Estimation of Monthly Flow for Ungauged Catchment (Case Study Baro - Akobo basin)
No Thumbnail Available
Date
2013-10
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
This paper deals with predicting discharge at ungauged catchments, the case of Baro-Akobo river basin, using the WATBAL conceptual lumped rainfall-runoff model. Parameters calibrated with the model are extrapolated from gauged catchments to ungauged catchments of similar physical characteristics with Regionalization technique. In the Baro-Akobo river basin, most of the catchments are ungauged. Hydro-meteorological data from fifteen metrological and eleven hydrological gauging stations of the basin are used to calibrate and validate the model parameters. Sizes of gauged catchments whose data are used vary from about 52.5km2 to 2800km2. Key model parameters considered consist of sub-surface runoff coefficient (α), surface runoff coefficient (ε), and maximum water holding capacity of catchments (Smax). These parameters are calibrated using an automatic optimizing routine of the WATBAL model and through routine iteration; Smax is found to vary from 3025mm to 138mm, α from 20.50mm/day to 0.004mm/day and ε from 2.91 to 0.08. Some more model parameters, namely, direct runoff coefficient (β), subsurface runoff coefficient (γ), and base flow (Rb) are calibrated manually. The hydrograph characteristics of observed and simulated events are compared using various evaluation criteria consisting of Coefficient of Efficiency (R2) greater than 0.7, relative volume error (E) between -10 and 10%, and coefficient of determination (r2) greater than 0.7. Among the catchments of the basin that are used in the modeling work, 80% fulfilled the criteria. Based on availability of required data, five physical catchment characteristics are selected among those commonly used in many regionalization studies. These are catchment area, mean annual precipitation, mean annual evapotranspiration, average catchment slope and mean catchment elevation. In addition to the physical catchment characteristics, input data including precipitation, observed flow, temperature, relative humidity and sunshine are used in the WATBAL rainfall-runoff model. The WatBal model combined with the Priestly-Taylor for climate impact assessment on river basins reveals the insensitivity of the basin to the existing precipitation changes, where precipitation was almost fairly uniform, and it behaves fairly well given its simplicity. After that, regionalization methods by developing regression equations, which is the most commonly used method, were applied to transfer model parameter values from the gauged to the ungauged catchments.
Description
Keywords
Hydrograph characteristics of observed