Water use

A. Overview of Water Use

The Lockyer Valley is a very important agricultural area, which is based on groundwater supply. The valley supplies the State with 35% of its irrigated vegetables, as both winter and spring crops, plus fodder and small crops.

Less than 8% of the area of the Lockyer catchment provides over 80% of all agricultural production for the area. This demand for high productivity puts agricultural land under substantial pressure.

Prior to 1936 irrigation in the Lockyer Valley was limited, but by 1956 the area under irrigation had increased to 3,500 ha. Many dairy farms still existed at that time. By 1969 the area irrigated increased to 12,000 ha, and by mid-1970’s had stabilised at around 13,000 ha. The population is now increasing around the edges of the catchment, where many rural residential blocks ("hobby farms") are developing.

A 1993 report (DPI-WR) estimated that groundwater accounted for 80% of irrigation supply. This corresponds to an average annual groundwater withdrawal of around 45,000 ML. The annual groundwater withdrawal ranges from 20,000 to 74,000 ML. [megalitres = 1 million litres = approx 1 olympic swimming pool].

Groundwater storage in the alluvial aquifers is estimated at a safe annual yield of approx. 25,000 ML.

These volumes of water indicate that the alluvial aquifers are over-exploited. This is evident in dry periods when many bores become dry. Demand for irrigation water therefore exceeds supply, except in the wettest seasons.

Continued unregulated extraction of groundwater both depletes the alluvial aquifers, and lowers the water quality. It also changes the chemistry of the water. These conditions become excessive during ongoing periods of low or no rainfall.

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B. 5 year scenario: Aquifer Response to Prolonged Drought

Monitoring of groundwater by state government and other organisations over the years has shown that prolonged dry period have substantial impacts on groundwater levels, and on groundwater salinity. These impacts are increased by continued extraction of groundwater for irrigation under these conditions.

Following is a series of figures based on a generalised concept of the Lockyer Valley hydrology using a south to north cross-section. (This is not to scale, and there is a large vertical exaggeration). The responses are based on recorded observations within our research projects. Salinity for each bore is shown as TDI (total dissolved ions) in mg/L.

The cross section shows 5 bores in different settings across the valley,

  1. middle-upper sub-catchment, within alluvium, screen stops at bedrock;
  2. lower sub-catchment, drilled into bedrock and screened within a confined aquifer within the sandstone;
  3. sub-catchment floodplain, screen stops at bedrock;
  4. main alluvial aquifer of valley floor, screen is within basal coarse sands and gravels, and intersects bedrock;
  5. main alluvial aquifer adjacent to river course, screen stops in finer grained alluvium;
  6. main alluvial aquifer, distant from river course, screen stops in finer grained alluvium and shallow depth.

The following scenarios are represented (with continued groundwater extraction),

  1. Regular summer rainfall as storms
    under these conditions there is recharge from the ranges, flow in the streams with subsequent recharge to the alluvial system.



  2. Little or no rain for 12 months
    water table is lowered to bed of main creek, some minor increases in salinity.



  3. No rain for 24 months
    water table well down, noticeable increases in salinity, Bore B not much change.



  4. No rain for 36 months
    water table is well below creek beds and some margin and shallower bores have gone dry. No recharge from the ranges. Bore D starts to draw on bedrock water.



  5. No rain for 48 months
    water table is pulled down into the basal gravels, and groundwater is being “mined”. Under these conditions of pumping Bore D is drawing water from both upstream and the bedrock, and has a substantial increase in salinity. Most marginal and shallow bores are dry; the confined aquifer retains some flow, but has an increase in salinity.



The processes that can be observed confirm that the potential of the alluvial aquifer system is highly variable in different locations. Also obvious is that some areas of the valley are more favourably located for recharge that others.

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C. Water Use by Crops

to be developed .............

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D. Water Use Models

to be developed ............