Two Bowen ratio-energy balance (BREB) systems for measuring
evapotranspiration (ET) under wet and dry conditions were compared. The
study sites were an irrigated grass sod near Kimberly, Idaho and a sagebrush-grass
range site on the Reynolds Creek Experimental Watershed near Boise,
Idaho. One Bowen ratio system used positive-head, ceramic-wick, aspirated
psychrometers. The other system measured the vapor gradients with a cooled-mirror,
dew-point hygrometer. Differences in the amounts of ET measured by
the two systems were very small and of little practical consequence. The system
using a single-mirror, dew-point hygrometer was the most reliable, required the
least maintenance, and was the easiest to use

Hanson, C.L.

Wight, J.R.

Wright, J.L.

USDA - ARS - NWISRL

Comparing Bowen Ratio-Energy Balance Systems for Measuring ETJ. Ross Wight, Clayton L. Hanson, and James L. Wright'AbstractTwo Bowen ratio-energy balance (BREB) systems for measuringevapotranspiration (ET) under wet and dry conditions were compared. Thestudy sites were an irrigated grass sod near Kimberly, Idaho and a sagebrush-grass range site on the Reynolds Creek Experimental Watershed near Boise,Idaho. One Bowen ratio system used positive-head, ceramic-wick, aspiratedpsychrometers. The other system measured the vapor gradients with a cooled-mirror, dew-point hygrometer. Differences in the amounts of ET measured bythe two systems were very small and of little practical consequence. The systemusing a single-mirror, dew-point hygrometer was the most reliable, required theleast maintenance, and was the easiest to use.jntroductionThe importance of evapotranspiration (ET) measurements is wellrecognized. In both rainfed and irrigated agriculture, knowledge of ET is usedto evaluate management systems. In irrigated agriculture, ET is the primaryinput into irrigation scheduling algorithms. On arid and semiarid rangelands, ETis a major component of the water-balance equation, and accurate measurementsor calculations of ET are essential to the development and application ofhydrologic models.Micrometeorological methods and lysimeters have been used extensivelyto measure ET in rainfed and irrigated cropping systems. On rangelands,articularly in arid and semiarid regions, water balance and, to some extent,imetry have been the primary method of determining ET. Only recently havemicrometerlogical methods such as the BREB been used in the drier rangelandenvironments.1 Range Scientist and Agricultural Engineer, respectively, AgriculturalResearch Service, 800 Park Blvd., Plaza IV, Suite 105, Boise, ID 83705; SoilScientist, Agricultural Research Service, 3793 N, 3600 E, Kimberly, ID 83341.9:)-1	 IRRIGATION AND DRAINAGE SYSTEMSThe Bowen ratio (Bowen, 1926) and the energy balance equation are thebasis for the BREB method of determining ET using micrometeorological andsoil heat flow measurements (Rosenberg et al. 1983; Fritschen, 1966; Tanner,1960). The basic energy balance model is: Rn + G + H + LE = 0, where Rnis net radiation, G is soil heat flux, H is sensible heat to the atmosphere, and LEis the evaporative flux. The Bowen ratio (B) is the ratio of sensible heat flux towater vapor flux (H/LE) and can be estimated from the temperature and vaporpressure gradients by B y (AT / se) where y is the psychrometric constant, A Tis the air temperature gradient and Ae is the vapor pressure gradient. LE is thencalculated by the equation LE = -(Rn + H) / (1 + B). Because temperatureand/or vapor pressure gradients are often very small, the BREB method was notused extensively to measure ET until field worthy instrumentation had beendeveloped that could measure these gradients. However, improvedinstrumentation that is powered by batteries and solar panels makes theapplication of the BREB method a viable option for measuring ET from bothcroplands and rangelands.Other studies (e.g., Dugas et al. 1991) have compared BREB methods.In this study, two BREB systems that utilize different methods for measuringvapor gradients, were compared under both wet and dry conditions.BREB SystemsA positive-head, ceramic-wick, aspirated psychrometer (PCAP) systemsimilar to that described by Gay and Greenberg (1985) and