25-Year Spatiotemporal Assessment of Vegetation Dynamics and Drought Severity in The Amu Darya Irrigated Zone Using Landsat Time Series and Mann-Kendall Trend Analysis
Abstract
This study presents a 25-year spatiotemporal assessment of vegetation dynamics and drought severity in the Khorezm irrigated zone of the Amu Darya Basin (59–62.5°E, 40.5–42.5°N) over the period 2000–2024. Annual Normalized Difference Vegetation Index (NDVI) values were derived from Landsat Collection 2 Surface Reflectance imagery accessed via the Microsoft Planetary Computer STAC API, filtered to cloud cover below 20% and growing season months (April–September). The Vegetation Condition Index (VCI) was computed to classify annual drought severity according to the five-class Kogan framework. The Mann-Kendall non-parametric trend test and Sen's slope estimator were applied to assess long-term vegetation trends. Results indicate a mean annual NDVI of 0.1094 across the study period, with a minimum of 0.0754 in 2001 (Extreme drought) and a maximum of 0.1442 in 2022 (No drought). VCI classification identifies two years of Extreme drought (2000–2001), one year of Moderate drought (2008), fourteen years of Mild drought (56%), and eight years of No drought (32%). The Mann-Kendall test detects a positive trend direction (Kendall's τ = +0.267, p = 0.065, Sen's slope = +0.000548 NDVI yr⁻¹) that does not reach statistical significance at p < 0.05, reflecting high interannual variability and non-monotonic dynamics. The findings characterize the Khorezm irrigated zone as a landscape under chronic structural water deficit, with the 2016–2022 period representing the best sustained vegetation conditions of the entire record.
Keywords
Downloads
References
- Saiko, T.A., & Zonn, I.S. (2000). Irrigation expansion and dynamics of desertification in the Circum-Aral region of Central Asia. Applied Geography, 20(4), 349–367.
- Statistical Yearbook of the Republic of Uzbekistan. (2025). Desertification, land degradation and drought affected area. Tashkent: State Statistics Committee of Uzbekistan.
- Tischbein, B., et al. (2012). Water management in Khorezm, Uzbekistan: Current situation and options for improvement. In Cotton, Water, Salts and Soums (pp. 69–92). Springer.
- Conrad, C., et al. (2013). Remote sensing and hydrological measurement based irrigation performance assessments in the upper Amu Darya Delta. Agricultural Water Management, 118, 1–10.
- Dubovyk, O., et al. (2013). Spatio-temporal analyses of cropland degradation in the irrigated lowlands of Uzbekistan using remote-sensing and logistic regression modeling. Environmental Monitoring and Assessment, 185(6), 4775–4790.
- Microsoft Planetary Computer. (2024). USGS Landsat Collection 2 Level-2 Science Products. https://planetarycomputer.microsoft.com/dataset/landsat-c2-l2
- Kogan, F.N. (1995). Application of vegetation index and brightness temperature for drought detection. Advances in Space Research, 15(11), 91–100.
- Hussain, M., & Mahmud, I. (2019). pymannkendall: A Python package for non-parametric Mann-Kendall family of trend tests. Journal of Open Source Software, 4(39), 1556.
- Kendall, M.G. (1975). Rank Correlation Methods (4th ed.). London: Charles Griffin.
- Zhang, Z., et al. (2023). The impact of climate change and human activities on the Aral Sea Basin over the past 50 years. Journal of Hydrology: Regional Studies, 44, 101248.