- Email: [email protected]
Applicability of the eddy correlation method to measure sensible heat transfer to forest under rainfall conditions
AGRICULTURAL AND FOREST METEOROLOGY Agricultural and Forest Meteorology 86 (1997) 193-203
Applicability of the eddy correlation method to meas,ure sensible heat transfer to forest under rainfall conditions K. Mizutani
a,*, K. Yamanoi a, T. Ikeda b, T. Watanabe a
a Forest Environment Division, Forestry and Forest Products Research Instimte, Tsukuba, Ibaraki 305, Japan, b Kansai Research Center, Forestry and Forest Products Research Institute, Kyoto 612, Japan Received 12 December 1995; accepted 20 January 1997
Abstract Evaporation of intercepted rainfall is maintained by sensible heat supplied from the atmosphere. However, no direct measurements have been reported for sensible heat during rainfall. Therefore, we first examined the accuracy of a superspnic anemometer thermometer during rainfall, and found that the sensible heat flux could be measured satisfactorily even under rainfall of low intensity (I 3 mm h-l). Second, we measured the sensible heat flux above a canopy during and af’ter rainfall with a supersonic anemometer thermometer, and estimated evaporation by the eddy correlation/energy balance method. These measurements co8nfirmed a downward flux of sensible heat used in evaporation of intercepted rainfall. Continuous evaporation even during rainfall and nighttime was estimated. Evaporation during rainfall was found to be larger than that after rainfall. 0 1997 Elkevier Science B.V. Keywords: Eddy correlation; Sensible heat; Rainfall; Evaporation; Energy balance
1.Introduction Evaporation of intercepted rainfall is known to be large in forests even if net radiation and vapor pressure deficit are small, because the aerodynamic resistance of a forest stand is low and the energy source for evaporation is a downward sensible heat flux from the atmosphere. Under the same level of radiation, the average rate of evaporation of intercepted rainfall has been found to be three times the average rate of transpiration from a dry canopy (Stewart, 1977). There are many reports in which the
* Corresponding author. E-mail: [email protected]
evaporation of intercepted rainfall increases in proportion to the amount of rainfall. Accordingly, it is likely that a large amount of intercepted rainfall evaporates during rainfall. Calder and Wright (1986) estimated canopy water storage by measuring the attenuation of a horizontal beam of gamma ray through the canopy. Their data indicated that evaporation of intercepted rainfall was higher during rainfall than after rainfall. Tsukamoto et al. (1988) made similar findings, including higher evaporation during higher rainfall intensities, by measuring the weight of a whole tree. In this study, we directly measured the sensible heat flux over a forest during rainfall using a supersonic anemometer thermometer, and estimated evaporation as a residual of energy budget.
0168-1923/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIZ SO168-1923(97)00012-9
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K. Mizutani et al. /Agricultural
and Forest Meteorology 86 (1997) 193-203
Wind tunnel experiments were used to examine interference to anemometer performance during rain.
2. Laboratory tfsts 2.1. Method Noise might occur during and after rainfall because the probe of the supersonic anemometer thermometer gets wet in rain or a raindrop crosses the path of the supersonic wave. To investigate the accuracy of the measurement during and after rainfall, we examined the effects of water sprinkling and wetness of the probe on measurements of three components of wind speed (u, u, w) and air temperature (T) using a wind tunnel. 1. First, we used the supersonic anemometer thermometer (Kaijyo Denki, DAT-3OO/TR-61C) that is used at the experimental site in Section 3, and put water on the probe head of the w-axis by a spout and measured u, v, w and T in the wind
‘^‘E IV
“3 b
tunnel. The surface of the sensor window is circular and flat. The dimensions of the wind tunnel are 1.2 m (W) X 1.6 m (H) X 10.0 m (L). In this study, the mean wind speed in the tunnel was 6 m SK’, and the turbulence intensity was less than 1%. The depths of water on the head were 0.0, 2.5, 1.5, 1.0 and 0.5 mm. 2. Two kinds of supersonic anemometer thermometers (Kaijyo Denki, DAT-3OO/TR-61C; Kaijyo Denki, DA-600/m-61A) which have the same path length but different sensor arrays, were mounted in a larger wind tunnel with dimensions of 4.0 m (W) X 3.0 m (H) X 20.0 m (L). We sprinkled water over one of the anemometers to compare the data from dry and wet anemometers. The intensities of sprinkled water were 0, 2, 3, 6, 9, 24, 33, 36 and 60 mm h- ‘. The mean wind speed in the large tunnel was 5 m s-’ and the turbulence intensity was less than 0.5%. Each measurement lasted for 9 min. The power spectra and cospectra were computed using the Fast Fourier Transform (FFT) technique without detrending.
