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A simple system for automated long-term Bowen ratio measurement
Agricultural and Forest Meteorology, 66 (1993) 81-92 0168-1923/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
81
A simple system for automated long-term Bowen ratio measurement Pierre Cellier *'a, Albert O l i o s o b aInstitut National de la Recherche Agronomique Station de Bioclimatologie, F78850 Thiverval-Grignon, France bInstitut National de la Recherche Agronomique Station de Bioclimatologie, BP 91, F84143 Montfavet, France (Received 10 August 1992; revision accepted 24 March 1993)
Abstract The Bowen ratio method is widely used to estimate fluxes of sensible heat and evaporation between a crop and the atmosphere. The most critical points of this method are the measurements of vertical humidity gradients, and long-term operation. We propose a simple system, using a capacitive hygrometer and alternate sampling of air at two levels with pumps to measure humidity gradients. It is well adapted to long-term measurements: low power consumption, low charge of maintenance, and low cost. It has been tested by comparison of hourly sensible and latent heat flux measurements performed by both eddy correlation and Bowen ratio, over a bare soil field and a fully evaporating canopy. Results are satisfactory over a large range of fluxes (0-400 W m -2) under daytime conditions. The main sources and magnitude of errors were investigated under the contrasted situations of bare soil and fully evaporating canopy. The system was shown to give good flux estimates even with very low humidity gradients.
Introduction Bowen ratio determination (Bowen, 1926) coupled with net radiation and ground heat flux measurements is often used, for research purposes and agricultural applications: estimation of sensible heat flux or evaporation from a crop (Black and McNaughton, 1971; Perrier et al., 1975; Ashktorab et al., 1989; Cellier and Brunet, 1992) or estimation of hourly averages of crop photosynthesis (Held et al., 1990), for example. The Bowen ratio method is widely used for different reasons: (1) it uses robust sensors allowing continuous measurements (Bingham et al., 1987; Gay, 1988; Fritschen and Simpson, 1989); (2) it requires only two measurement levels when classical aerodynamic method needs more; (3) its site requirements are less restrictive than most other micrometeorological methods: it can be * Corresponding author.
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P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
used with relatively low fetch-to-height ratios (Heilman et al., 1989) over moderate slopes (Fritschen and Qian, 1990) and even close to the canopy top in the roughness sublayer (Cellier and Brunet, 1992). Conditions of strong advection are the main limitation of this method, but their effects on the applicability of the method are controversial: while Motha et al. (1979) and Warhaft (1976) estimated that turbulent transfer of sensible heat were more enhanced that latent heat transfer, Lang et al. (1983) gave opposite conclusions. Apart this case, the above mentioned advantages contributed to a large use of this method and to the development of different automated systems for flux measurements (see e.g. Black and McNaughton, 1971; Perrier et al., 1975; Bingham et al., 1987; Gay, 1988; Tanner, 1988; Fritschen and Simpson, 1989). One critical problem deals with the ability to make long term measurements. This requires reliability in the of sensors and other parts of the system, a low energy consumption and a low charge of maintenance. We propose here an apparatus which meets these conditions, using a capacitive hygrometer to measure humidity gradients. Problem
Fluxes and humidity gradients calculations
The convective fluxes of sensible and latent heat from a natural surface (soil and/or vegetation) to the atmosphere can be estimated using the energy balance equation Rn - G = H + AE
(1)
and the Bowen ratio /3 = H / A E = (cp/A)(OT/Or)
(2a)
is the net radiation, G the soil heat flux, and H and AE respectively the sensible and latent heat fluxes (all in W m'2); Cp is air specific heat at constant pressure (J kg-1 K - l ) and A latent heat of vaporization of water (J kg-1). T is air potential temperature (K). For humidity gradients, the mixing ratio, r (kg kg- 1), is preferred in order to eliminate the effects of vertical density gradients on the Bowen ratio, which can be important under strong temperature gradients (Webb et al., 1980). For practical application, Eq. (2a) is generally approximated by finite differences Rn
/3 ~ (Cp/A)(AT/Ar)
(2b)
H and AE are then estimated from
H = (R. - G)/3/(1 +/3)
(3a)
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(3b)
AE = (Rn - G)/(1 + [3) This requires the knowledge of the fluxes gradients of air temperature and humidity.