Gay (1988) and acooled-mirror, dew-point hygrometer (CDH) system developed by CampbellScientific Inc., (Logan, UT) were used in this study. The photograph in Fig. 1shows the CDH system between the two towers of the PCAP system. In thePCAP system, two psychrometers were mounted on each of two towers tomeasure air temperature and vapor gradients. Dry- and wet-bulb temperatureswere measured with small nickel-iron resistance thermometers. The upper andlower psychrometers were exchanged each 6 minutes to cancel systematic sensorerrors. The sampling scheme consisted of 3 minutes for the instruments to cometo equilibrium with the environment, and a 3-minute sampling period followingwhich the psychrometers exchanged positions and continued the 12-minute cycle.The two 3-minute samples were combined to obtain a 6-minute sample for afrequency of five smples per hour. At each tower, Rn was measured by aSwissteco Type S-1 net radiometer and G was measured by a pair of soil heatflux disks placed at 1 cm below the soil surface and located within 2 m of eachother. The entire system was controlled by a battery operated computer anddata acquisition system. Solar panels were used to charge the batteries whichmade it possible to use the PCAP unit at remote locations.`Trade names are used in this publication solely to provide specificinformation. Mention of trade name does not constitute a guarantee of warrantyof the product by U. S. Department of Agriculture or an indorsement over otherproducts not mentioned.RX110-ENERGY BALANCE SYSTEMS 	 )55Figure 1. Photograph of the CDH system between the towers of the PCAPsystem on the Reynolds Creek site. 1992.The CDII system was similar to that described by Bingham et al. (1987),Tanner et al. (1987) and Malek et al. (1990). This system measured air anddew-point temperature at the same two heights above the soil surface as thePCAP system. Air temperature was measured by unshielded, nonaspirated, 76-µm diameter, chromel-constantan thermocouples. Dew-point was measured bya single cooled-mirror dew-point hygrometer. Air was continuously pumpedfrom both heights through 1 vm teflon filters. Air being moved across thecooled mirror was switched from one height to the other every 2 minutes withthe first 40 seconds used to stabilize the new dew-point temperature and 1minute and 20 seconds for measurements. Net  radiation was measured by aREBS Q6- net radiometer. Soil heat flux was determined by two parallel heatflux plates each located at a depth of 8 cm and the average soil temperatureabove the heat flux plates as measured by four parallel thermocouples locatedat 2- and 6-cm depths. Mean 20-minute values from each of the sensors wereused to calculate ET. This system was controlled by a battery powereddatalogger. Solar panels were used to charge the batteries permitting operationof the system in remote locations.Study SitesOne study site was in the center of a 2.6 ha rectangular field locatedabout 1 km south of the USDA, Agricultural Research Service research facilitynear Kimberly, Idaho, 42° 33'N, 114° 21'W (Wright, 1991). Site elevation isabout 1207 m and is surrounded by irrigated agricultural fields typical of theirrigated region. The Portneuf silt loam soil at the site was about 4 m deep andis underlain by fractured bedrock. The fescue grass was mowed regularly to9:-)6 	IRRIGA HON AND DRAINAGE: SYS FLNISmaintain a height of 8 to 15 cm. The leaf area index of the grass averaged about2.5 during the study period. The field was irrigated whenever the plant availablesoil water was reduced to 50 percent (about 65 kP tension at the 30-cm depth).The second study site was located on a gravelly loam, sagebrush-grassrange site on the U.S. Department of Agriculture-Agriculture Research Service'sReynolds Creek Experimental Watershed in southwestern Idaho, 43° 9'N, 116°44'W. Site elevation is about 2100 m. The slope varies from 3 to 5 percent.Average annual precipitation is 785 mm of which most occurs as winter snow.Sagebrush is the dominant plant species accounting for about half of thevegetation cover. Native grasses and forbs accounted for about equal amountsof the remaining vegetation. The average growing season is from