Incorporation of the Flow Duration Curve Method Within Digital Filtering Algorithms to Estimate the Base Flow Contribution to Total Runoff

The urgent need for rational use of water resources and the intense competition among most of the riparian countries with respect to the use of shared water sources have raised the importance of examining the contribution of base flow to total stream flow. This paper examines the Diyala transboundary river basin shared between Iraq and Iran. Daily unregulated flow records during the hydrologic years between 1955 and 1982 were considered at the upstream flow gauging station and daily unimpaired flow time series during the water years between 1931 and 1961 were examined at the most downstream hydrometric station. Two digital filtering algorithms and the flow duration curve method were applied to extract the base flow time series and compute its signature to total river flow. A generic methodology, which incorporates the flow duration curve method within digital filtering algorithms, has been introduced to obtain the filter parameter α in the filtering algorithms, and obtain the base flow contribution with a high level of confidence. Findings revealed that the base flow proportion is 38 % for the upper sub-basin and 31 % for the upper and middle combined sub-basins. The computed base flow signatures using the incorporated approach have very strong correlation coefficients of around 0.985. The driest year had the highest annual base flow index. The base flow index values started to increase in April and climbed to a peak by the end of May. A gradual decline between August and September was observed. The proposed methodology should be applied to unregulated watersheds.