Soil temperature and the energy balance of vegetative mulch in the semi-arid tropics. II. Dynamic analysis of the total energy balance

Abstract

Soil temperatures and water losses under killed vegetative mulch canopies are examined in the context of no-tillage crop production, using a numerical dynamic model of the soil, canopy and lower atmosphere. Both liquid and vapour movement in the soil are included, as are free and forced convection in the canopy. The predictions of the model for a clay loam soil are as follows. Medium and heavy mulches reduce the water loss over six days by 1.4 and 2.7 mm respectively, the reduction occurring while the soil surface is wet. This small effect is important in giving seedlings an extra 2 or 3 days for establishment. Water loss from bare soil and under a medium mulch is limited by soil resistance even on the first day of evaporation from initially wet soil. Mulch canopies that intercept 80 and 50% of incoming radiation can keep surface soil temperatures within 10 and 20°C respectively of ambient, whereas bare soil temperatures may rise 30°C above ambient. A moderate wind reduces soil temperatures under a mulch only a few degrees, but cools the canopy much more. A rough soil surface helps cool the soil. Water losses and soil temperatures are little affected by a 50% change in soil water diffusivity or thermal conductivity. An extensive mulched area results in temperatures well above those observed on small plots surrounded by transpiring vegetation, which maintains cool air above the mulch. A simplified form of the model, which incorporates only a single mulch layer and which ignores effects of wind, yields soil temperatures which are not greatly different from those generated by the more complex model for wind speeds below 1 ms-1 at canopy height.