Regional low-flow frequency relationships for north-western Bangladesh

Abstract

This thesis briefly deals with regional low-flow frequency analysis. Low-flow quanti lcs arc commonly used to describe stream characteristics and are used as parameters in various studies. Unfortunately, it can be difficult to estimate extreme low-flow quantiles, especially at sites with short record lengths. A great deal of research has been directed to the improvement of extreme quantile estimation procedure and regional frequency analysis has been found to be one of the best ways. The present study is undertaken with the objective of providing a method to obtain better estimates of low-flow of the streams in the north-west region of Bangladesh through regionalization. Three steps are involved in this study. Selection of low-flow data series that satisfy such statistical criteria as randomness, stationarity, independence and homogeneity; choosing a theoretical distribution for modelling the data sample; and estimation of low-flow. Two different distribution selection methods were applied for the selection of appropriate probability distributions, namely the method of L-moment ratio diagram and the method of product moment ratio diagram. Weibull and the log Pearson type 3 (LP3) distributions were found to fit the low-flow data series of the selected rivers. However, the procedure based on moment ratio diagrams depends on subjective judgement. These two distributions are further subjected to a quantitative statistical test, namely Kolmogorov-Smimov test for further screening. This test also suggested the Weibull and the LP3 as suitable probability distributions to fit the selected low-flow data. The shape parameters of both the distributions have been regionally estimated over the north-west region yielding a regionally estimated skewness of 0.21 for Weibull distribution and -0.88 for LP3 distribution. These regional estimates were used to calculate low-flow quantiles for recurrence intervals of 10, 25, 50 and 100 year. These low-flow quantiles are compared to the low-flow quantiles obtained by atsite approach. It is found that, in case of Weibull distribution, for recurrence interval 10 year, the maximum relative difference between regional and at-site quantile is about 40 percent whereas that for LP3 distribution is 13 percent. Generally regional estimates of low-flows showed better agreement with the corresponding values at sites with longer records. Moreover regionally estimated statistics have smaller sampling variations than those estimated only on the basis of at-site data. As such when low-flow estimates are not significantly affected in magnitude by process of regionalization, they are likely to exhibit smaller variances.