Calibrating the National Water Model V2.1 over the Contiguous United States

The NOAA/OWP's operational National Water Model (NWM) produces high-resolution hydrologic and land surface variables at short, medium and long forecast horizons with lead times ranging from 1 h up to 30 days across the Contiguous United States (CONUS) and Hawaii. Since its implementation in 2016, NWM underwent several versions of upgrades in physics and hydrofabric, leading to improved forecast guidance products which are increasingly utilized by various communities for operational support and research applications, particularly hazard preparation and mitigation.

The NWM V2.1, which will become operational in 2020, is in the process of development and it incorporates significant improvements, such as modified snow physics, upgraded level-pool and new reservoir treatments, use of a new land cover dataset and expansion of the geographical coverage to include the Puerto Rico and Great Lakes/Laurentian Basin domains. V2.1 calibration uses the Analysis of Record for Calibration (AORC) product, recently developed by the OWP, as the main meteorological forcings. Comparison of the AORC dataset against in-situ temperature and precipitation measurements showed that the AORC performs better than the NLDAS2, which was used as forcings in the calibration of previous NWM versions. To augment the list of suitable river basins for calibration, the River Forecast Centers (RFCs) were solicited to provide more than 130 natural basins with challenging hydrological behaviors, which are also valuable to monitor the behavior of NWM forecast. In addition, a series of screening steps are taken to select 1339 river basins over the CONUS with light anthropogenic alteration and at least 50% completeness of hourly/daily flow measurements.

Similar to previous versions, V2.1's key parameters related to soil, vegetation, runoff, snow and groundwater are calibrated using Dynamically Dimensioned Search algorithm (DDS) and then are transposed to the un-calibrated catchments with similar landscape properties. The calibration period spans from 2008-10 to 2013-10 and validation is from 2013-10 to 2016-10. The calibration is performed at the hourly time scale when hourly streamflow data is available and at daily time scale otherwise. We will present the performance of the NWM V2.1 over the calibration and validation periods and compare it against the previous versions of NWM.