Abstract:
This study describes and demonstrates the use of a probability model for representing
stochastic flow duration curves for selected rivers in the north-western region of
Bangladesh. Empirical flow duration curves are also generated, compared with the
stochastic flow duration curves, and finally regional flow duration curves are
generated.
The daily discharge records of 18 discharge gauging stations in the north-western
region of Bangladesh are used in this study. The analysis involves estimation of mean
daily discharge and their variance by rank, selection of homogeneous sites and a
suitable probability distribution for obtaining the stochastic flow duration curve by
combining the principle of order statistics. Based on homogeneity test using
discordancy measures all of the above selected stations were found to be
homogeneous. Four probability distributions: lognormal, gamma, generalized extreme
value (GEV) and mixture lognormal, which possess the ability to match the
combination of skewness and peakedness, were tested for modeling the stochastic
flow duration curve. Based on X2 and K-S goodness-of-fit tests five parameter mixture
lognormal distribution was found to be suitable for modeling the stochastic flow
duration curve.
The stochastic flow duration curves were derived applying five parameter mixture
lognormal distribution for the selected gauging stations. Following this, such curves
were separately obtained using observed flows and a comparison of the two showed
that the mixture lognormal distribution is adequate for modeling the flow duration
curve. Empirical flow duration curves for the selected stations were also generated
and compared with the stochastic flow duration curves. For a given station, discharge
obtained from the stochastic flow duration curve for a given rank compared
reasonably well with those obtained from the empirical flow duration curve at
corresponding exceedence probability. Finally, regional flow duration curves are
constructed using both stochastic and empirical approaches and their comparison
showed that these two regional curves also follow the similar relationship.