Rn
and G, and of the vertical
Problems involved in humidity measurements
Measuring humidity gradients is delicate because sensor-dependent errors are often of the same order of magnitude or even larger than the gradients themselves (see Table 1). To solve this difficulty, two approaches are currently used to remove the major part of the errors: (1) moving sensors between the different measurement levels in order to avoid too long return times for each level, sensors are generally exchanged in pairs; (2) routing successively air samples from each level to the humidity sensor through tubes. As a consequence, the main problem is no longer absolute precision, but rather resolution of the sensor and absence of short term drift. Presently, three main types of humidity sensors are used for outdoor routine measurement. (1) Psychrometers are widely used because of their good resolution, low cost and simplicity of conception, but they require constant attention (problems of wicks wetness and cleanness) and cannot work easily under temperatures below 0°C. They generally need strong ventilation to reach a stable psychrometric constant, which consumes a large amount of electric power (Table 1). This drawback can be lessened by using small probes, but they cannot in any case be used with air routed through tubes. However it is certainly the most widely used sensor for Bowen ratio determination (Fuchs and Tanner, 1970; Black and McNaughton, 1971; Perrier et al., 1975; Angus and Watts, 1984; Gay, 1988; Fritschen and Simpson, 1989; Heilman et al., 1989; Held et al., 1990; Cellier and Brunet, 1992). (2) Dew point hygrometers are often presented as reference systems. They give a direct estimate of absolute humidity with one single measurement, but few manufactured models are designed for field use and even in that case they Table 1 Characteristics of different humidity sensors (absolute precision is estimated with an air temperature of 20°C and a dew point of 10°C, either in the sensor unit or in mixing ratio) Sensor type
Psychrometer Dew point hygrometer (laboratory) (field use) Capacitive hygrometer
Absolute precision (sensor unit)
(kg kg- t
0.1 K 0.2 K 0.5 K 1% 2% (RH > 90%)
100 100 250 125 250
106)
Power consumption (mA)
100- 500 100- 200 1- 10
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have a noticeable energy consumption (Table 1). More, because of the permanent presence of liquid water on the mirror, they need regular checking of mirror cleanliness, if not equipped with auto-cleaning system (which increases cost and energy consumption). Several recent Bowen ratio apparatus use such hygrometers (Heimburg, 1982; Bingham et al., 1987; Tanner, 1988; Ashktorab et al., 1989). (3) Capacitive hygrometers have been more recently developed, but they arc now widely used for routine measurement on meteorological stations (Tanner, 1990), because of their simplicity of use and low power consumption (Table 1). They are generally cheaper than other hygrometers. The reduced size of the sensor and its rapid response time to humidity changes give them many applications in ecophysiological research (Day, 1988). They were known to have a significant drift with time, but most recent models only exhibit a drift of one to several percent per year (Skaar et al., 1989; Tanner, 1990) and are generally corrected in temperature; they then became quite reliable sensors. Despite these qualities, capacitive sensors are not usually employed for humidity gradient measurements. To our knowledge, the only experiment where this is mentioned, is that of Lang et al. (1983). We propose here to extend the use of this type of sensors to such applications. Method
Description of the system We used a HMP35A humidity sensor (VAISALA, Helsinki, Finland) whose most interesting characteristics for our concern were its low power consumption (1-3 mA), fast response time (about 1 s), temperature compensation and low cost. The air sample is routed from the two measurement levels to the sensor by two low power consumption (20 mA) pumps (ASF, Puchheim, Germany) providing a flow rate of about 1 1 min -1 through 1 m long Tygon tubes (Fig. 1). Tygon was chosen for its low permeability to water vapour. The internal diameter of tubes was 3.2 mm. The inlet of each tube was placed in a radiation shield (used for air temperature measurement) to prevent liquid water from being aspirated in case of rain. The sensor was placed inside a measurement chamber of about 1 cm 3 (Fig. 1). In order to avoid any condensation of water vapour in the tubes, a thin constantan wire was placed inside them; this heating resistance was switched on when the relative humidity at sensor level exceeded 90%. This generally occurred for short time periods during rainy days, at the end of nights with important dew, or just after sunrise when air humidity increases rapidly owing to evaporation, while the tubes remain cold. The power consumption required to obtain a temperature increase of 2 - 3 K is approximately 200 mA. It is sufficient, in
85
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92 Air p u m p s
Connection
9
panel
IIIIIII i
Valve "/;1~ _eating wire
¢
flow m e t e r
. 7,"-
I!
~-~_-~ .:/
Temperature sensor
¢ ~
Hygrometer
Tygon t u b e
I P u m p s and Heating
control
Fig. 1. Schematicrepresentation of the humidity gradient measurementsystem. most situations, to stabilize relative humidity at 90%, which remains in the range of best performances of capacitive hygrometers (Table 1). The pumps were operated alternately every 150 s, resulting in a complete cycle in 5 min. Their valves prevented air from going through the other nonoperating pump. To allow stabilization, measurements were excluded during the first 45 s following the inversion. Air temperature and humidity in the chamber were measured every 5 s and immediately converted into vapour pressure or mixing ratio. The humidities at both levels were then averaged over 60 min intervals. In some circumstances (low humidity gradients together with a strong increase of humidity over short time intervals: over bare soil just after sunrise for example) the shift in average humidity owing to the time lag between
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P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81 92
successive measurements can be of the same order of magnitude as the humidity gradient itself; it is then necessary to correct the gradient along with this humidity evolution. This could be avoided by using two humidity sensors which would perform simultaneous measurements at both levels. For sake of simplicity, this has not been done in the present study. F r o m Eqs. (2) and (3), it can be seen that flux estimates can be very erroneous when temperature and humidity gradients are small, and when the Bowen ratio is close to - 1 . In order to eliminate the questionable data sets, we used the objective criteria defined by Ohmura (1982), and rejected all the data where -1.25 < ,~ < -0.75 (Tanner, 1988).
Experimental sites This system has been tested under strongly contrasting conditions: over bare soil where evaporation and humidity gradients are very weak, and, in the opposite situation, over a fully evaporating canopy: (1) in Grignon, France (48°51tN, 1°58'E) during May and June 1990, over a bare soil field just after maize sowing; (2) in Montfavet, France (43°55'N, 4°51fE) from July to September 1990, over a soybean crop whose leaf area index was between 2.0 and 4.0. The aim of these experiments was to compare the sensible heat flux estimated by the previously described Bowen ratio system with an independent estimate by eddy correlation using a one-dimensional sonic anemometer (Tanner, 1988). Consequently, the following measurements were performed in both situations: (1) net radiation with a Swissteco (Oberriet, Switzerland) type S 1 net radiometer in Grignon, and with a Crouzet (Valence, France) net radiometer in Montfavet; (2) air temperature at two levels above the crop or soil surface using thermocouples placed into ventilated radiation shields; measurement levels were 0.5 and 1.50 m in Grignon, and two and three times the height of the canopy in Montfavet; (3) air humidity at two levels using the previously described system; air was sampled in the ventilated radiation shields used for air temperature measurements; (4) soil temperatures using thermocouples located at six levels between soil surface and a depth of 50 cm; they were used to estimate the soil heat flux G, by a calorimetric method using soil temperature measurements coupled with gravimetric estimations of soil density and humidity; (5) sensible heat flux using a onedimensional sonic anemometer CA27 (Campbell Scientific, Shepshed, UK) sampled at a frequency of 10 Hz. The sensor was placed at an intermediate level between air temperature measurement levels. For the Bowen ratio method, the measurements and all control operations (pump commutations, heating) were performed by a CR10 datalogger in Grignon and a 21X datalogger in Montfavet (both Campbell Scientific) and
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
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the data were averaged over time intervals of 30 min and 60 min respectively in Grignon and Montfavet. For eddy correlation, temperature and wind speed fluctuations measurements were performed on a 21X datalogger in both locations. Results and discussions Technical performances As can be seen on Figs. 2(a) and 2(b), our Bowen ratio measurement system is able to provide a good estimate of sensible and latent heat fluxes throughout the day. This is true for both contrasted situations of bare soil and fully evaporating vegetation. As could be expected, fluxes estimated by Bowen ratio exhibit a discontinuity near sunset and sunrise, owing to Bowen ratios close to 1. In Grignon (Fig. 2(a)), no eddy correlation measurements were performed during the night because of risks of rain or dew deposition. During daytime hours, both methods are in fair accordance, especially around noon, when fluxes reach their maximum values. The slight observed shifts during the early morning and late morning on Fig. 2(a) cannot be attributed to a unique evident cause such as advective conditions, errors on net radiation owing to a poor horizontality of the sensor, or poor estimate of soil heat flux (these are the hours where it changes very rapidly). It should be noticed that even during the night in Montfavet, in spite of very low humidity gradients, the agreement
v
600
600-
500
500-
400
400-
- 300
30O
200
200 •
100-
100
O"
0
(b)
-100
-100 0
4
8
12 UT
(hours)
16
20
24
0
4
8
12 UT
16
20
24
(hours)
Fig. 2. Example of energy fluxes measured either by eddy correlation (thin line) or Bowen ratio (thick line) at (a) Grignon on 23 May 1991 a n d (b) Montfavet on 2 September 1991: ( ~ ) net radiation; ( ) soil heat flux; ( - - ) sensible heat flux; ( . . . . . . ) latent heat flux.
