Helping dams hold each drop
From the November 2007 edition of Agriculture Today.
Good management of irrigated land is crucial in these current climatic conditions, and one area industry is addressing is deep drainage.
"Midkin", one of Auscott’s commercial cotton farms, north-west of Moree, estimated the farm’s existing dual cell reservoir was losing more than 3570 megalitres in a normal year, with 43 per cent due to deep drainage.
The losses were caused by three main features: The positioning of the dam over ancient water channels, the large surface area to volume ratio resulting in high levels of evaporation, and decaying roots from dead trees within the reservoir, leaving deep channels through the reservoir floor.
The reservoir was constructed in the early 1980s. Unlike then, Auscott has the advantage of using Electromagnetic (EM) technology to help identify a new site for their reservoir, to reduce losses.
EM surveys can be used to map variations in soil properties. This is extremely useful when trying to assess the suitability of a given site for irrigation fields, supply channels and reservoirs.
An EM survey, along with soil sampling or ground truthing, provides maps of soil characteristics and can be used to locate soil type variation and vulnerability to deep drainage.
Terry Haynes, water resources manager at "Midkin", first became interested in the potential of the technology when the University of NSW developed maps of deep drainage across the Ashley district from EM surveys, soil sampling and comput er based modelling.
An EM survey was conducted at several potential reservoir sites and soil pits were dug to ground truth the maps. Once all the parameters were assessed, a final EM survey was conducted at the preferred location.
Prior to the availability of EM technology, the entire reservoir would have been excavated to the same depth. However, with the aid of the EM surveys, vulnerable sites were only minimally disturbed so as not to expose more vulnerable soils to deep drainage.
Mr Haynes believes the use of this technology has been extremely valuable. "The survey even dictated where to extract suitable material for the planned reservoir walls and we reduced the surface area from 180 hectares to 60 ha, which dramatically reduces water losses by evaporation."
Together these changes during a more typical year will save over 2500 megalitres per year. This provides a substantial financial saving along with increased water use efficiency and environmental benefits for the greater catchment, including a reduction in deep drainage.
Contact Helen Squires, Narrabri, (02) 6799 1588.
