A numerical model of heat and water movement in furrow‐sown water repellent sandy soils

Abstract

Water repellent soils have thin layers of hydrophobic organic matter on the surfaces of soil particles. The hydrophobic organic matter makes these soils difficult to wet in opening rains after dry periods. This can cause losses of crop and pasture production and encourage land degradation by wind and water erosion. Wetting patterns in water repellent soils are irregular and incomplete, and no successful model of heat and water movement in water repellent soils has been reported. Furrow sowing improves crop production on water repellent sandy soils. Seeding machinery forms ridges and furrows. Rain is easily shed from dry ridges and runoff accumulates in the furrow (water harvesting). Ponded water infiltrates easily into the soil around the seed. The water movement in furrow‐sown water repellent sandy soils is more regular. This regularity makes it possible to develop a model. A numerical model of heat and water movement in furrow‐sown water repellent sand was developed and validated to help optimize the design of furrow sowing. The model used aerodynamic resistance, soil surface moisture and temperature, and surface evaporative resistance to estimate evaporation from each surface element. Wind speed and radiation was varied by calculation, according to the direction of the Sun or wind to the ridge direction and height. Movement of water and heat in the soil was determined by the initial conditions, boundary conditions, and coupled functions for heat and water movement, as used in many other models. Water repellency effects on drying were included by including the appropriate moisture and unsaturated hydraulic conductivity characteristics. Application of the finite element method to a two‐dimensional cross section of a ridge and furrow system allows water to be excluded from a soil region beneath a ridge. Field experiments were used to estimate surface evaporative resistance characteristics and to validate the model. The model has been successfully used to suggest optimized designs for ridge and furrow systems to minimize high soil temperatures and evaporation in Western Australia.