White, P.A. 1997 A Postaudit study of the Waimea Plains aquifer model. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 97/17 x, 91 p.
Abstract: Adjustment to the hydrogeological parameters used to model Waimea Plains groundwater is suggested by geological and water use data collected since the model was completed in 1985. The lateral boundaries of the two confined aquifers could be adjusted to better represent the presently-known aquifer extents by adding 75 ha and removing 500 ha from the Upper Confined Aquifer, and by adding 500 ha and removing 500 ha from the Lower Confined Aquifer. Recorded water use data show that the model assumptions of average irrigation application rates of 0.1 1.s-1.ha-1 is valid for the Waimea West and Upper Confined Aquifer water use zones. Waimea East zone uses the largest volume of water with an average water use of 2,159,361 cubic m in the irrigation seasons 1992/93, 1993/94 and 1994/95. The Lower Confined Aquifer zone is the zone of greatest groundwater use, with the Tasman District Council pumping on average 63 percent of total zone pumpage. Recorded water abstraction rates in all water use zones are less than the zone abstraction limit defined by the Tasman District Council. The groundwater model predicted groundwater level drawdowns due to domestic (only) pumpage in the Lower Confined Aquifer would cause a 3.5 m summer drawdown in the Chipmill well. Total pumpage of the Lower Confined Aquifer has caused an average summer drawdown of 2.9 m after 1985. The model predicted that pumpage of the Unconfined Aquifer and implementation of the Waimea East Irrigation Scheme would have a severe impact on summer river flows. Low-flow gauging data indicate that no change in the low flow characteristics of the Waimea/Wairoa rivers has been measured over the period of record, although bed level measurements have shown that the river has become more incised. Only one of the three irrigation schemes that were modelled has come into operation. The model predicted that pumpage from the Upper Confined Aquifer would reduce to 28 l.s-1 after implementation of the Waimea East Irrigation Scheme. Maximum weekly pumpage in the Upper Confined Aquifer water usage zone of 83 l.s-l has been measured since 1985. The model predicted Lower Confined Aquifer pumpage would be reduced by 37 percent, in the driest month, after the Waimea East Irrigation Scheme commenced operation. The average decrease in Lower Confined Aquifer pumpage after the commencement of the Waimea East Irrigation Scheme was 37 percent. The commencement of the Waimea East Irrigation Scheme, which caused a decrease in aquifer pumpage and possibly an increase in aquifer recharge, saw increases in the summer low level of the Rail Reserve well by 3.0 m, the Buschl1 well by 5.6 m, the Chipmill well by 1.4 m, and the Harford well by 1.6 m. These level rises were similar to the predicted increases in aquifer water level. The high water use scenario of 0.58 l.s-1.ha-1 used in the groundwater management simulations is significantly higher than the maximum record ed weekly pumpage of 0.33 l.s/ sup -l/.ha-l which occurred in the Lower Confined Aquifer usage zone during the 1982/83 irrigation season. The model predicted that the high usage scenario would cause a drawdown of 32 m at the Rail Reserve well. The measured drawdown in the period of maximum recorded pumpage was 5.4 m. (auth)