92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Monday, 23 January 2012
National Weather Service Forecast Reference Evapotranspiration Across the Western US
Hall E (New Orleans Convention Center )
Cynthia K. Palmer, NOAA/NWS, San Diego, CA; and H. D. Osborne

After a year of drought conditions across Northern California, the National Weather Service (NWS) Weather Forecast Office (WFO) in Sacramento started collaborating with the California Department of Water Resources (CA DWR) and the University of California Davis in the early spring of 2008, to explore the possibilities for the NWS to provide daily reference evapotranspiration (ETrc) forecasts. Such a product would then allow water managers, the agricultural community, and the public to make more informed water-use decisions and to permit operational planning. 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. With water restrictions and cutbacks in available water supply apparent across portions of Northern California in 2008, the Sacramento WFO started producing 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. The Sacramento WFO developed a smart tool that uses the Penman Monteith ETrc and 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. Following user feedback, 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. Early verification from CA DWR and NASA Ames Research Ecological Forecasting Laboratory indicate that forecasts for days one through three correlate well to observations of ETrc by the California Irrigation Management Information System. Work is ongoing in matching the climatology to the forecasts (e.g. bias correction, resolving the spatial resolutions). Since the initial implementation three years ago, the FRET product suite has expanded to 16 WFOs across the NWS Western Region, with some subregional modifications. For example, the Pacific Northwest uses the Kimberly Penman equation for alfalfa to match the standard set by the Bureau of Reclamation. It is publically available on their WFO websites (e.g. http://www.wrh.noaa.gov/forecast/evap/FRET/FRET.php?wfo=sto).

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