Table 4.4 Salinity risk assessments for hydrozones (numbers in parentheses relate to hydrozone numbers in Figure 4.1) Hydrozone name & number % of zone within Land Monitor A O C L P in 1996–98 % of zone within Land Monitor A H A V F Dominant groundwater trend Likelihood of future salinity Consequence of future salinity Timing of future salinity Salinity risk Comments Kalbarri Sandplain (1) Less than 1 the Land Monitor mapping product did not produce a reliable estimate for this hydrozone variable possible moderate medium term moderate Variable groundwater trends occur throughout this hydrozone. Rising trends occur where groundwater is less than 15m deep. Falling and stable trends occur where groundwater is deep (more than 15m). The salinity risk is moderate because of the extensive areas of flat plains. Northampton Block (2) the Land Monitor mapping product did not produce a reliable estimate for this hydrozone 17 mostly rising unlikely minor short low Prior to 2000, this hydrozone appeared to be in hydrological equilibrium. In 2001–07, drought led to significant groundwater decline. Since 2007, groundwater levels have been rising but generally remain below pre-2000 levels. Salinity is unlikely to expand, the consequences will be minor because the incised topography would restrict the extent. East Binnu Sandplain (3) less than 1 the Land Monitor mapping product did not mostly rising almost certain moderate medium term high Rising groundwater trends dominate. Consistent rising groundwater and extensive areas where groundwater is less than 10m deep make it almost certain
produce a reliable estimate for this hydrozone that salinity will continue to spread with moderate consequences, as there are extensive low-lying areas with salinity hazard. Irwin Terrace (4) 4 10 variable possible moderate medium term moderate Variable groundwater trends occur in this hydrozone. Rising trends are associated with areas of sandplain soils. Falling trends are associated with heavier soil types and stable trends occur where groundwater is shallow (less than 2m). The salinity risk is moderate because further salinity could develop with moderate consequences because of the high quality of the agricultural land at risk. Arrowsmith (5) 1 the Land Monitor mapping product did not produce a reliable estimate for this hydrozone variable possible minor medium term low Rising groundwater trends occur in central and eastern parts of this hydrozone, where localised perched watertables overlie the regional groundwater system. West of Eneabba, where extensive areas of salinity have developed, mostly falling trends now prevail. Salinity will possibly expand with minor consequence because the areas at risk are largely Pale deep
sands of low productivity. Dandaragan Plateau (6) 1 the Land Monitor mapping product did not produce a reliable estimate for this hydrozone mostly rising likely major long term high Rising groundwater trends occur throughout this hydrozone. At many sites, groundwater is less than 10m deep. Salinity is actively spreading in an area south-west of Moora (west Gillingarra). It is likely that salinity will continue to spread with major consequences in the long term. As well as high quality agricultural land, there are high value biodiversity assets at risk. Northern Zone of Ancient Drainage (7) 6 30 variable likely moderate medium term moderate The salinity risk is moderate because rising groundwater trends continue to occur throughout the hydrozone. Therefore, it is likely that salinity will continue to spread. The development and spread of salinity is slow and incremental with moderate consequences. Northern Zone of Rejuvenated Drainage (8) 6 24 variable likely moderate medium term moderate Most bores in low landscape positions have shallow, stable groundwater levels, fluctuating seasonally in response to rainfall and evaporation. In mid to upper landscape positions, most bores with groundwater deeper than 5m have falling or stable trends.
Leeuwin (17) the Land Monitor mapping product did not produce a reliable estimate for this hydrozone the Land Monitor mapping product did not produce a reliable estimate for this hydrozone Not monitored unlikely insignificant No data very low There are few areas of salinity within this hydrozone, hence DAFWA does not monitor groundwater levels. The salinity risk is very low because salinity is unlikely to expand and consequences are insignificant. Scott Coastal Plain (18) the Land Monitor mapping product did not produce a reliable estimate for this hydrozone the Land Monitor mapping product did not produce a reliable estimate for this hydrozone Not monitored unlikely insignificant No data very low Most of the hydrozone is forested or recently cleared and there are no large areas of salinity, hence DAFWA does not monitor groundwater levels. The hydrozone is probably close to hydrological equilibrium; therefore, the salinity risk is very low because salinity is unlikely to expand with insignificant consequences. Salinity associated with private irrigation systems has not been assessed. Southland (19) 1 11 mostly stable possible moderate short term moderate Aquifers in this hydrozone respond to changes in recharge and depending on land use and/or seasonal rainfall, groundwater can rise or fall accordingly. With many
hydrogeology. The risk assessment methodology was based on a matrix of likelihood and consequence adapted from Spies and Woodgate (2005). The groundwater trends and salinity risk assessments are reported for hydrozones that are regions of similar hydrogeological, climate, landscape and farming system attributes. The risk assessments are presented in Table 4.4. The salinity risk map is shown in Figure 4.1 and shows that the highest salinity risk occurs in: portions of the Northern and Central Agricultural Regions of the region, where groundwater levels have continued to rise, despite several decades of below average rainfall in the east of the Southern Agricultural Region, where rainfall has been above average for the past decade and watertables are already shallow. The significant proportion of bores with rising trends in and adjacent to areas of salinity hazard, indicates that, over most of the region, the impact of reduced rainfall on groundwater trends is still less than the impact of clearing. Climate variability therefore appears to be a moderating, rather than driving factor in the risk of dryland salinity in the south-west agricultural region. Appendix A Summary of groundwater trends by hydrozone Table A1 Summary of groundwater trends in each hydrozone in the 2001–07 analysis period (George et al. 2008) and the 2007–12 analysis period of bores of bores % of bores with falling trend 2001–07 % of bores with falling trend 2007–12 % of bores with stable trend 2001–07 % of bores with stable trend 2007–12 % of bores with rising trend 2001–07 % of bores with rising trend 2007–12 Kalbarri not 11 not 45 not 18 not 36 Block 31 32 90 3 6 6 3 91 East Binnu 1 25 not 0 100 16 not 84 Irwin Terrace 13 25 62 20 23 36 15 44 40 53 75 45 18 25 8 30