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

Abstract

The Australian 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 and northern Australian deep-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 100–250 mm year–1 were typical, with 3–900 mm year–1 observed, until water shortage in the 2000s and continued research and extension focussed attention on water-use efficiency (WUE). (ii) More recently, values of 50–100 mm year–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 m under irrigated fields, whereas profiles are dry under native vegetation in the same landscapes. (viii) Deep drainage leachate is typically saline and not a source of good quality groundwater recharge. Large losses of nitrate also occur in deep drainage. The consequences of deep drainage for groundwater and salinity are different where underlying groundwater can be used for pumping (fresh water, high yield; e.g. Condamine alluvia) and where it cannot (saline water or low yield; e.g. Border Rivers alluvia). Continuing improvements in WUE are needed to ensure long-term sustainability of irrigated cropping industries. Globally there is great potential for increased production using existing water supplies, given deep drainage of 10–25% of water delivered to fields and WUE of <50%. Future research priorities are to further characterise water movement through the unsaturated zone and the consequences of deep drainage.

Leaching of dissolved organic carbon and nitrogen under cotton farming systems in a Vertisol

“…In relation to water inputs, our values were <7% of applied irrigation water and <4.5% of applied irrigation (Table ) and rainfall (Figure ) per annum. These values and relative proportions are comparable to those reported by Silburn, Foley, Biggs, Montgomery, and Gunawardena () for Australian cotton farming systems. The relatively lower deep drainage in 2016–17 season compared with 2014–15 and 2015–16 seasons may be a consequence of the extended heat wave and hence greater evapotranspiration that occurred during 2016–17 (Figure ).…”

Potential contributions of surface and ground water to nitrous oxide emissions from irrigated cotton production systems

“…Cotton production system during 2001-2010 lost 20 kg NO 3 -N ha -1 below 1.2m due to deep drainage. It is unclear how much of this NO 3 -N reaches the production aquifer but certainly irrigated cropping systems can significantly increase the NO 3 -N concentration in the deep unsaturated zone (Silburn et al, 2013) and in the groundwater (Korbel et al, 2013). There is no recorded systematic monitoring of the NO 3 -N and total N concentration of the ground water resources within the Namoi Catchment.…”

Event‐based deep drainage and percolation dynamics in Vertosols and Chromosols

“…Drainage estimates at Theten Farm in relation to drainage estimates from other studies on (a) irrigated and non‐irrigated land, and (b) on Black Vertosol and Grey Vertosol across eastern Australia (After Connolly et al, ; Dalton et al, ; Hulugalle et al, , ; Radford et al, ; Ringrose‐Voase & Nadelko, ; Silburn et al, , Silburn, Foley, Biggs, Montgomery, & Gunawardena, ; Silburn & Montgomery, ; Thorburn et al, ; Tolmie et al, ; Weaver et al, ; Willis et al, ; Willis & Black, )…”

“…However, quantification of deep drainage is challenging, and rates can vary strongly over space and time. For example, while deep drainage is commonly considered to be less than 10% of total rainfall for native vegetation, estimates can be high as 70% under irrigated cotton cultivation (Dalton, Raine, & Broadfoot, 2001;Kurtzman & Scanlon, 2011); and deep drainage has been recorded as ranging from 0% to 50% of rainfall within the same general soil type (cracking clays) Dalton et al, 2001;Hulugalle, Weaver, & Finlay, 2010Radford et al, 2009;Ringrose-Voase & Nadelko, 2013;Silburn et al, 2011Silburn et al, , 2013; Thorburn, Cowie, & Lawrence, 1991;Tolmie et al, 2011;Weaver, Hulugalle, & Ghadiri, 2005;Willis, Black, & Meyer, 1997;.…”