Present-day and future contributions of glacier melt to the Upper Middle Fork Hood River : implications for water management

Abstract

Glaciers are effective reservoirs because they moderate variations in runoff and supply reliable flow during drought periods. Thus, there needs to be a clear understanding of the influence of glacier runoff at both the basin and catchment scale. The objectives of this study were to quantify the late summer contributions of glacier melt to the Upper Middle Fork Hood River and to simulate potential impacts of climate change on late summer streamflow. The Upper Middle Fork Hood River catchment (50.6 km²) is located on the northeast flanks of Mount Hood Oregon. Discharge measurements and isotope samples were used to calculate glacier meltwater contributions to the entire catchment, which feeds into a major water diversion used for farmland irrigation. Data were collected over the period August 10 - September 7, 2007. This late summer period was selected because there is typically little rain and suspected high glacier melt contributions. Discharge measurements taken at glacier termini, show that just two of the mountains glaciers, Eliot and Coe, contributed 41% of the total surface water in the catchment. The Eliot Glacier contributed 87% of the total flow in the Eliot Creek, while the Coe Glacier supplied 31% of the runoff in Coe Creek. Isotopic analyses, which include the inputs of all other glacier surfaces in the catchment, show a total glacier contribution of 88% from the Eliot Glacier to the Eliot Creek, in excellent agreement with the streamflow measurements. Isotopes also showed an 88% contribution from the Coe Glacier to the Coe Creek, higher than the amount measured from streamflow. This latter discrepancy is likely due to undersampling of streamflow from the Coe Glacier. During the isotope measurement period, overall contributions of both Coe and Eliot Glaciers to the Upper Middle Fork Hood River were 62 - 74% of catchment discharge. A temperature index model was used to simulate projected impacts of glacier recession and warmer temperatures on streamflow. The Snowmelt Runoff Model (SRM) was chosen for this task because it has been shown to effectively model runoff in glacierized catchments where there are limited meteorological records. SRM was calibrated using the 2007 discharge records to quantify August – September glacier runoff in the Upper Middle Fork catchment under a variety of glacier and temperature scenarios. SRM simulations indicate that runoff from the catchment glaciers are highly sensitive to changes in glacial area, glacier debris-cover, and air temperature. Model simulations show that glacier recession has a greater effect on runoff than do projected temperature increases. Thus, even without warmer summer temperatures, glacier contributions to streamflow will decrease as long as the glacier continues to lose mass. Applying both current glacier recession rates and a 2°C temperature forcing, the model predicts a decrease of 31% of late summer glacier runoff by 2059, most of which is lost in August. This study suggests that glaciers currently play a significant hydrological role in the headwater catchments of the Hood River Basin at a time when water is needed most, and that these contributions are projected to diminish over time.