Evaluation of a simple approach for crop evapotranspiration partitioning and analysis of the water budget distribution for several crop species

Climate variability and climate change induce important intra- and inter-annual variability of precipitation that significantly alters the hydrologic cycle. The surface water budgets and the plant or ecosystem water use efficiency (WUE) are in turn modified. Obtaining greater insight into how climatic variability and agricultural practices affect water budgets and regarding their components in croplands is, thus, important for adapting crop management and limiting water losses. Therefore, the principal objectives of this study are: (1) to assess the contribution of different components to the agro-ecosystem water budget and (2) to evaluate how agricultural practices and climate modify the components of the surface water budget. To achieve these goals, we tested a new method for partitioning evapotranspiration (ETR), measured by means of an eddy-covariance method, into soil evaporation (E) and plant transpiration (TR) based on marginal distribution sampling (MDS). The partitioning method proposed requires continuous flux recording and measurements of soil temperature and humidity close to the surface, global radiation above the canopy and assessment of leaf area index dynamics. This method is well suited for crops because it requires a dataset including long bare-soil periods alternating with vegetated periods for accurate partitioning estimation. We compared these estimations with calibrated simulations of the ICARE-SVAT double source mechanistic model. The results showed good agreement between the two partitioning methods, demonstrating that MDS is a convenient, simple and robust tool for estimating E with reasonable associated uncertainties. During the growing season, the proportion of E in ETR was approximately one-third and varied mainly with crop leaf area. When calculated on an annual time scale, the proportion of E in ETR reached more than 50%, depending on the crop leaf area and on the duration and distribution of bare soil within the year.