Temperature Modelling of Wet and Dry Desert Soils

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Abstract
(1) As part of a study on thermal relations of desert plants, soil temperatures at various depths, soil evaporative water loss, radiation components, wind speeds and air temperatures were measured hourly over a 24-h period for a naturally dry and an artificially wet soil in the north-western Sonoran Desert in California [USA]. Also, the thermal conductivity, volumetric heat capacity and water potential of the soil were determined in the laboratory for a wide range of soil-water contents. A computer model was then developed to predict soil temperature at various depths. (2) The measured soil temperature was generally within 1.degree. C of that predicted by the model, and the measured net energy exchange of the soil with the atmosphere closely agreed with the predicted heat storage in the soil. A sensitivity analysis showed that maximum soil surface temperatures were markedly influenced by shortwave radiation, wind speed and air temperature. (3) Model simulations showed that nearly all of the decrease in maximum temperature of a wet soil occurred above a soil-water potential of -0.2 MPa. The damping depth for daily temperature changes was about 10 cm for both the wet and the dry soils. (4) When the soil was shaded by a common bunchgrass, which influenced shortwave and longwave radiation as well as wind speed profiles, simulated maximum soil surface temperatures decreased about 2.degree. C for every 10% increase in shading, indicating that nurse plants can have a substantial effect on the local microclimate.