Insights for the Partitioning of Ecosystem Evaporation and Transpiration in Short-Statured Croplands

Reducing water losses in agriculture needs a solid understanding of when evaporation (E) losses occur and how much water is used through crop transpiration (T). Partitioning ecosystem T is however challenging, and even more so when it comes to short-statured crops, where many standard methods cannot be applied. In this study, we combined biometeorological measurements with a SPA-Crop model to estimate T and E at a Swiss cropland over two crop seasons with winter cereals. We compared our results with two recent data-driven approaches: the Transpiration Estimation Algorithm (TEA) and the underlying Water Use Efficiency (uWUE).

Our results showed that the available energy reaching the soil through the crop canopy can highly vary depending on growth and climatic conditions. Despite large differences in the productivity of both years, the T to evapotranspiration (ET) ratio had relatively similar seasonal and diurnal dynamics, and averaged to 0.72 and 0.73 for both crop seasons. Our measurements combined with a SPA-Crop model provided T estimates similar to the TEA method, while the uWUE method underestimated T even when the soil and leaves were dry. T was strongly related to the leaf area index, but additionally varying due to climatic conditions. The most important climatic drivers controlling T were found to be the photosynthetic photon flux density (R²=0.84 and 0.87), and vapor pressure deficit (R²=0.86 and 0.70). Our results suggest that site-specific studies can help establish T/ET ratios, as well as identify dominant climatic drivers, which could then be used to partition T from reliable ET measurements.