Quantifying the contributions of climate change and direct human interventions to streamflow alteration in the Hablehrood watershed using empirical approaches

Climate changes and human activities are the two main factors altering the hydrological cycle of watersheds causing changes in the spatial and temporal distribution of water availability. The river flow, as the most critical component of the hydrological cycle, is the most vulnerable component that is affected by these changes resulting in worrying consequences on water demand by different sectors. Since there is growing controversy on the contribution of two main factors (climate change and human interventions) affecting river flow alteration, assessment and quantification of their contribution is of utmost importance to the water resources managers. Quantification of climate change and direct human intervention contributions to streamflow alteration is a prerequisite for developing adaptation strategies and policies for regional water resources planning and management. To this end, various approaches and methods have been developed and proposed during the last decades in order to separate the contribution of natural and human-induced factors on river flow regime conditions. The current research investigates the contribution of climate change and direct human interventions on the discharge decline of the Hablehrood watershed.

In this research, the upstream area of the Hablehrood watershed draining to the Bonekouh hydrometry station, located within the Tehran Province jurisdiction, has been studied. The hydrological condition of the Hablehrood watershed, as a main drainage channel of the watershed, has drastically altered during recent years and its discharge has significantly decreased. In this study, first, the long-term statistics of Simindasht and Dalichay hydrometric stations were collected and subjected to pre-processing. Then, using the Pettit and Lanzante statistical tests, the significant change point in their annual discharge time series was identified. Then, the empirical methods of the slope change ratio of accumulative quantity (SCRAQ) and double mass curve (DMC) were applied to separate the effect of climate change and human interventions on the discharge decline of the Hablehrood watershed and its main tributaries. Since rainfall, temperature, and potential evapotranspiration are considered the main climatic elements in both the used methods, the relationship between discharge, and precipitation, between discharge and precipitation as well as temperature, between discharge and precipitation as well as potential evapotranspiration were analyzed in order to compute the contribution of climatic drivers first. According to the empirical methods, after calculating the contribution of climatic variables to the changes in water flow, the remaining changes in water flow are attributed to human interventions. In this study, the observed data from the hydrometry stations of Simindasht and Dalichay during 1980 – 2017 were used. For SCRAQ, the slope change of the ratio of cumulative discharge and cumulative values of rainfall and temperature as well as the slope change of the ratio of cumulative discharge and cumulative values of rainfall and evapotranspiration were computed to separate the effects of climate change and human interventions.

The results of both tests showed that the hydrological regime of the basin changed in the mid-1990s in both hydrometric stations and the average annual discharge drastically decreased in the period after the change point. Results of different calculation methods of SCRAQ (the slope change of the ratio of cumulative discharge and cumulative values of rainfall and temperature (first method) and the slope change of the ratio of cumulative discharge and cumulative values of rainfall and evapotranspiration (second method)) showed that human intervention is the main cause of discharge decline within the Hablehrood watershed. According to the first calculation method, the contribution of climate change on discharge decline at the Simindasht and Dalichay stations were, respectively, 15.53 % and -37.08 %, and for the second calculation method, 0.55 % and -39.72 %, respectively. The positive values for the contribution of climate change to the Dalichay station indicate that climate change has resulted in an increase in its discharge. Results of different calculation methods of DMC between cumulative values of discharge and climate variables showed that climate change has increasing effects on the discharge of both Simindasht and Dalichay sub-watersheds.

Human intervention is the main cause of discharge decline in the Hablehrood watershed and the contribution of climate change is very small and incremental in some cases, particularly for the Dalichay sub-watershed. Therefore, management policies and priorities should be focused on managing human interventions, promoting public awareness, optimal using of water resources, and preventing over-exploitation of water resources across the watershed. The results of this study can provide a scientific guidefor the development, utilization, and management of regional water resources and ecological environment protection.