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n u-w Fig. 1. Normalized power spectra of fluctuations measured by the two anemometers ( sprinkling). The sprinkling ,intensity is 3 mm h-l. (A) u, (B) U, (C) w, (D) T.
n 0-m under the sprinkling, --- outside the
K. Mizutani et al. /Agricultuml
and Forest Meteorology 86 (1997) 193-203
2.2. Results 2.2.1. Effects of covering the probe head with water First, the standard deviation and power spectra of wind speed remained unchanged as a result of covering the probe head with water. Since the speed of sound is about 340 m s-l in air and about 1430 m s-r in water, a water cover of 1 mm thick on the bottom probe causes only 0.5% error in arrival time of sonic waves for the 200~mm span between probes. Measurements of wind speed were not greatly affected by water cover on the probe. Second, no changes were observed in the standard deviation and power spectra of air temperature, whereas apparent air temperature rose about 2°C mm-’ of water depth. This was caused by the above-mentioned overestimation in sonic velocity because the absolute value of apparent temperature is very sensitive to the estimation of sonic velocity, but the increase of 2°C causes an error of less than 0.7% compared with the absolute temperature (292 K). Since the sensible heat flux is calculated from instan-
195
taneous deviation from the mean air temperature, the water covering is responsible only for an error of less than 0.7% in the estimation of sensible heat flux. Hence, we conclude that there was little effect of covering the probe head with water on measurements of sensible heat flux. 2.2.2. Effects of sprinkling water We examined the effects of water sprinkling on measured wind speed and air temperature. First, T was calculated using the sonic velocity of the u-axis, since the water scarcely adhered to the head of the horizontal probes. When water sprinkling intensity was 3 mm h- ’, the level of the normalized power spectra of u, v, w and T were almost the same as those given by the dry instrument (Fig. 1). When the sprinkling intensity was more than 6 mm h- ‘, however, the difference between wet and dry measurements was serious for the low frequency range (Fig. 2), and it increased with water sprinkling intensity. The same tendency can be seen in Fig. 3 for standard deviation of w and T, exhibiting a consid-
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Fig. 2. Same as Fig. 1, except for higher sprinkling intensity (36 mm h-’ 1.
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196
K. Mizutani et al. /Agricultural
and Forest Meteorology 86 (1997) 193-203
sonic anemometer thermometer even in rainfall of low intensity. In addition, if T was calculated using the sonic velocity of the w-axis, T was hampered by noise caused by fluctuation in the depth of water cover and strike by water drops on the head (Fig. 4). Therefore, the use of the u-axis to calculate T is more advantageous for rainfall conditions.
3. Application to an actual forest
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Effects
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erable increase when the sprinkling intensity is higher than 3 mm h- ’. The sudden increase in a. observed for the sprinkling intensity of 36 mm h-’ was caused by water drops falling from the top probe. For lower intensities ( I 3 mm h- ’), however, standard deviations were relatively invariant. Judging from these measurements (Figs. l-31, the sensible heat flux can be measured satisfactorily with a super-
Fluctuation
in
the
depth
of
water
9 ;: Strike
on
the
head
0
60
30 Time
(set)
Fig. 4. Commonly observed noise in T, when T is calculated using tbe sonic velocity of the w-axis. The sprinkling intensity is 4Ommh-‘.