P. Cellier, A. Otioso / Agricultural and Forest Meteorology 66 (1993) 81 .-92
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is satisfactory; the occurrence of evaporation throughout the night certainly results from the limited size of the experimental field and from an irrigation performed several days before. These effects are also enhanced by the strong northern winds (called 'Mistral') which prevailed during the experimental period. A significant advantage of this system is its ability to run with minimum maintenance: during the two experiments presented above, and five others performed in 1991 and 1992, the systems remained in the field for periods of several months with no other maintenance than checking the air flow in tubes. The only problems encountered dealt with pump functioning: a poor valve air-tightness owing to a too long working period could drastically diminish the air inflow rate, or make it go through the other pump rather than to the humidity sensor. This type of problem could have been avoided by using vanes preventing air flow from going from one pump to the other.
Compar•on withfluxes estimated by other methods In Fig. 3 hourly values of sensible heat fluxes measured by Bowen ratio, are compared with those estimated by eddy correlation (in Grignon hourly datasets were reconstructed from the average of two successive datasets). The analysis was limited to daytime values, when available energy (Rn - G) was more than 50 W m -2, to avoid errors as a result of too low gradients and available energy. Moreover, so that important uncertainties resulting from 500
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100
200
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300
400
500
H eddy c o r r e l a t i o n (W/m 2 ) Fig. 3. Comparison between sensible heat fluxes measured by eddy correlation (X-axis) and Bowen ratio (Y-axis) in Grignon (D) and Montfavet (,).
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89
advective conditions do not affect this analysis (Motha et al., 1979; Lang et al., 1983), we excluded the days following irrigation over the soybean crop, where the sensible heat flux was strongly negative during day. The agreement is satisfactory over a wide range of fluxes: over bare soil in Grignon the sensible heat fluxes reached up to 400 W m -2. A linear fit calculated on these data (H by Bowen ratio as a function of H by eddy correlation) gives a slope of 1.02 and an offset of 10 W m -2, and corresponding standard deviations of 0.02 and 25, respectively. The average difference between both estimates is 12 W m -2 with a standard deviation of 25 W m -2. This general overestimate can therefore be observed over the whole range of fluxes. It can result from an overestimate of available energy or from the influence of possible advection, which would increase sensible heat transfer rather than latent heat transfer. In the case of Montfavet data, this would meet the conclusions of Motha et al. (1979) obtained from similar conditions to ours (irrigated alfalfa). Some part of this overestimate could also be the consequence of measurement errors, under both situations. Therefore, we analysed the behaviour of the system according to probable sources of measurement errors (Fuchs and Tanner, 1970; Angus and Watts, 1984). The relative error on sensible heat flux can be written as
5g/g
=
(4a)
(SR n q- 5a)l[Rn - al + 53/131/(1 + 131)
with (4b)
5 3 / 3 = 5 A T / [ A T I + 5Ar/lAr]
The notation 5 X refers to the error on operator X. The expected errors on each term or Eq. (4) are listed in Table 2. Measurement errors of 0.05K on temperature gradients and 5 Pa on humidity gradients were assumed. The errors on the gradients were estimated according to the average values of the related quantities for each experimental site: temperature gradients of 1.0 and 0.25K, and humidity gradient of 50 and 300 for Grignon and Montfavet, respectively. A larger error on R n - G is assumed for the bare soil situation, owing to the higher G values. It can be seen that the large error on humidity gradient over bare soil (Grignon), resulting from low humidity gradients, does not strongly affect the Bowen ratio and sensible heat flux estimates. As most of the available energy is dissipated into sensible heat an error on Bowen ratio would only Table 2 Estimated errors on the different terms of Eq. (4)
Grignon Montfavet
6AT
5Ar
5Rn + 5G
6~3
6H
[AT t
[Ar I
]R n - G I
]/3[(1 + ]~3[)
H
0.05 0.20
0.30 0.10
0.10 0.05
0.05 0.20
0.15 0.25
P. Cellier, A. Olioso i Agricultural and Forest Meteorology 66 (1993) 81-92
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P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
91
be significant for the low evaporation flux. As expected, this method is not well adapted to evaporation estimates under very dry situations. On the contrary, over the soybean crop the main uncertainty results from the temperature gradient measurement. The resulting relative errors on Bowen ratio and sensible heat flux are large. Nevertheless, owing to the low values of sensible heat fluxes this does not induce very large absolute errors on this flux. As can be seen in Fig. 4(a), the differences between both methods exhibit no trend with humidity gradient values. Of course, the scatter increases at low humidity gradients (above -300 x 10-6 kg kg-1), but it remains quite acceptable if one considers the very low values of Ar presented here. In the same way, flux estimates are not really affected even under very large Bowen ratios (Fig. 4(b)). Yet, these conditions of large Bowen ratio are generally considered as the limit of applicability of this method. These results can be interpreted as an indication of the reliability of this system until very low humidity gradients. Finally, it could also be remarked on Fig. 4(c) that the differences between both methods do not evidently increase with flux values.
