Transpirational supply and demand: Plant, soil, and atmospheric effects evaluated by simulation

Abstract

The assumption that transpiration is the lesser of an atmospheric demand function and a water supply function was tested by simulation with Federer's (1979) soil-plant-atmosphere model. The best estimate of atmospheric demand is called unstressed transpiration, defined as the transpiration that would occur in ambient conditions if stomata were unaffected by plant-water potential. For practical purposes the Penman equation provides a good estimate of unstressed transpiration for short vegetation but not for forests. Even when atmospheric variables and the Penman estimate are held constant among forest canopies, unstressed transpiration can vary by a factor of two because of variation both in the maximum value of leaf conductance and in the ratio of canopy conductance to leaf conductance. The best water supply function incorporates depth variation of soil water potential and of root and soil properties. A more practical supply function uses the ratio of available water in the root zone, W, to maximum available water WM. The maximum available water is soil water held at potentials less than that at which the hydraulic conductivity is 2 mm/d and greater than the critical leaf water potential at which stomata are completely closed. Using a mature hardwood forest as a standard, various parameters were varied to examine their effects on a water supply function defined as a supply constant times W/WM. The supply constant was found to be independent of soil texture and physical properties. Root density and the internal resistance of the plant to water flow were the most important determinants of the supply constant. Reasonable variation of root density and internal resistance produced variation in the constant from 1.9 mm/h, which implies that supply is less than demand only when soil is very dry, to 0.5 mm/hr, which implies that supply cannot meet the demand even when the soil is wet. ß eO e e• ß ß ß o/ ß ß / ß ß ß o¸ ß ß ß ß e 0 ß ß

Description and validation of the Simple, Efficient, Dynamic, Global, Ecological Simulator (SEDGES v.1.0)

“…In contrast, in SEDGES, f leaf is created by the covering of bare soil by green leaves when looking down from above the canopy. As such, it is not obvious why the simple supply rate formulation in Federer (1982) is multiplied by f leaf in SEDGES. The answer lies in a close examination of the trmax parameter of the original model.…”

“…The aforementioned simple supply rate model of Federer (1982) is tested, evaluated, and calibrated in the same paper against a more sophisticated "Type I" (Guswa et al, 2002) water uptake model that is forced using site-specific atmospheric observational data. Doing so reveals that the maximum transpiration rate (i.e., the trmax parameter) in the simple model depends on the following input parameters for the Type I model (in decreasing order of strength): rooting density, root internal resistance, depth of the rooting zone, and vegetation height/surface roughness.…”

“…To simulate the maximum supply rate of water for transpiration, SEDGES adapts the simple model of Federer (1982). The original Federer maximum supply rate is directly proportional to the soil wetness fraction multiplied by a fixed constant parameter.…”

Transient soil‐moisture dynamics and climate change in Mediterranean ecosystems

“…E max andÊ max for trees and grasses were then obtained using the Penman-Monteith equation [Federer, 1979[Federer, , 1982Brutsaert, 1982;Dingman, 1994]. The values of these parameters are listed in Table 3.…”

Critical Loads of Sulphur and Nitrogen for Terrestrial Ecosystems in Europe and Northern Asia Using Different Soil Chemical Criteria

“…The effect of snow cover on evapotranspiration was included by simulating accumulation and melting of a snow layer at each site using the temperature and precipitation data. Actual evapotranspiration was then computed using a reduction function for potential evapotranspiration based on the available water content in the soil described by Federer (1982). Soil water content is in turn estimated using a simple bucket-like model that uses water holding capacity and precipitation data.…”