3.1. Site description and measurement The experimental site is an evergreen broad-leaved forest (Castanopsis cuspidata Schottky) located in the Mt Tatsuta experimental forest in Kumamoto, Japan (33”50’N, 130’45’E). The forest covers 125 ha and surrounds the instrument tower to a radius of 300 m. The dip of the study site is about 18” to the southwest. The mean height of predominant trees is 14.3 m, maximum tree height is 17.8 m and the density of predominant trees is 710 trees ha-‘. The subordinate trees are Castanopsis cuspia’ata Schottky, Mcrotropis japonica H. Hallier, Cleyera ochnacea DC., Gardenia jasminoides Ellis, Ilex oldhami Miq., Quercus glauca Thunb., Rhus silvestris Sieb. et Zucc and Gilibertia @da Makino. The mean height of subordinate trees is 2.3 m with a density of 9440 trees ha- ‘. A meteorological observation tower 21.5 m high was used. To measure sensible heat flux by the eddy correlation method, U, v, w and T were measured at 20 m every 0.1 s with a supersonic anemometer thermometer (Kaijyo Denki Inc., Tokyo, DAT300/TR-61C). Each measurement lasted for 9 min. The data which were not filtered, were recorded using a personal computer (NEC Inc., Tokyo, PC98OlIJV). Net radiation at 20.5 m, dry and wet bulb temperatures (resolution of O.l”C), wind speed at 18.5 m and 21.5 m and soil heat flux at a depth of 2 cm were measured every 10 min with a net radiometer (Eiko Seiki Inc., Tokyo, CN-111, ventilated psychrometers (Eiko Se&i, MH-020T), anemometers (Makino Oyosokki Inc., Tokyo) and a heat flow plate (Eiko Se&i, CN-811, respectively. A tipping bucket located about 380 m from the observation
K. Miwtani et al. /Agricultural and Forest Meteorology 86 (1997) 193-203
197
tower measured amount of rainfall with OS-mm accuracy. There was also a 4-m X l-m metal throughfall trough. Stemflow gauges were installed at breast height on three trees in the throughfall plots. In this paper we analyze data from 7 April 1991 and 23 April 1991.
specified. During and after rainfall, however, there will be unavoidable errors in measurements of net radiation due to wet net-radiometer dome. The estimated evaporation, therefore, may have some errors but will nonetheless demonstrate the occurrence of evaporation during rainfall.
3.2. Method of analysis
3.2.2.1. The eddy correlation / energy balance method. Using Eqs. (1) and (21, the water vapor flux was calculated by the following equation:
3.2.1. Calculation of eddy correlation The sensible heat flux was calculated using the same method as McMillen (1988). To minimize possible errors caused by turbulent wind flow over the inclined experimental site, the coordinates were rotated such that W was equal to zero. When the inclination of the z-axis was detected as 8, the sensible heat flux in the vertical direction was then calculated by the following equation: H, = pCpm.
sin t9
3.2.2. Calculation of evaporation rate Evaporation rate was estimated by three different methods-the eddy correlation/energy balance method, the Bowen ratio/energy balance method and the Penman-Monteith equation. Hattori (1985) estimated that the ratio of advection and heat storage in gaseous phase of forest and tree to the net radiation is 8.6% and below 5%, respectively. The ratio of photosynthetic energy to solar radiation is commonly below 1%. The common basic equation of these three methods is the energy balance equation, i.e. (2)
where R, is the net radiation, A is the latent heat of vaporization, E is the water vapor flux, H is the sensible heat flux and G is the soil heat flux. In the following methods, R, and G must be
(3)
3.2.2.2. The Bowen ratio/energy balance method. Using the Bowen ratio ( /?I, the net available energy (R, - G) was divided into the latent and sensible heat fluxes as
(4)
(1)
where H, is the sensible heat flux by the eddy correlation method, p is the density of air, Cr is the specific heat of air at constant pressure, w’ is instantaneous wind speed in the direction of z-axis, T’ is the instantaneous deviation from the mean air temperature and the over bar denotes the mean value. One calculation was made for every 9-min sampling period.
R,=hE+H+G
hE,, = R, - H, - G
and R, - G Hb=P--l+P
(5)
respectively. The Bowen ratio was calculated from AT
(6)
P=Yz
where AT and he are differences in air temperature and vapor pressure, respectively, measured at two heights above the canopy, and y is the psychromettic constant. 3.2.2.3. Penman-Monteith equation. Evaporation was also calculated from the Penman-Monte&h equation (Monteith, 1965): hEP =
A(R, - G) +.&(e: - 4/r, A + Y( 1+ rc/rJ
(7)
where A is the slope of the saturation water vapor pressure curve at air temperature, ei is the saturated vapor pressure at air temperature, e, is vapor pressure of the air, r, is aerodynamic resistance and r, is canopy resistance. Since the value of r, depends on canopy wetness, an interception tank model (Suzuki, 1980) was adopted to determine canopy wetness. The intercepted model consists of two tanks of water
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and Forest Meteorology 86 (1997) 193-203
O.* v
I
c$O.Ol t
Fig. 5. The power spectra of w (A) and T (B), and the cospectra of w’T (C) before rainfall (23 April ll:OO-11:09x
Fig. 6. The power spectra of w (A) and T (B), and the cospectra of W’T’ (C) when rainfall intensity was 0.5 mm h-’ (23 April 14:30-14:39).
