RAINFALL RUNOFF MODELLING -USING TOPOGRAPHY BASED DISTRIBUTED MODEL (TOPMODEL)

Abstract

The formation of runoff occurs, in most cases as the result of combination of causes including rainfall intensity exceeding the infiltration capacity and available water capacity in the soil being exceeded by the total amount of rainfall. Topography is recognized as an important factor in determining stream flow response of watershed to precipitation. The TOPMODEL, Topography based Hydrological model, is a variable contributing area conceptual model. Due to it's structural simplicity and a few number of parameterization, TOPMODEL has become more popular for land surface process study using digital elevation models (DEMs). TOPMODEL simulates runoff at the catchment outlet based on the concept of saturation excess overland flow and subsurface flow. It utilizes topographic index as an indicator of likely spatial distribution of rainfall excess generation in the catchment. The present work aims at to evaluate the TOPMODEL applicability to the forest and sub-Himalayan watershed. Such an evaluation does not appear to have been reported in literature. In this study TOPMODEL was applied to simulate continuously the runoff hydrograph of Chaukhutia watershed of Ramganga catchment. The objectives were to relate hydrological responses to runoff generation mechanisms, operating in the catchment and to estimate the uncertainty associated with runoff prediction. The Topographic Index values within the catchment were determined using digital elevation (DEM) data. Select parameters in TOPMODEL were calibrated using an iterative procedure to obtain iii a set of parameters to produce minimum sum of square of errors between observed and computed hydrograph. The calibrated parameters are the transmissivity decay parameter, m, the surface transmissivity ln(To), the root zone water available capacity, SRmax, initial moisture deficit in root zone, SRinit and channel velocity, ChVel. Observed data for the period (1975 — 78) was used for calibrating model parameters and response of the model with calibrated parameters was verified using data for (1979 — 81).The generalized likelihood (GLUE) framework was used to assess the performance of the model with randomly selected parameter sets. The TOPMODEL did not perform as a good simulator for Chaukhutia watershed, which is characterized by its land use as: forest=50%, agriculture and pasture=27%, and barren land=8%. Efficiency of the model in calibration period was of the order of 0.58 and in validation period was of the order of 0.649. Although model simulated well the base flow portion of the observed hydrograph but the model efficiency has been deteriorated due to the underestimation of peaks by the model, which is not unusual as most of the long-term rainfall-runoff models perform poorly, especially in forested watersheds and often underestimate peak flows.