Groundwater Leakage, Resolved—The Hidden Flux Connecting Ridge to Reef in the Hawaiian Islands

In Hawaiʻi, groundwater draining from upland areas of high recharge provides virtually all drinking water and delivers freshwater nutrients (as well as contaminants) to ecosystems in the nearshore environment. Despite the importance of groundwater flow, its hidden nature makes it difficult to observe and quantify, which results in a poor understanding of its spatiotemporal dynamics from the uplands to the coast. We combine a storage‐discharge approach with a long‐term (7+ years) water balance to quantify the relationship between catchment storage and stream discharge in 11 (USGS‐gaged) watersheds across the Hawaiian Islands. We then use each watershed's storage‐discharge relationship to resolve daily estimates of catchment storage and the ‘groundwater leakage’ flux emanating from the watersheds. The resolved mean specific daily leakage is consistent with compiled measurements of submarine groundwater discharge (SGD) at the coast downstream of the 11 watersheds and upland leakage appears to be hydraulically connected to head responses in a basal aquifer. Example groundwater leakage hydrographs from Hālawa (Oʻahu) and Waiākea (Hawaiʻi) watersheds resolve temporal dynamics of groundwater flowing beneath the urban centres of Honolulu and Hilo, where contaminants enter aquifers, threaten drinking water supply, and degrade coastal environments. Although additional data are needed to validate results across space and time, resolving the groundwater leakage flux reveals a previously hidden portion of the hydrologic cycle that can inform modelling of water resources and water quality in leaky watersheds in Hawaiʻi and beyond.