Conclusions
We presented a simple system using a capacitive hygrometer to measure humidity gradients. This system is designed for long-term measurements with such applications as flux estimation by Bowen ratio method. When compared with other existing equivalent systems, it presents three main advantages: (1) its very low power requirement (20-30 mA, including hygrometer and pump power consumptions); (2) the absence of maintenance: capacitive hygrometers do not require any regular supply of water as psychrometers and are not so exposed to dust deposition than some other humidity sensors; (3) its low cost. The good performances of this system show that this type of hygrometer is full of promise for long-term humidity gradient measurements in natural environment. Thus, it allows an easy continuous recording of energy fluxes under any climatic conditions. Up to now they have been operated with success during five long term experiments, in order to estimate energy exchanges of various natural surfaces (wheat, millet, vegetative mulch, bare soil). References Angus, D. and Watts P.J., 1984. Evapotranspiration - How good is the Bowen ratio method? Agric. Water Manage., 8:133 150. Ashktorab, H., Pruitt, W.O., Paw U, K.T. and George, W.V., 1989. Energy balance determinations
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P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
close to the soil surface using a micro-Bowen ratio system. Agric. For. Meteorol., 46: 259274. Bingham, G.E., Tanner, B.D., Green, J. and Tanner, M., 1987. A Bowen ratio system for long term remote measurement of evapotranspiration. In: 18th Conf. Agric. and For. Meteorol. and 8th Conf. Biometeorol. and Aerobiol. 15-18 September, Am. Meteorol. Soc., Boston, MA, preprint volume, pp. 63-66. Black, T.A. and McNaughton, K.G., 1971. Psychrometric apparatus for Bowen-ratio determination over forests. Boundary-Layer Meteorol., 2: 246-254. Bowen, I.S., 1926. The ratio of heat losses by conduction and by evaporation for any water surface. Physiol. Rev., 27: 779-787. Cellier, P. and Brunet, Y., 1992: Flux-gradient relationships above tall plant canopies. Agric. For. Meteorol., 57:93-117. Day, W., 1988. Water vapour measurement and control. In: B. Marshall and F.I. Woodward (Editors), Instrumentation for Environmental Physiology. Cambridge University Press, Cambridge, pp. 59-78. Fritschen, L.J. and Simpson, J.R., 1989. Surface energy and radiation systems: general description and improvements. J. Appl. Meteorol., 28: 680-689. Fritschen, L.J. and Qian, P., 1990. Net radiation; sensible and latent heat flux densities on slopes computed by the energy balance method. Boundary-Layer Meteorol., 53:163-171. Fuchs, M. and Tanner, C.B., 1970. Error analysis of Bowen ratios measured by differential psych rometry. Agric. For. Meteorol., 7:329 334. Gay, L.W., 1988. A portable Bowen ratio system for ET measurements. In: Proc. Nat. Conf. Irrig., Drain. Am. Soc. Civil Eng., NY, pp. 625-632. Heilman, J.L., Brittin, C.L. and Neale, C.M.U., 1989. Fetch requirements for Bowen ratio measurements of latent and sensible heat fluxes. Agric. For. Meteorol., 44: 261-273. Heimburg, K.F., 1982. Evapotranspiration: an automated measurement system and a remote-sensing method for regional estimates. Thesis University of Florida, 211 pp. Held, A., Steduto, P., Orgaz, F., Matista, A. and Hsiao, T.C., 1990. Bowen ratio-energy balance technique for estimating crop net CO2 assimilation and comparison with a canopy chamber. Theor. Appl. Climatol., 42: 203-214. Lang, A.R.G., McNaughton, K.G., Chen, F., Bradley, E.F. and Ohtaki, E., 1983. An experimental appraisal of the terms in the heat and moisture flux equations for local advection. BoundaryLayer Meteorol., 25: 89--102. Motha, R.P., Verma, S.B. and Rosenberg, N.J., 1979. Exchange coefficients under sensible heat advection determined by eddy correlation. Agric. Meteorol., 20: 273-280. Ohmura, A., 1982. Objective criteria for rejecting data for Bowen ratio flux calculations. J. Appl. Meteorol., 21: 595-598. Perrier, A., Itier, B., Bertolini, J.-M. and Blanco de Pablos, A., 1975. Mesure automatique du bilan d'6nergie d'une culture. Exemples d'application. Ann. Agron., 26: 19-40. Skaar, J., Hegg, K., Moe, T. and Smedrud, K., 1989. WMO International Hygrometer Intercomparison. World Meteorological Organization No. 480, Geneva, Switzerland. Tanner, B.D., 1988. Use requirement for Bowen ratio and eddy correlation determination of evapotranspiration. In: DeLynn R. Hay (Editor), Planning Now for Irrigation and Drainage in the 21st Century. Irrig. and Drain. Div., Am. Soc. Civil Eng., NY, pp. 605-616. Tanner, B.D., 1990. Automated weather stations. Remote Sensing Rev., 5: 73-98. Warhaft, Z., 1976. Heat and moisture flux in the stratified boundary layer. Q. J. R. Meteorol, Soc.. 102: 703-707. Webb, E.K., Pearman, G.I. and Leuning, R., 1980. Correction of flux measurements for density effects due to heat and water vapour transfer. Q. J. R. Meteorol. Soc., 106: 85-100.
