Assessing the ramifications of climate change for the purpose of modeling streamflow within the Upper Merced basin in California: 1984-2099

Abstract

Hydrological responses within the Upper Merced Basin of California to future climate variations were assessed over a continuous 60-yr period. Using a calibrated SWAT (Soil and Water Assessment Tool) basin model, future monthly peak flow and streamflow discharge sensitivity was tested under climate schemes from 2040-2099. Two general circulation models (GCMs) utilized in this study, the Met Office Hadley Centre Model 2 -- Carbon Cycle (HadGEM2-CC) and The National Center for Atmospheric Research (CCSM4), were inputted into the Upper Merced SWAT model under IPCC forcing scenarios RCP 6.5 and 8.0. Probabilities of discharge peaks were determined using the respective climate model's temperature, precipitation, and solar radiation. Results indicate that times of projected peak flow may occur one to three months earlier than the studied historical time period (1984-2013). Under the historical observed time period of 1984-2007, average Upper Merced peak flow occurred most often in May (22%) and April (20%) (Figure 6.4). Under future climate scenarios tested from 2040-2099, streamflow peak flow shifted to earlier in the winter/spring seasons. HadGem2-CC RCP6.0 scenario shows peak flow most likely occurring in January (33.8%) and February (35.6%). During the same time period under CCSM4, peak flow is shown to occur during December (33.8%) and February (23.7%). HadGem2-CC RCP 8.5 peak flow stretches across January (27.1%), December (22%), and February (22%), while CCSM4 RCP 8.5 similarly shows peak flow shared between January (28.8%), February (25.4%), and December (27.1%). Modeled future streamflows exhibit high relative sensitivities to temperature and precipitation change due to warming conditions. These findings in collaboration with Upper Merced basin characteristics of high elevation, low groundwater influence, and late season snowmelt, indicate high hydrological sensitivity to snowpack reduction. Results underscore the importance of a flexible management style for water resources dependent on snowpack. Increased and thorough climate monitoring is necessary to assess climate impacts within vulnerable high-elevation mountainous regions.