As Auckland's population grows, demand for groundwater resources increases. Quantifying the amount of groundwater available is an essential element of sustainable groundwater management. Groundwater recharge is the most important water balance component for resource quantification, yet is also the most difficult to estimate.
Council’s Research, Investigations and Monitoring Unit is currently setting up a groundwater recharge monitoring programme using drainage lysimeters. Lysimeters are large containers holding soil and plants, buried in the ground. The amount of rainfall entering the container and drainage exiting the container is continuously measured to determine groundwater recharge. The difference between rainfall and drainage is water returning to the atmosphere through evapotranspiration.
In May 2016, three lysimeters were installed at Karaka North. Each lysimeter has a diameter of 50cm and a depth of 70cm. There is also a rain gauge at the site and a solar-powered data logger which transmits data back to our database systems. Initial results indicate that over the last four months, between 20 and 80 per cent of rainfall became groundwater recharge. However, monitoring at this site is in the early stages and we have only captured data over a single winter season so far.
Recharge rates may be very different during summer and highly variable from year to year. We have recently installed a second site in the Pukekohe area and are looking to continue data collection from these two sites over a period of ten years in order to quantify groundwater recharge and assess the variability overtime.
The knowledge gained from this programme will help establish efficient allocation and efficient use of water in the Auckland region as required by the National Policy Statement of Freshwater Management.