Monitoring water level changes in Hashilan wetland using remote sensing

Introduction
There are two approaches in dealing with environmental changes. The first approach is based on ignoring these changes and continuing the current situation, which will result in further environmental destruction. The second approach is based on identifying these changes from the past to the present and formulating an environmental management plan to control these changes and plan for improving the environmental situation. Wetlands are one of the most important natural habitats in the world and have many values and benefits, especially in energy circulation in the environment. In the last 20 years, the area of many wetlands has decreased due to the increase in industrial and agricultural development and the lack of water in arid and semi-arid regions (Cui et al., 2009). One of the effective tools in the field of wetland area changes is the use of remote sensing technology and satellite data. The use of satellite images, due to their wide spatial coverage, high resolution, low cost, temporal archive, free access to satellite images, and the availability of practical software and spectral indices, has become very important in estimating the area of aquatic zones. Monitoring and assessing the water level of wetlands using remote sensing to protect this natural heritage has become a focus for many countries in recent years, and this is of particular importance as the first step in identifying problems and attempting to solve them. Heshilan wetland plays a significant role in nourishing and increasing the stability of underground waters, preventing floods, and promoting agriculture in the region, as well as providing a unique habitat for native and migratory birds to easily pass winter. The aim of this research is to monitor changes in the water level of Heshilan wetland over a 22-year period using remote sensing and geographic information systems.

Methodology
Landsat satellite images were downloaded from the United States Geological Survey website. After downloading the Landsat satellite images, the Heshilan wetland area was delineated, and the images were radiometrically and atmospherically corrected. To enhance the spatial resolution of the study area, the 30-meter pixel size image was converted to a 15-meter pixel size image for better spatial resolution (the 15-meter resolution provides more accurate information than approximate surface area estimates of wetlands), then we normalized the surface reflectance values. The Target Detection Wizard tool in ENVI 5.6 software was used to determine the aquatic zone area of the wetland in Murch of each year during (2000-2022), and the output file was imported into ArcMap 10.8.2 to calculate the aquatic zone area of Heshilan wetland in square kilometers on the date of image acquisition.

Results and discussion
The results of interpreting satellite images over the years under study indicate that in 2018, the wetland had its lowest surface area in two decades, with an area of 0.34 square kilometers in that year. Additionally, the highest area of the aquatic zone of the wetland occurred in 2005, with an area of 3.83 square kilometers. The results show that over the past 22 years, the wetland has faced a decrease in water level by 0.79 square kilometers for the specific month.

Conclusions
The research results indicate that analyzing satellite images using remote sensing techniques can effectively show changes in wetland areas over time intervals. The results of interpreting satellite images over the years under study show a reduction in the extent of Heshilan wetland, especially in the western and southwestern regions, which can be attributed to natural phenomena such as drying up some springs due to droughts and periods of low water levels, as well as undesirable effects of human activities such as increased human access to wetlands through access roads in this area, land grabbing by villagers converting them into agricultural lands, deep wells around the wetlands, and utilizing wetland water through agricultural canals without considering proper water inflow. In these conditions, the wetland ecosystem has completely changed, and other aquatic birds no longer live in this area, with many migratory birds moving to tropical regions, depending on their habitats along the migration route. Given the unfavorable trends in Heshilan wetland developments, continuing the current conditions could jeopardize the existence and integrity of Heshilan wetland irreversibly. Ecosystem management of wetlands by developing a management plan to preserve and revitalize the structure and functions of this ecosystem with long-term stability can be a solution.