Submitted to: Decennial National Irrigation Symposium
Publication Type: Proceedings
Publication Acceptance Date: November 14, 2000
Publication Date: N/A
Interpretive Summary: Scientific irrigation scheduling is used to calculate how much water to apply and when to apply it. One important goal is to obtain the most yield possible per unit of water applied. Other goals include avoiding deep drainage and the consequent transport of nutrients to ground water and streams. So, scientific irrigation scheduling has the dual purposes of conserving water and avoiding pollution. The most common scientific irrigation scheduling method is based on equations that should reliably predict a reference crop's water use from weather data such as daily sunshine strength, wind speed, and the temperature and humidity of the air. Then, the water use of a given crop at a given growth stage can be predicted by multiplying the reference crop water use by a crop coefficient that has previously been measured for that particular crop and growth stage. Alfalfa water use is one of two widely used reference water use crops; but there is evidence that the equations predicting alfalfa water use under well-watered conditions do not work the same in different climatic zones of the United States. We tested three methods of calculating daily alfalfa reference water use from weather data and compared the results with direct measurements of the water use of well-watered alfalfa grown at Bushland, TX, for four years (1996-1999). An equation known as the Penman-Monteith (PM) equation worked well when using half-hourly weather data, but no so well using daily weather data. The Kimberly-Penman equation also did not work as well. The results show that the PM equation works well to predict alfalfa reference water use for accurate irrigation scheduling if half-hourly or hourly weather data are used. Automatic weather station networks should be established and maintained to provide such data.
Technical Abstract: Alfalfa evapotranspiration (ET) is one of two common reference ET (ETR) values. The other is grass ET. We tested Penman-Monteith (PM) and the 1982 Kimberly Penman (KP) equation predictions of ETR against measured alfalfa ET under reference conditions. Alfalfa (Medicago sativa, var. Pioneer 5454) was grown in 1996 through 1999 on Pullman clay loam (Torrertic Paleustoll) at Bushland, TX. The crop was well-watered with a lateral move sprinkler. Monolithic weighing lysimeters (3-m by 3-m by 2.4-m deep) measured ET every half hour to 0.05 mm precision. Yields were 16.5, 16.4, 20.6, and 15.2 dry Mg/ha in 1996 through 1999. Crop water use averaged 1.01 m per year in 1996 and 1997, and was 1.16 m in 1998. Daily ET in this windy, semi-arid environment occasionally exceeded 14 mm. Daily alfalfa ETR predicted using PM methods and half-hourly weather data compared well with our measurements (regression r**2 of 0.94, SE of 0.6 mm, slope of 0.94, and intercept of 0.2 2mm). Use of daily weather data increased the SE to 0.8 mm (r**2 of 0.90, slope of 0.98) and introduced a positive offset of 0.7 mm. The KP equation used with daily weather data produced more biased predictions (r**2 of 0.91, SE of 0.7 mm, intercept of 0.9 mm, and slope of 0.88). The ASCE Manual 70 methods for predicting net radiation from solar irradiance worked well when applied to half-hourly data (r**2 of 0.97, SE of 0.6 MJ/m**2, and slope of 1.03). But these methods applied to daily data produced biased results (r**2 of 0.94, SE of 0.8 MJ/m**2, intercept of 1.5 MJ/m**2, and slope of 0.85). Use of the KP net radiation equations with daily data produced slightly less biased results (r**2 of 0.97, SE of 0.6 MJ/m**2, intercept of 0.7 MJ/m**2, and slope of 0.87). Alfalfa ET was 1.15 times grass ET.