storage on foliage and on stems. We determined storage capacity of each tank from the measurements of rainfall, throughfall and stemflow, as 0.784 mm for foliage and 0.987 mm for stems. When rainfall
occurred, water is first stored in the foliage tank. If the foliage tank becomes full, then water overflows to the stem tank. During the period when tanks store
Fig. 7. Time courses of sensible heat flux and evaporation of intercepted rainfall (23 April 1991). (A) Vapor pressure deficit at 18.5 m. (B) Mean wind speed at 20 m. (C) Air temperaturqat 18.5 m. (D) The difference in air temperatare between hvo levels (21.5 rn, 18.5 m) above the canopy (T,s,S - ‘f,,,,). (E) Time courses of the evaporation estimated by the eddy correlation/energy balance method (A, E,), the Penman-Monteith equrrtion (m, I?,,)and the Bowen ratio/energy balance method (0, IT,,).(F) Time courses of the net radiation (0, R,), the sensible heat flux measm’ed by the eddy cormlation method (a, I&) and evaporation estimated by the eddy correlation/energy balance method (A, E, 1. (G) Rainfall intensity.
K. Mizutani et al. /Agriculhual
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and Forest Meteorology 86 (1997) 193-203
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and Forest Meteorology 86 (1997) 193-203
any water, rC is assumed to be zero even when the canopy is not completely wet. The stored water diminishes according to evaporation intensity. When both tanks are empty, the canopy is assumed to be completely dry and transpiration resumes. Aerodynamic resistance (r,) can be calculated by the following equation: 1 r=-lnz-dZ a
k2U( z) !
20 1
where z is the height of the anemometer, d is zero plane displacement, z. is roughness length, k is von Karman’s constant and U(z) is mean wind speed at height z. Parameters z. (1.42 m) and d (14.3 m) were determined from wind profile observation above the canopy using the following equations: ln(z-d)
=aU(z)
+b
(9)
z. = exp( b)
(10)
where a and b are constants. The value of d producing the highest correlation coefficient in Eq. (9) was taken as the correct value, and z. was determined from Eq. (10). 3.3. Time courses evaporation
of the sensible
heat jlux
and
First, we present the results of 23 April 1991 when there was 2 mm of rainfall. According to the theory of turbulent wind flow, the power spectra of both wind speed and air temperature show a -2/3 slope in the inertial subrange (Kaimal et al., 1972). For a period before the rainfall, the power spectra of w and T showed a -2/3 slope in the inertial subrange (Fig. 5(A) and (B)). The cospectra of w’T’ indicates upward sensible heat flux occurred (Fig. 5(C)). When rainfall intensity was 0.5 mm h- ‘, the power spectra of T deviated from a slope of -2/3 in the high frequency range over 3 Hz (Fig. 6(B)). The data in the high frequency range, however, have little effect on the sensible heat flux, because the cospectra in the same range is close to zero (Fig. 6(C)). The cospectra indicate that a downward sensible heat flux occurred. The time course of the sensible heat flux estimated by the eddy correlation method is shown in
Fig. 7(F). When it began to rain, the sensible heat flux was about - 200 W m-*, and remained negative during rainfall. After the rainfall, it increased to about - 100 W m-‘. Fig. 7(D) shows the difference in air temperature between two levels above the canopy. During and after rainfall, air temperature measured at the upper level was generally higher than that of the lower level. When evaporation of intercepted rainfall estimated by the eddy correlation/energy balance method peaked at 13: 10 and 19: 10 (Fig. 7(F)), air temperature above the canopy decreased (Fig. 7(C)). Energy consumption by the evaporation of intercepted rainfall made the temperature of the canopy lower than air temperature, which induced a downward flux of sensible heat. Evaporation estimated by the eddy correlation/energy balance method was larger during rainfall than aftcrwards. This agrees with the results of Tsukamoto et al. (1988) and Calder and Wright (1986). Fig. 7(E) shows time courses of evaporation estimated by the three methods. Since the gradient of air temperature above the canopy was very small as was accordingly the Bowen ratio, the estimated evaporation of intercepted rainfall was almost the same as the net radiation. Thus, the calculated latent