**5. Groundwater level response**

The results of the HARTT analysis for the selected bores are summarised into a Table in Appendix I. The groundwater trends determined in this analysis are comparable for both the Werribee Plain Delta aquifer and the Cape Flats aquifer. There is little or no delay in response to rainfall events. Although most of the bores showed a generally slight decline, few bores have a rising trend yet the rise is much less than 20 cm/yr. Most of the bores screened in the Werribee Delta aquifer have groundwater trend ranging -3 to 4 cm/yr (in exception of B112802 with more positive trend, 12 cm/yr). All of these bores showed no delayed response and a quick rise in response to the wet year 2010 (after lower than average rainfall from 2007-2009) (Appendix II a-d). The selected bores in the Werribee mostly showed the lowest groundwater levels (i.e. highest drawdown) in late 2003 and early 2004 except B59536 whose highest drawdown was in February 2007 (Appendix II d).

The groundwater trend of bores within the Cape Flats aquifer ranges from -8 to 8 cm/yr (except BA232 with more positive trend, 14 cm/yr; which is within the Philippi allotment portion). The bores on the Cape Flats showed marked seasonal fluctuations and a more slightly downward trend in comparison to the Werribee bores (see Table in Appendix I). The Cape Flats bores are examples of good data records with missing gaps (Appendix II e-j). Most of the bores selected along the south coast on the Cape Flats also showed no delayed response except BA002 (Appendix II e). Although there are no lithologic logs for most of these bores, there are reports of occurrence of thick lenses of clay within the Cape Flats sand aquifer (Adelana, 2011; Gerber, 1976) that may contribute to delayed response of bores to rainfall events.

There are no significant negative trends (groundwater trend all < -9 cm/yr) in both study area, even though a few bores were also selected from the intensively irrigated Atlantis area of Western Cape. Examples of bores from the Cape Flats sand in north-western Cape (Atlantis) showing influence of pumping in the 1990s are presented in Appendix II (k-n) with summary table in Appendix I). The Cape Flats aquifer in the Atlantis area has been under the Managed Aquifer Recharge (MAR) program since the last 20 years. Although the extent to which this has influenced the response of the bores is not known since the data are not accessible, it is expected to contribute to a more positive trend. Irrigation is intense in the Werribee area but the conjunctive groundwater use (with surface water, recycle water) may have been responsible for no significant negative trend.

However, the Philippi-Mitchells Plain bores are still more negative relative to both Atlantis and Werribee Irrigation districts even though both have longer history of groundwater use for irrigation. This may be in response to groundwater usage. It was estimated that approximately 13 million m3 are abstracted from the Cape Flats aquifer by commercial farmers in the Philippi area of Cape Town (Colvin & Saayman, 2007), and an additional 5 million m3 are abstracted by the City of Cape Town administration to irrigate sports elds at Strandfontein and Mitchell's Plain (Wright & Conrad, 1995). Moreover, another 20 million m3 was abstracted from wellelds in the southern part of the aquifer during the Pilot Abstraction Scheme to understudy the Cape Flats aquifer response to stress conditions (Gerber, 1981; Vandoolaeghe, 1989).

The bores examined across the Werribee Plain showed declines in groundwater levels occurring from 1996 to 1999, 2003 to 2004 and in late 2006 to early 2007. This tends to follow the downward trend in the frequency and amount of rainfall and is consistent with the general groundwater trend observed across Victoria during this period (Hekmeijer et al., 2008; Reid, 2010). The groundwater level drawdown of Werribee Delta aquifer shows that during the early 1990s seasonal drawdown was less than 0.5 m but in 1996, the seasonal fall increased up to 2 m. This indicates more use of groundwater for irrigation due to the lack of supply from the Werribee River. Therefore, the seasonal fluctuations are mostly influenced by rainfall and usage; however, some observation bores show seasonal fluctuation that is believed to align with the pattern of channel deliveries (i.e. due to enhanced channel leakage) and groundwater pumping (SKM, 2009a, 2009b).

The observed groundwater trends and behaviour in the South African example (i.e. bores screened in the Cape Flats aquifer) are equally consistent with the fluctuations in rainfall pattern. It is obvious that the groundwater level falls due to less rain and possibly higher use from production bores, while rainfall recharge and recovery take place in wetter times when there is conversely less pumping. Some of the Cape Flats bores in Atlantis showed a marked response to pumping influences and have recorded higher groundwater level changes within short time. For example, WP167 with groundwater level decline of 3.5 m from August 1993-June 1995 and continuous decrease into the early 2000s. WP184 also show declines of 5.5m (September 1994-April 1995) and 4.8 m (October 1999-August 2000). Such high declines have influenced spring flows and base flow, and hence, the implications on groundwater management. Therefore, the groundwater declines are discussed in the context of groundwater resource sustainability and its implications on water security and resource management plans, including consideration of water conservation measures or conjunctive water use.
