results are contrasted to rainfall data for five stations, one from each region, where Accumulated
Monthly Residual Rainfall (AMRR 1975-1999) is compared pre and post 2000.
Region Bores Pre 2000 Post 2000
# Rising Falling Stable Rising Falling Stable
Northern 109-170 66% 6% 27% 18% 69% 13%
Central 299-479 47% 5% 47% 23% 37% 40%
South-West 331-370 53% 3% 44% 37% 12% 52%
South Coast (West) 76-80 74% 17% 9% 50% 31% 17%
South Coast (East) 175-219 72% 5% 23% 71% 7% 22%
Table 1. Bores analysed for groundwater trends 1990-2000 (n=990) & post 2000 (n= 1318 bores).
Bores qualified for inclusion in the analyses if they were located in cleared agricultural land, remote
from the effects of any salinity management treatment (drains, trees, perennial pastures) and met
minimum standards (e.g. 5 years duration and/or 20 monitoring observations). The average catchment
had 14 bores and 50 groundwater observations. Trend analyses were conducted on between 266 and
1318 bores [<1990 (n=266), 1990-2000 (n=990), 2000-07 (n=1198), and ALL (n=1318)].
Bores were not sorted by soil-landscape zone. For example, in some catchments the proportion of
bores in valleys with shallow watertables dominated (eg Central Region where Wallatin and Beacon
catchments had 72-83 bores in areas where average watertables were < 3m), while other catchments
were dominated by bores with deep watertables (e.g. NE Yilgarn = 9 m and East Belka = 12m).
The relative proportions of groundwater bores with rising trends changed after 2000, both in terms of
the amount and degree of rise/fall, and also spatially across the five regions. Prior to 2000, in four
regions, between 53-74% of all bores had rising trends and <6% had a falling trend. About 9-47% had
no trend (stable). Pre 2000, the western South Coast had the greatest number of falling trends (17%)
and also had fewest stable trends (9%). After 2000, the number of bores with rising trends decreased
in 4 out of the 5 regions. This was most pronounced in the Northern region (18%), and progressively
reduced towards the eastern South Coast where the number pre & post 2000 was unchanged (71-72%).
Groundwater trends differ depending on the depth to watertable. Plots of trend by depth for each time
period (<1990, 1990-2000, All time) for the five regions (Figure 1) show that prior to 2000, nearly all
bores displayed a rising trend whether the watertable was shallow or deep. However, after 2000 (b
plots) trend appeared dependant on depth to watertable. In the Northern region, downward trends, to -
0.5 m/yr, were most common in bores with shallow watertables (<10m), less significant falls (-0.2
m/yr) were apparent for deeper watertables, to >30m. In the Central region, falls were less (-0.2 m/yr)
and all but three were observed where watertables were <10m. In the South West and Western South
Coast falls were less (<-0.1 m/yr), and were observed at <5m watertable depths. By contrast, in the
Eastern South Coast, the trends pre and post 2000 remained the same; upwards (>0.2 m/yr) or stable.
In addition to a reduction in the number of post 2000 bores with rising trends, the magnitude of rise
diminished from north to south (Figure 2c). In some Northern region bores, rises in the 1990s were
offset by falls post 2000. However, this wasn’t the case in the entire Northern region (eg not Perth
Basin), nor in most other regions, where watertables rose (<1990-2007). In the Central, South West
and Western South Coast regions, it was usually only ‘discharge’ bores that demonstrated falling
trends (<-0.2m/yr >2000). Bores in areas of valley hazard and uplands, or those remote from
discharge zones, continued to rise.
After 1975, South West annual rainfalls reduced. However, since 2000 in the North and West
(Morowa, Narrogin), rainfall has been further reduced (Figure 1). By contrast, it has increased in the
south-east (Esperance) and slightly in the Central (Merredin) region, the later due to three large events.