Abstract |
Research has shown that the effects of air movement and air compressibility in soil columns are important. For soils underlain by a relatively impervious layer or by a shallow water table, methods based on Richards' equation would overpredict infiltration rates by factors of 2,3 or more. Even when air compressibility effects are insignificant as in the case y of an open semi-infinite column, air viscous effects are important. The formula of Green and Ampt underestimates the viscous resistance to flow behind the wetting front from 20 to 70 percent depending on soil type. The use of a theory that considers the movement of water and air in the unsaturated zone has the advantage of accounting for observed experimental results that cannot be modeled by the one-phase flow theory. In addition the mathematical problem is actually simplified, not complicated, by the more complete approach. The fact that the problem of Green and Ampt could be solved simply in a few lines whereas it had eluded solution since 1911 is conclusive evidence. Comparison with experimental results shows that the approximations in the methods of solution yield highly accurate and practical estimates of the infiltration quantities of interest. (Modified author abstract) |