Parameter Sensitivity of the National Hydrologic Model Alaska Domain

Parameter sensitivity analyses guide calibration and development of hydrologic models. We present the results of a parameter sensitivity analysis using the Precipitation-Runoff Modeling System (PRMS) on 15,000 hydrologic response units representing Alaska and the Canadian watersheds draining to the Yukon River and southeast Alaska, hereafter the Alaska Domain. The Alaska Domain is an extension of the U.S. Geological Survey National Hydrologic Model (NHM) to Alaska and the surrounding region. This data-sparse domain covers 2,180,000 km² and includes glacial, coastal, and permafrost-dominated regions as well as significant elevation, temperature, and precipitation gradients which require complex and spatially variable model representations that are difficult to parameterize. We used the Fourier Amplitude Sensitivity Test (FAST) to explore the sensitivity of PRMS flux and state variables to model parameters across the Alaska Domain. Both calibration and derived parameters were included in the FAST analysis to explore (1) where to focus future calibration efforts and (2) which high-value derived parameters could be improved in subsequent versions of the Alaska Domain NHM. As Alaska is data-sparse, initial values for parameters derived from elevation and land cover were generated from remotely sensed data sets. Initial values for parameters derived from soil, rooting depth, and subsurface geology datasets were generated from global simulations and interpolations. Calibration parameters were initially set using published default values and varied according to the FAST methodology for the sensitivity analysis. Simulations to perform the FAST analysis were forced with fifteen years of Climate Forecast System Reanalysis (CFSR) air temperature and precipitation data from the National Center for Environmental Prediction (NCEP). The first five years of the dataset were used to spin-up model states and fluxes and the subsequent ten years of the dataset were used for the FAST analysis. Results from this analysis will guide calibration and refinement of model physics and parameters as well as indicate where in the Alaska Domain different model processes dominate hydrologic response.