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

Vapor movement is often an important part in the total water flux in the vadose zone of arid or semiarid regions because the soil moisture is relatively low. The twomajor objectives of this study were to develop a numerical model in the HYDRUS-1D code that (i) solves the coupled equations governing liquid water, water vapor, and heat transport, together with the surface water and energy balance, and (ii) provides flexibility in accommodating various types of meteorological information to solve the surface energy balance. The code considers the movement of liquid water and water vapor in the subsurface to be driven by both pressure head and temperature gradients. The heat transport module considers movement of soil heat by conduction, convection of sensible heat by liquid water flow, transfer of latent heat by diffusion of water vapor, and transfer of sensible heat by diffusion of water vapor. The modifications allow a very flexible way of using various types of meteorological information at the soil–atmosphere interface for evaluating the surface water and energy balance. The coupled model was evaluated using field soil temperature and water content data collected at a field site. We demonstrate the use of standard daily meteorological variables in generating diurnal changes in these variables and their subsequent use for calculating continuous changes in water contents and temperatures in the soil profile. Simulated temperatures and water contents were in good agreement with measured values. Analyses of the distributions of the liquid and vapor fluxes vs. depth showed that soil water dynamics are strongly associated with the soil temperature regime. THE simultaneous movement of liquid water, water vapor, and heat in the vadose zone plays a critical role in the overall water and energy balance of the nearsurface environment of arid or semiarid regions in many agricultural and engineering applications. Moisture near the soil surface is influenced by evaporation, precipitation, liquid water flow, and water vapor flow, most of which are strongly coupled. In arid regions, vapor movement is often an important part of the total water flux since soil moisture contents near the soil surface usually are very low (Milly, 1984). In agricultural applications, spatial and temporal changes in surface soil moisture need to be well understood to achieve efficient and optimum water management. Vapor transport is also important since the actual contact area between liquid water and seeds is often very small such that seeds need to imbibe water from vapor to germinate (Wuest et al., 1999). In engineering applications, accurate assessment of the volume of leachate from landfills, which is essential for long-term landfill management, requires estimates of water and heat movement through engineered landfill covers (e.g., Khire et al., 1997). Understanding surface energy and water balances as well as liquid water, water vapor, and heat movement in soils is critical for the performance evaluation of engineered surface covers for waste containment in landfills in arid or semiarid regions (Scanlon et al., 2005). Traditional engineered covers typically consist of multilayered resistive barriers with low hydraulic conductivities to minimize water movement into the underlying waste. Recently, evapotranspiration (ET) covers have been developed (e.g., Hauser and Gimon, 2004) to increase the water storage capacity of landfill covers and their protective function by removing water through evapotranspiration. These ET cover designs thus need to be assessed mainly in terms of the near surface water and energy balance. Another engineering application requiring an assessment of coupled liquid water, water vapor, and heat transport involves nuclear waste repositories. Radioactive decay in repositories may create steep temperature gradients in adjacent soils and rocks, which can lead to unwanted consequences. Generated heat may cause evaporation of soil water, and subsequent migration and condensation of water vapor in cooler areas. This process may significantly change the physical characteristics of surroundingmaterials due to the precipitation and dissolution of various minerals (e.g., Spycher et al., 2003). Finally, in military applications, an accurate prediction of water contents and temperatures near the soil surface may allow improved detection of buried land mines since their presence can significantly change these two variables (Šimůnek et al., 2001). Early pioneering studies on interactions between liquid water, water vapor, and heat movement were reported by Philip and de Vries (1957), who provided a mathematical description of liquid water and water vapor fluxes in soils driven by both pressure head (isothermal) and soil temperature (thermal) gradients. They derived the governing flow equation for nonisothermal flow as an extension of the Richards equation, which originally considered only the pressure head gradient. The theory of Philip and de Vries (1957) was later extended by Nassar and Horton (1989), who additionally considered the effect of an osmotic potential gradient on the simultaneous movement of water, solute, and heat in soils. Heat transport and water flow are coupled by the movement of water vapor, which can account for significant transfer of latent energy of vaporization. Soil temperatures may be significantly underestimated when the movement of energy associated with vapor transport is not considered. For example, Cahill and Parlange H. Saito and J. Šimůnek, Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521; B.P. Mohanty, Biological and Agricultural Engineering, Texas A&M Univ., College Station, TX 778432117. Received 12 Jan. 2006. *Corresponding author (hirotaka.saito@ ucr.edu). Published in Vadose Zone Journal 5:784–800 (2006). Original Research doi:10.2136/vzj2006.0007 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations:DOY, day of the year; TDR, time domain reflectometry. R e p ro d u c e d fr o m V a d o s e Z o n e J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 784 Published online May 26, 2006