Modeling leaf and crop water use efficiency of rainfed Cotton: Interplay of environmental and biophysical drivers

Being the key eco-hydrologic trait linking carbon fixation (photosynthesis) and water consumption (transpiration), water use efficiency (WUE) plays a vital role in the plant-atmosphere continuum. However, ecological drivers and scaling relations on WUE represented at different scales is poorly understood. This study is aimed at simulating WUE of rainfed Cotton at leaf ‘WUE_L’ and crop ‘WUE_C’ scales, and further investigates the role of environmental and biophysical factors on WUE dynamics. The simulated leaf and crop WUE fluxes were used to establish the scaling relations. Stomatal conductance ‘g_s’ and WUE_L of Cotton leaves exposed to ambient CO₂ are simulated using modified Ball-Berry model with instantaneous gas exchanges measured around noon used to validate the model. We observed a large diurnal (4.3 ± 1.9 mmolCO₂ mol⁻¹H₂O) and seasonal (5.16 ± 1.51 mmolCO₂ mol⁻¹H₂O) variations in WUE_L. Model simulated g_s and WUE_L are in agreement with measurements (ρ>0.5, RMSE<0.3). Meteorological, phenologic, and management conditions during the crop cycle were used to simulate WUE_C using FAO-AquaCrop. Model calibration was performed with gravimetric measurements of soil water content (ρ = 0.95, RMSE = 21.8) and eddy covariance (EC) measurements of evapotranspiration (ρ = 0.57, RMSE = 0.88). Seasonal variations in WUE_C ranged from 1.7 to 7.3 kg/m³ during crop cycle. We observed strong linear relations between WUE_L and WUE_C during maturity stage (ρ = 0.84) followed by growth stage (ρ = 0.75). Eight climate and biophysical factors were considered to see their dependence on WUE at both scales. Our results conclude that WUE_L is governed by soil moisture content (ρ = −0.75) and leaf area index (ρ = 0.57), whereas WUE_C is controlled by canopy cover (ρ = 0.86) and vapor pressure deficit (ρ = −0.63). Our findings can help in developing effective water management strategies to improve WUE in rainfed Cotton.