81
A simple system for automated long-term Bowen ratio measurement Pierre Cellier *'a, Albert O l i o s o b aInstitut National de la Recherche Agronomique Station de Bioclimatologie, F78850 Thiverval-Grignon, France bInstitut National de la Recherche Agronomique Station de Bioclimatologie, BP 91, F84143 Montfavet, France (Received 10 August 1992; revision accepted 24 March 1993)
Abstract The Bowen ratio method is widely used to estimate fluxes of sensible heat and evaporation between a crop and the atmosphere. The most critical points of this method are the measurements of vertical humidity gradients, and long-term operation. We propose a simple system, using a capacitive hygrometer and alternate sampling of air at two levels with pumps to measure humidity gradients. It is well adapted to long-term measurements: low power consumption, low charge of maintenance, and low cost. It has been tested by comparison of hourly sensible and latent heat flux measurements performed by both eddy correlation and Bowen ratio, over a bare soil field and a fully evaporating canopy. Results are satisfactory over a large range of fluxes (0-400 W m -2) under daytime conditions. The main sources and magnitude of errors were investigated under the contrasted situations of bare soil and fully evaporating canopy. The system was shown to give good flux estimates even with very low humidity gradients.
Introduction Bowen ratio determination (Bowen, 1926) coupled with net radiation and ground heat flux measurements is often used, for research purposes and agricultural applications: estimation of sensible heat flux or evaporation from a crop (Black and McNaughton, 1971; Perrier et al., 1975; Ashktorab et al., 1989; Cellier and Brunet, 1992) or estimation of hourly averages of crop photosynthesis (Held et al., 1990), for example. The Bowen ratio method is widely used for different reasons: (1) it uses robust sensors allowing continuous measurements (Bingham et al., 1987; Gay, 1988; Fritschen and Simpson, 1989); (2) it requires only two measurement levels when classical aerodynamic method needs more; (3) its site requirements are less restrictive than most other micrometeorological methods: it can be * Corresponding author.
82
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
used with relatively low fetch-to-height ratios (Heilman et al., 1989) over moderate slopes (Fritschen and Qian, 1990) and even close to the canopy top in the roughness sublayer (Cellier and Brunet, 1992). Conditions of strong advection are the main limitation of this method, but their effects on the applicability of the method are controversial: while Motha et al. (1979) and Warhaft (1976) estimated that turbulent transfer of sensible heat were more enhanced that latent heat transfer, Lang et al. (1983) gave opposite conclusions. Apart this case, the above mentioned advantages contributed to a large use of this method and to the development of different automated systems for flux measurements (see e.g. Black and McNaughton, 1971; Perrier et al., 1975; Bingham et al., 1987; Gay, 1988; Tanner, 1988; Fritschen and Simpson, 1989). One critical problem deals with the ability to make long term measurements. This requires reliability in the of sensors and other parts of the system, a low energy consumption and a low charge of maintenance. We propose here an apparatus which meets these conditions, using a capacitive hygrometer to measure humidity gradients. Problem
Fluxes and humidity gradients calculations
The convective fluxes of sensible and latent heat from a natural surface (soil and/or vegetation) to the atmosphere can be estimated using the energy balance equation Rn - G = H + AE
(1)
and the Bowen ratio /3 = H / A E = (cp/A)(OT/Or)
(2a)
is the net radiation, G the soil heat flux, and H and AE respectively the sensible and latent heat fluxes (all in W m'2); Cp is air specific heat at constant pressure (J kg-1 K - l ) and A latent heat of vaporization of water (J kg-1). T is air potential temperature (K). For humidity gradients, the mixing ratio, r (kg kg- 1), is preferred in order to eliminate the effects of vertical density gradients on the Bowen ratio, which can be important under strong temperature gradients (Webb et al., 1980). For practical application, Eq. (2a) is generally approximated by finite differences Rn
/3 ~ (Cp/A)(AT/Ar)
(2b)
H and AE are then estimated from
H = (R. - G)/3/(1 +/3)
(3a)
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
83
(3b)
AE = (Rn - G)/(1 + [3) This requires the knowledge of the fluxes gradients of air temperature and humidity.
Rn
and G, and of the vertical
Problems involved in humidity measurements
Measuring humidity gradients is delicate because sensor-dependent errors are often of the same order of magnitude or even larger than the gradients themselves (see Table 1). To solve this difficulty, two approaches are currently used to remove the major part of the errors: (1) moving sensors between the different measurement levels in order to avoid too long return times for each level, sensors are generally exchanged in pairs; (2) routing successively air samples from each level to the humidity sensor through tubes. As a consequence, the main problem is no longer absolute precision, but rather resolution of the sensor and absence of short term drift. Presently, three main types of humidity sensors are used for outdoor routine measurement. (1) Psychrometers are widely used because of their good resolution, low cost and simplicity of conception, but they require constant attention (problems of wicks wetness and cleanness) and cannot work easily under temperatures below 0°C. They generally need strong ventilation to reach a stable psychrometric constant, which consumes a large amount of electric power (Table 1). This drawback can be lessened by using small probes, but they cannot in any case be used with air routed through tubes. However it is certainly the most widely used sensor for Bowen ratio determination (Fuchs and Tanner, 1970; Black and McNaughton, 1971; Perrier et al., 1975; Angus and Watts, 1984; Gay, 1988; Fritschen and Simpson, 1989; Heilman et al., 1989; Held et al., 1990; Cellier and Brunet, 1992). (2) Dew point hygrometers are often presented as reference systems. They give a direct estimate of absolute humidity with one single measurement, but few manufactured models are designed for field use and even in that case they Table 1 Characteristics of different humidity sensors (absolute precision is estimated with an air temperature of 20°C and a dew point of 10°C, either in the sensor unit or in mixing ratio) Sensor type
Psychrometer Dew point hygrometer (laboratory) (field use) Capacitive hygrometer
Absolute precision (sensor unit)
(kg kg- t
0.1 K 0.2 K 0.5 K 1% 2% (RH > 90%)
100 100 250 125 250
106)
Power consumption (mA)
100- 500 100- 200 1- 10
84
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81 -92
have a noticeable energy consumption (Table 1). More, because of the permanent presence of liquid water on the mirror, they need regular checking of mirror cleanliness, if not equipped with auto-cleaning system (which increases cost and energy consumption). Several recent Bowen ratio apparatus use such hygrometers (Heimburg, 1982; Bingham et al., 1987; Tanner, 1988; Ashktorab et al., 1989). (3) Capacitive hygrometers have been more recently developed, but they arc now widely used for routine measurement on meteorological stations (Tanner, 1990), because of their simplicity of use and low power consumption (Table 1). They are generally cheaper than other hygrometers. The reduced size of the sensor and its rapid response time to humidity changes give them many applications in ecophysiological research (Day, 1988). They were known to have a significant drift with time, but most recent models only exhibit a drift of one to several percent per year (Skaar et al., 1989; Tanner, 1990) and are generally corrected in temperature; they then became quite reliable sensors. Despite these qualities, capacitive sensors are not usually employed for humidity gradient measurements. To our knowledge, the only experiment where this is mentioned, is that of Lang et al. (1983). We propose here to extend the use of this type of sensors to such applications. Method
