Quantifying the contributions of climate change and direct human interventions on streamflow alteration using the GSFLOW hydrological model

Abstract
Background and objectives:
Climate variations and human activities are two major factors influencing the trends of hydrological cycle components in watersheds. It is essential to understand how flow behavior is separately affected by climate change and human activities in order to formulate effective adaptation policies and strategies for water resource planning and management. In recent years, the reduction in the discharge of the Hablehroud River has led to tensions among stakeholders of the basin’s water resources in Semnan and Tehran provinces, as well as between upstream and downstream farmers. Therefore, the aim of this research is to quantify the impacts of climate change and human interventions on the discharge of the Hablehroud River and to determine the relative contribution of each influencing factor. Additionally, this study investigates objectives such as identifying trends in climatic variables and the status of surface water resources, quantifying the effects of various natural and anthropogenic factors on these trends, predicting the future status of water resources in the study area under the continuation of current policies and strategies, and forecasting water resource conditions under different climate change scenario. The study area encompasses the upstream section of the Hablehroud watershed, extending to the Bonekouh hydrometric station, which is situated at the administrative boundary between Tehran and Semnan provinces. The Hablehroud River, serving as the main drainage of the watershed, has been increasingly affected in recent years by human activities and climate change, resulting in significant alterations to its hydrological status over the past decades.
Materials and Methods:
In this study, long-term statistics from hydrometric stations were collected and pre-processed. A significant change point in the annual discharge time series was then identified. The GSFLOW hydrologic model was applied to distinguish the effects of climate change and human interventions on the declining discharge of the Hablehroud River. In addition to analyzing the impact of influencing factors during the observed period, the region’s future climate conditions were projected using the Canadian Earth System Model (CanESM2) under RCP 2.6, RCP 4.5, and RCP 8.5 scenarios. Land-use changes and their dynamics in the study area were simulated using the Markov chain and CA-Markov models within the MOLUSCE module, an extension of the QGIS spatial information system. Future flow regime modeling under the combined influence of climate change and human activities was conducted using the GSFLOW model. Ultimately, the study provides a long-term perspective on the impacts of climate change and anthropogenic factors on the discharge of the Hablehroud River.
Results:
The results indicate that land-use changes in the Hablehroud basin have generally shifted toward an increase in agricultural lands, gardens, and residential areas, accompanied by a decline in pasture quality and continued degradation of natural vegetation. Climatic indicators also reveal an upward trend in the average temperature of the basin, along with changes in several precipitation indices. According to the findings, a significant abrupt change occurred in the annual discharge time series of the Bonekouh hydrometric station in the mid-1990s, leading to a decline in most hydrological indices of the Hablehroud River. The separation of climate change and human intervention effects showed that human activities have had a greater impact on the reduction of discharge in the basin than climate change. Hydrological modeling using the GSFLOW model suggests that climate change and human interventions contributed approximately -50.2% and 150.2%, respectively, to changes in the discharge of the Hablehroud basin. Climate projections for the basin indicate that, for most stations and months in future periods, total monthly precipitation will show a slight increasing trend, while temperature variations will exhibit a significant rise in both average monthly maximum and minimum temperatures. Furthermore, land-use map predictions suggest an expansion of irrigated agricultural lands and a reduction in pasture vegetation cover in the future. Based on climate projections and hydrological modeling with GSFLOW, future hydrological changes in the rivers of the Hablehroud basin are expected to follow the same trends observed in the past.
Conclusion:
Human intervention plays the most significant and influential role in the decline of the Hablehroud River’s flow, primarily due to land-use changes, which serve as key indicators of such interventions. These changes are largely driven by inappropriate administrative policies implemented across the basin. Therefore, reforming management practices and implementing appropriate policies to prevent unsustainable land-use changes can have a substantial impact on stabilizing the river’s discharge rate. Given the consequences of human interventions in the Hablehroud watershed such as the sharp decline in discharge rates in its tributaries and the intensification of conflicts among stakeholders it is recommended that management policies and interventions prioritize effective planning. This includes the regulation and optimization of human activities through the development and implementation of land preparation programs, as well as water consumption optimization and conservation strategies. The findings of this study can serve as a valuable reference for the future development, utilization, and management of water resources throughout the study area.

Conclusion:
Human intervention plays the most significant and influential role in the decline of the Hablehroud River’s flow, primarily due to land-use changes, which serve as key indicators of such interventions. These changes are largely driven by inappropriate administrative policies implemented across the basin. Therefore, reforming management practices and implementing appropriate policies to prevent unsustainable land-use changes can have a substantial impact on stabilizing the river’s discharge rate. Given the consequences of human interventions in the Hablehroud watershed such as the sharp decline in discharge rates in its tributaries and the intensification of conflicts among stakeholders it is recommended that management policies and interventions prioritize effective planning. This includes the regulation and optimization of human activities through the development and implementation of land preparation programs, as well as water consumption optimization and conservation strategies. The findings of this study can serve as a valuable reference for the future development, utilization, and management of water resources throughout the study area.