Numerical Analysis of Coupled Water, Vapor and Heat Transport in the Vadose Zone using HYDRUS

Simultaneous movement of water, vapor and heat in the vadose zone of arid or semi-arid regions is of great interest in evaluating water and energy balance of engineering covers of landfills for their performance assessment. Vapor movement often dominates the water movement in landfill covers because the soil moisture near the soil surface is usually very low. Since it is well known that water and/or vapor flow and heat transport processes are closely coupled and strongly affect each other, their simultaneous interactions have to be considered. In this paper, we present the analyses of coupled movement of water, vapor, and heat in the subsurface, as well as interactions of these subsurface processes with the mass and energy balance at the soil surface using the modified HYDRUS-1D program. The code considers the movement of liquid water and water vapor in the subsurface to be driven by both pressure head (isothermal transport) and temperature (thermal transport) gradients. The heat transport module considers movement of soil heat by conduction, convection of sensible heat by water flow, transfer of latent heat by diffusion of water vapor, and transfer of sensible heat by diffusion of water vapor. Available meteorological information often varies from site to site and it does not always match the needs of a particular application. For example, although hourly variations of meteorological data may be required, only daily values of meteorological variables are often available. The modified HYDRUS code therefore allows a very flexible way of using various meteorological models to take different data formats into account at the soil-atmosphere interface for evaluating surface water and energy balance equations. A number of meteorological models are evaluated using field soil temperature data collected at seven depths at a proposed low-level radioactive-waste disposal site in Chihuahuan Desert in West Texas, 10 km east of Sierra Blanca, where prototype engineering covers were installed (Scanlon et al., 2005). Different meteorological models are compared in terms of prediction errors of soil temperatures at seven observation points. The choice of the air emissivity model has a great impact on temperature predictions, while remaining meteorological models have only a minor impact on results. The results indicate that most available and accepted meteorological models can be used to solve the energy balance equation at the soil-atmosphere interface in coupled water, vapor, and heat transport models.