Reigning in the Rain with Satellite and Radar

By Marguerite Huber

When it rains, it pours!background_water_puddle

Actually, that phrase is not necessarily true. A rain shower can consist of just drizzle, a steady rain, a downpour, or even all three! Either way, accurate rain totals are the basis of watershed modeling for evaluating the water cycle.

Meteorological data (precipitation, temperature, humidity, etc.) required for watershed assessments have traditionally come from land-based weather gauge stations. They collect weather data from all over the country. Unfortunately, not all watersheds have meteorological stations. Some watersheds have too few, are too far away, or aren’t working properly to correctly represent precipitation totals or their distribution within the watershed. You can check the National Oceanic and Atmospheric Administration’s website to see how many weather gauge stations are in your watershed!

For accuracy, the best options for watershed modeling applications in the U.S. are rain gauges and weather radar data, but precipitation amounts can vary throughout the watershed. Where land-based stations are lacking, remote sensing and radar satellite data are increasingly being used to augment data in space and time.

EPA scientists were involved in a study aimed at providing options for watershed modelers. They did this by comparing precipitation data from radar-based stations to data from ground-based stations to see the effectiveness of using either one for watershed modeling, especially at locations where gauge stations were insufficient.

Because ground-based gauges are the norm, the scientists evaluated the efficacy of using radar or gauge precipitation data to support watershed modeling.

Researchers evaluated two areas in Wisconsin using hourly precipitation data from 2002-2011: the Manitowoc River Basin and Milwaukee area, which are approximately 84 miles apart.

National Climatic Data Center precipitation data from gauges on the ground were compared to two different types of satellite and radar data: North American Land Data Assimilation System and NEXt generation RADar Multi sensor Precipitation Estimates. Both were used to evaluate the reliability of radar and gauge precipitation data.

Results showed gauge and radar data at Milwaukee to be similar, while the Manitowoc River Basin had large differences in precipitation occurrence and totals, which strongly suggest radar data as being more reliable.The gauged precipitation at Manitowoc River Basin also poorly correlated with radar data, which can detect more frequent precipitation, drizzle, and small storms.

In the end, the researchers concluded that the use of radar precipitation data can be an acceptable alternative to the gauged data in Manitowoc River Basin. The results also show benefits from automating the collection process of radar data as an additional option in watershed modeling.

With options of using land-based or radar data, scientists will be able to conduct more accurate watershed assessments, providing important information for keeping our watersheds healthy.

About the Author: Marguerite Huber is a Student Contractor with EPA’s Science Communications Team

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