Shift in streamflow seasonality and regime due to precipitation variability in a snow-influenced mountainous watershed

Abstract

In mountainous regions, water availability and flood hazards are tightly linked to the timing and seasonality of streamflow, making their analysis indispensable for sustainable resource planning. In this regard, this study examines changes in seasonality and precipitation in the Khiav Chai River watershed, a snow-influenced mountainous region in northwest Iran, over the 1970–2022 period, using Innovative Trend Analysis (ITA), Pettitt change-point detection, and circular statistical analysis. Contrasting results were observed in relation to the hydro-climatic variables studied in the research period. While annual precipitation followed an upward trend (ITA slope = 0.022), the streamflow data indicated a decreasing trend (ITA slope = -0.00093). The Pettitt test found that statistically significant changes occurred in 1984 and 1995 in streamflow and precipitation, respectively, implying that changes in the hydrological regime occurred prior to changes in precipitation. According to the circular statistics, the Center of Mass Timing showed only a minor difference between precipitation and streamflow (precipitation: 5.09 months; streamflow: 4.45 months) between these two hydro-climatic variables over time. On the contrary, seasonality index showed opposite directions. Decreasing seasonality was observed in precipitation (τ=-0.23, p  = 0.01) implying an even distribution of rainfall throughout the year, while increasing seasonality was observed in streamflow (τ = 0.34, p  < 0.001) indicating a higher amount of streamflow occurring at specific periods during the hydrological year. Based on polar vector analysis, clustering and seasonal concentration were significantly more pronounced in streamflow than precipitation, showing the effect of snow accumulation and melt processes in watersheds. The results indicate a growing trend of separation between the seasonality of precipitation and streamflow, implying the non-stationarity of the hydrological regime within the watershed. This study demonstrates that precipitation and streamflow seasonality can decouple in snow-influenced watersheds, where snow dynamics mediate hydrological responses to climate change. These results suggest that the effects on the hydrological system cannot be solely attributed to modifications in precipitation patterns and that seasonal flow adjustments should be taken into account for adaptation purposes.