Statistical Comparison of the National Water Model Streamflow Guidance with non-USGS Stream Gages on the Cottonwood River in Minnesota

The National Water Model (NWM) provides streamflow guidance to the NWS RFC and WFO forecasters. The literature publicizes 2.7 million river reaches of forecast information available from the NWM within the continental United States. There are just over 10,000 U.S. Geological Service (USGS) river gages in service across the country, of which a subset will be used to calibrate the NWM. Calibration statistics will provide forecasters with a measure of how accurate NWM simulations will be at that given reach. However, millions of reaches do not have a USGS river gage for model calibration. To assess how accurate the NWM may or may not be at any given location, NWS forecasters will need to compare it with additional data sources. This project uses an independent stream gage data source to develop a baseline set of statistics indicating relative performance of NWM streamflow guidance at non-USGS gaging locations. This baseline study will provide valuable insight as to how NWS forecasters can best use NWM output at select non-USGS gaging locations. The state of Minnesota has strategically invested in ecological and water resource monitoring equipment over the past several decades. The Minnesota Department of Natural Resources (DNR) and Minnesota Pollution Control Agency (MPCA) have installed stream gages and water quality gages on streams throughout the state to supplement the United States Geological Survey (USGS) network and provide localized data on rivers and streams. The Cooperative Stream Gaging (CSG) website brings together data sources from the Minnesota DNR, MPCA, National Weather Service (NWS) and the USGS providing access to near real-time and historical streamflow, water quality data, lake data, and ground water data for the state of Minnesota. The focus of this presentation will be on the Cottonwood River in central/southern Minnesota. From a data perspective, the Cottonwood River is a prime initial candidate for this study, as no reaches along the river have been included in direct NWM calibration efforts to date. This river regularly poses forecasting challenges for the NWS North Central River Forecast (NCRFC) office with ice jams and prolonged spring flooding. The growing agricultural drainage infrastructure has amplified forecasting challenges on the Cottonwood River and many other rivers in the NCRFC region. In the spring of 2019, the Cottonwood River was in flood stage from mid-March to late-April. It peaked at a stage of 17.91 feet, nearly 2 feet over major flood stage and ranking third on the Historical Crests Record for the Cottonwood River. The combination of snowmelt, ice jams, and spring rain lead to the recent prolonged flood event, making news headlines throughout the state with road closures and flood damage. The second ranked flood on this river occurred less than one year earlier in July 2018, and the fourth ranked event was in April 2018. The variety of local, state and federal stream gaging sites on the Cottonwood River will be used to compare and statistically quantify performance of the National Water Model (NWM) for these recent flood events.