EVALUATING CROP BIOMASS PRODUCTIVITY AND WATER USE EFFICIENCY USING SEBAL-DERIVED EVAPOTRANSPIRATION: A CASE STUDY OF NISHON DISTRICT, KASHKADARYA REGION, UZBEKISTAN

Abstract

This paper studies how evapotranspiration (ET) and biomass production change over time in Nishon district, Kashkadarya region, Uzbekistan, between March and October 2025. Actual daily evapotranspiration (ET24) was calculated by the SEBAL energy balance model using Landsat 8/9 satellite images at 30 m resolution. Crop biomass production was estimated with a light-use efficiency approach (Monteith model) driven by NDVI. The monthly NDVI was highest in March (0.275) and lowest in October (0.165). Biomass production followed almost the same pattern, falling from 39.3 kg/ha/day in March to a minimum of 1.0 kg/ha/day in July, then rising again to 14.5 kg/ha/day in September. The link between NDVI and biomass was strong (R² = 0.87), but the link between ET24 and biomass was weaker (R² = 0.55). A pixel-level check also found a group of pixels with very high ET24 (up to 6.6 mm/day) but almost zero biomass, which most likely come from the Tallimarjon reservoir and wet bare soil. Another small group used very little water (around 0.2 mm/day) but reached water productivity above 3.6 kg/m³. These findings show that part of the water used in the district does not turn into crop growth, and that NDVI alone gives a better picture of biomass than ET24.