Penman's sink-strength model as an improved approach to estimating plant canopy transpiration

Abstract

The Penman energy balance equation has become a common approach to estimating the evaporation of plant canopies. Unfortunately, implementation of the energy balance equation requires various assumptions and empiricisms, including “crop coefficients”, so that in practice the Penman equation is used as a framework in which to apply empirical approximations. These limitations are reviewed in this paper. An alternative to the energy balance approach suggested by Penman was a “sink strength” model in which the gradient in vapor pressure was considered the prime driver of plant water loss. In this paper, the sink strength model is developed for determining plant canopy transpiration based on explicitly defined properties of the plants. The key variables are the carbon assimilation pathway of photosynthesis and the biochemical composition of the plant material being synthesized. These two parameters define the transpiration efficiency coefficient and radiation use efficiency that are required in the calculation of transpiration. Also, it is necessary to define a weighted vapor pressure gradient for transpiration when making estimates of the amount of transpiration on a daily basis. The derived expression of the sink strength model was used to compare predicted transpiration with measured values of four turf grass species measured over three years. A close linear correlation was observed for all grasses (P<0.0001). However, there was an indication that the grasses may vary slightly in their radiation use efficiencies, which with measurement of photosynthetic capacity would further improve the predicted transpiration rates. The ability to directly compute transpiration from explicitly defined variables based on plant traits offers an attractive alternative to the Penman energy balance approach.