Deep Drainage Under Irrigated Cotton Surface & Groundwater Implications

Abstract

Deep drainage (DD) - water that passes beyond the root zone - can be an important contributor in terms of recharging ground water as well as leaching salts from the root zone. Excessive DD is economic poor practice and a potential source of rising ground water tables with increased solute concentrations; potentially challenging issues for irrigated agricultural landscapes and the communities therein. The project’s prime aim was the direct quantification of DD across a wide, yet representative range of cotton soils and management systems, while concurrently assessing both salt balances of collected leachates and around-lysimeter soils, as well as crosschecking the measured DD data with less expensive, indirect methods of predicting DD (eg SODICS, SaLF and ET/SIRMOD). Secondly, to monitor irrigation efficiencies in terms of recent technology utilisation in the cotton industry, specifically the comparative efficiency of a lateral move irrigator (LM) vs adjoining furrow irrigation, in terms of lessened water applied and DD; LM considered as having great potential for positive impacts on water savings. Thirdly, to investigate linkages (if any) between surface water events (DD, irrigation, river flows, etc) with historic and current (logged) groundwater levels in the St George irrigation area (SGIA); checking for rising water and salinity risk. Instrumentation was 35 drainage lysimeters (constant suction) at three locations in one field on each of 10 commercial farms and at the Australian Cotton Research Institute. Up to five irrigation seasons have been monitored (2002 to present). Results show a maximum DD of 310 mm (3.1 ML/ha) in one season has been measured (representing ~39% of the applied irrigation water). However, of 69 sampling occasions across four growing seasons and all the lysimeter sites, only 14 occasions (~20%) provided DD values of >100 mm (1 ML/ha). Additionally, DD has been found to vary strongly across fields - from head to tail ditches, and there is strong between-seasons’ variation in the lysimeter data at any one site, apparently linked to during-season weather and water (irrigation water) availability. Some sites that provided >150 mm (1.5 ML/ha) of DD in one season, gave a zero reading the following season. These unexpected variabilities in the DD data, though important to know and to begin to rationalise in terms of site practice and seasonal weather, cause difficulties in rationalisation of the main drivers (of DD) towards the development of industry-applicable BMPs. Water quality analysis of the DD leachates apparently shows salt loads being mobilised under all sites. Soil chloride analysis (over five years at some of the DD sites) shows increased salts in the root zone of certain fields. Close investigation of both data sets is current. The indirect methods of predicting DD have proven most poor in providing matches to the measured lysimeter DD values; in terms of both magnitude (of DD volumes) and correspondence with (at times large) measured infield and seasonal variability in DD. Preliminary analysis of the historic borehole logs and real-time logging of groundwater levels suggests that the shape of the groundwater contours does not particularly illustrate the presence of a broad groundwater mound in the SGIA, but rather the development of more localised groundwater mounds probably reflecting zones of locally preferential accession of DD (most probably due to channel leakage and leakage from on farm storages). The depth to groundwater data suggest that groundwater levels have not yet approached the 2 m bGL level that is commonly viewed as posing a risk for soil salinisation via capillary rise of groundwater. Currently there are 28 operational lysimeters (2 sites having been recently de-commissioned) and 18 borehole loggers (logging aquifer level twice daily) that will continue monitoring to 2011. These additional data will aid clarity in the drivers of DD and associated groundwater response. Further DD leachate and soil salinity data will be collected to continue the salt mass balance study.