Assessment of the Impact of Watershed Management Practices on Hydrological Variables in the Mohammadabad Watershed using the SWAT Model

Abstract

Introduction

Hydrological processes in watersheds are dependent on atmospheric processes and the physical characteristics of the watershed, playing a key role in the planning and management of water resources. In recent years, watershed management practices have gained attention as a managerial approach to improve ecosystem performance. These practices positively impact hydrological parameters of water bodies, including reducing flood risk, increasing soil permeability, and enhancing water quality. Additionally, by decreasing surface runoff, groundwater resources are also bolstered. Furthermore, reducing evaporation and improving water distribution throughout different seasons aids in optimizing water resource management, contributing to the sustainability of the environment and ecosystems. Therefore, assessing the impacts of these practices is essential for enhancing their efficiency. However, due to the complexity and high costs of these evaluations, the process can be challenging. Consequently, hydrological models are often utilized to simplify natural conditions and assist in water resource management, enabling the planning and prediction of changes in the hydrological cycle. In this study, the effectiveness of these practices was evaluated using the SWAT model in the Mohammadabad watershed of Golestan.

Materials and Methods

For this study, the required data, including climatic, physiographic, hydrological, land use, geological, and soil data, were obtained from relevant authorities and organizations. After gathering the necessary data, a database was created and data was analyzed using Excel 2019, SPSS 26, ArcGIS 10.8, and TerrSet 18.21 software. To simulate the climatic variables of the study area, daily data on rainfall, minimum and maximum temperatures, solar radiation, relative humidity, and wind speed from the Mohammadabad station for the years 1988 to 2017 were utilized. To examine the elevation of the study area, contour lines from 1:25,000 scale maps were imported into the GIS system, and a digital elevation model (DEM) with 10×10 m pixels was created, correcting any potential outlier cells using a LOW filter. The land use of the study area was initially prepared using 1:25,000 scale maps from the National Cartographic Organization, then refined and finalized using satellite images and field visits for high accuracy. For the SWAT model implementation, some necessary soil parameter data were obtained from the Golestan Natural Resources and Watershed Management Department, and due to the unavailability of some parameters, data for those parameters were sourced from FAO soil maps. Various algorithms and objective functions for calibration and validation were utilized using the SWAT-CUP software, and after evaluating the model performance in simulation and obtaining acceptable results, the model was prepared for the implementation of water and soil conservation measures.

Results and Discussion

The results indicate that the calibration and validation results demonstrate the acceptable performance of the SWAT model for simulating the hydrology of the Mohammadabad watershed. Specifically, the coefficient of determination during calibration and validation was found to be 0.93 and 0.98, respectively. Additionally, the Nash-Sutcliffe efficiency index was 0.90 for calibration and 0.96 for validation. Based on the findings, surface runoff would decrease significantly with the implementation of watershed management practices such as stone check dams, gabion walls, dry stone walls, afforestation, and seeding, by 36.36%, 22.73%, 18.18%, 18.18%, and 4.55%, respectively, compared to the scenario without these practices. Moreover, evaporation and transpiration were observed to increase by 85.69%, 53.87%, 29.87%, 10.11%, and 1.64% respectively when these interventions were applied. The implementation of watershed management practices in the Mohammadabad watershed has led to a reduction in peak flood discharge, decreasing from 181.5 m2 .s before these practices to 156 m2 .s per second afterward. The analysis of the 25-year flood discharge in the study watershed revealed a decrease of 13.13 m2 .s second, dropping from 93.47 m2 .s before the management practices to 80.34 m2 .s after. According to the study results, mechanical projects in the Mohammadabad watershed have successfully reduced surface runoff and flooding, subsequently decreasing soil material loss, erosion, and the transport of eroded particles from channels. This has also led to a reduction in sediment transfer from slopes to waterways, and from waterways to rivers and eventually to reservoirs and agricultural lands. In this context, the implementation of all watershed management practices in the Mohammadabad watershed has resulted in a 16.34% reduction in the 25-year flood discharge.

Conclusion

Overall, the findings suggest a significant positive impact of these practices within the examined watershed. The results showed the impact of watershed management measures on hydrological variables including surface runoff, peak discharge and flood volume of the Mohammadabad watershed, which will reduce the destruction and damage caused by floods in the study area. Based on the results of the watershed management operations in the study area, it was determined that biological operations are difficult to implement due to the high slope and shallow soil depth (mountainousness of the area). Therefore, since the Mohammadabad watershed is near the city of Fazelabad and has a mostly touristic aspect and the local economy is not dependent on livestock farming, therefore, management operations will be more effective. Also, mechanical operations will be prioritized over mechanical programs due to conditions such as high slope, shallow soil depth (mountainousness of the area), low concentration time, high peak discharge (basin shape coefficient) and consequently the presence of debris sediments and, most importantly, the presence of a flood history in the study watershed. Finally, it can be acknowledged that watershed management measures, in addition to helping reduce damage caused by floods and sediment, can also turn the threat of floods into an opportunity to nourish groundwater aquifers and provide water in springs, canals, and wells during droughts, at the lowest cost and in accordance with the ecological conditions of the region.