Streamflow characteristics of Sangu-Matamuhuri watershed in the southeastern part of Bangladesh.

Quantification of streamflow chatacteristics is considered crucial for designing effective management practices in a watershed. Sangu and Matamuhuri are two major rivers of Chittagong Hill Tracts (CHT's) and main sources of upland freshwater inflows to the south-eastern part of Bangladesh. This study was performed to evaluate the streamflow characteristics of Sangu-Matamuhuri watershed based on 25 years historical observed streamflow and rainfall records. Mean daily discharge data recorded by Bangladesh Water Development Board were collected and analyzed for the period of 1995-2019. Stream flow characteristics were determined in terms of different parameters including mean annual runoff discharge, specific water yield, run-off ratio, peak flow, seasonal flow and flow duration. Various statistical measures including mean, standard deviation, co-efficient of variation, lag-1 serial correlation, Mann Kendall test and Sen's slope estimator were applied to justify these parameters. The mean annual runoff discharges of 2167.77 and 1490.61 million cubic meters (mcm) were measured at the gaging stations at Sangu and Matamuhuri watershed, respectively. Matamuhuri basin showed higher value of specific water yield (1.52 mcm/sq. km) compared to the Sangu basin (1.02 mcm/sq. km). The average annual runoff coefficient value of Sangu watershed was found 0.36 while in case of Matamuhuri watershed it was about 0.49. The mean annual peak discharges of 25 years were found 607.63 ms and 628.42 m s at Sangu and Matamuhuri watershed, respectively. Mann-Kendall Test (MK) and Sen's Slope Estimator method were applied for temporal trend analysis. An increasing trend in runoff observed during wet season which may cause flash floods and a decreasing trend observed during dry season that may cause water scarcity in Sangu-Matamuhuri watershed area. Flow duration curves based on 25 years' hydrological data were developed which showed progressive reduction in flows over time in both the watersheds for three periods 1995-2002, 2003-2010 and 2011-2019 with major changes occurred in Matamuhuri watershed during 2011-2020 which might be due to the impact of land use and land cover changes as well as changes in precipitation pattern. Implications of these findings indicate baseline information that can be used to evaluate the impacts of land use changes and climate change, and thereby may facilitate the planning and local management of water resources in the long term.