Program Brief

Statement of Problem: The client required an efficient, integrated energy and mass transport code capable of modeling shallow infiltration processes for decade time scales in arid and semiarid environments for both present-day and future climatic conditions. Modeling shallow infiltration in arid and semiarid environments must focus on the short time periods when significant precipitation occurs. A code to satisfy these needs must (i) efficiently solve Richards equation for liquid flow, (ii) solve for energy transport in the liquid and vapor phases, (iii) incorporate efficient time-stepping algorithms between temporally sparse precipitation events, and (iv) allow the transpiration algorithm to change with climatic conditions.

Approach and Accomplishments: The code BREATH was developed to address this problem. Although BREATH can be used as a general unsaturated flow simulator for humid environments, it has features that make it particularly well suited for long-time simulations of shallow infiltration in arid and semiarid environments. In arid environments, evaporation and transpiration are important components of the water balance, with annual potential evapotranspiration often being greater than annual precipitation; yet, water can still infiltrate and percolate to the water table, particularly in areas of thin soils over porous or fractured bedrock. During the sparse precipitation events, capillary and gravity driven flow requires the use of a numerical solution of the Richards equation. After a short period of redistribution, timestep size can be significantly relaxed until the next precipitation event.

BREATH is a 1-dimensional flow simulator for coupled heat and moisture flow. The simulator couples the Richards equation with diffusive vapor transport. Heat flow is modeled as diffusive flux in the matrix and advective flux in the liquid and vapor phases. BREATH has a transpiration model that allows for changes in vegetation, as would be expected for past and future climatic conditions. The simulator is capable of using constant or finely resolved temporal meteorological data as an upper boundary condition. A free drainage condition is also included along with other types for the lower boundary condition to facilitate modeling of large unsaturated domains.

Client Benefits: BREATH provides a fast, general purpose tool for modeling unsaturated heat and moisture flow particularly suited for arid and semiarid climates. The efficient solution techniques employed by BREATH enable sensitivity and uncertainty analyses to be performed.