GROUNDWATER CONTRIBUTIONS TO STREAMFLOW IN A GLACIERIZED CATCHMENT OF THE CORDILLERA BLANCA, PERU

Meltwater from tropical glaciers supplies approximately half of dry-season stream discharge in glacierized valleys of the Cordillera Blanca, Peru, but glacial contributions to discharge are decreasing in many valleys as glaciers lose mass due to climate change. Besides melting ice, groundwater is the only other significant source of streamflow during the dry season. A better understanding of groundwater sources and groundwater-surface water interactions is necessary to predict how surface water resources might continue to change in the future.

We present the results of a study of groundwater-surface water interactions in the Quilcay watershed, which is typical of the western-facing valleys of the central Cordillera Blanca in that the stream flows through a series of low-gradient wet meadows, punctuated by steeper moraine and landslide units. We performed a series of artificial tracer dilution experiments along the stream in both meadow and moraine reaches, including a 4-km-long constant-rate tracer injection, which covered two moraine-to-meadow sequences. We were able to estimate both gross and net inputs of groundwater to the stream along this length and characterize the geochemistry of inputs. We found that groundwater contributions were significant as the stream flowed through both meadow and moraine units; however, we observed concentrated zones of groundwater discharge to the stream where it transitioned from steep moraines into shallower meadow reaches. In these locations, we measured net increases in stream discharge of 10-to-20 percent over relatively short reaches. We discuss the implications of these findings for our conceptual model of valley hydrogeology, as well as for downstream water quantity and quality.