Environmental isotope (δ18O) and hydrological data to assess water flow in unsaturated soils planted with different crops: Case study lysimeter station “Wagna” (Austria)

Modelling variable soil water fluxes and transport processes under natural conditions in the unsaturated zone is often restricted and difficult to perform. Highly sophisticated numerical models are required and numerous parameters are needed. However, these parameters are difficult to estimate especially for heterogeneous systems. Therefore, the aim of this study was to investigate whether a simple lumped parameter approach is applicable in the unsaturated zone of cropped soils. The environmental isotope contents of δ18O and δ2H were used as tracers to study the flow processes in two lysimeters containing the same soil and planted with different crops (maize monoculture and crop rotation). The results from lumped parameter modelling were compared with those obtained with a numerical model approach. It was shown that the lumped parameter approach is, to some extent, applicable in the unsaturated zone. The simulations of the isotope contents in the discharge gave a general trend of mean transit times when the estimation of the tracer concentration in the inflow (input function) was weighted according to the amount of precipitation as well as when the observation period was separated into single vegetation periods. The resulting flow and transport parameters were similar to those from numerical modelling. The lumped model yielded adequate results particularly during the winter periods with grass cover. Although filled with the same soil material, different hydraulic flow systems were established in both lysimeters due to the differences in crop growth. In general, smaller fluxes were observed during crop rotation compared to maize monoculture whereas the smallest fluxes were found for canola.