Description of the system We used a HMP35A humidity sensor (VAISALA, Helsinki, Finland) whose most interesting characteristics for our concern were its low power consumption (1-3 mA), fast response time (about 1 s), temperature compensation and low cost. The air sample is routed from the two measurement levels to the sensor by two low power consumption (20 mA) pumps (ASF, Puchheim, Germany) providing a flow rate of about 1 1 min -1 through 1 m long Tygon tubes (Fig. 1). Tygon was chosen for its low permeability to water vapour. The internal diameter of tubes was 3.2 mm. The inlet of each tube was placed in a radiation shield (used for air temperature measurement) to prevent liquid water from being aspirated in case of rain. The sensor was placed inside a measurement chamber of about 1 cm 3 (Fig. 1). In order to avoid any condensation of water vapour in the tubes, a thin constantan wire was placed inside them; this heating resistance was switched on when the relative humidity at sensor level exceeded 90%. This generally occurred for short time periods during rainy days, at the end of nights with important dew, or just after sunrise when air humidity increases rapidly owing to evaporation, while the tubes remain cold. The power consumption required to obtain a temperature increase of 2 - 3 K is approximately 200 mA. It is sufficient, in
85
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92 Air p u m p s
Connection
9
panel
IIIIIII i
Valve "/;1~ _eating wire
¢
flow m e t e r
. 7,"-
I!
~-~_-~ .:/
Temperature sensor
¢ ~
Hygrometer
Tygon t u b e
I P u m p s and Heating
control
Fig. 1. Schematicrepresentation of the humidity gradient measurementsystem. most situations, to stabilize relative humidity at 90%, which remains in the range of best performances of capacitive hygrometers (Table 1). The pumps were operated alternately every 150 s, resulting in a complete cycle in 5 min. Their valves prevented air from going through the other nonoperating pump. To allow stabilization, measurements were excluded during the first 45 s following the inversion. Air temperature and humidity in the chamber were measured every 5 s and immediately converted into vapour pressure or mixing ratio. The humidities at both levels were then averaged over 60 min intervals. In some circumstances (low humidity gradients together with a strong increase of humidity over short time intervals: over bare soil just after sunrise for example) the shift in average humidity owing to the time lag between
86
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81 92
successive measurements can be of the same order of magnitude as the humidity gradient itself; it is then necessary to correct the gradient along with this humidity evolution. This could be avoided by using two humidity sensors which would perform simultaneous measurements at both levels. For sake of simplicity, this has not been done in the present study. F r o m Eqs. (2) and (3), it can be seen that flux estimates can be very erroneous when temperature and humidity gradients are small, and when the Bowen ratio is close to - 1 . In order to eliminate the questionable data sets, we used the objective criteria defined by Ohmura (1982), and rejected all the data where -1.25 < ,~ < -0.75 (Tanner, 1988).
Experimental sites This system has been tested under strongly contrasting conditions: over bare soil where evaporation and humidity gradients are very weak, and, in the opposite situation, over a fully evaporating canopy: (1) in Grignon, France (48°51tN, 1°58'E) during May and June 1990, over a bare soil field just after maize sowing; (2) in Montfavet, France (43°55'N, 4°51fE) from July to September 1990, over a soybean crop whose leaf area index was between 2.0 and 4.0. The aim of these experiments was to compare the sensible heat flux estimated by the previously described Bowen ratio system with an independent estimate by eddy correlation using a one-dimensional sonic anemometer (Tanner, 1988). Consequently, the following measurements were performed in both situations: (1) net radiation with a Swissteco (Oberriet, Switzerland) type S 1 net radiometer in Grignon, and with a Crouzet (Valence, France) net radiometer in Montfavet; (2) air temperature at two levels above the crop or soil surface using thermocouples placed into ventilated radiation shields; measurement levels were 0.5 and 1.50 m in Grignon, and two and three times the height of the canopy in Montfavet; (3) air humidity at two levels using the previously described system; air was sampled in the ventilated radiation shields used for air temperature measurements; (4) soil temperatures using thermocouples located at six levels between soil surface and a depth of 50 cm; they were used to estimate the soil heat flux G, by a calorimetric method using soil temperature measurements coupled with gravimetric estimations of soil density and humidity; (5) sensible heat flux using a onedimensional sonic anemometer CA27 (Campbell Scientific, Shepshed, UK) sampled at a frequency of 10 Hz. The sensor was placed at an intermediate level between air temperature measurement levels. For the Bowen ratio method, the measurements and all control operations (pump commutations, heating) were performed by a CR10 datalogger in Grignon and a 21X datalogger in Montfavet (both Campbell Scientific) and
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
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the data were averaged over time intervals of 30 min and 60 min respectively in Grignon and Montfavet. For eddy correlation, temperature and wind speed fluctuations measurements were performed on a 21X datalogger in both locations. Results and discussions Technical performances As can be seen on Figs. 2(a) and 2(b), our Bowen ratio measurement system is able to provide a good estimate of sensible and latent heat fluxes throughout the day. This is true for both contrasted situations of bare soil and fully evaporating vegetation. As could be expected, fluxes estimated by Bowen ratio exhibit a discontinuity near sunset and sunrise, owing to Bowen ratios close to 1. In Grignon (Fig. 2(a)), no eddy correlation measurements were performed during the night because of risks of rain or dew deposition. During daytime hours, both methods are in fair accordance, especially around noon, when fluxes reach their maximum values. The slight observed shifts during the early morning and late morning on Fig. 2(a) cannot be attributed to a unique evident cause such as advective conditions, errors on net radiation owing to a poor horizontality of the sensor, or poor estimate of soil heat flux (these are the hours where it changes very rapidly). It should be noticed that even during the night in Montfavet, in spite of very low humidity gradients, the agreement
v
600
600-
500
500-
400
400-
- 300
30O
200
200 •
100-
100
O"
0
(b)
-100
-100 0
4
8
12 UT
(hours)
16
20
24
0
4
8
12 UT
16
20
24
(hours)
Fig. 2. Example of energy fluxes measured either by eddy correlation (thin line) or Bowen ratio (thick line) at (a) Grignon on 23 May 1991 a n d (b) Montfavet on 2 September 1991: ( ~ ) net radiation; ( ) soil heat flux; ( - - ) sensible heat flux; ( . . . . . . ) latent heat flux.
