Hydrological modelling of the west coast of India

Abstract

A hydrological modelling framework was assembled for simulating discharges of the west-coast rivers of India. The framework is applied to the Mandovi river, Goa, a typical west-coast river. The modelling framework consisted of a digital elevation model (Global Land One kilometre Base Elevation (GLOBE); resolution ~ 1 km), a hydrological routing algorithm (Terrestrial Hydrological Model with Biogeochemistry; THMB), and a rainfall-runoff model (Soil Conservation Service Curve Number (SCS-CN)). The SCS-CN method is incorporated into THMB framework and called as SCS-THMB model. The rainfall forcing maps for SCS-THMB were obtained by a multivariate interpolation scheme to capture the sharp increase in rainfall on the windward slopes and the exponential decay in rainfall on the leeward side of the Sahyadri range on the west coast of India. The key result is that treating the windward and leeward sides separately by an a priori subjective extraction of the ridge line can reduce the underestimation of rainfall that is common in data-sparse mountainous terrain. This algorithm is incorporated online in SCS-THMB. A series of simulations are carried out to build the model parameterisation using three of 18 years of data (rainfall and discharge) during 1981–1998; the other 15 years were used to validate the model. Simulations, including spatial and temporal variation of the SCS parameters such as CN and initial abstraction, match quite well with the observed daily discharges. Spatio-temporal variability is incorporated by an objective definition of different regimes during the monsoon season to better assess antecedent moisture conditions. The strength of our method lies in the low demand it makes on hydrological data. Apart from information on the average soil type in a region, the entire parameterisation is built on the basis of the rainfall, apart from some easily available data on soil and land-cover, that is used to force the model. The data and results suggest that the framework should work for other basins too on the Indian west coast. That the model does not need to be calibrated separately for each river is important because most of these basins are ungauged. Hence, though the model has been validated only for the Mandovi, its potential application for rest of the west-coast rivers is considerable.