Root Water Uptake as Simulated by Three Soil Water Flow Models

Abstract

We compared four root water uptake (RWU) models of diff erent complexity that are all embedded in greater soil water fl ow models. The soil models used were SWAP (one-dimensional), FUSSIM2 (two-dimensional), and RSWMS (three-dimensional). Within SWAP, two RWU func ons were u lized (SWAP-macro and SWAP-micro). The complexity of the processes considered in RWU increases from SWAP-macro, to SWAP-micro, to FUSSIM2, to RSWMS. The objec ve of our study was to determine to what extent the RWU models diff ered when tested under extreme condi ons: low root length density, high transpira on rate, and low water content. Comparison 1 looked at the results of the models for a scenario of transpira on and uptake and Comparison 2 studied compensa on mechanisms of water uptake. The uptake scenario pertained to a long dry period with constant transpiraon and a single rainfall event. As could be expected, the models yielded diff erent results in Comparison 1, but the diff erences in cumula ve transpira on were modest due to various feedback mechanisms. In Comparison 2, the cumula ve eff ect of diff erent feedback processes were studied. Redistribu on of water due to soil pressure head gradients generated by water uptake led to an increase in cumula ve transpira on of 32%, and the inclusion of compensa on in uptake by the roots resulted in a further increase of 10%. Going from one-to three-dimensional modeling, the horizontal gradients in the soil and root system increased, which reduced the actual transpira on. The analysis showed that both soil physical and root physiological factors are important for proper determinis c modeling of RWU.Abbrevia ons: RWU, root water uptake.

Modelling hourly evapotranspiration and soil water content at the grass-covered boundary-layer field site Falkenberg, Germany

“…Such an impact of the application of compensatory root-water uptake models has also been described in other studies and was claimed to be unrealistic and inconsistent with plant behaviour (e.g. Skaggs et al 2006, de Jong van Lier et al 2008, Shouse et al 2011, de Willigen et al 2012. However, in these studies, field verification of such compensatory root-water uptake models (e.g.…”

“…However, in these studies, field verification of such compensatory root-water uptake models (e.g. ETr measurements by EC-systems with higher measurement frequency) was recommended, but not carried out (de Jong van Lier et al 2008, Shouse et al 2011, de Willigen et al 2012.…”

“…Compensatory means that reduced root-water uptake in upper stressed parts of the soil is compensated by an increased uptake from deeper unstressed soil layers. Such approaches are described by, for example, Skaggs (2006), Šimůnek and Hopmans (2009), Jarvis (2011) andde Willigen (2012). However, few studies using experimental field data for the validation of such root-water uptake models have been published to date and most of the experimental data used in these studies are based on a daily time-step and are for investigation periods shorter than 1 year (Skaggs et al 2006, De Jong van Lier 2008, Shouse et al 2011.…”

“…However, few studies using experimental field data for the validation of such root-water uptake models have been published to date and most of the experimental data used in these studies are based on a daily time-step and are for investigation periods shorter than 1 year (Skaggs et al 2006, De Jong van Lier 2008, Shouse et al 2011. Therefore, further verification of such compensatory root-water uptake approaches using field experiments with a higher measurement frequency, such as hourly time-steps, and longer investigation periods was recommended in these studies (Skaggs et al 2006, de Jong van Lier et al 2008, Shouse et al 2011, de Willigen et al 2012. Despite the many improvements in measurement techniques and modelling approaches, accurate determination of ETr is still a scientific challenge (Faharani et al 2007, Verstraeten et al 2008, Castelvi and Snyder 2010, Izadifar and Elshorbagy 2010, Wohlfahrt et al 2010.…”

Benchmarking test of empirical root water uptake models

“…Braud et al, 2005;Yadav et al, 2009;Dong et al, 2010). Comparisons with physically based models (Jarvis, 2011;de Willigen et al, 2012) that implicitly account for compensation showed that models not including compensation, like Feddes et al (1978), are less accurate with respect to crop transpiration and soil water content predictions under some circumstances, e.g. at a high root length density.…”