Principal Investigators

Gretchen R. Miller;  Collaborators: Brendan Roark, Tony Cahill, and Kelly Brumbelow, - Texas A&M University.

Abstract

Hydrologic, geologic, and biologic processes are critical to understanding the ecosystem in the tropical premontane transitional forests of Costa Rica. Precipitation is significantly lower during the dry season, and incoming rainfall can be completely intercepted and re-evaporated by the forest canopy during light events. This leaves groundwater as a main source of stream flow throughout the watershed which is used downstream for drinking water and hydropower uses.  By using stable isotope signatures we were able to reliably and precisely characterize the nature of these ecohydrologic processes to determine the influences on the groundwater system in a challenging environment with limited accessibility and complex subsurface conditions.  This study focuses on research conducted at the Texas A&M Soltis Center for Education and Research, near San Ramón, Costa Rica. We have monitored a 2.2 ha watershed there, measuring precipitation and transpiration rates for over two years, and groundwater levels and stream flow rates for nearly one year. Precipitation rates for the watershed averaged 4.4 m/yr since 2010. Stream flow (runoff, spring flow, and baseflow) averaged 0.09 m3/sec during the 2012-2013 wet seasons. At 1.2 mm/day, transpiration was a relatively minor component of the water budget. Over a 40-day span during summer 2013, we collected a combination of daily and rain-event based samples from locations throughout the watershed. Sources included: the main stream and two small tributaries, groundwater from piezometers, pore water from suction lysimeters, throughfall and stemflow from under canopy collection systems, and xylem water from 8 tree species across the watershed. We then measured stable isotope fractions (δ18O and δD) in the water using a Picarro L2120i CRDS. Isotope ratios for surface water averaged -5.50‰ for δ18O and -28.00‰ for δD; baseflow was measured at -5.45‰ for δ18O and -29.18‰ for δD.  Results of baseflow partitioning confirm that groundwater is the dominant source for stream water even in the wet season.  We additionally conclude that in this watershed, groundwater transport to the stream is characterized by short residence times attributed to macropore flow in the subsurface, despite a low permeability of the andisol clay.

Funding Sources

National Groundwater Research and Education Foundation

Associated Activities

One M.S. student, one undergraduate REU student

Publications

Landon, A.C., Van Riper, C.J., Angeli, N.F., Fitzgerald, D.B., Neam, K.D. 2013. In review. Growing Transdisciplinary Roots in the Peruvian Amazon. Target Journal: The Journal of Transdisciplinary Environmental Studies.

Neam, K.D., Wood, M.A. 2014. Collaboratory blogging: Many minds are better than one. Applied  Biodiversity Sciences Perspectives Series.

Neam, K. D., Petriello, M., Wood, M. (2014). [Blog] Central America: Applied Biodiversity Science. Available at: http://centralamericaabs.wordpress.com/.

Presentations

DuMont, A. and G.R. Miller, (2014), Groundwater dynamics in a premontane transitional rainforest using stable isotope techniques, oral presentation at the National Ground Water Association Summit 2014, Boulder, Colorado.
Miller, G.R., DuMont, A., Roarke, B., K. Cahill, A. T., Brumbelow, (2013), Baseflow separation in a premontane transitional rainforest using stable isotope techniques, poster presentation at the AGU Fall Meeting, San Francisco, California.
  
Paper in preparation:
DuMont, A., Miller, G.R., Roarke, B., Brumbelow, K. and Cahill, A. T., “Using stable isotope tracers to quantify baseflow in a Costa Rican watershed,” for submission to Groundwater.