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Diffuse vadose zone soil-water flux is minuscule in semi-arid environments. Measurements of soil suction and hydraulic conductivity are prone to errors and their use leads to unreliable flux estimates. Often, unambiguous environmental tracer profiles are available in the unsaturated zone soil profile. They can be used to obtain robust estimates of the soil-water flux by applying simple analytical models. Environmental tracers are very suitable because they normally occur in small concentrations and do not disturb the natural flow system significantly. They also provide integrated values of the flux over longer periods of time and in balance with the local vegetation or land use patterns. Current techniques enable small concentrations of these tracers to be measured reliably. In the present instance three different tracer profiles were used to obtain flux estimates by using very simple models requiring minimal input data. If tracer profiles suggest that diffusion dominates the transport process then the use of simple models is justified.

The shape of the tracer profiles in conjunction with the water contents and porosity data suggest that tracer advection and dispersion was of minor importance. Soil-water flux was first estimated by using the tritium peak-migration and tritium mass-balance methods. A heat diffusion analogy was also used to match the observed tritium profile. Chloride mass balance of the profile was used to obtain another flux estimate. A modification of the chloride mass-balance method was also used to obtain advective and diffusive components of moisture flux. Simple piston flow models based on peak migration were used to obtain additional estimates from chloride and nitrate profiles. The estimates obtained were of the same order of magnitude and within the range reported for semi-arid regions in other parts of the world. Copyright (C) 2000 John Wiley & Sons, Ltd.