growers@sgcotton.com.au Roger Tomkins - Greenmount Press

Mapping estimated deep drainage
in the lower Namoi Valley
■ By Alice Woodforth and John Triantafilis
THE Murray Darling Basin (MDB) is a prime agricultural region
of southeastern Australia. It accounts for half of all water
used for irrigation in Australia. But there are increasing
pressures on irrigators to improve water use efficiency (WUE)
owing to increasing demands on water for environmental flows,
new mining industries (such as coal seam gas production) and
in some instances to manage shallow water tables. In addition,
climate change modeling suggests that not only will rainfall
decrease but it will also become more variable in the MDB.
One way to improve WUE in irrigated areas is to reduce
deep drainage (DD). This is because DD is synonymous with the
network of prior stream channels that characterise the Riverine
Plains of the MDB. The most accurate way to measure DD is to
use a lysimeter. But these are expensive to install and require
significant time to equilibrate. More commonly, DD is estimated
using chloride mass balance (CMB) models.
Despite this advantage, the labour in the field and laboratory
time and expense of gaining estimates of DD means the spatial
variability across a district scale is usually still not attainable.
Electromagnetic (EM) induction instruments are useful in valueadding
to the limited number of estimates. This is because an EM
instrument measures many soil properties that can affect the rate
of DD (such as clay content).
In this research project, we explored the use of a CMB model
coupled with EM34 data to map the spatial distribution of DD
across the predominantly irrigated cotton growing area around
Wee Waa. The main features of the valley are the clay plains,
prior stream formations, the Pilliga scrub and a coarse-textured
dissected flood plain (Figure 1).
EM Survey, CMB modeling and calibration
EM34 measurements were made on an approximate 1 km
grid, with a total of 1869 sites visited (Figure 2). In order to
complement this data, a soil sampling scheme was developed
from the EM34 data and approximately 36 soil sample locations
were selected across the study area. At each site soil samples
FIGuRE 1: Map of physiographic units
(Stannard and Kelly, 1977)
were obtained at one metre depths and to a maximum depth of
9–18 metres across the study area.
The soil samples were analysed for chloride ion concentration.
The data was then entered into a simple CMB model along with
other information obtained from previous research. This included
an estimate of the concentration of chloride in irrigation water
and rainfall, an estimate of irrigation water application (i.e. 600
mm/year) and the average annual rainfall (i.e. 584 mm/year)
around Wee Waa.
Figure 3 shows the relationships between DD and EM34
data. The relationship can approximately be described as curvelinear.
Using a slightly more statistically rigorous analysis of this
relationship (for example, we log transformed the estimates of
DD), we used the subsequent relationship to estimate DD from
the EM34 data.
Map of estimated DD
Figure 4 shows the spatial distribution of estimated DD (mm/
year) generated from our calibrated EM34 data. The largest
estimates are associated with the Pilliga Scrub (around >450
mm) which is located to the south of Wee Waa. Some caution is
required with these estimates of DD because only one soil sample
location was collected in this area. Nevertheless, estimated DD is
consistent with the fact that few irrigated cotton growing farms
have been developed for furrow irrigation here.
Where fields have been developed for furrow irrigation, the
length of the field is short (about 300 metres). More commonly,
irrigation is limited to sprinkler or trickle irrigation and for the
purpose of vegetable (e.g. potatoes) or horticultural (e.g. table
grapes) production, respectively. The former is grown under
centre pivot irrigation.
In terms of physiographic units upon which fields have been
developed for irrigated cotton production, the largest estimates
of DD (350-450 mm/year) correspond with the low dissected
floodplain west of the ACRI and either side of the Spring Plain
road. Here, the irrigated fields are small – the length of the
FIGuRE 2: Location of EM34 measurement sites
and soil sample locations
44 — The Australian Cottongrower August–September 2012