Analysis of river flow regime changes using the indicators of hydrologic alteration (Case study: Hableroud watershed)

Introduction
Hydrological regimes play a major role in changing the structure and function of ecological processes and river ecosystems. Significant changes in the hydrological regimes of river flow cause the spatial and temporal heterogeneity of river systems and the degradation of natural ecosystem services and threaten biodiversity. Trend analysis and change point detection are important topics in the analysis of hydrological time series. The study area in this research includes the upstream part of the Hablehroud river basin draining to the Bonekoh hydrometry station, located within the jurisdiction of the Tehran province. The Habaleroud river as the main drain of this watershed has recently encountered the pressures induced by human interventions and climate change, resulting in significant changes in its hydrological status.
 
Materials and Methods
In this research, using the sequential Mann-Kendall, Pettitt, Buishand Range, Buishand U, Standard Normal Homogeneity, and double mass curve tests, the significant change point in the annual discharge time series (1980–2017) of the Bonekoh hydrometry station at the outlet of the Hableroud watershed was detected. Then, using the Indicators of Hydrologic Alteration (IHA), the alterations in the hydrological condition in the period after the change point (Altered flow regime) compared to the period before the change point (the natural flow regime) were analyzed using the daily observed discharge data of the Hableroud watershed.
 
Results and Discussion
According to the research results, in the mid-1990s, a statistically significant change point in the annual discharge time series of the Bonekoh hydrometry station occurred, and most of the hydrological indicators show a deterioration in the condition of the Habaleroud watershed flow regime. Whereas for most of the hydrological indicators, after the change point, the frequency of the low values category has increased and the frequencies of the middle and high values categories, have decreased. These reductions have not only occurred for high extreme values, but also for low extreme values. In addition, the mean monthly discharge for all months of the year and the base flow of the basin have decreased. Both the frequency and duration of low-flow pulses have increased. On the contrary, both the frequency and duration of the high-flow pulses have decreased. For this reason, the frequency and magnitude of high extreme events such as medium and large floods have decreased. The long-term trend analysis indicated that 25 out of the 33 IHA have experienced a statistically significant decreasing trend. Therefore, the mean annual discharge of the watershed at the Bonekoh station has declined from 8.43 m³/s during the pre-impact period to 5.43 m³/s during the post-impact period, which is equivalent to about 35 % decline in the watershed outflow. While the watershed’s mean annual precipitation shows a negligible long-term increasing trend. Therefore, it seems that human interventions across the watershed play a major role in the hydrologic regime alteration of the watershed.
 
Conclusion
In the Benkoh hydrometric station in the mid-1990s, the hydrological regime of the basin has changed significantly. Then, using special software, the hydrological change indicators and key environmental flow indicators were analyzed in the periods before and after the change point. Unfortunately, most of the hydrological indicators show a downward trend in the Habaleroud river flow. So that the average discharge has decreased in all months of the year. Base current values are reduced. Both the frequency and duration of minimum current pulses are increased. On the contrary, both the frequency and duration of maximum current pulses are reduced. For this reason, the frequency and magnitude of extreme events such as medium and large floods have decreased. The results of the analysis of the trend of several indicators of the environmental flow also indicate the regressive course in the ecohydrological conditions of the Hableroud watershed. So that the minimum monthly flows for all months of the year show a downward trend. On the one hand, the continuity and frequency of periods of water shortage has increased, and on the other hand, the frequency of high flow pulses has decreased. The consequence of these changes will be creating tension and threatening riverside plant and animal communities that live in the flood plains of rivers and provide many ecological services. On the other hand, with the destruction of these riverside communities, the hydraulic conditions of the floodplains have changed and the vulnerability of river ecosystems and infrastructure facilities around the river increases against possible floods and causes a lot of damage. With the continuation of the existing process of managing water resources of the basin, stakeholders and beneficiaries of the basin will face many challenges in the future. Due to the fact that the average annual rainfall of Hableroud basin does not show a decreasing trend, it seems that human interventions are one of the main factors affecting the hydrological changes of this basin. Therefore, it is suggested that the main focus of management policies and measures should be focused on the management and optimization of human interventions in Hableroud watershed. In other words, instead of focusing on the top-down management approach and (hard) structural engineering measures, the focus should be on the participatory management approach and (soft) management engineering measures, and the water and soil resources of this basin should be used optimally and in accordance with the principles of sustainable development, so that at the same time Reducing the conflicts between the beneficiaries and the stakeholders upstream and downstream of the watershed (social challenges and threats) which currently occurred on a larger scale between the two provinces of Tehran and Semnan, and also preventing these conflicts from occurring on a smaller scale between the smaller communities upstream and downstream in The extension of waterways and rivers in the basin prevented the occurrence and spread of diverse environmental challenges and threats and vulnerability to natural hazards such as sudden floods and droughts. Also, it is suggested that the future changes in discharge of the studied watershed should be predicted according to the results of climate change models and land use changes, and suitable solutions should be formulated and implemented in order to deal with or adapt to these changes.