Application of InVEST Water Supply Ecosystem Service Model to Predict Water Erosion in Bandar Abbas Basin

Instead of focusing on how development affects ecosystems, the ecosystem service approach focuses on how ecosystems affect development. By quantifying the water production service in a basin, it is possible to determine the amount of water production in different uses of the land and use this information to apply better management decisions in line with the potential of the region. The provision of this service depends on the characteristics of the watershed such as topography, vegetation and land use, climate and other parameters governing the provision of services. Various tools and models have been developed to study these services to help authorities make appropriate decisions for ecosystem management. These tools are divided into two general categories, traditional hydrological tools and ecosystem service tools, which are service-specific tools. InVEST tools focus on the final services of the ecosystem and map the services and in this research has been used to serve the water production of the study area.

The study area of the coastal strip of Bandar Abbas from Bustano (west) to Bandar Abbas airport (east) is 69 km along the coastline. Therefore, to make the research comprehensive, the smallest hydrology unit in the area or catchment area where this area of Bandar Abbas city is located was selected. In this research, the water production model from the model set, InVEST 3.8.9 was used with the aim of plotting the possible future of the study area. This process consists of two general steps, first processing and preparation of satellite images, then modeling water production with InVEST model. Landsat satellite images (5 and 8) were used to prepare mapping images of the study area and after performing radiometric, atmospheric and ground reference corrections (by Ye and Grimm method, 2013) And the classification of the images was achieved with the maximum likelihood similarity algorithm (by Shrestha et al method. 2018).

The watershed area that has been selected as the study area includes three sub-basins with a total area of 272,806 hectares. In this section, the land use of the study area was divided into 5 categories: water, urban, agricultural, arid and grassland. And modeling was done first in the year 2000 as the base year of comparison for subsequent years and then in 2020. In the year 2000, the area of human-made floor is 10721 hectares, water is 671 hectares, agriculture is 11285 hectares, arid lands are 150123 hectares and grassland cover is 100003 hectares. The percentages of each land use are 3.9, 0.24, 4.1, 55 and 36.6, respectively. These figures for 2020 are: human-made land uses with 6.2% and water, agriculture, arid and grassland land uses with 0.62, 8.7, 50 and 34%, respectively. It can be seen that human-made, water and agricultural uses have increased and pasture and wasteland uses have decreased. Examination of the water production map shows that it increased from a maximum of 81 mm in 2000 to 70 mm in 2020. Also, by comparing the maps, the amount of water production in the study area had decreased from 7.143.433 cubic meters to 5.463.450 cubic meters in 2020, which indicates a decrease of 23%. The results are also visible at the sub-basin level. In 2020, the reduction of water production in sub-basins 12, 11 and 13 demonstrated a decrease of 24, 21 and 27%, respectively.

Comparing the land use cover of the base conditions in 2000 and 2020 revealed that man-made, water and agricultural land uses have increased and grassland and arid land uses have decreased. Comparing the rainfall maps indicated that the rainfall situation in the study area is increasing from south to north and the lowest rainfall in the southwestern part and the highest rainfall in the northern and eastern parts of the study area and for both periods of rainfall status are almost the similar.The InVEST water production model estimated the ecosystem water production volume in the whole basin to be more than 7 million cubic meters in 2000 and more than 5 million cubic meters in 2020, which represents a decrease of 23%. The map of water production by sub-basin shows a decrease in all sub-basins in 2020 compared to 2000, and among sub-basins, sub-basin has the highest and one lowest production, and this is the same in both periods.
According to the obtained results, the study area has severe water stress based on water production and basin area. In adiiton, by considering the importance of water and taking into account other factors such as climate change, rainfall reduction, change in rainfall type, global warming, increase in evaporation, and population increase, we need to decide and invest in supply, transmission, and increase of water efficiency. This is an important endeavor considering the factors changing the type of rainfall, which means the intensity and aggressive nature of local rainfall in the study area and its role to start the separation and transfer of soil particles by runoff. The results of the present study also shows lower overall rainfall in 2020 compared to the year 2000, which affects the inherent sensitivity of the soil and makes it more susceptible to erosion, which, in turn, doubles the importance of addressing this issue.
Due to the fact that water production in the whole basin has severe water stress, and the overall decrease in rainfall in 2020 compared to 2000, this situation will have a greater impact on the inherent sensitivity of the soil and predisposes it to erosion.This is an important issue considering the intensity factor and the aggressive nature of local rainfall in the study area and its role in initiating the separation and transfer of soil particles by runoff which exposes the study area to more severe water erosion. Also, considering the spatial pattern of precipitation changes in the northern parts of the watershed, this area will be more erodible, and due to the impact of wind erosion on water erosion, the results of investigations for this section are useful to help study wind erosion and increase fine dust.