Using measured soil water contents to estimate evapotranspiration and root water uptake profiles – a comparative study

Abstract

Understanding the role of plants in soil water relations, and thus ecosystem functioning, requires information about root water uptake. We evaluated four different complex water balance methods to estimate sink term patterns and evapotranspiration directly from soil moisture measurements. We tested four methods. The first two take the difference between two measurement intervals as evapotranspiration, thus neglecting vertical flow. The third uses regression on the soil water content time series and differences between day and night to account for vertical flow. The fourth accounts for vertical flow using a numerical model and iteratively solves for the sink term. None of these methods requires any a priori information of root distribution parameters or evapotranspiration, which is an advantage compared to common root water uptake models. To test the methods, a synthetic experiment with numerical simulations for a grassland ecosystem was conducted. Additionally, the time series were perturbed to simulate common sensor errors, like those due to measurement precision and inaccurate sensor calibration. We tested each method for a range of measurement frequencies and applied performance criteria to evaluate the suitability of each method. In general, we show that methods accounting for vertical flow predict evapotranspiration and the sink term distribution more accurately than the simpler approaches. Under consideration of possible measurement uncertainties, the method based on regression and differentiating between day and night cycles leads to the best and most robust estimation of sink term patterns. It is thus an alternative to more complex inverse numerical methods. This study demonstrates that highly resolved (temporally and spatially) soil water content measurements may be used to estimate the sink term profiles when the appropriate approach is used

Dynamic niche partitioning in root water uptake facilitates efficient water use in more diverse grassland plant communities

“…The water balance method was validated by estimating the coefficient of determination (R 2 ) of the regression between the measured ET (predictor variable) and the modelled ET (response variable) on 25th, 28th, and 29th June 2012. Since the modelled ET is calculated from the integrated root water uptake profile, good fit between the measured and modelled ET indicates that the predicted root water uptake profile represents the true profile, which was shown in a prior study by Guderle and Hildebrandt (2015). A detailed description and evaluation of the method can be found in Guderle and Hildebrandt (2015) and Guderle (2015).…”

“…Although root water uptake profiles provide detailed information on resource uptake, plant growth, nutrient cycling, and species coexistence (Kulmatiski, Adler, Stark, & Tredennick, 2017), the majority of studies measured only total water fluxes, and did not differentiate it according to depth. We address this knowledge gap by using a data-driven method to estimate root water uptake profiles, such as by combining measurements of short-term fluctuations of soil water content (Guderle & Hildebrandt, 2015) with lysimeter-measured evapotranspiration.…”

Isotopic approaches to quantifying root water uptake and redistribution: a review and comparison of methods

“…Yet, as change of soil water content with time is not due to root extraction only (i.e., soil water redistribution can also occur), the assessment of RWU based on water content 15 distribution alone is not possible in conductive soils . Rather, the full soil water flow equation accounting for root uptake and soil water redistribution must be solved in an inverse mode, and, with an accurate knowledge of soil and root properties RWU distribution can be inferred (Guderle and Hildebrandt, 2015;Hupet et al, 2002;Musters and Bouten, 1999;Vandoorne et al, 2012). Nuclear magnetic resonance (NMR) imaging has been suggested as an adequate technique to measure water flow velocity in xylem vessels but no application exists yet on living roots in soils (Scheenen et 20 al., 2000).…”

A Dynamic Variation of Root Water Uptake in Drip Irrigated Paddy

“…The distribution of soil water content contains information of RWU distribution which is easily measured by adopting advanced technologies like Time Domain Reflectometry (TDR). The higher spatial and temporal resolution of soil water content provides a better information on root zone process (Guderle and Hildebrandt, 2015).…”