Assessment of Instream Flow Requirement of Dudhkumar River

Abstract

This study deals with the assessment of Instream Flow Requirement (IFR) of Dudhkumar river using three methods of hydrological approach. Methods used are (i) Mean Annual Flow (MAF) method, (ii) Flow Duration Curve (FDC) method and (iii) Constant Yield (CY) method. Dudhkumar river is located in the north-east corner of north-west region of Bangladesh. It is an international river shared by Bhutan, India and Bangladesh. The present study is a preliminary level desk-top analysis using historical river flow data. Instream flows are defined as those flows which are needed to be maintained in the river for sustaining the various functions of a river, e.g. carrier function, production function, regulation function etc. Instream flows are essential within a stream to maintain its natural resources at desired level. Knowledge of instream flow requirement assists in planning of new development projects, in evaluating existing projects as well as for proper understanding of the issues relating to the natural environment of the stream.According to the MAF method, IFR has been computed for eight habitat quality classes, the choice of which depends on the management objectives. The computed IFR varies from 48 m3/s (which represents the ‘poor’ habitat quality) to 950 m3/s (which represents the ‘flushing’ habitat quality). For the sake of the present study, out of eight habitat quality, two habitat quality namely ‘good’ and ‘outstanding’ have been considered for dry (low flow) season. For the monsoon (high flow) season only ‘flushing’ habitat quality has been considered. The corresponding IFR for ‘good’ and ‘outstanding’ habitat quality is found to be 95 m3/s and 190 m3/s respectively for the low flow season. IFR for the ‘flushing’ habitat quality is found to be 950 m3 According to the FDC method, computed IFR during the low flow season varies from 72 m/s for both low and high flow season. 3/s (in March) to 190 m3/s (in November) having an average of 113 m3/s. In the high flow season it varies from 580 m3/s (in June) to 1190 m3/s (in July) having an average of 937 m3/s. According to the CY method, the computed IFR during the low flow season varies from 90 m3/s (in March) to 234 m3/s (in November) having an ii average of 147 m3/s. In the high flow season it varies from 435 m3/s (in October) to 1032 m3/s (in July) having an average of 744 m3/s. Possible water demand that could be met from Dudhkumar river has been assessed considering irrigation demand for Kurigram Irrigation Project, North Unit and domestic water demand in the area. It is to be mentioned here that there is no classified waterway in Dudhkumar river. Peak diversion requirement at Pateswari is found to be 35.55 m3/s occurring in the month of January. The 2nd peak diversion requirement occurs in the month of March. This diversion requirement includes the irrigation and domestic water demand in the project area. For comparing demand availability scenario, IFR determined by the MAF method for two options have been considered. Option-I consists of ‘good’ and ‘flushing’ habitat quality for the low and high flow season while Option-II consists of ‘outstanding’ and ‘flushing’ habitat quality for low and high flow season respectively. Flow availability at Pateswari has been assessed for 75% and 90% dependability. It is found that 75% dependable flow in low flow season varies from 80 m3/s (March) to 220 m3/s (November). On the other hand 90% dependable flow in low flow season is found to vary from 72 m3/s to 190 m3/s. In the high flow season 75% dependable flow varies from 360 m3/s to 950 m3/s, while the 90% dependable flow vary from 250 m3/s to 780 m3 From a comparison of water availability and various demands, it is found that during the months of April, June, July, August, September, October, January, February and March there is shortage of water to meet the total requirement if 90% dependable flow, IFR under Option-I and other demands are considered. If 75% dependable flow availability is considered, the duration of deficit period shortens and April, June, July, September, October, January, February and March remain as the deficit period. If IFR under Option-II is considered, the shortage of water to meet the demands increases. Given the nature and details of environmental issues that influence the instream flow requirement and the limitations of the present study, further investigation may be carried out encompassing wider aspect of the environment.