Quantifying groundwater-surface water interactions using a stream energy balance model and dye tracing in a proglacial valley of the Cordillera Blanca, Peru

Abstract:

Streams in many Andean regions provide downstream communities and industries with water sourced from both groundwater and glacier melt, such as the streams of the Cordillera Blanca, Peru, which has the highest density of glaciers in the tropics. From May to September, when precipitation is minimal, approximately half the discharge in the region’s proglacial streams comes from groundwater. However, due to the remote nature of the region, there are few effective field methods to identify the spatial distribution of groundwater discharge at the reach scale. An energy balance model, Rhodamine WT dye tracing, and high-definition kite-borne imagery were used to determine gross and net groundwater inputs to a 4 km reach of the Quilcay River within Huascaran National Park, Peru.

The HFLUX computer program (http://hydrology.syr.edu/hflux.html) was used to simulate the Quilcay River’s energy balance using stream temperature observations, meteorological measurements, and kite-borne areal photography. Model results indicate 29% of stream discharge at the reach outlet was contributed by groundwater discharge over the study section. A constant rate Rhodamine-WT dye tracing experiment, coupled with the energy-balance model, shows that approximately 49% of stream water is exchanged (ie. no net gain) with the subsurface as gross gains and losses.

The energy balance simulations suggest the largest net groundwater gains in streamflow occur in reaches situated in low gradient meadows, likely a product of the abundant springs that flow into the main channel. Dye tracing results indicate significant groundwater-surface water exchange occurs in stream sections that traverse cross-valley moraines, where connectivity between the subsurface and the stream is highest. These insights into pathways of groundwater-surface water interaction can be applied to improve hydrological modeling in proglacial catchments throughout South America.