Simple physics-based models of compensatory plant water uptake: concepts and eco-hydrological consequences
Open Access
- 16 November 2011
- journal article
- Published by Copernicus GmbH in Hydrology and Earth System Sciences
- Vol. 15 (11), 3431-3446
- https://doi.org/10.5194/hess-15-3431-2011
Abstract
Many land surface schemes and simulation models of plant growth designed for practical use employ simple empirical sub-models of root water uptake that cannot adequately reflect the critical role water uptake from sparsely rooted deep subsoil plays in meeting atmospheric transpiration demand in water-limited environments, especially in the presence of shallow groundwater. A failure to account for this so-called "compensatory" water uptake may have serious consequences for both local and global modeling of water and energy fluxes, carbon balances and climate. Some purely empirical compensatory root water uptake models have been proposed, but they are of limited use in global modeling exercises since their parameters cannot be related to measurable soil and vegetation properties. A parsimonious physics-based model of uptake compensation has been developed that requires no more parameters than empirical approaches. This model is described and some aspects of its behavior are illustrated with the help of example simulations. These analyses demonstrate that hydraulic lift can be considered as an extreme form of compensation and that the degree of compensation is principally a function of soil capillarity and the ratio of total effective root length to potential transpiration. Thus, uptake compensation increases as root to leaf area ratios increase, since potential transpiration depends on leaf area. Results of "scenario" simulations for two case studies, one at the local scale (riparian vegetation growing above shallow water tables in seasonally dry or arid climates) and one at a global scale (water balances across an aridity gradient in the continental USA), are presented to illustrate biases in model predictions that arise when water uptake compensation is neglected. In the first case, it is shown that only a compensated model can match the strong relationships between water table depth and leaf area and transpiration observed in riparian forest ecosystems, where sparse roots in the capillary fringe contribute a significant proportion of the water uptake during extended dry periods. The results of the second case study suggest that uncompensated models may give biased estimates of long-term evapotranspiration at the continental scale. In the example presented here, the uncompensated model underestimated total evapotranspiration by 5–7% in climates of intermediate aridity, while the ratio of transpiration to evaporation was also smaller than for the compensated model, especially in arid climates. It is concluded that the parsimonious physics-based model concepts described here may be useful in the context of eco-hydrological modeling at local, regional and global scales.Keywords
This publication has 89 references indexed in Scilit:
- Simulations of global evapotranspiration using semiempirical and mechanistic schemes of plant hydrologyGlobal Biogeochemical Cycles, 2009
- Cover crop evapotranspiration under semi-arid conditions using FAO dual crop coefficient method with water stress compensationAgricultural Water Management, 2007
- Water deficit effects on root distribution of soybean, field pea and chickpeaField Crops Research, 2006
- Comparison of root water uptake modules using either the surface energy balance or potential transpirationJournal of Hydrology, 2004
- Plant parameter values for models in temperate climatesEcological Modelling, 2003
- Global synthesis of leaf area index observations: implications for ecological and remote sensing studiesGlobal Ecology and Biogeography, 2003
- Interception of a Dense Spruce Forest, Performance of a Simplified Canopy Water Balance ModelHydrology Research, 2001
- Prophecy, reality and uncertainty in distributed hydrological modellingAdvances in Water Resources, 1993
- A Field Study of Soil Water Depletion Patterns in Presence of Growing Soybean Roots: III. Rooting Characteristics and Root Extraction of Soil WaterSoil Science Society of America Journal, 1975
- A Field Study of Soil Water Depletion Patterns in Presence of Growing Soybean Roots: II. Effect of Plant Growth on Soil Water Pressure and Water Loss PatternsSoil Science Society of America Journal, 1975