Droplets to Deluges: Combining Stable-Isotope and Meteorological Data to Resolve Climate Controls on Recharge and Runoff in Tropical Watersheds

Abstract:

Determining which rain-producing weather patterns contribute most to water supply is important in areas where climate change is expected to influence large-scale atmospheric parameters controlling precipitation. Tropical mountain watersheds receive high rainfall, but their resilience to drought conditions is not well understood. Their hydrology may include extremely frequent small precipitation events as well as infrequent events with extreme amounts. Isotope hydrology studies in the Luquillo Mountains, Puerto Rico (LUQ) and the Hawaiian Islands compared rain, cloud water, stream and groundwater measurements to build understanding of the climate patterns that contribute to recharge and baseflow. Despite small annual fluctuation in land surface temperature, precipitation stable isotope composition varied seasonally, had a large range (δ ¹⁸O = −0.73 to −20.4‰, δ ²H = +12 to −154‰) and correlated with cloud altitude (atmospheric temperature) and storm history in both places. Permeability, storage capacity and antecedent moisture control how rainfall is partitioned into overland flow, plant-available soil moisture, and shallow and deep groundwater pathways on its way to the stream channel. In LUQ, orographic rain and cloud water were more important than convective rainfall in maintaining baseflow; a significant proportion of convective rainfall became runoff. In contrast, stream isotopic composition showed that a large storm supplied baseflow for 18 months in Hawaiian volcanic terrane. Over 3 years in LUQ, seasonal variation in deuterium excess indicated runoff in the 1780 ha Mameyes basin was about 25% quickflow (transit time < 7 days) and 75% groundwater. In a 1.5 ha headwater catchment, nightly cloud water deposition sustained stream water levels, and cloud water contribution to streamflow was evident in isotopic samples. Further research linking weather patterns, precipitation, and subsurface flow partitioning will help with water management in a changing climate.