Hydrologic regime alteration and seasonal flow redistribution in a dam-induced mountainous river: an integrated analytical framework

Abstract Reservoir regulation poses a major challenge to ecological and hydrologic sustainability in river systems, especially in semi-arid mountainous river basins, via alteration of the hydrologic regime. This study investigated long-term alterations in the hydrologic regime and seasonal redistribution of streamflow in the Baliqlu Chay River following the operation of the Yamchi Dam. To investigate these hydrologic changes, a daily and monthly discharge data review from 1991 until 2022 was conducted using an Integrated Analytical Framework (IAF), which included three steps. The first step was to utilize Innovative Trend Analysis (ITA), Pettitt’s Change-Point Test, and Indicators of Hydrologic Alteration (IHA) to analyze the data. Subsequently, Range of Variability Approach (RVA) and Flow Duration Curve (FDC) analyses were applied to the monthly discharge data. Finally, the third step was to examine the Data using Directional (Circular Statistics). The ITA results show a continuous declining trend in streamflow (ITA slope=-0.01), whereas the Pettitt test found a statistically significant hydrologic breakpoint in August 2008 ( p < 0.001), indicating a marked post-regulation regime shift. The IHA and RVA analyses indicated substantial alteration of the flow regime in the post-dam period, specifically during spring snowmelt. The mean April discharge dropped from roughly 7.4 m³/s pre-dam, to less than 1 m³/s post-dam (an 88% reduction). The IHA and RVA analyses also indicated that substantial hydrologic alterations were observed across multiple months. In many months, alteration factors approached − 100%. Likewise, the FDC analyses showed that, on average, high flows, medium flows, and low flows decreased systematically with reservoir regulation indicating decreasing hydrologic stability and increasing downstream hydrologic vulnerability. Temporal redistribution of streamflow seasonality occurred following the construction of the analyzed dam using directional statistical analyses. Streamflow timing exhibited a winter-centered timing pattern prior to regulation, whereas streamflow timing shifted toward spring/summer timing in several post-regulation years. In several of the post-regulated years the resultant vector length (R) was reduced, with a minimum value of 0.05 (minimum year was 2015). This supports the results that streamflow seasonality was reduced in the magnitude of seasonal concentration of streamflow and increased temporal dispersion of streamflow. Although reservoir regulation is consistent with the observed changes, concurrent hydroclimatic variability and watershed-scale changes may also have contributed to the hydrologic response. The study highlights the value of directional hydrologic statistics as a complementary tool alongside conventional metrics for diagnosing seasonal flow redistribution in regulated rivers.