|Evett, Steven - Steve|
Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Other
Publication Acceptance Date: 7/12/1998
Publication Date: N/A
Technical Abstract: Evapotranspiration (ET) from a cool-season type grass with a height of 0.12 m and a surface resistance of 70 s/m has become a standard reference surface as the basis for crop coefficients and for modeling water use from many crops. Although many data sets were used in developing and verifying this standard, few of them are subject strong regional advection like that found in the Southern High Plains of the U.S. Our objectives were to measure water use from a tall fescue grass (Festuca arundinacea Schreb.), to determine if differences existed in computed reference grass ET from several widely used methods in this environment, and to examine methods for estimating irrigated fescue grass water use rates. ET of the subsurface irrigated fescue was measured at Bushland, TX, using a weighing lysimeter (a square 2.25 m**2 container in area and 2.4 m deep) with an ET resolution exceeding 0.1 mm in the latter half of 1995 through 1997. The Food and Agriculture Organization (FAO) Penman-Monteith (PM) methods estimated net radiation well, but an assumption of zero soil heat flux was better than using the FAO-PM daily air temperature method. The data set contained a few days with ET rates exceeding 10 mm/d, but most of the midsummer ET rates were in the 6-10 mm/d range. Winter time ET rates varied, but often were in the 1-3 mm/d range. The 1948 Penman combination equation closely paralleled the more recent FAO (Food and Agricultural Organization of the United Nations) Penman-Monteith (PM) equation calculated ET. The FAO-PM estimated ET was larger than the measured at low rates (mainly in the late fall, winter, and early spring) and tended to underestimate the ET for the higher rates (> 8 mm/d). Surprisingly, the 1948 Penman combination appeared to be slightly better correlated to the measurements.