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.