COMPARISON OF SINGLE AND MULTIPLE FLOW DIRECTION ALGORITHMS FOR COMPUTING TOPOGRAPHIC PARAMETERS IN TOPMODEL

Single flow direction (sfd) and multiple flow direction (mfd) algorith ms were used to compute the spatial and statistical distributions of t he topographic index used in the watershed model TOPMODEL. An sfd algo rithm assumes that subsurface flow occurs only in the steepest downslo pe direction from any given point; an mfd algorithm assumes that subsu rface flow occurs in all downslope directions from any given point. Th e topographic index in TOPMODEL is ln (alpha/tan beta), where ln is th e Napierian logarithm, a is the upslope area per unit contour length, and tan beta is the slope gradient. The ln (alpha/tan beta) distributi ons were computed from digital elevation model (DEM) data for location s with diverse topography in Arizona, Colorado, Louisiana, Nebraska, N orth Carolina, Oregon, Pennsylvania, Tennessee, Vermont, and Virginia. The means of the ln (alpha/tan beta) distributions were higher when t he mfd algorithm was used for computation compared to when the sfd alg orithm was used. The variances and skews of the distributions were low er for the mfd algorithm compared to the sfd algorithm. The difference s between the mfd and sfd algorithms in the mean, variance, and skew o f the ln (alpha/tan beta) distribution Were almost identical for the v arious DEMs and were not affected by DEM resolution or watershed size. TOPMODEL model efficiency and simulated flow paths were affected only slightly when the ln (alpha/tan beta) distribution was computed with the sfd algorithm instead of the mfd algorithm. Any difference in the model efficiency and simulated flow paths between the sfd and mfd algo rithms essentially disappeared when the model was calibrated by adjust ing subsurface hydraulic parameters.