Introduction
The location of the unique ecological area of Hamoon in the Sistan plain has given it a special environmental importance, and according to the Ramsar Convention, the Hamoon area as an international wetland has special ecological values for the world. However, neglecting the ecological water requirement has led to some natural ecosystems being seriously damaged and at the same time turning into dust sources. The need to determine the minimum water requirement of aquatic ecosystems and the regulation of environmental flows has received global attention and is one of the important challenges of water management and wetland ecosystems. Available free satellite data can reveal long-term surface water changes, improve modeling, and inform changes for water management decision-making. Using of the Google Earth Engine (GEE) platform has solved the need for speed in processing and access to various data. The use of the world surface water layer product (JRC Global Surface Water Mapping Layers), by providing the Transition band and Seasonality band which is the result of changes in water areas from 1983 to 2021, can provide a suitable base level. In order to calculate the water requirement, it should provide based areas with less time and cost. In this study, the base level of Hamoons was extracted using the mentioned product in Google Earth Engine and interpreted in ArcGIS software. Finally, the water requirement of Hamoons was calculated using the hydrological method.
Materials and Methods
In the Sistan region, located in the east of Iran, there are three main Hamoons named Pozak, Sabori and Hirmand. In order to estimate the water required for wetland restoration, using the hydrological water balance method, the water losses in one year will be compensated through precipitation and discharge. In order to estimate the pattern of wetting of the wetland area, the global surface water layer product was used in the Google Earth Engine environment.
Considering that this product is derived from the changes of water zones between 1984 and 2019, it can be zoning the wetland based on the hydrological trend. First, the changes of the aqueous zone were investigated using the Transition band. Then the classes in this band were interpreted using the Seasonality band. The final map was created by merging the classes that followed the same behavior. In order to reveal the trend of the water body area of the wetland in different seasons, a number of plots were extracted from each class and the trend of the NDWI index was monitored over five years (from the beginning of 2016 to the end of 2020). Then, using the monthly long-term average (fifty years) of precipitation, evaporation and transpiration of the area, which was converted into the seasonal average, the water losses and the amount of discharge required for the stability of the water body of the wetland were estimated.
Results and Discussion
The " Permanent to Seasonal " and " Lost Permanent " classes were wet in the past and currently receive less water. Therefore, they can be first priority. At the same time, the "Seasonal" class shows the most presence of water during the year. As a result, it can be second priority. The first priority area is 993.7 square kilometers, the second priority area is 871.7 square kilometers, and the third priority area is 1370.6 square kilometers. Therefore, by merging the above-mentioned classes, the base level map for the estimation of Hamoon's water requirement was obtained. After calculating the median of NDWI index for each pixel, the 5-year trend was monitored for each zone (class of each priority). The monitoring of NDWI changes in each of the zones shows that the surface with the first priority in the period of April to August (spring and summer) and the surface with the second priority in the period of February to June (winter and spring) is wet. The surface with the third priority is often dry and in some years it is rarely wet in April (spring season). As a result, the water requirement of Hamoons with an area of 3237 square kilometers is equivalent to 3804 million cubic meters per year. Conclusion: According to the results of this research, the water requirement of Hamoon is 3804.3 million cubic meters per year with a base area of 3237 square kilometers.
Conclusion
Using the pattern of the number of months of water presence in the wetland, which is extracted from the seasonality band, the eleven classes of the transition band were interpreted. As a result, it can be second priority. The results show that the pattern of long-term behavior of water in the Hamoon area indicates the absence of a permanent water surface in these ecosystems. Examining the trend of changes shows that the areas are wet in three seasons: spring, summer and winter. The results of this research, using the GEE product and the change of the trend of NDWI index, could help reveal the hydrological system and lead to a more realistic estimate of the ecosystem's water requirement. Perhaps the best advantage of this method compared to the traditional types of using satellite images, in addition to spending less time, is a more comprehensive view of the unique changes of each water zone compared to other zones. In fact, using the time series of images helps to plan for each water area according to its nature and will prevent prejudice and repetition of a fixed pattern. Estimating water requirement along with its seasonal distribution, can help managers and policy makers to demand the required amount of water and increase productivity by managing water distribution.
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