Streamflow forecasts are critical for water and environmental management, especially in the water-short Western U.S.. Land Surface Models (LSMs), such as the Variable Infiltration Capacity (VIC) model and the Noah-Multiparameterization (Noah-MP) play an essential role in providing comprehensive runoff forecasts across the region. Virtually all LSMs require parameter estimation to optimize their predictive capabilities. We describe a systematic calibration of parameters for VIC and Noah-MP over 263 river basins in the Western U.S., and distribution of the calibrated parameters over the entire region. Post-calibration results showed a notable improvement in model accuracy in the calibration basins: the median daily streamflow Kling-Gupta Efficiency (KGE) for VIC rose from 0.37 to 0.70, and for Noah-MP, from 0.22 to 0.54. Employing the donor-basin regionalization method, we developed transfer relationships to hydrologically similar basins and extended the calibrated parameters to ungauged basins and the entire region. We assessed factors that influence calibration efficiency and model performance using regional parameter estimates. We evaluated high and low flow simulation capabilities of the two models and observed marked improvements after calibration and regionalization. We also generated gridded parameter sets for both models across all 4816 HUC-10 basins in the Western U.S., a data set that is intended to support regional hydrologic studies and hydrologic climate change assessments.