Estimation of diffuse vadose zone soil-water flux in a semi-arid region

Abstract

The two primary objectives of this research project were to determine quantitative estimates for soil-water flux in the vadose zone and to develop an understanding of the possible mechanisms for vadose zone soil-water flow at the study site. Field data for determining the soil water fluxes and characterizing the flow mechanisms in the vadose zone of a glacio-lacustrine silty clay soil in a semi-arid Prairie environment are presented and discussed. The data suggest that the study site lies in a general recharge area of the larger landscape. Measurements of hydraulic head and total dissolved solids indicate a lateral flow component in the saturated zone. Because the movement of water through the vadose zone of soils in semi-arid regions is extremely slow, it is preferable to employ a variety of measurement techniques in attempts to establish reliable estimates of the water flux. The effects of the plant root zone on the movement of soil water movement is manifested in the existence of a zero flux plane. Using information on the positioning of the plane, the short term fluxes of water through the zone were calculated to range from 11 to 17 mm y$\sp{-1}.$ Values for the long term-average flux by the chloride mass balance method were 0.2 to 2 mm y$\sp{-1}.$ Medium term flux estimates using chloride, nitrate and tritium peak migration were 9 to 48 mm y$\sp{-1}.$ Tritium mass balance estimates over the period 1953 to 1994 gave estimates of about 15 mm y$\sp{-1}.$ Interpretation of stable isotope data showed that soil water at depth below the root zone is comprised of a mixture of some summer and mainly winter/meltwater precipitation. A conceptual model of the flow and transport mechanism has been constructed on the basis of interpretation of the field data. It suggests that rapid transport of water and solutes occurs to a depth of about 1.5 m primarily during "episodes" like snowmelt and thunder storms. Low Peclet numbers below this depth indicate that water may move largely by "non dispersive" piston flow mechanism and solute transport may be diffusion dominated. The physical properties of the geological media are such that diffusional equilibrium may prevail in the wetted pore space.