Parameterising the impact of shelter on crop microclimates and evaporation fluxes

This paper presents the modelling framework and parameterisations adopted to enable the effects of wind shelter on crop microclimates and growth to be simulated. Two modelling components were required: the first, SCAM (Soil Canopy Atmosphere Model), is a biophysical land surface model used to simulate the effects of shelter on the plant canopy and soil energy and water exchanges. The second is the Agricultural Production Systems Simulator (APSIM) configured for crop modelling (Carberry et al. 2002). This paper describes the link between the two and the parameterisations developed to simulate the effects of shelter and provide potential evaporation estimates for input to APSIM. A series of sensitivity analyses using SCAM demonstrate, as expected, that over the course of the growing season and during meteorological conditions typical of agricultural regions in south-eastern Australia, plant evaporation is relatively insensitive to wind speed. This is consistent with theory and illustrates that reducing wind speeds will only reduce evaporation fluxes when the (unsheltered) canopy evaporation rate exceeds the equilibrium evaporation rate. In other words, reductions in water loss as a result of shelter will arise from one or a combination of the following: (i) reduced soil evaporation following rainfall; (ii) reduced evaporation of rainfall intercepted by the plant canopy; and (iii) reduced plant transpiration during wet periods and/or periods of very high demand when transpiration exceeds the equilibrium evaporation rate. Increased storage of soil moisture as a result of shelter early in the season may sustain crop transpiration later in the growing season. While this may not modify seasonal water use, increased soil water availability for the growing crop may confer an advantage in terms of final yield. The paper then describes the development and validation of a bulk scalar transfer coefficient, or resistance, based on wind tunnel experiments investigating scalar (heat and water vapour) transport in windbreak flows. This coefficient is used in SCAM to parameterise the effect of shelter on the exchanges of heat and water between the atmosphere and the soil and/or plant canopy. A simple parameterisation for this coefficient is developed and implemented. The potential effects of shelter on evaporative demand and hence plant growth are then quantified by using the predictions of plant potential evaporation, calculated from a modified version of SCAM as input to APSIM. The simulations show that events where reduced evaporative demand benefits plant growth can occur intermittently in the latter part of the growing season (i.e. post-anthesis for cereals) for some climatic regimes, as illustrated by the simulations for the Roseworthy, Esperance and Atherton sites used in the National Windbreaks Program.