Climate driven changes to rainfall and streamflow patterns in a model tropical island hydrological system

summary Rising atmospheric CO2 and resulting warming are expected to impact freshwater resources in the tropics, but few studies have documented how natural stream flow regimes in tropical watersheds will respond to changing rainfall patterns. To address this data gap, we utilized a space-for-time substitution across a naturally occurring and highly constrained (i.e., similar geomorphic, abiotic, and biotic features) model hydrological system encompassing a 3000 mm mean annual rainfall (MAR) gradient on Hawai‘i Island. We monitored stream flow at 15 min intervals in 12 streams across these watersheds for two years (one normal and one dry) and calculated flow metrics describing the flow magnitude, flow variability (e.g., flow flashiness, zero flow days), and flow stability (e.g., deviations from Q90, daily flow range). A decrease in watershed MAR was associated with increased relative rainfall intensity, a greater number of days with zero rainfall resulting in more days with zero flow, and a decrease in Q90:Q50. Flow yield metrics increased with increasing MAR and correlations with MAR were generally stronger in the normal rainfall year compared to the dry year, suggesting that stream flow metrics are less predictable in drier conditions. Compared to the normal rainfall year, during the dry year, Q50 declined and the number of zero flow days increased, while coefficient of variation increased in most streams despite a decrease in stream flashiness due to fewer high flow events. This suggests that if MAR changes, stream flow regimes in tropical watersheds will also shift, with implications for water supply to downstream users and in stream habitat quality for aquatic organisms.