Submitted to: Decennial National Irrigation Symposium
Publication Type: Proceedings
Publication Acceptance Date: November 14, 2000
Publication Date: N/A
Interpretive Summary: Water use from a well-watered grass, usually a cool-season type, that is 4 to 5 inches tall, has become a world-wide standard for use in estimating water use from other crop types. Few measurements of grass water use have been made in environments with high winds, low humidity, and clear skies. We measured irrigated grass water use at Bushland, TX, using a precise method and compared it with computed grass water use using many different equations and with other measurements of alfalfa water use. Generally, the measured grass water use agreed with the computed values, except the computed values were under-predicted on higher water use days and over-predicted on lower water use days. The measured alfalfa to grass water use ratio was high in 1998, a season with hot, dry winds, but closer to predictions in 1999, a more typical year. Additional research will be needed to improve the irrigated grass water use predictions for this environment to insure accurate water use estimates.
Technical Abstract: Cool-season, short, and well-watered grass is the world-wide standard reference for crop evapotranspiration (ET) research and practice. Fescue grass (Festuca arundinacea Schreb.) has been grown at Bushland, TX, since 1995, and its water use measured with a monolithic weighing lysimeter. The grass was mowed (to 0.11 m) and irrigated frequently and managed for vigorous growth. It was irrigated with a subsurface drip irrigation (SDI) system. Data were analyzed for the period 1 July 1995 through December 1999 that included a wide diversity in climatic regimes. Several grass reference ET models including FAO-56, ASCE Penman-Monteith (PM), FAO-24 Penman, Kimberly-96 Penman, SCS-93 PM, Penman-48, and the Hargreaves-Samani equations all for grass; the ASCE PM and Kimberly-Penman equations for alfalfa (Medicago sativa L.); and the general Priestley-Taylor equation for non-advected conditions were evaluated and contrasted with the daily grass ET measurements. Measured fescue daily ET rates exceeded 10 mm d**-1 occasionally. The FAO-56 and ASCE Penman Monteith equations tended to over-estimate during spring and fall and under-estimate during summer and especially on high ET days (> 8 mm d**-1). The older Penman formula ET correlated well to the measured daily data. The Hargreaves-Samani and Priestley-Taylor equations substantially under estimated grass ET in this environment.