Determining the long-term changes in the flow regime of the Khiavchai mountainous river using the the Range of Variability Approach (RVA)

The changes in the river flow regime are the main factor in the stability of the river ecosystem and may change due to factors such as dam construction, water abstraction, flow diversion, and climate change. The hydrological regime of the river is the main driving force of ecosystem dynamics and river functioning. In addition, hydrological regimes play a major role in changing the structure and ecological processes of river ecosystems. On the other hand, the morphological characteristics are constantly changing over time as a dynamic system and these changes will be different due to changes in river flow and sediment transport capacity. The management of the watershed according to the distribution of the annual values of each of the 33 parameters of hydrological changes within the range of natural changes of the parameters is the basis of the range of variability approach. When the purpose of river flow change analysis is different between two time periods, the IHA software allows users to use RVA change methods to calculate indicators and compare results. The effective management of the river ecosystem requires the description of the parameters of the hydrological regime of the natural flow and the determination of the degree of changes in the flow rate of the regulated flow compared to the natural flow. Based on the literature review, the assessment of changes in the regime of rivers in mountainous areas requires comprehensive research. Khiavchai River of Meshgin shahr is a snow-fed rivers and a typical mountainous river, which has been selected as the study area. The purpose of this study is to investigate the changes in hydrological flow indicators in Khiavchai River, in this regard, long-term discharge data and range of variability (RVA) are used.

First, the changes in annual discharge statistics were evaluated in the periods of 1969 to 2019 and then the studied periods were analyzed based on the change point analysis software. The range of variability approach is a type of hydrologically-based methods for river flow analysis. In this regard, management goals should be determined based on available ecological information regarding the river flow regime. In the absence of appropriate ecological information, it is recommended that the standard deviation range can be considered as a default for the initial determination of targets. In other words, the normal values of each of the IHA parameters in the normal state should be considered within the range of standard deviation (± STD) from the average values, or the 25% and 75% quartiles should be considered as the lower and upper limits of the parameters, respectively. The results of the changes in five groups of parameters of hydrological changes (IHA) in the range of range of variability changes were obtained at the Pol-Soltani hydrometric station, Khiavchai River using IHA software. In this regard, hydrologic change indicators are calculated using Indicator of Hydrologic Alteration software and compared and analyzed in different periods.

The results showed that the annual runoff series changes in two consequent periods. The changes in the average discharge variables in different months in both study periods of the year have been decreasing and the rate of change of the index has been negative compared to the previous periods. The results showed that the low flow discharge minimum 1, 3, 7, 30 and 90 days decreased and decreased to zero in the third period compared to the second period, to a value of 0.01. The mean changes of the base flow index also showed a decrease from 0.02 to zero. It is worth noting that the base flow index can lead to the reduction of snow storage or the destruction of vegetation, and as a result, it is upstream of the study area. The increasing trend of low flow pulses and recession rate also decreases considerably.

As a concluding remark, the pattern of changes in river flow indicators based on the changes of hydrologic indices in both study periods had a similar trend, which indicates the decreasing trend of changes in river flow regime components, and these changes have occurred as a result of water abstraction and river flow diversion. Although a part of the changes in discharge values can be related to the change in hydroclimatic variables, but due to lack of sufficient studies on changes in climatic variables, it is not possible to draw a firm conclusion in this regard. Determining changes in climate generators for the river flow regime can be suggested as one of influencing factors in river behavior changes as well as better management of surface water