P. Cellier, A. Otioso / Agricultural and Forest Meteorology 66 (1993) 81 .-92
88
is satisfactory; the occurrence of evaporation throughout the night certainly results from the limited size of the experimental field and from an irrigation performed several days before. These effects are also enhanced by the strong northern winds (called 'Mistral') which prevailed during the experimental period. A significant advantage of this system is its ability to run with minimum maintenance: during the two experiments presented above, and five others performed in 1991 and 1992, the systems remained in the field for periods of several months with no other maintenance than checking the air flow in tubes. The only problems encountered dealt with pump functioning: a poor valve air-tightness owing to a too long working period could drastically diminish the air inflow rate, or make it go through the other pump rather than to the humidity sensor. This type of problem could have been avoided by using vanes preventing air flow from going from one pump to the other.
Compar•on withfluxes estimated by other methods In Fig. 3 hourly values of sensible heat fluxes measured by Bowen ratio, are compared with those estimated by eddy correlation (in Grignon hourly datasets were reconstructed from the average of two successive datasets). The analysis was limited to daytime values, when available energy (Rn - G) was more than 50 W m -2, to avoid errors as a result of too low gradients and available energy. Moreover, so that important uncertainties resulting from 500
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H eddy c o r r e l a t i o n (W/m 2 ) Fig. 3. Comparison between sensible heat fluxes measured by eddy correlation (X-axis) and Bowen ratio (Y-axis) in Grignon (D) and Montfavet (,).
P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81 92
89
advective conditions do not affect this analysis (Motha et al., 1979; Lang et al., 1983), we excluded the days following irrigation over the soybean crop, where the sensible heat flux was strongly negative during day. The agreement is satisfactory over a wide range of fluxes: over bare soil in Grignon the sensible heat fluxes reached up to 400 W m -2. A linear fit calculated on these data (H by Bowen ratio as a function of H by eddy correlation) gives a slope of 1.02 and an offset of 10 W m -2, and corresponding standard deviations of 0.02 and 25, respectively. The average difference between both estimates is 12 W m -2 with a standard deviation of 25 W m -2. This general overestimate can therefore be observed over the whole range of fluxes. It can result from an overestimate of available energy or from the influence of possible advection, which would increase sensible heat transfer rather than latent heat transfer. In the case of Montfavet data, this would meet the conclusions of Motha et al. (1979) obtained from similar conditions to ours (irrigated alfalfa). Some part of this overestimate could also be the consequence of measurement errors, under both situations. Therefore, we analysed the behaviour of the system according to probable sources of measurement errors (Fuchs and Tanner, 1970; Angus and Watts, 1984). The relative error on sensible heat flux can be written as
5g/g
=
(4a)
(SR n q- 5a)l[Rn - al + 53/131/(1 + 131)
with (4b)
5 3 / 3 = 5 A T / [ A T I + 5Ar/lAr]
The notation 5 X refers to the error on operator X. The expected errors on each term or Eq. (4) are listed in Table 2. Measurement errors of 0.05K on temperature gradients and 5 Pa on humidity gradients were assumed. The errors on the gradients were estimated according to the average values of the related quantities for each experimental site: temperature gradients of 1.0 and 0.25K, and humidity gradient of 50 and 300 for Grignon and Montfavet, respectively. A larger error on R n - G is assumed for the bare soil situation, owing to the higher G values. It can be seen that the large error on humidity gradient over bare soil (Grignon), resulting from low humidity gradients, does not strongly affect the Bowen ratio and sensible heat flux estimates. As most of the available energy is dissipated into sensible heat an error on Bowen ratio would only Table 2 Estimated errors on the different terms of Eq. (4)
Grignon Montfavet
6AT
5Ar
5Rn + 5G
6~3
6H
[AT t
[Ar I
]R n - G I
]/3[(1 + ]~3[)
H
0.05 0.20
0.30 0.10
0.10 0.05
0.05 0.20
0.15 0.25
P. Cellier, A. Olioso i Agricultural and Forest Meteorology 66 (1993) 81-92
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P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
91
be significant for the low evaporation flux. As expected, this method is not well adapted to evaporation estimates under very dry situations. On the contrary, over the soybean crop the main uncertainty results from the temperature gradient measurement. The resulting relative errors on Bowen ratio and sensible heat flux are large. Nevertheless, owing to the low values of sensible heat fluxes this does not induce very large absolute errors on this flux. As can be seen in Fig. 4(a), the differences between both methods exhibit no trend with humidity gradient values. Of course, the scatter increases at low humidity gradients (above -300 x 10-6 kg kg-1), but it remains quite acceptable if one considers the very low values of Ar presented here. In the same way, flux estimates are not really affected even under very large Bowen ratios (Fig. 4(b)). Yet, these conditions of large Bowen ratio are generally considered as the limit of applicability of this method. These results can be interpreted as an indication of the reliability of this system until very low humidity gradients. Finally, it could also be remarked on Fig. 4(c) that the differences between both methods do not evidently increase with flux values.
