Assessment of evaporation and transpiration ratios under varying moisture conditions in a soybean field.

Freshwater is a scarce resource facing a growing demand. One aspect of this growing demand arises from the expansion and intensification of crop production on irrigated land. To preserve valuable water resources, agricultural water management must aim at an efficient use of water. This can be approached by facilitating sufficient water supply for optimal crop transpiration (T) and thus crop production, and at the same time reduce unproductive water losses due to soil evaporation (E). In this regard, knowing the ratios of E and T and how they are affected by environmental and management conditions is required to develop, adapt, and evaluate agricultural practices with respect to efficient water use.

The study aimed at applying a modified E-T-partitioning method to evaluate irrigation and how varying water availability affected E and T ratios. Field experiments were conducted 2019 in Groß-Enzersdorf in the agricultural region Marchfeld east of Vienna, Austria (48°12’ N, 16°34’ E; 157 m elevation a.s.l., average annual precipitation of approx. 540 mm). A conventionally managed field was planted with soybean (glycine max l. merr) and irrigated twice with a hose reel irrigation machine. Partitioning of evapotranspiration (ET) was analyzed using an adapted water balance and stable isotope mass balance method. Monitoring throughout the soybean vegetation period comprised weekly analyses of the isotopic composition of soil samples, the profile water content in 10 cm steps down to 80 cm, weather data, the isotopic signatures of precipitation and irrigation water, ET, and crop growing stages. ET was measured with eddy covariance technique, and isotopic fractionation for determining E and T ratios was calculated from measured boundary conditions.

Weekly T ratios from blossom to beginning of maturation of soybean ranged from 56 to 84 %, which is in agreement with studies based on comparable partitioning methods. The relation between E and T did not only progress according to the canopy development but also responded to water availability in the rooting zone. During the vegetative growth stage, for example, the proportion of T was larger at partial canopy cover and sufficient water availability (from spring precipitation) compared to full canopy cover and when facing soil water stress. When soil surface was dry, E dropped to almost zero. On the other hand, a wetted surface substantially raised the E, even under closed canopy. Multiple small rain events during full canopy cover mainly contributed to E. As the analyses sufficiently revealed the relations of E and T ratios to changing boundary conditions, the method proved useful to evaluate irrigation events and strategies and deduce further improvements. In case of using a hose reel irrigation machine, the results suggest an intensification of the individual watering. However, a quantitative relationship between irrigation and the amount of water used for T would require shorter evaluation intervals of 2-3 days.