The cotton industry and other investors have spent millions of dollars on ‘water balance’

related research over the past two decades. While soil water balance models can be used

today to answer critical questions affecting the industry, little of this water balance

research has been fully captured or insufficient data are available to model these sites. A

lack of basic soil physical data limits the use of water balance models across many areas of

the cotton industry.

Models are commonly used as decision support tools in agriculture. This project captured

data from various past and current studies of water balance and deep drainage under

irrigation in N NSW and Qld in a form useful for modelling, and ‘filled in’ some to the

gaps with additional soil hydraulic measurements at key sites. These data were used to

test the models, derive parameters, enhance understanding and so provide insights across

landscapes and over longer time periods, especially in CRC groundwater/catchment

modelling studies.

Electromagnetic Induction (EM38) technology (with depth slicing) was developed and

used throughout a cotton growing season to monitor soil water at a range of depths and

provided input to the model HowLeaky (Mills et al. 2008). The data agreed to a large

degree with the model, which estimated that during two periods in the season, a

significant amount of deep drainage occurred (100 mm). This is of concern both in terms

of wasting water and potential groundwater recharge.

Geophysical surveys and deep soil coring were taken in both irrigated and uncleared

landscapes throughout the Condamine (co-funded by Condamine Alliance) and Border

Alluvia (co-funded by Border-Gwydir CMA), to define the moisture status below the root

zone. That is, is there a moisture deficit buffer which can store increased deep drainage

from irrigation or is the regolith nearly full? Transects imaged across naturally vegetated

landscapes into irrigated paddocks found all soils under native vegetation to be very dry

(low conductivity) even when only sparsely populated by trees. In contrast, significant

long-term migration of water has occurred to deep within the regolith (up to 15 m) in

most irrigated paddocks. A wet (close to saturated) zone was found in the upper 6 m of

soil in the irrigated paddocks. The imaging results confirmed that the unsaturated zone is

filled to near saturation under all irrigated sites and remains very dry under all native

vegetation sites. Deep coring has also be used at a number of sites to collect data to

improve geophysics model parameterisation for clay soils, so that resistivity imaging can

be used with more confidence in the future to estimate the amount of water to deep in the

soil regolith.

Finally, a monitoring bore network was installed across the Border-Moonie catchments to

allow continued observation of groundwater behaviour in irrigated and dryland areas.

This area was found to have a high potential risk of future salinity problems but had little

monitoring installed. A “20-point plan” was developed and has been funded in stages by

NRM Regional Bodies (QMDC & Border-Gwydir CMA) and DERM.