| Water table depth is the most widely used criterion for drainage system design. However, according to van Schilfgaarde and Williamson (1965) "crops do not respond to water table depth per se however to the aeration status in the root zone".;A finite element simulation model describing the transient transfer of soil water, soil air, and gas constituents was developed to analyze the effects of varying soil water distributions on convective and diffusive soil gas constituent transfer. The model results are in agreement with literature reported numerical and analytical analysis, and reported experimental results.;Simulation experiments were conducted to evaluate the modes of gaseous transfer, namely, convective and diffusive. Additional analyses consisted of studying the effects of soil water distribution above the water table, and the effect of soil heterogeneity on gaseous transfer. A scenario defining optimal condition for crop growth was developed. Various scenarios were compared to this condition to characterize the soil environment for nonoptimal conditions in terms of a parameter called Oxygen Deficit Index (ODI).;Soil texture, soil layering, source/sink term, location of water table, and initial conditions have significant effects on gas constituent transfer in the soil profile. Treatment of gas constituent transfer as purely diffusive, which follows from the assumption of static air phase, is a poor assumption. In fact, model simulations indicated that during transient infiltration and/or drainage processes the convection of both water and air phases play a significant role in gas constituent transfer. The location and thickness of a restrictive soil layer dictates the amount of gas transfer.;The ODI concept developed shows promise in investigating relationships between soil water distribution and corresponding oxygen content distribution in the soil profile. There is a need to develop relationships between ideal crop growth conditions subject to various environmental and soil scenarios, so that the ODI concept can be developed and used in drainage design. |
