Estimation of Low Flow Quantiles In Ungauged River Catchments (Case Study of Dedessa River Basin)
No Thumbnail Available
Date
2015-02
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
AAU
Abstract
Estimates of low flow statistics are required for a variety of water resource applications. At
gauged river sites, the estimation of low flow statistics requires estimation of annual n-day
minimum stream flows, low flow estimates, as determined by probabilistic modeling of observed
data sequences, are commonly used to describe certain stream flow characteristics.
Unfortunately, reliable low-flow estimates can be difficult to come by, particularly for gauging
sites with short record lengths. The shortness of records leads to uncertainties not only in the
selection of a distribution for modeling purposes but also in the estimation of the parameters of a
chosen model. In flood frequency analysis, the common approach to mitigate some of these
problems is through the regionalization of frequency behavior. The same general approach is
applied here to the case of low-flow estimation, with the general intent of not only improving
low-flow estimates but also illustrating the gains that might be attained in doing so.
In this study low flow regionalization has been performed using three approaches: flow duration
curves, base flow index and low flow frequency analysis. Low flow frequency analysis of
Dedessa river basin have been carried out using 7-day annual minimum flow series extracted
from the average daily stream flows observed at 10 stream flow gauging sites across Dedessa
river basin. L- Moment ratio diagrams were used for identifying and grouping of stations in to
hydrological homogeneous regions and hence the underlying statistical distribution.
Accordingly, the basin was delineated in to two homogeneous regions. The homogeneity of the
delineated regions was tested using the Cv-based and discordance tests. The goodness of fit test
(Z-statistics) and the minimum standard error of estimates have been used for assessing the
suitability of the selected distribution. Consequently, the Wakebay lower boundary distribution is
found to be a suitable candidate for region one and the generalized extreme value distribution for
region two. The method of probability weighted moments was considered as the best parameter
estimation procedure as compared with the method of moments and the maximum likelihood
method.
Flow duration curves of 1, 7, 10, 30 days have been derived for 10 stations. The basins flow
duration curves are characterized in to two homogeneous classes based on the characteristics
shown on the right tail end of the basins flow duration curves. Base flow separation has been made by the standard method called the institute of hydrology method. Accordingly the base
flow index is calculated and the basin is grouped in to three classes based on their range of BFI.
Regression analyses were conducted on the delineated regions to predict the index low flows
(mean annual 7-day minimum flow) for ungauged catchments using catchment characteristics.
The multiple coefficients of determinations R
2
was used as a measure of the ability of the
regression model to describe variations in the dependent variables. The value of R
was found to
be 0.997 for region one and 0.84 for region two. In addition the standardized low flow frequency
curves which will be used for estimating low flow quantiles at ungauged catchments have been
established for the two delineated regions using the standardized flow data.
Description
Keywords
Flow Quantiles, Ungauged River Catchments, Dedessa River Basin