Spectral characteristics of saline arable land in the foothill catchment Zailiisky Alatau ridge (Northern Tien-Shan) during 2016–2022 using Sentinel-2 data
Authors:   Institute of  information and Computational Technologies, Terekhov Alexey , Sagatdinova Gulshat ,   Institute of  information and Computational Technologies
Published: May 2024

Introduction. The foothill catchment of the Zailiisky Alatau (Northern Tien Shan), adjacent to the ridge from the north, occupies an altitude range of 475–925 m above sea level and has an area of about 8500 km2. The Almaty urban agglomeration with a population of 2.5 million people is located here and there is a developed agriculture. Crop production in the sharply continental, arid and hot climate of Central Asia is based on irrigation agriculture. For irrigation in the foothill catchment, river runoff from the Zailiysky Alatau Ridge is used. In some parts of the foothill catchment there are salt stores and salt outbreaks in the landscape, which contribute to the salinization of arable land. Secondary salinization of arable land is a negative factor that reduces their quality and productivity. Monitoring the salinity of arable land is practically an important task. The most effective technologies for detailed assessment of soil salinity are based on the processing of satellite data. The purpose of the work. The purpose of this study was to analyze the long-term spectral characteristics of agricultural land potentially susceptible to secondary salinization. This information is necessary to determine optimal strategies in the task of remote assessment of salinization of arable land in the foothill catchment of the Zailiysky Alatau Ridge. Materials and research methodology. The 323 cloudless Sentinel-2 images (10 m resolution) of the period 2016–2022 were selected. Four spectral indices: Normalized Difference Vegetation Index (NDVI); Soil Adjusted Vegetation Index (SAVI); Vegetation Soil Salinity Index (VSSI); and Simple Ratio Index (SRI); were used to monitor the test site, which was located in the northeastern part of the foothill catchment of the Trans-Ili Alatau Ridge. The test site included three polygons of irrigated arable land subjected to secondary salinization, and two polygons of saline abandoned land. The satellite monitoring of spectral indices was used to determine the most informative periods within the season, as well as to assess the inter-seasonal stability of the results. Results and discussion. The dynamics of the considered vegetation indices (NDVI, SAVI and SRI) differed most significantly in July-August, i.e., in conditions of the most developed vegetation. The VSSI index was the most informative in April-May and December-February. The vegetation condition of saline arable lands is controlled by the moisture content and salinity of the rootinhabited soil layer. The precipitation deficit leads to deterioration in the state of vegetation, which masks the negative impact of secondary salinization of fields. Thus, the assessment of the significance of the factor of secondary salinization in the state of vegetation requires annual ground sub-satellite surveys. Conclusion. The variations in the values of the spectral indices of test polygons are very large and depend on the seasonal weather characteristics. Therefore, practically significant satellite methods for assessing the salinity of arable land in the foothill catchment of the Trans-Ili Alatau Ridge should be based on annual sub-satellite surveys. This limits the range of significant remote sensing technologies, methods of extrapolation of sub-satellite surveys data to regular grids, within territories with similar hydrological characteristics and land-use types.