SPATIALLY DISTRIBUTED RAINFALL-RUNOFF MODELLING USING REMOTE SENSING AND GIS - A CASE STUDY

Abstract

Ever since its conception, the Sherman unit hydrograph (UH) has been widely used in rainfall-runoff modeling. Unit hydrograph of a watershed is defined as direct runoff hydrograph resulting from a unit depth of rainfall-excess generated uniformly over the drainage area at a constant rate for an effective duration. The unit hydrograph is a lumped linear model of a watershed where it is assumed that a catchment acts on an input of effective precipitation in a linear and time invariant manner to produce an output of direct storm runoff. Methods for determining a UH from storm events with observed direct runoff hydrograph and effective rainfall hyetograph are umpteen. The unit hydrograph theory suffers from limitation that the response function is lumped over the whole watershed properties. Efforts have been made to overcome this limitation of unit hydrograph theory by introducing of geographical information system for spatial discretization of watershed into an inter-linked system of grid cells. It is now becoming common to represent land surface elevation over the watershed by grid based digital elevation model (DEM). Standardized algorithm are available in geographical information systems, which use the local terrain slope to link each cell with one of its neighbor along the line of steepest descent, thus creating a one-dimensional flow network over the entire land surface. It had been shown that this terrain representation can be utilized for runoff computation under spatially varying, but time and discharge invariant velocity field. The linear system response at the watershed outlet can be spatially decomposed into a set of cell based linear systems whose individual response functions summation give the watershed response function. This study derives spatially distributed unit hydrograph using the isochrones varying in time with rainfall intensity, overcoming the stationary constraint of unit hydrograph concept. The model uses digital elevation model (DEM) data, ground slope, flow direction and flow accumulation maps to characterize the watershed terrain in a geographical information system (GIS). A raster based approach deals with spatial domain discretization and supports rainfall-runoff simulation in a modular distributed model. The translation characteristics of the catchment are accounted by time area (TA) concept. The TA diagram is a graph of cumulative drainage area contributing to discharge at the watershed outlet iii within a specified time of travel. The isochrones of travel time are allowed to vary with temporal changes in intensity of rainfall-excess. The storage-induced time delay and catchment diffusion effects on the resultant flood hydrograph are accounted by routing the time variant TA diagram through a conceptual reservoir. Overland flow travel times are calculated by the kinematic wave equation for time to equilibrium, and channel flow times are based on Manning and continuity equations, which is an improvement over the existing isochrone extraction technique. The time series of travel time (or isochrones) maps constitute the basis for incremental and total runoff hydrograph computations. The data of a meso-scale catchment located in Madya Predesh (Temur river, tributary of Narmada river) were used to test the proposed method. Results obtained indicate that introduction of two reservoirs in series to account for catchment-induced storage effects on runoff hydrograph has significantly improved model computed results indicating use of simple kinematic wave derived travel time histogram for derivation of unit hydrograph is not sufficient for a mid-sized catchment exhibiting storage induced diffusion. The overall very high values of R2 and Nash-Sutcliffe efficiency for most of the storm events indicate suitability of the proposed model to Temur catchment which exerts substantial amount of storage-induced diffusion effects on runoff hydrographs due to flatter topography in the middle to southern parts of the catchment.