Shuttleworth-Wallace Model Research Articles

Evapotranspiration Modeling of Partial Canopy/Residue-covered Fields

Knowledge of soil evaporation (E) and crop transpiration (T), collectively referred to as evapotranspiration (ET), is central to water budget analysis and modeling of agricultural cropping systems. The double layer Shuttleworth-Wallace (S-W) model provides the potential of modeling ET for the entire range of canopy cover. This model is extended herein to include the effects of surface crop residues on soil evaporation by explicitly specifying a partially covered soil area and partitioning evaporation between the bare and residue-covered areas. Although the extended S-W model is physically based, simplifying assumptions are made for the sake of functionality. Model derivation and potential uses are discussed. The main drawback to the model’s field application is parameter estimation. In this article, the concepts of soil and residue evaporative resistances are explored leading to proposed methods for estimating these difficult-to-determine parameters. An immediate use of the model is in predicting daily E and T rates in system-wide agricultural simulation models.

The estimation of transpiration from sparse dryland millet using stomatal conductance and vegetation area indices

The stomatal conductances of the leaves and panicles of a sparse dryland millet crop grown at ICRISAT Sahelian Center, Sadoré, in southwest Niger were measured using a diffusion porometer and an infrared gas analyser, respectively. Leaf conductances were found to be high, up to 12 mm s −1 or 480 mmol m −2 s −1, and varied according to the leaf surface, age and position in the canopy. These data were combined with measurements of leaf area index to calculate canopy conductance. Because of the low leaf area (maximum 1.3), canopy conductances were low and varied both diurnally and seasonally. Transpiration was calculated using the above canopy conductance values using the Penman-Monteith and Shuttleworth-Wallace models. In comparison to the Shuttleworth-Wallace model, it was found that the Penman-Monteith equation underestimated transpiration when the soil was dry and overestimated it when the soil was wet. These differences in transpiration arise because of the modification of the in-canopy vapour pressure deficit caused by heat and water vapour fluxes from the soil, a mechanism which is only present in the Shuttleworth-Wallace model.

Application of an energy combination model for evaporation from sparse canopies

This study tests the evaporation energy combination model developed by Shuttleworth and Wallace (1985) with data collected in a subarctic wetland. The modelled evaporation was compared with evaporation calculated from the Bowen ratio energy balance technique over a range of leaf area indices (LAI) from non-vegetated to fully vegetated conditions. The Shuttleworth-Wallace (SW) model was in excellent agreement with the measured evaporation for hourly and day-time totals for all values of LAI. This gives a particular advantage to the SW model compared to the simple Penman-Monteith combination equation. A comparison of measured and modelled total evaporation for all days yielded a root mean square error and mean bias error of 0.98 and −0.13 MJ m −2 day −1, respectively. The model also shows good agreement with the measured evaporation on an hourly basis. Although the results of this study are encouraging, we are cautious because this test is not truly independent. The need for additional investigation and testing of certain model parameters is recognized. In this study, we assume that eddy diffusivity within the canopy decreased exponentially and was controlled by a decay constant which varied with LAI. However, there is little information available to validate this treatment. Net radiation at the soil surface was computed from net radiation over the canopy and an exponential function of LAI, which was held constant over the course of the growing season.