Investigation of water movement in the unsaturated zone under an irrigated area using environmental tritium

A 14-a year record of convective-dispersive flow in the unsaturated zone was reconstructed using the difference between the environmental tritium content of rain and of irrigation water. Samples were taken from a loess profile in the northern Negev, Israel, in an area where precipitation is 200 mm/winter and irrigation is 650 mm/summer. The difference between the measured tritium profile and the input tritium profile was interpreted in terms of mobile and immobile water domains. The percentage of the total water which is immobile was estimated to be about 40% at the surface and 55% at 8.5 m depth. This leads to the estimate that 8 ± 1% of the rain and irrigation water percolated downward, with a velocity of 0.66 ± 0.03 m yr⁻¹ and a maximal effective dispersion coefficient of 5 × 10⁻¹¹ m²s⁻¹. Using these values, we determined the limits of dispersivity (0.5–1.5 mm) and of tortuosity (0.05–0.11) in the medium studied. (author)