Dynamics of Ecosystem Water Use Efficiency in Citrus Orchards of Central India Using Eddy Covariance and Landsat Measurements

Accurate quantification of water use efficiency (WUE) and characterization of its variability across multiple time scales can help to initiate appropriate management measures for cropland ecosystems in response to natural and anthropogenic changes. This study is aimed to understand the diurnal and seasonal patterns in WUE and its dominant controls in the citrus orchards of central India. We used eddy covariance measurements to estimate the evapotranspiration (ET) and gross primary product (GPP) fluxes from two crop cycles (2016 and 2017). On a daily scale, ET and GPP exhibited similar patterns, with peaks occurring during the fruit development stage. The daily WUE ranged from 0.22 to 3.39 g C kg(-1) H2O with a mean of 1.77 g C kg(-1) H2O. We observed high interseasonal variability in WUE, which emphasized the need to partition the fluxes between the growth stages. Landsat images were then acquired to characterize the spatiotemporal variability in WUE at the regional scale. Satellite-derived ET, GP and WUE (= GPP/ET) estimates were consistent with ground-based measurements (R-2 > 0.80, n = 16). Eight biophysical indices derived from Landsat were then regressed with WUE estimates to see whether these indices, either independently or in combination, can explain the WUE dynamics in citrus orchards. Our results indicated that the enhanced vegetation index and soil-adjusted vegetation index are strongly related to WUE with correlation strengths greater than 0.75 at all growth stages. We then developed the constitutive relations between WUE and biophysical indices that could be utilized by water managers to improve crop water productivity in response to changing agro-climatic conditions.