Skip to main content
ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #195314
Title: SENSITIVITY ANALYSES AND SENSITIVITY COEFFICIENTS OF STANDARDIZED DAILY ASCE-PENMAN-MONTEITH EQUATION

Author
item IRMAK, S - UNIV. OF NEBRASKA
item PAYERO, J - UNIV. OF NEBRASKA
item MARTIN, D - UNIV. OF NEBRASKA
item IRMAK, A - UNIV. OF NEBRASKA
item Howell, Terry

Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2006
Publication Date: 11/4/2006
Citation: Irmak, S., Payero, J.O., Martin, D.L., Irmak, A., Howell, T.A. 2006. Sensitivity analyses and sensitivity coefficients of standardized daily ASCE-Penman-Monteith equation. Journal of Irrigation and Drainage Engineering. 132(6):564-578.

Interpretive Summary: The Penman-Monteith equation to estimate water use from standard crops is becoming widely used worldwide. It was adopted by the United Nations Food and Agriculture Organization (FAO) in their handbook, FAO-56, now widely used since 1998. This work analyzed the relative errors or sensitivity in the computed reference crop water use from small changes in the weather data used in the equation for a diversity of climates. In all locations, crop water use was most sensitive to the relative humidity when expressed in its vapor pressure deficit form. In semiarid locations, reference crop water use was sensitive to wind speed next. Solar radiation (amount of sunshine) was a dominate factor during summer in more humid climates. These sensitivity factors are useful in designing weather station instrument accuracy specifications and in estimating important future water use changes from climatic change.

Technical Abstract: The sensitivity of the standardized American Society of Civil Engineers grass-reference Penman-Monteith evapotranspiration (ASCE-PM ETo) equation to climate variables in different regions has not yet been studied. Sensitivity analyses for the standardized daily form of the ASCE-PM equation were conducted on wind speed at 2-m height (U2), maximum and minimum air temperatures (Tmax and Tmin), vapor pressure deficit (VPD), and solar radiation (Rs) in semi-arid (Scottsbluff, NE, and Bushland, TX), a Mediterranean-type climate (Santa Barbara, CA), coastal humid (Fort Pierce, FL), inland humid and semi-humid (Rockport, MO, and Clay Center, NE), and an island (Twitchell Island, CA) regions of the United States. The sensitivity coefficients were derived for each variable on a daily basis. In general, ETo was most sensitive to VPD at all locations. However, sensitivity of ETo to the same variable showed significant variation from one location to another and at the same location within the year. After VPD, ETo was most sensitive to U2 at semi-arid regions (Scottsbluff, Clay Center, and Bushland) during the summer months. The Rs was the dominant deriving force of ETo at humid locations (Fort Pierce and Rockport) during the summer months. At Santa Barbara, the sensitivity of ETo to U2 was minimal during the summer months. At Bushland, Scottsbluff, and Twitchell Island, ETo was more sensitive to Tmax than Rs in summer months, whereas it was equally sensitive to Tmax and Rs at Clay Center. The ETo was not sensitive to Tmin at any of the locations. The change in ETo was linearly related to change in climate variables (with r2>/=0.96 in most cases) with the exception of Tmin, at all sites. Increase in ETo with respect to increase in climate variable varied with month considerably. On an annual average, a 1 deg C of increase in Tmax resulted in 0.11, 0.06, 0.16, 0.07, 0.11, 0.08, and 0.10 mm increases in ETo at Scottsbluff, Santa Barbara, Bushland, Fort Pierce, Twitchell Island, Rockport, and Clay Center. A 1 m s-1 increase in U2 resulted in 0.42, 0.18, 0.37, 0.28, 0.31, 0.20, and 0.26 mm increases in ETo at the same locations. A unit increase in Tmax resulted in largest increase in ETo at Bushland, and a unit increase in Rs caused the largest increase in ETo at Fort Pierce. A 1 MJ m-2 d-1 increase in Rs resulted in 0.05, 0.08, 0.06, 0.11, 0.05, 0.06, and 0.06 mm increase in ETo at the same locations. A 0.4 kPa increase in VPD resulted in 1.13, 0.54, 1.29, 0.57, 1.04, 1.10, and 1.22 mm increase in ETo at the same locations. U2 had the most effect on ETo at Scottsbluff and Bushland, the two locations where dry and strong winds are common during the growing season. The sensitivity coefficient for Rs was higher during the summer months and lower during the winter months and the opposite was observed for VPD (except for Twitchell Island). The decrease of the sensitivity coefficients for Rs corresponding to an increase in the sensitivity coefficient for VPD is due to a decrease in the energy term in favor of the increase in significance of the aerodynamic term of the standardized ASCE-PM equation in summer versus winter months. Because the ASCE-PM and the Food and Agriculture Organization Paper No. 56 Penman-Monteith (FAO56-PM) equations are identical when applied on a daily time step, the results of the sensitivity analyses and sensitivity coefficients of this study should be directly applicable to the FAO56-PM equation.