National Weather Service Forecast Reference Evapotranspiration and Verification across the Western US

The National Weather Service (NWS) Weather Forecast Offices (WFOs) in Western Region are now producing daily reference evapotranspiration (ETrc) forecasts. Such a product allows water managers, the agricultural community, and the public to make more informed water-use decisions and to permit operational planning, especially with a loaming drought across much of the West. For example, rice paddies across the Sacramento Valley are typically kept at a depth of 6 inches. If the farmer knows that the forecast ETrc is 0.25 inches a day, then he can calculate how much water needs to be added to the paddy to keep it at a constant depth of 6 inches.

Most Western Region WFOs produce daily ETrc forecasts, 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) as the national standard for calculating ETrc. For example, the Sacramento WFO, which developed the Penman – Monteith ETrc smart tool, ingests their forecast grids for temperatures, relative humidity, wind, and cloud cover to produce these daily-forecast reference evapotranspiration (FRET) products out for seven days. The sensitivity of these daily calculations to fluctuations in temperatures, humidity, winds, and sky cover allows forecasters with knowledge of local terrain and weather patterns to better forecast changes in the ETrc inputs. Meanwhile, the Pacific Northwest uses a subregional modification, the Kimberly – Penman equation for alfalfa, to match the standard set by the Bureau of Reclamation.

Following feedback from the California Department of Water Resources and the University of California Davis, the daily FRET product evolved into a suite of products, including a weekly ETrc forecast for better water planning and a tabular point forecast for easy ingest into local water management models. With the assistance of NWS's Western Region Headquarters and Colorado Basin River Forecast Center (CBRFC), a twice monthly Penman – Monteith ETrc climatology was derived using 30 years of daily data from the North American Land Data Assimilation System data set, permitting a daily departure from normal to be calculated as a reference to supplement the forecast product.

Using the California's Central Valley for verification, the NASA Ames Research Ecological Forecasting Laboratory and a student from California State University Monterey Bay are comparing the NWS FRET forecasts to observations of ETo from the California Irrigation Management Information System (CIMIS), a network of 139 agricultural weather stations in California. They are also comparing the FRET forecasts to the 2 km daily interpolated ETo data products from the Spatial CIMIS model over the period from September 1, 2011 to August 31, 2012. The 6 month CIMIS verification between August 2011 and April 2012 suggests that forecast bias over the grid is -0.012 and 0.017 inches, when compared to the ETrc observations. It is publically available on their WFO websites, where sid is the site identification for your local office (e.g. http://www.wrh.noaa.gov/forecast/evap/FRET/FRET.php?wfo=sid).