Implications of deep drainage through saline clay for groundwater recharge and sustainable cropping in a semi-arid catchment, Australia

Abstract

Abstract. The magnitude and timing of deep drainage and salt leaching through clay soils is a critical issue for dryland agriculture in semi-arid regions (<500 mm yr −1 rainfall, potential evapotranspiration >2000 mm yr −1 ) such as parts of Australia's Murray-Darling Basin (MDB). In this rare study, hydrogeological measurements and estimations of the historic water balance of crops grown on overlying Grey Vertosols were combined to estimate the contribution of deep drainage below crop roots to recharge and salinization of shallow groundwater. Soil sampling at two sites on the alluvial flood plain of the Lower Namoi catchment revealed significant peaks in chloride concentrations at 0.8-1.2 m depth under perennial vegetation and at 2.0-2.5 m depth under continuous cropping indicating deep drainage and salt leaching since conversion to cropping. Total salt loads of 91-229 t ha −1 NaCl equivalent were measured for perennial vegetation and cropping, with salinity to ≥10 m depth that was not detected by shallow soil surveys. Groundwater salinity varied spatially from 910 to 2430 mS m −1 at 21 to 37 m depth (N = 5), whereas deeper groundwater was less saline (290 mS m −1 ) with use restricted to livestock and rural domestic supplies in this area. The Agricultural Production Systems Simulator (APSIM) software package predicted deep drainage of 3.3-9.5 mm yr −1 (0.7-2.1 % rainfall) based on site records of grain yields, rainfall, salt leaching and soil properties. Predicted deep drainage was highly episodic, dependent on rainfall and antecedent soil water content, and over a 39 yr period was restricted mainly to the record wet winter of 1998. During the study period, groundwater levels were unresponsive to major rainfall events (70 and 190 mm total), and most piezometers at about 18 m depth remained dry. In this area, at this time, recharge appears to be negligible due to low rainfall and large potential evapotranspiration, transient hydrological conditions after changes in land use and a thick clay dominated vadose zone. This is in contrast to regional groundwater modelling that assumes annual recharge of 0.5 % of rainfall. Importantly, it was found that leaching from episodic deep drainage could not cause discharge of saline groundwater in the area, since the water table was several meters below the incised river bed.

Soil–aquifer phenomena affecting groundwater under vertisols: a review

“…4). Similar to the Murray-Darling Basin (Timms et al, 2012), the upper groundwater under vertisols in this aquifer were more saline than the deep groundwater (e.g., Fig. 1 in Baram et al, 2014).…”

“…A large bulk of literature from eastern Australia has reported increased deep-drainage and leaching of salts, and in some cases, salinization of aquifers under cultivated vertisols. A typical increase in deep drainage from < 1 mm yr −1 under native conditions to 10-20 mm yr −1 under rain-fed cropping were reported by Silburn et al (2009);Timms et al (2012) and Young et al (2014); whereas variable deep fluxes often in the 100's mm yr −1 range were reported for irrigated fields (mostly furrow-irrigated cotton; Gunwardena et al, 2011;Silburn et al, 2013;Weaver et al, 2013). These deep fluxes desolate salts that accumulated in the vadose zone in the native-vegetation period, moving them down towards the water table (Fig.…”

“…Low water content in the deeper unsaturated zone results in low hydraulic conductivities and makes aquifer recharge through matrix flow very small year-round. Matrix fluxes on the order of 1 mm yr −1 under the root/crack zone were reported in a number of studies (e.g., Silburn et al, 2009;Kurtzman and Scanlon, 2011;Timms et al, 2012). These very low water fluxes contain the conservative ions (e.g., chloride) originating from 200-600 mm yr −1 of precipitation (with salts from wet and dry fallout) that enter the matrix at soil surface.…”

Groundwater dynamics under water-saving irrigation and implications for sustainable water management in an oasis: Tarim River basin of western China

“…The key control on regional water table dynamics is the exchange flux (EF), which is defined as the water flux into or out of the lower boundary of the soil control volume representing the vadose zone (Tang et al, 2007a;Timms et al, 2012). Since EF is substantially influenced by irrigation and the conversion from flood irrigation to drip irrigation, any study of groundwater table dynamics in this region requires that EF be appropriately quantified under these different circumstances.…”

Estimating groundwater residence time and recharge patterns in a saline coastal aquifer

“…The occurrence of saline and brackish groundwater in reclaimed coastal aquifers worldwide is often a serious treat for agriculture (Da Lio et al, 2015;De Louw et al, 2011;Shi and Jiao, 2014;Timms et al, 2012). For this reason, the integrated management of water resources in coastal regions must be accomplished taking into account the exchanges between surface and groundwater bodies (Henriksen et al, 2008;Jønch-Clausen and Fugl, 2001).…”