrological simulations. It was found that the number of blocks which results the minimum standard deviation of block average slope or soil topographic index produces the best hydrological simulation.