Abstract
Baseflow is one of the most important components in the streamflow regime of any ecosystem. Understanding which key factors control baseflow dynamics is an essential mission to pursue future decisions involving the accelerated population growth and climate variability which are currently altering water distribution and availability worldwide. This study combines a series of analyses in natural and human-altered watersheds:
(i) Baseflow recession analysis in the inland Pacific Northwest, USA:
Storage-discharge relationships were analyzed in 26 watersheds in the inland Pacific Northwest of United States (PNW) across a variety of drainage areas (100 - 103 km2), land uses, geology units, and climate gradients. Overall, baseflow recession characteristics are not strongly correlated with watershed area. The mean basin slope and the aridity index are the best estimators of baseflow coefficients. Main underlying geology plays a significant role on controlling the characteristic baseflow recession time scales.
(ii) Stable isotope hydrology and geochemical analysis in the inland PNW:
Isotope ratios in precipitation resulted in a regional meteoric water line of d2H = 7.42∙d18O + 0.88 (n = 316; r2 = 0.97). Isotope values showed a clear temperature-dependent seasonality. Despite the observed seasonal variation in precipitation, the annual stream d18O variation was very small except for one large snowmelt event. Baseflow mean transit times ranged from 0.42 − 0.62 years (human-altered and basalt-dominated landscape), 0.73 - 1.7 years (mining-altered and predominantly sedimentary rocks), and 0.67 − 3.2 years (forested and granitic-dominated watersheds). The best fits between observed and d18O ratios were obtained with the exponential and dispersion weighting functions. Baseflow geochemical data supported the relative water age estimates. Greater residence times were represented by more homogenous water chemistry meanwhile smaller residence times resulted in more dynamic composition
(iii) Baseflow geochemical evidence of active serpentinization in a tropical environment:
A baseflow geochemical analysis conducted in the Santa Elena Ophiolite (Costa Rica) provided novel evidence of active serpentinization. Natural springs fluids are characterized by basic conditions (pH>11), high hydroxide, carbonates, and relative low magnesium concentrations. Active methane vents coupled with carbonate deposits (d13CCO2= −27 / to −14 /; d18OCO2= −17 / to − 6 /) also provide evidence for active serpentinization and carbonation. Isotope ratios of the alkaline fluids (d18O= −7.9 /, d2H= −51.4 /) and groundwater (d18O= −7.6 /; d2H= −48.0 /) suggests that, during baseflow recession, springs are fed by deep groundwater circulation. Methanogenic archaea, which comprises a relatively high percentage of the RNA tag sequences, suggests that biological methanogenesis may play a significant role in the system. Santa Elena's extreme varying weather results in a scenario that could be of significant importance for a) improving the knowledge of conditions on a humid early Earth or Mars that had periodic changes in water supply, b) revealing new insights on serpentinizing solute transport, and c) modeling hydrogeochemical responses as a function of meteoric recharge.