10.1  Groundwater levels

10.1.1  Groundwater data

The CIA has a large network of piezometers, which is used to monitor groundwater in the irrigation area and surrounds.  A map showing the location of these piezometers is shown in Figure 10.1.  As per licence conditions these piezometers are read twice yearly in February/March and August/September.  This data is compiled in databases to be analysed using ArcView GIS and Spatial Analyst.  The information in this section has been extracted from this analysis.

Figure 10.1 Location of piezometers

Due to large variations in data recovery over a period of time a smaller sub-set of piezometers has been selected for use in trend analysis.  This sub-set is also shown in Figure 10.1.  The large variation in data recovery is illustrated in Figure 10.2.  It is considered important to use a continuous sub-set of piezometers when conducting trend analysis for the integrity of the results.  The sub-set contains 144 piezometers in the 5-12m range (upper Shepparton aquifer) and 296 piezometers in the 12-35m range (lower Shepparton aquifer).

Figure 10.2  Data recovery from piezometer monitoring

To maintain consistency with annual reporting, the same years have been used for trend analysis as were used in the 1999 and 2000 AERs.  Hence the years used for analysis are 1986, 1987, 1990, 1992, 1994, 1996, 1997, 1998, 1999 and 2000.

10.1.1  Groundwater mapping

Figure 10.3 and Figure 10.4 are contour maps of the piezometric levels in the CIA for August/September over the years 1986 to 2000.  These maps were produced using the inverse distance weighted method of interpolation.  This method of interpolation requires input in the form of x and y coordinates for location and a z coordinate for the groundwater piezometric level.  An output grid cell size of 100 metres was used.  The number of neighbours sampled was 12 and a power of two was used as the exponent of distance.

Figure 10.3 Depth to piezometric level (5-12m)

Figure 10.4 Depth to piezometric level (12-35m)

These figures along with the following tables show that in 2000 there has been relatively little change in the piezometric levels of both aquifers from 1999, when piezometric levels rose.  Table 10.1and Table 10.2 show the data extracted from the contour maps in terms of hectares within various piezometric ranges. There has been a small decline in the area with a piezometric level of zero to two metres in the lower Shepparton aquifer.

Table 10.1 Areas of the CIA with piezometric level in various ranges (5-12m)

Table 10.2 Areas of the CIA with piezometric level in various ranges (12-35m)

Table 10.3 and Table 10.4 illustrate the variation between the four regions of the CIA.  To create these tables all previous analyses were divided into the four new regions of the CIA; Boona (B), Coly (C), Argoon (A) and Yamma (Y).  These regions can be seen in Figure 10.1.

Table 10.3  Areas with piezometric level in various ranges (5-12m) in the four regions of the CIA

Table 10.4  Areas with piezometric level in various ranges (12-35m) in the four regions of the CIA

The previous maps were produced using the smaller sub-set of piezometers. It is also useful to analyse the entire set of piezometers for September 2000 to show a snapshot in time of the piezometric levels in the two aquifers.  Figure 10.5 shows the contour maps produced by analysing the complete data set.  It is apparent when comparing these maps to the maps generated from the smaller sub-set that it is essential to consider the complete data set when interpreting maps.

Figure 10.5  Depth to piezometric level for all piezometers, September 2000

The piezometric levels have also been converted to Australian Height Datum (AHD) and have been mapped for the two aquifers.  These maps are shown in Figure 10.6.  The AHD piezometric levels represent the heights of the watertables in each aquifer above sea level.  This data may be used to identify the direction of groundwater flow.  The groundwater flow under gravity will flow perpendicular to the groundwater contours.  The trends shown in this year’s maps are similar to those evident in the 2000 AER maps.  The deeper aquifer (12-35m) shows three distinct groundwater mounds from which groundwater flows away in all directions.  The shallower aquifer (5-12m) also exhibits these trends however the mounds are not as distinct.

Figure 10.6  AHD piezometric level

10.1.3  Hydrographs

Hydrographs of the piezometers in the CIA have been created from the information in the GIS database.  Geometric means from each data set have been used to produce the hydrographs.  When the data is examined, most sets show a skewed distribution.  For this reason, the geometric mean is believed to be a more appropriate descriptor of the datasets then the arithmetic mean.

Figure 10.7 is a summary of the September depths for the entire CIA.  It shows that overall; the September 2000 piezometric levels for both the lower and upper Shepparton aquifers have shown little change from September 1999.  More detailed information can be viewed below in the hydrographs for each region.

