Estimation of Root Zone Drainage from a Chardonnay Vineyard: A Comparison of Methods

In the Murray Mallee region of Australia, intensive irrigation of horticultural crops has led to the salinisation of farmland and the discharge of saline water into the Murray River system, leading to adverse economic and environmental impacts. The primary cause of this process is excess drainage of water beyond the root zone of crops. Quantification of root zone drainage (RZD) is therefore necessary for irrigators to benchmark their environmental performance and increase their water use efficiencies. Previously, root zone drainage has been estimated using readily available water (RAW) values and seasonal water balances. The increasing trend of growers to schedule irrigations using capacitance probes that provide real time soil water data allows a unique opportunity for continuous monitoring and management of RZD. Three methods of estimating root zone drainage using capacitance probe data were trialled on a 0.66 ha drip irrigated vineyard (Vitis vinifera cv. 'Chardonnay'). Estimates were based on a water balance, Darcy's flux equation and a simple method of measuring the leaching fraction from the apparent field capacity of the soil profile. Estimates of RZD over a nine month period were 0.43, 0.15 and 0.54 ML/ha for the water balance, Darcy's flux equation and field capacity methods, respectively. These methods need further testing before they can be incorporated into irrigation management packages relevant to the region.