River flow regimes play a major role in changing the structure and function of ecological processes in river ecosystems. Significant changes in the hydrological regimes of river flow cause spatial and temporal heterogeneity of river systems and the destruction of natural ecosystem services and threats to biodiversity. In addition, the evaluation of the changes in the river flow regimes will be used to determine the dominant times in terms of sediment transport. Estimation of natural river flow regime is very important for planning water resources in watersheds. Determining the temporal and spatial distribution of the river flow can help policy makers and decision makers in managing water resources.
Evaluating the temporal distribution of river discharge based on long-term data can be useful in determining the dominant regime of rivers and planning the use of surface water. In this regard, the current research is planned with the aim of analyzing the changes in the water yield concentration index (WYCI) in 31 hydrometry stations of Ardabil province (1974-2018) over different time scales and its spatial changes using GIS.
Toward this attempt, the WYCI index was calculated in all selected river gauge stations for available recorded data and the spatial variation of the river flow regime has been prepared using the inverse distance weighted method. In this regard, in order to analyze the distribution of water yield concentration index in different time scales in the selected stations of the study area, the WYCI index was calculated in wet and dry periods, seasonal and annual time scales. It should be mentioned that dry and wet periods were considered from June to November and December to May, respectively, according to the long-term river flow regime and the hydroclimatic condition of the study area.
According to the box-plot of the river discharge values, the highest range of seasonal changes has been observed in spring season, and the lowest changes is related to summer season. According to the results, the highest median amount of flow discharge was related to the spring season (3.7 cubic meters per second). In addition, the median discharge values of the winter, and autumn seasons were 2.5 and 1.3 cubic meters per second, respectively. Also, the lowest median value of the flow was observed in the summer season with a value of 0.3 cubic meters per second. The results showed that the values of WYCI index in autumn, winter, spring and summer are 11.9, 10.7, 13.8 and 15.2, respectively. In addition, the values of the WYCI for dry and wet periods was detected as very irregular (21.6) and moderate irregularity (14.3). According to the results, the stations located in the southern hillslopes of Sablan (Lai, Nir, Atashgah and Yamchi river gauge stations) had a regular river flow regime, which are fed by snow dominant precipitation. While, the stations located in the eastern part of the province are distinguished from the autumn season towards the summer season with increasing WYCI values and as a result more irregularity of the river flow regime.
As a concluding remark, it can be said that the analysis of spatial changes of WYCI indicates the existence of a unique spatial pattern in different time scales and it needs to be considered in the planning of water use in Ardabil province. The evaluation of temporal and spatial changes of water yield can be effective in determining areas with high water yield and sound management based on water supply. In this regard, while comparing the flow regimes in different rivers, it is possible to prioritize months or different times of the year in terms of water flow. In addition, predicting future changes using hydrological modeling and climate change scenarios is suggested to a better understanding of river regime changes in the future. Finally, it is suggested to differentiate the climatic and human effects in changing the river flow regime due to their distinct in terms of magnitude and timing.