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 lead to inaccurate measurements. In this study, we combined biometeorological measurements with a Soil-Plant-Atmosphere Crop (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 data-driven approaches: The Transpiration Estimation Algorithm (TEA) and the underlying Water Use Efficiency (uWUE). 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. Our measurements combined with a SPA-Crop model provided T estimates similar to the TEA method, while the uWUE method produced systematically lower T even when the soil and leaves were dry. T was strongly related to the leaf area index, but additionally varied due to climatic conditions. The most important climatic drivers controlling T were found to be the photosynthetic photon flux density (R-2 = 0.84 and 0.87), and vapor pressure deficit (R-2 = 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. Moreover, our results suggest that the TEA method is a suitable tool for ET partitioning in short-statured croplands.

Automated summary

To reduce water losses in agriculture, understanding the timing of evaporation losses and the amount of water used through crop transpiration is essential. This study estimated transpiration and evaporation using biometeorological measurements and a SPA-Crop model in a Swiss cropland. The results showed that the ratio of transpiration to evapotranspiration had similar seasonal and diurnal dynamics, and was influenced by leaf area index and climatic conditions.