The Australian Cotton Industry and four decades of deep drainage research: a review

Abstract. TheAustralian cotton industry and governments have funded research into the deep-drainage component of the soil–water balance for several decades. Cotton is dominantly grown in the northern Murray–Darling and Fitzroy Basins, using furrow irrigation on cracking clays. Previously, it was held that furrow irrigation on cracking clays was inherently efficient and there was little deep drainage. This has been shown to be simplistic and generally incorrect. This paper reviews global andnorthernAustraliandeep-drainage studies in irrigation, generally at point- or paddock-scale, and the consequences of deep drainage. For furrow-irrigated fields in Australia, key findings are as follows. (i) Deep drainage varies considerably depending on soil properties and irrigation management, and is not necessarily ‘very small’. Historically, values of 100–250mm year–1 were typical, with 3–900mm year–1 observed, until water shortage in the 2000s and continued research and extension focussed attention onwater-use efficiency (WUE). (ii)More recently, values of 50–100mmyear–1 have been observed, with no deep drainage in drier years; these levels are lower than global values. (iii) Optimisation (flow rate, field length, cut-off time) of furrow irrigation can at least halve deep drainage. (iv) Cotton is grown on soils with a wide range in texture, sodicity and structure. (v) Deep drainage is moderately to strongly related to total rainfall plus irrigation, as it is globally. (vi) A leaching fraction, to avoid salt build-up in the soil profile, is only needed for irrigation where more saline water is used. Drainage from rainfall often provides an adequate leaching fraction. (vii) Near-saturated conditions occur for at least 2–6