Temporal changes and flow pattern analysis using Colwell indices in mountainous rivers

The objective of the present study was to investigate the temporal changes of the river flow pattern using Colwell Indices (CIs) in rivers originating from the Sabalan mountainous. The simultaneous hydro-climatic and river flow data were used to determine the distinct periods of flow regimes. Then the flow statistics and CIs (Predictability, Constancy, and Contingency) were calculated annually. The results reveal that the Max statistics had a decreasing trend in Lai, Nir, and Pole-Soltani river gauge stations with 75.7, 89.3, and 81.4%, respectively. The difference in Lane index values in Ahl-e-Iman, Lai, Nir, Pol-e-Soltani, and Viladragh river gauge stations was −38.00, 20.8, 16.1, −1.8, and −35.3%, respectively, which its variations attributed to the precipitation variable. The Zero-days index increased considerably in Ahle-Iman and Pole-Soltani river gauge stations with the amounts of 201 and 947%, respectively. According to the Constancy index, the monthly stability of the flow in Ahl-e-Iman and Pole-Soltani river gauge stations was lower than the other stations, and the Nir station had the highest flow stability. The value of the contingency index at Lai and Villadaragh stations was shallow, meaning the minimal probability of occurrence of monthly river flow. As a concluding remark, the hydrological regime and the river flow indices in studied rivers had altered under the influence of precipitation and temperature change. Different aspects of altered patterns of river flow regimes can be captured by the Colwell’s indices in terms of predictability, constancy, and contingency of in mountainous virgin rivers. Therefore, the proper management decisions should be taken in accordance with the change in the river flow regime, regarding maintaining river flow and the climate change impacts.