[Introduction]:
Most of Australia’s dryland cropping is characterised by unreliable rainfall with frequent long gaps between falls. Stored soil water is therefore essential to support crop growth during the growing season while water stored during fallows has varying importance, depending on soil type and rainfall patterns in relation to cropping periods. For example, a winter crop at Walpeup in Victoria derives 10% of its water supply from soil water at planting while a winter crop at Emerald will access 80% of its water supply from stored soil water (Thomas et al 2007). Even when dependence on stored water is small, extra water can make a valuable difference to crop yield and profitability, especially in typical dry-finish seasons (Kirkegaard et al 2014). An understanding of available water before a crop is planted can influence management decisions (area planted, fertilizer rates). Estimating plant available water (PAW) also requires an estimate of a soils ability to store water, its plant available water capacity (PAWC).
This paper presents some observations of soil water from a 17-year study comparing water balances (runoff, evaporation and deep drainage) for a set of small contour bay catchments near Roma in southern Queensland. Our aim is to demonstrate some of the challenges associated with field measurement of both PAWC and PAW. This analysis is an extension of a detailed description of the development of SoilWaterApp (Freebairn et al. 2018)
