Plant Properties (e-W)

Capabilities have been added to eSTOMP-W for modeling evapotranspiration and water-uptake by plant roots based on user-provided values of potential evapotranspiration (PET). The actual (computed) evapotranspiration (ET) is determined as the PET scaled by root-length-density (RLD) functions, plant limiting functions, and crop coefficients. ET is partitioned into sink terms according to user-specified RLD functions, with the actual withdrawal being dependent on matric potentials or water contents in the root zone and the wilting point of the plant. Crop coefficients can also be specified, to provide additional flexibility in modulating simulated root-water-uptake behavior.  

These capabilities involve the use of a Plant Properties Card that provides options for specifying RLD functions, root stress models, and crop coefficients (Zeng, 1993; Vrugt et al., 2001; Jarvis, 1989; Ward et al. 2005). The RLD functions of Zeng (1993) and Vrugt et al. (2001) are implemented. The original two-parameter RLD function of Zeng (1993) was modified to include a maximum rooting depth. The Zeng model implementation also requires specification of two soil moisture tension (or matric potential) values that correspond to plant stomata being fully closed and fully open. The Vrugt et al. (2001) and Jarvis (1989) models follow the descriptions by Ward et al. (2005) and require the specification of four matric potential, or four water content values, respectively, that correspond with root stress factors of 0, 1, 1, and 0. The root-water-uptake model of Jarvis (1987) utilizes the RLD function of Vrugt et al. (2001). The four specified water content values for the Jarvis (1989) model correspond to the plant wilting point, a critical water content value at which the plant begins to experience stress, the so-called field capacity of the soil, and a water content near saturation above which respiration ceases due to the soil being too wet.

The capabilities for plant root-water-uptake utilize user-specified values of PET which are defined from the Boundary Conditions Card.

Typical applications of these PET and root-water-uptake capabilities will also require the use of the ~Source Card, to specify water inputs to the system that represent precipitation. This is accomplished using source terms in the upper-most grid cells at the top of a model domain, instead of using boundary condition specifications.

If plant root-water-uptake is modeled, and if multiple processors are used, the domain decomposition must only be in the X-Y plane. The root-water-uptake functionality will not work if domain decomposition is performed in the Z direction 

• Descriptions of the options of plant properties card are provided on the Card Options page.

• Formatting instructions for plant properties are provided on the Card Syntax page.

• Examples of Plant Properties Cards for eSTOMP-W are provided on the Card Examples page.

References

Jarvis NJ. 1989. A simple empirical model of root water uptake. Journal of Hydrology 107: 57-72.

Vrugt JA, MT van Wijk, JW Hopmans, and J Simunek. 2001. One-, two-, and three-dimensional root water uptake functions for transient modeling. Water Resources Research 37(10): 2457-2470.  

Ward AL, MD White, EJ Freeman, and ZF Zhang. 2005. STOMP: Subsurface Transport Over Multiple Phases, Version 1.0, Addendum: Sparse Vegetation Evapotranspiration Model for the Water-Air-Energy Operational Mode. PNNL-15465, Pacific Northwest National Laboratory, Richland, Washington.

Zeng X. 2001. Global vegetation root distribution for land modeling. American Meteorological Society 2:525-530.