Estimating Rainfall Recharge and Soil Water Residence Times in Pukekohe, New Zealand, by Combining Geophysical, Chemical, and Isotopic Methods

The combined use of geophysical (time domain reflectometry [TDR]), isotopic (δ18 O and δ2H) and chemical (NO 3 ‐N, Cl) techniques indicates that residence times in the unsaturated zone of a fractured basalt aquifer in the Pukekohe region of the North Island, New Zealand, are at least six months. Each technique provides useful information on specific aspects of recharge and residence times, but combined they provide a basis for determining the mechanisms of water movement in the unsaturated zone. TDR soil moisture measurements provide similar rainfall recharge estimates as model calculations over longer time periods (monthly or seasonal) but may be more accurate for shorter time scales (weekly or daily).