A helicopter‐borne electromagnetic survey to delineate groundwater recharge rates

Groundwater recharge is one of the most difficult components of the water balance to measure. For this reason, electromagnetic methods have been used to infer its variability from measurements of apparent electrical conductivity. In this study, groundwater recharge was estimated at 20 sites using unsaturated zone chloride methods. Interpolation between drill sites was accomplished with the aid of a helicopter‐borne electromagnetic survey (DIGHEM^IV). Correlations between recharge and apparent electrical conductivity were only significant (R² = 65%) at the highest frequency (56,000 Hz). Using these single‐frequency data, variations in recharge were mapped over an area of 32 km². Recharge, as inferred from the electromagnetic data, appears to be lognormally distributed, and varies from less than 1 to more than 50 mm yr⁻¹. Within the study region, spatially averaged recharge can be estimated from the electromagnetic data, with an accuracy of approximately −60%, +140% (90% probability). This is comparable to the estimation accuracy when surface electromagnetic methods are used. Aerial electromagnetic methods appear very useful for identifying areas of high and low recharge over large regions.