Elsevier

Advances in Water Resources

Volume 23, Issue 4, 11 January 2000, Pages 427-439
Advances in Water Resources

The dynamic role of root-water uptake in coupling potential to actual transpiration

https://doi.org/10.1016/S0309-1708(99)00023-8Get rights and content

Abstract

The relationship between actual (Eact) and potential (Ep) transpiration above a grass-covered forest clearing was investigated numerically and experimentally from simultaneous measurements of soil moisture content profiles, mean meteorological conditions, turbulent heat and water vapor fluxes in the atmospheric surface layer, and soil hydraulic properties for two drying periods. The relationship between Eact/Ep was found to be approximately constant and insensitive to variability in near-surface soil moisture content. To explore this near-constant Eact/Ep, a model that relates potential and actual transpiration and accounts for root-uptake efficiency, potential transpiration rate, and root-density distribution was proposed and field-tested. The total amount of water consumed by the root system was integrated and compared with eddy-correlation latent heat flux measurements (field scale) and total water storage changes (local scale). Model calculations suggested that the deeper and more efficient roots are primarily responsible for the total water loss within the root zone when the near-surface soil layer approaches their wilting point.

Introduction

Quantifying soil moisture content redistribution within a stratified soil in the presence of a rooting system continues to be a practical problem in surface hydrology and climate research 14, 30, 34, 46, 58. The basic approach to describe soil moisture content dynamics in the root-zone follows a modified one-dimensional continuity equation 18, 36, given byθ(z,t)t=−q(z,t)z−S(z,t),where θ is the soil moisture content at depth z (positive downward) and time t, q is the Darcian water flux, and S is the water sink by the plant-root system in a soil layer of thickness dz, and is related to the actual transpiration (Eact) viaEact(t)=0LS(z,t)dz,where L is the root-zone depth. In order to model θ(z,t) within the root-zone, a complete description of S(z,t) is necessary. Typically, S(z,t) depends on root-density distribution 19, 24, 31, 43, 45, 55, relative proportion of active roots responsible for water uptake [50], soil moisture content 18, 57, and atmospheric demand [43], among other things 20, 51, 52, and is difficult to implement in practical soil-plant-atmosphere models. In addition, much of the root properties (e.g. root-density, proportion of active roots, etc.) are not stationary 3, 4, 16, 21, 33. In climate and hydrologic models, it is difficult to explicitly describe (or resolve) all the complex processes influencing S(z,t). Hence, in such models, Eact is typically modeled or estimated usingEact=β(θ)Ep,where β is an empirical reduction function β(θ)∈[0,1] [2]. More elaborate resistance formulations such as the Penman–Monteith (see 5, 32 for reviews) are widely used to estimate Eact; however, estimating Eact alone only permits computing depth-averaged S(z,t) over the entire L. That is, with such Eact model estimates, only depth-averaged soil moisture content over the entire L can be calculated usingt0Lθ(z,t)dz=−q(L,t)−q(0,t)−Eact(t).Such an approach cannot permit any detailed accounting for the large vertical variability in observed θ within the root-zone. To permit elementary accounting of such large vertical gradients in soil moisture in climate models, Dickinson et al. [14] proposed that relative rates of water extraction from different soil layers be strictly represented in terms of root-density distribution. Whether such a representation is realistic remains to be investigated given the strong experimental evidence against the Dickinson et al. [14] proposition 22, 26, 37, 52. On the other extreme, Clausnitzer and Hopmans [12] and Somma et al. [53] proposed a transient model for root growth as a function of mechanical soil strength, soil temperature, solute concentration, and a branching pattern described by root age and empirically specified impedance factors. Such an approach can reproduce observed patterns in root-growth and changes in root-density; however, the model parameters of such an approach are not known a priori and cannot be determined from quantities resolved by hydrologic and climate models. Hence, there is a clear need for an intermediate class of models that combine many observed features about root-water uptake yet are sufficiently simple to incorporate in practical hydrologic and climate models.

In this study, a model for S(z,t) that accounts for vertical variability in root-density distribution, a newly proposed root-uptake efficiency, and atmospheric demand is developed. This model is then combined with measured soil moisture content time series at multiple depths to investigate mechanisms responsible for root activity at different soil layers in relation to soil moisture. The outcome of this study will help guide future multi-level soil-vegetation-atmosphere models in which vertical variability of root-zone soil moisture content is an intrinsic component.

Section snippets

Water movement in soil

In order to describe θ(z,t) within the root-zone, the two terms on the right-hand side of Eq. (1)require parameterizations. The first term can be quantified usingq=−kθ1+Ψ(θ)z,where kθ is the hydraulic conductivity function, and Ψ(θ) is the soil matric potential.

The Clapp and Hornberger [11] formulation for Ψ(θ) and kθ, given byΨθsθθs−b,kθ=ksθθs2b+3are used, where ks, Ψs are saturated hydraulic conductivity and air-entry soil tension, θs is the saturated moisture content, and b is an

Study site

The study site is an Alta Fescue grass-covered forest clearing at the Blackwood division of the Duke Forest in Durham, NC. The site is a 480 m × 305 m grass site surrounded by a 10–12 m Loblolly pine stand. The average elevation of the site is 163 m above mean sea level. The measurements were collected from May to July 1997. The grass height (hc) was 0.7 m at the beginning of the experiment and kept growing up to 1.05 m.

Meteorological measurements

The required meteorological inputs (mean wind speed V, air temperature Ta,

Results and discussion

The experiment duration (22 May–10 July 1997) coincided with maximum grass growth for which increase in grass height can be visually noted. Two drying cycles (22–31 May) and (1–10 July) were selected to evaluate the proposed theoretical framework because these periods had complete soil moisture content measurements and continuous meteorological and eddy-correlation measurements. Additionally, both periods experienced comparable total precipitation input (9.91 and 8.89 mm, respectively).

In this

Conclusion

This study investigated the role of root-water uptake on the Eact/Ep relationship experimentally and numerically. A model linking atmospheric demand (via potential evaporation) to soil moisture redistribution in the presence of a root system was developed. The Penman–Brutsaert potential evaporation model, a moisture-dependent root efficiency function and a linear root-density function were used to model water movement within the soil-vegetation-atmosphere continuum. Based on the measurements

Acknowledgements

The authors would like to thank Fred Mowry and David Ellsworth for lending us some of the micrometeorological instruments. The authors would also like to thank Shang-Shiou Li for his help in setting up the experiment. This study was supported, in part, by the Department of Energy FACTS-I project (DOE Cooperative Agreement No. DE-FG05-95 ER 62083) and National Science Foundation (NSF-BIR-9512333).

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