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Title: Stable-isotope and solute-chemistry approaches to flow characterization in a forested tropical watershed, Luquillo Mountains, Puerto Rico

Author: Scholl, Martha A.; Shanley, James B.; Murphy, Sheila F.; Willenbring, Jane K.; Gonzalez, Grizelle;

Date: 2015

Source: Applied Geochemistry, Volume 63, December 2015, Pages 484-497

Publication Series: Scientific Journal (JRNL)

Description: The prospect of changing climate has led to uncertainty about the resilience of forested mountain watersheds in the tropics. In watersheds where frequent, high rainfall provides ample runoff, we often lack understanding of how the system will respond under conditions of decreased rainfall or drought. Factors that govern water supply, such as recharge rates and groundwater storage capacity, may be poorly quantified. This paper describes 8-year data sets of water stable isotope composition (d2H and d18O) of precipitation (4 sites) and a stream (1 site), and four contemporaneous stream sample sets of solute chemistry and isotopes, used to investigate watershed response to precipitation inputs in the 1780-ha Río Mameyes basin in the Luquillo Mountains of northeastern Puerto Rico. Extreme d2H and d18O values from low-pressure storm systems and the deuterium excess (d-excess) were useful tracers of watershed response in this tropical system. A hydrograph separation experiment performed in June 2011 yielded different but complementary information from stable isotope and solute chemistry data. The hydrograph separation results indicated that 36% of the storm rain that reached the soil surface left the watershed in a very short time as runoff. Weathering-derived solutes indicated near-stream groundwater was displaced into the stream at the beginning of the event, followed by significant dilution. The more biologically active solutes exhibited a net flushing behavior. The d-excess analysis suggested that streamflow typically has a recent rainfall component (~25%) with transit time less than the sampling resolution of 7 days, and a more well-mixed groundwater component (~75%). The contemporaneous stream sample sets showed an overall increase in dissolved solute concentrations with decreasing elevation that may be related to groundwater inputs, different geology, and slope position. A consider-able amount of water from rain events runs off as quickflow and bypasses subsurface watershed flow-paths, and better understanding of shallow hillslope and deeper groundwater processes in the watershed will require sub-weekly data and detailed transit time modeling. A combined isotopic and solute chemistry approach can guide further studies to a more comprehensive model of the hydrology, and inform decisions for managing water supply with future changes in climate and land use.

Keywords: Climate change, water supply management, stable-isotope approach, solute-chemistry approach, flow characterization, tropical watershed, Luquillo Mountains, Puerto Rico

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  • This article was written and prepared by U.S. Government employees on official time, and is therefore in the public domain.

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Scholl, Martha A.; Shanley, James B.; Murphy, Sheila F.; Willenbring, Jane K.; Occhi, Marcie; Gonzalez, Grizelle. 2015. Stable-isotope and solute-chemistry approaches to flow characterization in a forested tropical watershed, Luquillo Mountains, Puerto Rico, Applied Geochemistry. 63:484-497.

 


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