Deep drainage through Vertosols in irrigated fields measured with drainage lysimeters

Abstract

Rising groundwater and salinity are potential risks across irrigated agricultural landscapes. Water is scarce in many areas that will benefit from efficient water use. Excessive deep drainage (DD, mm) beneath irrigated crops is undesirable because it may cause salinity and decrease water-use efficiency. Nine irrigated, commercial cotton fields (eight furrow-irrigated and one spray, lateral-move irrigated) were selected in the upper Murray–Darling Basin, on Vertosols with a wide range of clay contents (38–75%). The lysimeters used, described as ‘confined, undisturbed, constant tension, non-weighing’, were installed to capture water passing 1.5 m depth at three in-field positions: (i) near the head ditch, (ii) mid-way between head and tail ditches, and (iii) close to the tail ditch. At two sites, infiltration along the length of the field was monitored in two seasons using furrow advance-SIRMOD methods. Seasonal DD values of up to 235 mm (2.4 ML/ha.season) were measured (range 1–235 mm), equivalent to 27% of the irrigation applied at that location in that season. Individual DD events >90 mm accounted for 15 of 66 measured values from 26 furrow irrigations. DD varied strongly along the length of each field, with DD commonly reducing from the head ditch to the tail ditch. SIRMOD simulation mirrored this trend, with large decreases in infiltration amounts from head to tail. Greater DD at head locations was attributed to long periods of inundation, especially early in the season when siphons (in-flows) were allowed to run for up to 24 h. Most of the DD occurred during pre-irrigation and the first two or three in-crop irrigations. Inter-season variation in DD was large; limited water supply in drought years led to fewer irrigations with smaller volumes, resulting in little or no DD. The DD under lateral-move, spray irrigation was almost zero; only one irrigation event in 4 years resulted in DD. Control of DD under furrow irrigation can be achieved by changing irrigation management to lateral-move, spray irrigation, which minimises DD and greatly increases water-use efficiency with no yield (cotton) penalty. Across all of the lysimetry sites, high salinities of the DD leachate indicated that large amounts of salt were being mobilised. The fate and impacts of this mobilised and leached salt are uncertain.