Soil temperature and the energy balance of vegetative mulch in the semi-arid tropics. I. Static analysis of the radiation balance

Abstract

Planting crops into chemically killed pasture protects seedlings from high soil temperature. In this paper the radiation balance and convective heat loss of such dead vegetative mulch canopies are analysed mathematically for windless conditions, and maximum soil temperatures are calculated, together with their sensitivities to mulch and soil parameters. An approximate expression for soil heat flux, and assumptions about rates of change of radiation and other inputs with time, allow calculations where actual data are unavailable. A simplified analysis regards the mulch as a radiation filter which acts independently of the soil surface. Agreement with experimental data is good. The results show that energy losses by reflection, conduction, convection, radiation and evaporative cooling are all important in balancing incoming radiation and determining soil surface temperature, so that quantitative analysis is essential for a proper understanding of the system. .Mulch can reduce soil surface temperature by up to 20°C by intercepting incoming radiation; it dissipates this intercepted energy quite efficiently by free convection without concomitant increase in the temperature of the underlying soil surface. Penetration of radiation through the mulch is its most important characteristic. Forward scattering of shortwave radiation from the mulch to the soil surface has a big effect on soil temperature, but backward scattering has little effect, hence lighter-coloured more reflective mulches may be less efficient than darker ones. Evaporation of soil water is very efficient in reducing soil temperature, and the mulch prolongs the process of slow evaporation from the soil surface. The resulting higher soil water content also decreases soil surface temperature through its effects on soil thermal properties.