Conclusions
We presented a simple system using a capacitive hygrometer to measure humidity gradients. This system is designed for long-term measurements with such applications as flux estimation by Bowen ratio method. When compared with other existing equivalent systems, it presents three main advantages: (1) its very low power requirement (20-30 mA, including hygrometer and pump power consumptions); (2) the absence of maintenance: capacitive hygrometers do not require any regular supply of water as psychrometers and are not so exposed to dust deposition than some other humidity sensors; (3) its low cost. The good performances of this system show that this type of hygrometer is full of promise for long-term humidity gradient measurements in natural environment. Thus, it allows an easy continuous recording of energy fluxes under any climatic conditions. Up to now they have been operated with success during five long term experiments, in order to estimate energy exchanges of various natural surfaces (wheat, millet, vegetative mulch, bare soil). References Angus, D. and Watts P.J., 1984. Evapotranspiration - How good is the Bowen ratio method? Agric. Water Manage., 8:133 150. Ashktorab, H., Pruitt, W.O., Paw U, K.T. and George, W.V., 1989. Energy balance determinations
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P. Cellier, A. Olioso / Agricultural and Forest Meteorology 66 (1993) 81-92
close to the soil surface using a micro-Bowen ratio system. Agric. For. Meteorol., 46: 259274. Bingham, G.E., Tanner, B.D., Green, J. and Tanner, M., 1987. A Bowen ratio system for long term remote measurement of evapotranspiration. In: 18th Conf. Agric. and For. Meteorol. and 8th Conf. Biometeorol. and Aerobiol. 15-18 September, Am. Meteorol. Soc., Boston, MA, preprint volume, pp. 63-66. Black, T.A. and McNaughton, K.G., 1971. Psychrometric apparatus for Bowen-ratio determination over forests. Boundary-Layer Meteorol., 2: 246-254. Bowen, I.S., 1926. The ratio of heat losses by conduction and by evaporation for any water surface. Physiol. Rev., 27: 779-787. Cellier, P. and Brunet, Y., 1992: Flux-gradient relationships above tall plant canopies. Agric. For. Meteorol., 57:93-117. Day, W., 1988. Water vapour measurement and control. In: B. Marshall and F.I. Woodward (Editors), Instrumentation for Environmental Physiology. Cambridge University Press, Cambridge, pp. 59-78. Fritschen, L.J. and Simpson, J.R., 1989. Surface energy and radiation systems: general description and improvements. J. Appl. Meteorol., 28: 680-689. Fritschen, L.J. and Qian, P., 1990. Net radiation; sensible and latent heat flux densities on slopes computed by the energy balance method. Boundary-Layer Meteorol., 53:163-171. Fuchs, M. and Tanner, C.B., 1970. Error analysis of Bowen ratios measured by differential psych rometry. Agric. For. Meteorol., 7:329 334. Gay, L.W., 1988. A portable Bowen ratio system for ET measurements. In: Proc. Nat. Conf. Irrig., Drain. Am. Soc. Civil Eng., NY, pp. 625-632. Heilman, J.L., Brittin, C.L. and Neale, C.M.U., 1989. Fetch requirements for Bowen ratio measurements of latent and sensible heat fluxes. Agric. For. Meteorol., 44: 261-273. Heimburg, K.F., 1982. Evapotranspiration: an automated measurement system and a remote-sensing method for regional estimates. Thesis University of Florida, 211 pp. Held, A., Steduto, P., Orgaz, F., Matista, A. and Hsiao, T.C., 1990. Bowen ratio-energy balance technique for estimating crop net CO2 assimilation and comparison with a canopy chamber. Theor. Appl. Climatol., 42: 203-214. Lang, A.R.G., McNaughton, K.G., Chen, F., Bradley, E.F. and Ohtaki, E., 1983. An experimental appraisal of the terms in the heat and moisture flux equations for local advection. BoundaryLayer Meteorol., 25: 89--102. Motha, R.P., Verma, S.B. and Rosenberg, N.J., 1979. Exchange coefficients under sensible heat advection determined by eddy correlation. Agric. Meteorol., 20: 273-280. Ohmura, A., 1982. Objective criteria for rejecting data for Bowen ratio flux calculations. J. Appl. Meteorol., 21: 595-598. Perrier, A., Itier, B., Bertolini, J.-M. and Blanco de Pablos, A., 1975. Mesure automatique du bilan d'6nergie d'une culture. Exemples d'application. Ann. Agron., 26: 19-40. Skaar, J., Hegg, K., Moe, T. and Smedrud, K., 1989. WMO International Hygrometer Intercomparison. World Meteorological Organization No. 480, Geneva, Switzerland. Tanner, B.D., 1988. Use requirement for Bowen ratio and eddy correlation determination of evapotranspiration. In: DeLynn R. Hay (Editor), Planning Now for Irrigation and Drainage in the 21st Century. Irrig. and Drain. Div., Am. Soc. Civil Eng., NY, pp. 605-616. Tanner, B.D., 1990. Automated weather stations. Remote Sensing Rev., 5: 73-98. Warhaft, Z., 1976. Heat and moisture flux in the stratified boundary layer. Q. J. R. Meteorol, Soc.. 102: 703-707. Webb, E.K., Pearman, G.I. and Leuning, R., 1980. Correction of flux measurements for density effects due to heat and water vapour transfer. Q. J. R. Meteorol. Soc., 106: 85-100.