May 1 toOctober 15. During the study period, maximum leaf area index was about 0.77and the plant available soil water was less than 50 percent.MethodsET was measured with two BREB systems July 18-August 8, 1991 andJuly 10-29, 1992 at the Kimberly and Reynolds Creek sites, respectively. Thetwo systems were run side-by-side at each site. Dew point was measured at 53and 153 cm above the ground at the Kimberly site and 73 and 173 cm above theground at the Reynolds Creek site which placed the lower sensors about 10 cmabove the vegetation canopy. Net radiation was measured 140 cm above theground at each site by placing the radiometers parallel to the ground surface.Daily precipitation and pan evaporation were also measured at each site. At theReynolds Creek site, soil water was measured periodically during the study withgravimetric samples for the 0-15 cm soil layer and by the neutron scatter methodfor the rooting zone. Leaf area index was measured by the point interceptmethod (Warren-Wilson 1959). Regression analyses were performed on thedaily ET values from the two study sites. Statistical significance were determinedat the p s .05 confidence level.Results and DiscussionBoth systems operated effectively at remote locations. More power wasrequired by the PCAP system because of the psychrometer fans and motors usedto exchange the position of the psychrometers at 12-minute intervals. The PCAPsystem required almost daily service to check water levels in the psychrometersand lubricate the psychrometer masts. The CDH system only required weeklyservice to clean the mirror and change air filters. In other studies, we found thatcold and snowy conditions affected the performance of the PCAP system. Undersuch conditions, alcohol had to be added to the psychrometer water, and snowoften prevented the psychrometers from exchanging positions. Insects gettinginto and stopping psychrometer fan motors was an occasional problem with thePCAP system. The temperature sensing unit on the CDH system is very thinand fragile and susceptible to breakage. The CDH system is more compact andm REYNOLDS CREEK SITE, KIMBERLY SITE1	 2	 3	 4	 5	 6	 7	 8CDH SYSTEM MEASURED ET (mm/d)a7EE6wCI 5L.L.1crcf) 4M 3F->— 2Cfl<C>00RA Ik1-LNGK(,S liAl,ANCL SYS ELNIS	 9) /easier to transport and set up than is the PCAP system. Both units have to beprotected from intruding livestock.Regression analyses of the daily ET values (Table 1) indicated that therewere no significant differences in the relationship of PCAP-measured ET toCDH-measured ET between the two sites. The data from the two sites weretherefore pooled and analyzed (Fig. 2). The slope of the regression equation issignificantly different from 1.0 indicating a small difference in the performanceof the two BREB systems when run side by side. The PCAP system tended tomeasure slightly more ET at the high range of measurements and less at the lowrange than did the CDH system. This is especially noticeable at the Kimberlysite which also had the most uniform vegetation surface. From a practicalperspective, these differences are probably well within instrument error and siteheterogeneity.Figure 2. Daily ET measured by the PCAP and CDH systems at the Kimberlyand Reynolds Creek sites. The 1:1 line is shown.The daily energy balance plots in Fig. 3 are representative of clear daysat each study site. Partitioning of the energy balance within each system wasconsistent from day-to-day. At the Kimberly site there was very little differencebetween the two systems, and ET accounted for nearly all of the Rn. At thedrier Reynolds Creek site there were small differences in the way the twosystems partitioned the energy balance and ET accounted for less than half theRn. In addition to slightly higher Rn values, the CDH system recorded less ETand lower G values in the morning and more ET and higher G values in the958	 IRRIGATION AND DRAINAGE SYSTEMSafternoon as compared to the PCAP system. Differences between the energybalance measurements by the two systems reflect the heterogeneous nature ofthe vegetation and soil surfaces of this site. As would be expected, G was muchmore variable in the dry environment.Table 1. Daily pan evaporation (Ep) (mm) and ET (mm) from the Kimberly(1991) and Reynolds Mountain (1992) sites as measured by two BREB Systems.Kimber y Site	 Reynolds Creek SiteDate Ep_	 - -CDH PCAP Date Ep CDH PCAP7-18 9.14 2.96 2.85 7-10 7.11 2.97 2.7019 8.13 4.73 4.76 11 3.05 1.61 1.6720 5.84 5.41 5.30 12 5.59 3.07 3.2921 7.87 6.10 6.54 13 9.14 3.33 3.4122 7.11 6.07 6.30 14 9.40 3.24 3.1123 8.13 4.93 4.74 15 8.13 3.10 3.0424 6.86 6.07 5.78 16 8.13 3.13 3.0225 7.26 3.50 2.86 17 8.64 3.25 3.1626 5.33 6.79 7.65 18 5.59 3.10 2.9927 8.38 7.00 7.57 19 5.59 3.58 3.8728* - 3.77 3.43 20 5.59 2.54 2.4429 8.89 5.85 6.21 21 4.83 1.83 1.9030* - 3.68 3.39 22 8.64 2.57 2.4631* - 0.85 0.68 23 5.84 1.92 1.708-1 6.86 7.12 7.36 24 5.08 2.04 2.002 10.41 6.08 6.29 25 6.10 2.37 2.233 8.64 5.36 5.29 26 9.14 2.64 2.594 6.60 4.40 4.30 27 7.87 2.53 2.425 6.35 5.95 6.10 28 9.91 2.61 2.526 7.11 6.35 6.08 29 8.64 2.42 2.377 9.90 5.63 5.59Total 138.81 108.60 109.07 1 142.01 53.85 52.89* Part of day	 Rn (CDH)G	  LE	  RnRn (PCAP)	  LEn4	 Rn (CDH)- e	 e G	  LE	  Rn (PCAP)	  LEnRATIO-ENERGY BALANCE SYSTEMS 	 959KIMBERLY–IRRIGATED GRASS (7/27/91)2	 4	 6	 B	 10	 12	 14	 16	 18	 20	 22	 24TIME (MST)REYNOLDS CREEK–RANGELAND	 (7/29/92)I	 I	 1	 I	 I 10	 12	 14	 16	 18	 20	 22	 24TIME (MST)Figure 3. Energy balance comparisons of the PCAP and CDH systems on theKimberly and Reynolds Creek sites.In conclusion, the two systems measured ET equally well under both wetand dry conditions. It appears that the cooled-mirror, dew-point hygrometer isa viable alternative to the aspirated wet- and dry-bulb psychrometer formeasuring ET with the BREB method on both cropland and rangeland.BOON 600E400>- 200M• 0z-200X-J -400- 6000BOO600E400>- 2001-1- 200X-400LA_- 6000 2	 4	 6960	 IRRIGATION AND DRAINAGE SYSTEMSAppendix I. ReferencesBingham, G.E., Tanner, B.D., Greene, J., and Tanner, M. (1987). "A Bowenratio system for long term remote measurement of evapotranspiration,"18th Conference on Agricultural and Forest Meteorology. Amer. Meteor.Soc., pp. 63-66.Bowen, I.S. (1926). "The ratio of heat losses by conduction and evaporationfrom any surface," Phys. Rev., 27:779-789.Dugas, W.A., Fritschen, L.J., Gay, L.W., Held, A.A., Matthias, A.D., Reicosky,D.C., Steduto, P. and Steiner, J.L. (1991). "Bowen ratio, eddycorrelation, and portable chamber measurements of sensible and latentheat flux over irrigated spring wheat," Agric. For. Meteor., 56:1-20.Fritschen, L.J. (1966). "Energy balance method," pp. 34-37. Proc.Evapotranspiration and Its Role in Water Resources Management,ASAE, St. Joseph, MI.Gay, L.W. (1988). "A portable Bowen ratio system for ET measurements," pp.625-632, In: L.C. James and M.J. English (ed.), Proc. Planning Now forIrrigation and Drainage in the 21st Century, ASCE, New York, NY.Gay, L.W. and Greenberg, R.J. (1985). "The AZET battery-powered Bowenratio system," pp. 181-182, Proc. 17th Conference, Agricultural and ForestMeteorology. Amer. Meteor. Soc, Boston, MS.Malek, E., Bingham, G.E. and McCurdy, G.D. (1990). "Evapotranspiration fromthe margin and moist playa of a closed desert valley," J. of Hydrol.,120:15-34.Rosenberg, N.J., Blad, B.L. and Verma, S.B. (1983). 	 "Microclimate: TheBiological Environment," John Wiley & Sons, New York. 495 p.Tanner, C.B. (1960). "Energy balance approach to evapotranspiration fromcrops," Soil Sci. Soc. of Amer., Proc., 24:1-9.Tanner, B.D., Greene, J.P. and Bingham, G.E. (1987). "A Bowen ratio designfor long-term measurements," ASAE Paper No. 87-2503. ASAE, St.Joseph, MI.Warren-Wilson, J. (1959).	 "Analysis of the distribution of foliage area ingrassland," p. 51-61.	 In: J.D. Ivins., The measurement of grasslandproductivity. Butterworths Sci. Publ., London.Wright, J.L. (1991). "Using weighing lysimeters to develop crop coefficients," p.191-199. In: R.G. Allen, T.A. Howell, W.O. Pruitt, I. Walter, and M.E.Jensen (ed). Lysimeters for evapotranspiration and environmentalmeasurements. Proc. of the International Symp. on Lysimetery. ASCE,New York, NY.

Comparing Bowen ratio-energy balance systems for measuring ET

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Comparing Bowen ratio-energy balance systems for measuring ET

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