National Weather Service Forecast Reference Evapotranspiration Goes Nationwide

The National Weather Service (NWS) is producing daily reference evapotranspiration (ETrc) forecasts (FRET) across the United States using the Penman – Monteith Reference Evapotranspiration equation for a short canopy (12 cm grasses), adopted by the Environmental Water Resources Institute of the American Society of Civil Engineers (ASCE-EWRI, 2004). The FRET expanded from the Western U.S. to the whole nation in late 2014 when it became an experimental part of the NWS National Digital Forecast Database (NDFD), and will be adopted as an operational product by late 2015. The FRET is calculated with the temperatures, humidity, winds, and sky cover input by forecasters across the U.S. who have knowledge of the local terrain and weather patterns to provide better forecasts for the ETrc inputs. A weekly ETrc forecast is also available to assist with water planning decision, which can be critical during drought situations. Daily FRET departure from normal are calculated to help put the ETrc values into perspective for the time of year. The FRET products are available in Grib2 and XML formats and in graphics on the web at digital.weather.gov. Daily FRET values for particular locations are also available in a text product (ETT) from some NWS Weather Forecast Offices (WFOs).

The ETrc forecast product suite allows water managers, the agricultural community, and the public to make more informed water-use decisions. The FRET can assist with decision making on scheduling irrigation (e.g., farms, golf courses) and when to apply fertilizer and chemicals for farmers. By calculating the irrigation needs on a daily basis with the support of FRET, one can reduce water waste. These products also permit operational planning, especially with the drought across portions of the West. For example, the California Department of Water Resources not only ingests the FRET into their soil moisture models, but uses the FRET calculations when determining the reservoir releases for the Sacramento and American Rivers.