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

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.

Landsat, Mann-Kendall trend test, drought monitoring

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.