Separating the attributions of anthropogenic activities and climate change to streamflow and multivariate dependence analysis

Streamflow changes in basins are influenced by two significant factors: anthropogenic activities and climate change (AACC). Separating the attribution of each factor is crucial for managing water resources and economic, political, and social activities. In this study, the Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) model is used to separate annual AACC attributions to streamflow changes in Iran's Karkheh basin. The trend-free pre-whitening Mann–Kendall (TFPW-MK) test is used to determine trends in annual precipitation, streamflow, and air temperature. Multivariate dependence analysis is performed for precipitation and streamflow changes and attributions of AACC to streamflow changes using different copula functions. The point of change in the annual streamflow series, identified by the double cumulative curve (DCC) and Pettitt test, occurred in 1999. Compared to the pre-change period, the average annual streamflow decreased by 42.3%. The results of the hydrologic model simulation showed that climate change and anthropogenic activities contributed to streamflow reduction by 36.9% and 63.1%, respectively. The results showed that the attributions of AACC to streamflow each year could be obtained based on the dependence analysis between precipitation changes, streamflow changes, and the attributions of AACC to streamflow changes with copula functions. The results showed that for a joint probability of 0.5, the values of attributions of AACC to streamflow with the maximum joint density are equal to −42 and −36 mm, respectively.