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Over the years growers in the Lower Murray-Darling irrigation districts have consistently reported salinity damage to horticultural crops despite the moderate irrigation water salinity. There is also anecdotal information that after leaching the salinity in the root zone remains higher than expected, perceived due to incomplete mixing of the soil solution. A collaborative, Tri-State Salinity research project is being undertaken in the Riverland and Sunraysia to investigate the mechanism of soil salinity build up in the root zone and the resultant economic loss (Schrale and Biswas 2004).

One of the aims of this project is to estimate the rate of deep drainage (DD) from the root zone. In this paper we present two approaches for assessing DD from data generated by multi-sensor capacitance probes, which are now widely used for irrigation scheduling in those horticultural districts. Capacitance probes utilise Frequency Domain Reflectometry (FDR), which is based on dielectric properties of soil water. In the first method, DD was estimated by converting raw data for separate layers into a total soil profile water content and then used in a soil water balance model.

The second, new method of DD estimation is by using Darcy’s flux equation whereby a q-y relationship was developed from soil water content data generated by two sensors, located just below the root zone. The annual drainage volumes calculated from field capacities and water balance method varied from 181 to 569 mm or 19 to 43% of applied water. The DD derived by Darcy flux method showed both daily and seasonal variations of drainage rate; the data collected from a citrus property between 2002 and 2003 shows that a cumulative drainage of 250 mm or about 2.5 ML of leaching occurred during a period of 1 year. In general the water extraction pattern followed an inverse relationship with the soil depth whereby a maximum of 28 to 48% of total water was extracted from within the top 15 cm layer.