Figure 10.7  Piezometric level Coleambally Irrigation Area

Piezometers for hydrograph analysis in each of the four regions of the CIA have been selected based on their presence in the smaller sub-set of piezometers and their location within or close to the boundary of the CIA. The figures from Figure 10.8 through to Figure 10.11 show the September depths for the four regions. 

Figure 10.8 Piezometric level Boona region

Figure 10.9  Piezometric level Coly region

Figure 10.10  Piezometric level Argoon region

Figure 10.11 Piezometric level Yamma region

The Coly and Argoon hydrographs show that there has been very little change in the piezometric levels of both the upper and lower Shepparton aquifers.  For the Yamma hydrograph there has been little change in the lower Shepparton aquifer, but a noticeable rise in the upper Shepparton aquifer. The Boona hydrograph shows decreases in the piezometric levels for both aquifers. The hydrographs also show that there is a direct relationship between the two aquifers with both depth ranges following similar trends in all of the regions.

From the data presented so far, the September 2000 readings show little overall change in the piezometric levels of the upper and lower Shepparton aquifers. Table 10.5 summarises the rate of change in upper and lower Shepparton piezometric levels between years for both the February-March and August-September readings.  This table shows that there has been little change in the piezometric levels for the upper and lower Shepparton aquifers in September and the lower Shepparton aquifer in March, but a noticeable drop in the piezometric level for the upper Shepparton aquifer in March.

Table 10.5  Rate of rise in piezometric levels in the CIA

Figure 10.12 also illustrates that there has been little overall change in the piezometric level since 1999, although some areas of noticeable change do tend to occur near the edges of the CIA.  These maps were created by contouring the change in piezometric level between 1999 and 2000.  The negative numbers show the areas that have experienced a decrease in piezometric levels and the positive numbers show those areas that have experienced an increase in piezometric level.

Figure 10.12  Piezometric level change from September 1999 to September 2000

The piezometric levels in September of each year are generally lower then those in March of the same year.  Table 10.6 shows that there is substantial variation between September and March levels for all the regions. The most noticeable divergences from the long-term averages for 2000 occurred in the upper Shepparton aquifer of the Boona region and the lower Shepparton aquifer of the Argoon region.  The Boona region continues to show the largest variation between the March and September readings.

Table 10.6  Variation between September and March piezometric levels (metres)

10.1.4  Seasonal variation in piezometric level

Since the beginning of 1997, 30 piezometers within the CIA have been measured approximately every two months in order to track seasonal piezometric changes more closely.  Figure 10.13 shows the location of these piezometers.  The 30 piezometers are distributed throughout the four regions and occur in both the upper and lower Shepparton aquifers.

Figure 10.13 Locations of 30 piezometers measured every two months

Figure 10.14 and Figure 10.15 show the geometric means of the piezometric levels in the four regions for the 5-12m and 12-35m depth ranges respectively.  All regions display the same pattern of rise in levels from September to February and fall in levels in the remaining months of the year.  For the most part, this trend is consistent for both the 5-12m and 12-35m ranges.  The apparent anomaly for the Coly lower Shepparton aquifer in January 1998 is probably due to data availability.  Despite the same overall pattern occurring in the Boona region, it is known that many piezometers in its northern-most area do not exhibit this pattern. This may be due to a number of factors, including; higher ground surface elevation, high distribution of self mulching clays, close proximity to the deep bore pumping area, the narrow shape of the CIA in the northern area and greater potential for high rice water use on farms.

Figure 10.14  Seasonal variation in piezometric level in the four regions of the CIA (5-12m)

Figure 10.15  Seasonal variation in piezometric level in the four regions of the CIA (12-35m)

10.1.5  Piezometric levels in Kerarbury Channel Area

The piezometers in the KCA are generally deeper then those found within the CIA.  The area has shown continuing increases in lower Shepparton piezometric levels since the scheme began in 1994. The “>15” column in Table 10.7 illustrates this. 

Table 10.7  Areas with piezometric level in various ranges in the Kerarbury Channel Area (12-35m)

Figure 10.16 is a hydrograph that has been produced using the geometric mean of the piezometric levels measured in the Kerarbury area.  However, this graph does not indicate the level of watertable rise that is known to be occurring within the KCA. This is due to an insufficient number of the piezometers analysed actually occurring within the boundaries of the Kerarbury farms. Analysis of piezometers occurring within the area has shown a greater rate of watertable rise than is evident in the hydrograph. The selection of Kerarbury piezometers for analysis in the AER will be revised in 2001/2002.

Figure 10.16  Piezometric level in the Kerarbury Channel Area