Temporal Refinement of Water Budgets in Small Pacific Island Drainage Basins from Daily Rainfall and Temperature Maps

Abstract

Understanding the movement, distribution, and replenishment of groundwater is critical to sustainably managing this vital resource in Hawai‘i, USA, where groundwater meets more than 90% of freshwater needs statewide. Geochemical methods have been used increasingly to understand and model groundwater flow, but these methods are limited by the resolution of recharge measurements and the availability of precipitation chemistry data. Recent increases in climate data availability in Hawaiʻi allow water budgets to be examined with greater spatio-temporal resolution than ever before. While the current estimates of groundwater recharge are limited to annual long-term averages provided by the USGS, this paper provides a method for producing island-wide, monthly estimates of groundwater recharge using daily precipitation and temperature raster data. Recharge is calculated using a simple water budget: recharge ≈ precipitation – evapotranspiration – quickflow. Monthly precipitation, quickflow, evapotranspiration, and recharge are estimated for the island of Oʻahu between 1990 and 2014. Precipitation data are aggregated from recently published daily rainfall maps. Quickflow is calculated using simple regional regressions created by the USGS and compared with estimates from two different stream hydrograph baseflow separations. Evapotranspiration is calculated using a modification of the Thornthwaite equation, which was validated with local weather station data. The resulting estimates are compared with standards to quantitatively assess uncertainty and agreement. Our results show strong agreement with long-term average recharge estimates given by the USGS, and interannual climate trends further corroborate this agreement. These findings indicate that monthly estimates of groundwater recharge may be produced on Oʻahu via a low data intensity method with sufficient accuracy to better constrain recharge patterns for the island, and potentially improve groundwater management. These findings further suggest that the model presented here can be applied in any area where data on monthly precipitation, monthly temperature, and general quickflow trends are available.