Quantitative assessment of cotton evapotranspiration, irrigation requirements and water productivity from weather-based estimations

This study investigates the water requirements, evapotranspiration dynamics and water productivity of cotton (CO 17) under semi-arid climatic conditions across three cropping seasons (2023-2025) at Agricultural Research Station, Bhavanisagar. Field experiments were conducted on sandy loam soils with continuous monitoring of weather, soil moisture and crop parameters. The standardized FAO Penman-Monteith equation was used to compute daily reference evapotranspiration (ETo), while actual crop evapotranspiration (ETc) was estimated using both locally developed and FAO-based crop coefficient (Kc) curves derived as functions of thermal units. Seasonal ETc ranged from 386 to 607 mm and irrigation applied varied between 170 and 336 mm. Seed cotton yield ranged from 1997 to 2237 kg/ha and lint yield from 678.8 to 761.7 kg/ha, with corresponding water use efficiency (WUE) values between 0.107 and 0.155 kg/m³ and water productivity (WP) from 0.32 to 0.46 kg/m³. Statistical analyses, including paired t-tests and regression, confirmed no significant difference (p > 0.05) between local and FAO-based ETc estimates, while a strong linear relationship (R² = 0.93) was observed between adjusted ETc and irrigation applied. The locally calibrated Kc improved ETc estimation by better representing canopy development and climatic variability. Precisely scheduled irrigation and site-specific Kc calibration enhanced yield, water productivity and resource-use efficiency, emphasizing the importance of ET-based irrigation management for sustainable cotton production in water-limited environments.