Abstract
In glacier-fed systems climate change may have various effects over a range of time scales, including increasing river discharge, flood frequency and magnitude. This study uses a combination of empirical monitoring and modelling to project the impacts of climate change on the glacial-fed Middle Fork Toklat River, Denali National Park, Alaska. We use a regional calibration of the model HBV to account for a paucity of long term observed flow data, validating a local application using glacial mass balance data and summer flow records. Two Global Climate Models (HADCM3 and CGCM2) and two IPCC scenarios (A2 and B2) are used to ascertain potential changes in meteorological conditions, river discharge, flood frequency and flood magnitude. Using remote sensing methods this study refines existing estimates of glacial recession rates, finding that since 2000, rates have increased from 24m per year to 68.5m per year, with associated increases in ablation zone ice loss. GCM projections indicate that over the 21st century these rates will increase still further, most extensively under the CGCM2 model, and A2 scenarios. Due to greater winter precipitation and ice and snow accumulation, glaciers release increasing meltwater quantities throughout the 21st century. Despite increases in glacial melt, results indicate that it is predominantly precipitation that affects river discharge. Three of the four IPCC scenarios project increases in flood frequency and magnitude, events which were primarily associated with changing precipitation patterns, rather than extreme temperature increases or meltwater release. Results suggest that although increasing temperatures will significantly increase glacial melt and winter baseflow, meltwater alone does not pose a significant flood hazard to the Toklat River catchment. Projected changes in precipitation are the primary concern, both through changing snow volumes available for melt, and more directly through increasing catchment runoff.
Highlights
Greenhouse gases, including CO2, alter the radiative balance of the atmosphere, increasing global temperatures, and altering precipitation patterns [1,2]
It has been projected that increasing temperatures and altered precipitation patterns will initially lead to a general increase in rates of glacial recession and meltwater release [6], alter the timing and magnitude of soil saturation and runoff, lead to thinning of permafrost [7] and change lake levels and groundwater availability [8], affecting water quality [9,10]
All future projections under the global climate models (GCMs) explored in this study indicate that the small headwater glaciers of the Middle Fork Toklat (MFT) catchment will continue to melt throughout the 21st century, and at a higher rate than under the current climate
Summary
Greenhouse gases, including CO2, alter the radiative balance of the atmosphere, increasing global temperatures, and altering precipitation patterns [1,2]. There is, a general lack of long term data available for glacierised catchments [20], where efforts to establish permanent flow gauging stations are confounded by the remote nature of their locations, high energy braided rivers, and high rates of sediment transport Despite their large geographical extent and significant ecological and economical value, catchments fed by headwater glaciers are often neglected in monitoring programmes [21]. This study uses a combination of empirical monitoring and modelling to a) establish a strong flow rating curve and the first flow record for the MFT River Basin, b) calculate a long term glacial mass balance for the headwater glaciers, and c) project potential future changes in river discharge, with a focus on extreme flow events, under a series of future IPCC climate scenarios. By characterizing the catchment using a comprehensive hydrological model, based upon empirical measurements, the likely effects of various future climate scenarios can be assessed, including quantifying the influence that glacial recession will have upon the frequency and severity of flood events
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