Evaluating the soil retention ecosystem service using the InVEST model in Northeastern Iran

Introduction

Soil plays an important role in providing essential services needed by humans and other organisms and provides a variety of ecosystem services. Soil retention as an ecosystem service refers to the ability of an ecosystem to maintain soil and prevent erosion. This service is crucial as soil plays a vital role in providing essential goods for humans and other organisms, as well as a range of ecosystem services. Effective soil and water conservation management practices, particularly in enhancing vegetation cover for soil stabilization, are key. Therefore, safeguarding soil function against natural and human-induced threats is a critical regulatory function of ecosystems. In arid and semi-arid regions of Iran, ongoing degradation and significant changes in climate have impacted land cover, especially rangelands, affecting water conservation and ecosystem services provision. The Food and Agriculture Organization of the United Nations highlights that sustainable soil management can help prevent land degradation. By implementing measures such as conservation agriculture, reducing soil erosion, and enhancing water management, the risk of land degradation can be significantly reduced. Soil erosion and the consequent depletion of soil resources pose serious ecological and environmental challenges globally, hindering sustainable human development. The InVEST sediment delivery ratio model, built on the RUSLE erosion model, is a valuable tool for evaluating soil erosion, sediment export, and soil conservation across various spatial and temporal scales, widely utilized on an international scale.

Materials and Methods

Sarakhs Township, covering an area of approximately 5471 square kilometers, is a small part of the vast Qaraqom basin. Due to its proximity to the Qaraqom desert, this region has the potential for desert development based on its natural conditions. The cold and dry climate, low rainfall, minimal land use changes, and soil erosion have led to critical conditions for land degradation in the area. In this study, soil retention and erosion were quantified using the Sediment Retention model in the InVEST software. The necessary inputs for the model, including digital elevation model maps, land use data, rainfall erosivity, soil erodibility, and a biophysical table in raster format, were prepared in the ArcGIS 10.8 environment and inputted into the model. The DEM map of the study area was downloaded from Iran's weather and climatology website and extracted in ArcGIS version 10.8. The R factor was calculated using equation (5) based on index stations, and the R index was interpolated for the entire study area using the IDW method to create a rain erosion spatial layer in GIS. Data on the percentage of sand, silt, and clay were obtained from global soil information, converted to percentages, and used to calculate the soil erodibility index in the Raster calculator tool of the GIS environment. The NDVI index was calculated from Landsat satellite images in the Google Earth Engine platform. The P coefficient was determined using the slope-based Wenner method. A csv table with integer codes for each land use class in the land use map was required to run the model.

Results and Discussion

The findings indicated a wide range of soil retention levels, ranging from 0.47 to 0.53 tons per pixel per year, across different land uses. Pasture lands exhibited the highest soil retention rate at 5.98 tons per hectare per year, while residential areas had the lowest rate at 0.08 tons per pixel per year. The southwest region of the study area demonstrated the highest sediment retention capacity, likely due to the superior condition and quality of pastures in that area compared to other regions in the basin. Conversely, the central and eastern areas experienced increased erosion rates due to frequent land use changes and reduced vegetation cover. Developing a model to assess the ecosystem service of soil retention is crucial for effective ecosystem management. By utilizing spatial modeling, land managers can strategically plan to reduce sediment load at the watershed level by identifying conservation-worthy areas with high sediment retention capacity and optimizing land use practices.

Conclusion

To sum up, soil erosion and sediment production are significant issues in developing countries, leading to the destruction of agricultural lands, dam reservoir filling, and water pollution. This study highlights the importance of rangeland cover in soil conservation, with areas of low vegetation cover experiencing higher rates of erosion. The decline in sediment conservation in rangeland lands is attributed to reduced vegetation cover, caused by factors such as population growth, excessive livestock grazing, and prolonged land use. Land use change emerges as the primary factor influencing erosion rates and sediment conservation values. Given Sarakhs township's potential for desertification and its cold, arid climate with minimal rainfall, future studies should also assess wind erosion alongside water erosion. The results of this study can be useful in developing an ecosystem services management plan for land managers to have more effective spatial planning by identifying areas with conservation value due to the high supply of soil conservation and also paying attention to land use.