Comparison of real evapotranspiration measured by weighing lysimeters with simulations based on the Penman formula and a crop growth model

Abstract

Although the quantification of real evapotranspiration (ETr) is a prerequisite for an appropriate estimation of the water balance, precision and uncertainty of such a quantification are often unknown. In our study, we tested a combined growth and soil water balance model for analysing the temporal dynamics of ETr. Simulated ETr, soil water storage and drainage rates were compared with those measured by 8 grass-covered weighable lysimeters for a 3-year period (January 1, 1996 to December 31, 1998). For the simulations, a soil water balance model based on the Darcy-equation and a physiological-based growth model for grass cover for the calculation of root water uptake were used. Four lysimeters represented undisturbed sandy soil monoliths and the other four were undisturbed silty-clay soil monoliths. The simulated ETr-rates underestimated the higher ETr-rates observed in the summer periods. For some periods in early and late summer, the results were indicative for oasis effects with lysimeter-measured ETr-rates higher than corresponding calculated rates of potential grass reference evapotranspiration. Despite discrepancies between simulated and observed lysimeter drainage, the simulation quality for ETr and soil water storage was sufficient in terms of the Nash-Sutcliffe index, the modelling efficiency index, and the root mean squared error. The use of a physiological-based growth model improved the ETr estimations significantly.

Water table effects on measured and simulated fluxes in weighing lysimeters for differently-textured soils

“…Daily rainfall, and measured and simulated cumulative soil water storage change (storage) and outflow rates for lysimeters 1-2 (water table at 210 cm depth, first and second graphs) and 3-4 (water table at 135 cm depth, third and fourth graphs) using first (Model first set) and second (Model second set) sets of soil hydraulic parameters (Table 2a, 3a). ted previously (Wegehenkel and Gerke, 2013) with an IA from 0.48 to 0.77 and NS between -0.37 and 0.15. In contrast, daily drainage rates simulated by using the second set of soil hydraulic parameters were mainly overestimated and a large amount of simulated outflow peaks were not confirmed by the experimental data obtained from the lysimeters 9-12 (Fig.…”

“…In comparison with these results, our model performance for outflow was in the upper range, but for ETa, it was only in the middle range of the model performances reported in the other studies (Table 4). In the previous lysimeter study by Wegehenkel and Gerke (2013), IA from 0.64 to 0.92 and NS from 0.32 to 0.71 were obtained for outflow at the lysimeters 1-4 and corresponding model performance for ETa was described by an IA from 0.87 to 0.91 and NS ranging from 0.55 to 0.62.…”

“…The soil profiles of the lysimeters were spatially discretized in 150 computational layers of 1 cm thickness. The atmospheric boundary condition for the model application was defined by measured daily rates of rainfall; transpiration (T) and evaporation (E) rates of the grass cover were obtained from a physiologically-based grass cover growth model (Wegehenkel and Gerke, 2013). The ETa-rate time-series obtained from the lysimeter data directly could not be used here for the simulations because of important data gaps (c.f., Wegehenkel and Gerke, 2013).…”

“…Temperature boundary conditions were selected for the simulation of heat transport with measured soil surface temperature as upper and an assumed constant soil temperature of 5 ⁰C in 150 cm depth as lower boundary conditions. Maximum rooting depth of grass was set at 50 cm (e.g., Wegehenkel and Gerke, 2013). For the simulation of root water uptake according to Eq.…”

“…Mismatch between simulated and measured lysimeter drainage was explained by Loos et al (2007), Mertens et al (2005b), and Kelleners et al (2005), among others, with inaccurate parameterization of soil hydraulic properties and the use of an inappropriate ET model. In order to explain the underestimation of simulated drainage, a more sophisticated ET model was tested that included a physiologically-based vegetation growth model (Wegehenkel and Gerke, 2013). Still, some observed percolation events and cumulative drainage rates could not adequately be simulated despite an improved description of actual evapotranspiration (ETa) rates.…”