Trends and shifts in streamflow in Hawai‘i, 1913–2008

Abstract

AbstractThis study addresses a need to document changes in streamflow and base flow (groundwater discharge to streams) in Hawai‘i during the past century. Statistically significant long‐term (1913–2008) downward trends were detected (using the nonparametric Mann–Kendall test) in low‐streamflow and base‐flow records. These long‐term downward trends are likely related to a statistically significant downward shift around 1943 detected (using the nonparametric Pettitt test) in index records of streamflow and base flow. The downward shift corresponds to a decrease of 22% in median streamflow and a decrease of 23% in median base flow between the periods 1913–1943 and 1943–2008. The shift coincides with other local and regional factors, including a change from a positive to a negative phase in the Pacific Decadal Oscillation, shifts in the direction of the trade winds over Hawai‘i, and a reforestation programme. The detected shift and long‐term trends reflect region‐wide changes in climatic and land‐cover factors. A weak pattern of downward trends in base flows during the period 1943–2008 may indicate a continued decrease in base flows after the 1943 shift.Downward trends were detected more commonly in base‐flow records than in high‐streamflow, peak‐flow, and rainfall records. The decrease in base flow is likely related to a decrease in groundwater storage and recharge and therefore is a valuable indicator of decreasing water availability and watershed vulnerability to hydrologic changes. Whether the downward trends will continue is largely uncertain given the uncertainty in climate‐change projections and watershed responses to changes. Copyright © 2012 John Wiley & Sons, Ltd.