week3 - 533 Figure 5.69: Class A Evaporation Pan pan are...

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Unformatted text preview: 533 Figure 5.69: Class A Evaporation Pan pan are not standard equipment and should preferably not be used. Pans should be protected by fences to keep animals from drinking. Pan-evaporation rates are generally greater than evaporation rates from large bodies of water, where numerous studies have shown that large water bodies have evaporation rates far higher near the edge of the water than towards the center where the air is more saturated and able to absorb less water vapor. The small size of an evaporation pan means that the whole pan is effectively an ‘edge’ and will have higher evaporation rates than a much larger bodies of water (Davie, 2002). A second, smaller, problem is that sides of the pan and the water inside will absorb radiation and warm up quicker than a much larger lake, providing an extra energy source and greater evaporation rate. Reference-crop evapotranspiration, ETo , is estimated from pan measurements by multiplicative factors called pan coefficients. Therefore ETo = kp Ep (5.363) where kp is the pan coefficient and Ep is the measured pan evaporation. Pan coefficients vary seasonally, are typically in the range of 0.35 to 0.85 (Doorenbos and Pruitt, 1975; 1977), and representative pan coefficients for various site conditions are given in Table 5.47. Key factors affecting the pan coefficient are the average wind speed, upwind fetch characteristics, and the ambient humidity. The estimation of evapotranspiration using the pan coefficients in Table 5.47 is commonly called the FAO-24 pan evaporation method. This method was published by the Food and Agriculture Organization (FAO) in paper number 24, hence the name. The pan coefficients given in Table 5.47 are described by the following regression equations (Allen and Pruitt, 1991; Allen et al., 1988) Surrounded by short green crop: kp = 0.108 − 0.0286u2 + 0.0422 ln(FET) + 0.1434 ln(RHmean ) − 0.000631[ln(FET)]2 ln(RHmean ) Surrounded by dry area: kp = 0.61 + 0.00341RHmean − 0.000162u2 RHmean − 0.00000959u2 FET + (5.364) 534 Wind Light (< 2 m/s) Table 5.47: Pan Coefficients for Various Site Conditions Case A: Pan Case B: Pan surrounded by Surrounded by short green crop dry, bare area Mean relative Mean relative humidity, % humidity, % Upwind Upwind fetch of fetch green Low Med High of dry Low Med crop, m < 40 40–70 > 70 fallow, m < 40 40–70 1 0.55 0.65 0.75 1 0.70 0.80 10 0.65 0.75 0.85 10 0.60 0.70 100 0.70 0.80 0.85 100 0.55 0.65 1000 0.75 0.85 0.85 1000 0.50 0.60 High > 70 0.85 0.80 0.75 0.70 Moderate (2–5 m/s) 1 10 100 1000 0.50 0.60 0.65 0.70 0.60 0.70 0.75 0.80 0.65 0.75 0.80 0.80 1 10 100 1000 0.65 0.55 0.50 0.45 0.75 0.65 0.60 0.55 0.80 0.70 0.65 0.60 Strong (5–8 m/s) 1 10 100 1000 0.45 0.55 0.60 0.65 0.50 0.60 0.65 0.70 0.60 0.65 0.70 0.75 1 10 100 1000 0.60 0.50 0.45 0.40 0.65 0.55 0.50 0.45 0.70 0.65 0.60 0.55 Very strong (> 8 m/s) 1 10 100 1000 0.40 0.45 0.50 0.55 0.45 0.55 0.60 0.60 0.50 0.60 0.65 0.65 1 10 100 1000 0.50 0.45 0.40 0.35 0.60 0.50 0.45 0.40 0.65 0.55 0.50 0.45 Source: Doorenbos and Pruitt (1977). 0.00327u2 ln(FET) − 0.00289u2 ln(86.4u2 ) − 0.0106 ln(86.4u2 ) ln(FET) + 0.00063[ln(FET)]2 ln(86.4u2 ) (5.365) where u2 is the average daily wind speed at 2 m height (m/s), RHmean is the average daily relative humidity (%) calculated by averaging the maximum and minimum relative humidity over the course of a day, and FET is the upwind fetch (m). In applying Equations 5.364 and 5.365 the following data ranges must be strictly observed (Allen et al., 1998) 1m≤ FET ≤ 1000 m (5.366) ≤ 8 m/s (5.368) 30% ≤ RHmean ≤ 84% 1 m/s ≤ u2 (5.367) The pan coefficients given in Table 5.47 are applicable to short-irrigated grass turf. For taller and aerodynamically rougher crops, the values of kp would be higher and vary less with differences in weather conditions (ASCE, 1990). It is recommended that the pan be installed inside a short green ...
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