IntroductionWe have studied soil as a physical system with particular attention to the movement of water in the soil and the diffusion and dispersion of chemicals. To understand the fundamentals of water flow and contaminant transport we have developed and used analytical and numerical models of flow and transport in unsaturated soils. An example of water flow that we have not explicitly examined is the case of soil water evaporation. Evaporation is an important component of the surface energy balance. The availability of energy and water are the general principles that dominate the evaporation process. Evaporation affects plant available water content in the soil and causes salinization in irrigated soils. If evaporation is not controlled, a significant amount of water can be lost from both irrigated and rain-fed croplands. The evaporation of soil moisture involves not only the loss of water but also the danger of soil salinization. This danger is greatest in arid areas, where annual rainfall is low, irrigation water is brackish and the water table is high. The rate and amount of evaporation are influenced by many soil characteristics, tillage, and environmental interactions. In the absence of vegetation and when the soil surface is subject to the effects of radiation and wind, evaporation occurs entirely from the soil. In annual field crops, the soil surface may remain largely bare during periods of tillage, planting, germination, and initial seedling growth, periods in which evaporation may reduce surface soil moisture and thus hinder the growth of young plants during their most intense phase. vulnerable phase. The problem can be acute even in young orchards, where the soil surface is often kept bare continuously for several years, as well as in arid areas far... middle of the paper... decrease in soil conductivity. This results in a decrease in evaporation rate with respect to time. The duration of the initial drying phase depends on evaporation. Low evaporation will increase the duration of the first drying phase. The relationship between hydraulic conductivity K and water content can be seen below: Further topics - Flow processes can be isothermal or non-isothermal - There can be interactions between liquid flow and temperature gradient, heat and vapor conduction in the domain of soil. - Cracks within the soil matrix form secondary evaporation planesDepends on the physical and conductance properties of the soil:-water content of the soil body-soil water potential-soil conductivity-soil structure-compaction-stratification- depth of the water tableDepends on regularity (diurnal, seasonal) or irregular