heat flux by the Bowen ratio method was negative for nighttime. Evaporation estimated by the PenmanMonteith equation was larger than that estimated by the other methods when the mean wind speed (Fig. 7(B)) or the vapor pressure deficit (Fig. 7(A)) was large. In this case, the Penman-Monteith equation seemed to be too sensitive to the wind speed and the vapor pressure deficit. After the storage tanks had dried out at 19:40, the evaporation rate estimated by the Penman-Monteith method decreased to zero, while the eddy correlation/energy balance method showed continuous evaporation even during nighttime. Next, we show the results of 7 April 1991, when rainfall intensity was greater than the previous case. When rainfall intensity was higher than about 4 mm h-‘, the data were variable because rainwater dropped from the top probe of the anemometer. Thus, noise-free data for this day were only analyzed, and the sensible heat flux was estimated only for the period before 12:30. The average rainfall intensity between 0:OOand 12:00 was 1.625 mm h-’ (Fig. 8(G)). In this period, the vapor pressure deficit
K. Mizutani et al. /Agricultural
and Forest Meteorology 86 (1997) 193-203
Fig. S.-Same as Fig. 7, except for 7 April 1991.
201
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K. Mizutani et al. /Agricultural
and Forest Meteorology 86 (1997) 193-203
was not equal to zero (Fig. 8(A)). The downward flux of sensible heat measured by the eddy correlation method between 1:00 and 7:30 was about 100 W rnw2, after which it increased to about 200 W rnp2 (Fig. 8(F)). Th e time course of the downward flux of sensible heat corresponds well to those of the mean wind speed (Fig. 8(B)), indicating that evaporation from wet forest canopies is largely dependent on wind speed and vapor pressure deficit. Evaporation of intercepted rainfall estimated by the eddy correlation/energy balance method began to increase at 8:00 and approached 400 W me2 at 1O:lO. During the period between 8:00 and 12:00, the mean rainfall intensity was 1.75 mm h- ’ and the mean evaporation intensity was 254 W rne2. The data of Calder and Wright (1986) showed that when the mean rainfall intensity was about 2.4 mm h- ‘, the mean evaporation intensity was about 280 W rnp2 for 2 h. Our observations indicated that evaporation of intercepted rainfall under high rainfall intensity was higher than that under low rainfall intensity. Evaporation estimated by the Penman-Monteith equation was approximately the same as that by the eddy correlation/energy balance method, but the Bowen ratio method gave smaller values than the other methods. It is really difficult to estimate the evaporation during and after rainfall using the Bowen ratio/energy balance method, because it requires quite accurate measurements of the temperature and humidity gradients above the canopy which are often very small under the conditions discussed here. The Penman-Monteith equation needs values of ra and rc, which are also difficult to quantify accurately. An error of r, directly results in an incorrect estimation of evaporation from a fully wet canopy. The assumption that r, is zero in spite of a partially wet canopy is not suitable for a rainfall of low intensity, and leads to an overestimation of evaporation when the wind speed and the vapor pressure deficit are large. More studies should be done on modeling the evaporation from a partially wet canopy. The eddy correlation/energy balance method will provide useful dam. 4. Conclusions We examined the effects of water sprinkling and wetness of the probe of a supersonic anemometer
thermometer using a wind tunnel to investigate the accuracy during and after rainfall. This investigation showed that the sensible heat flux could be measured satisfactorily with a supersonic anemometer thermometer even under rainfall of low intensity ( < 3 mm h- ’>. During rainfall intensity of more than 4 mm h- ‘, however, the data contained much noise because rainwater continuously dropped from the probe head of the anemometer. The sensible heat flux over a forest was measured with the supersonic anemometer thermometer during and after rainfall, and the evaporation of intercepted rainfall was estimated by the eddy correlation/energy balance method. It was observed during rainfall that a large amount of sensible heat was transferred to the forest, supporting the occurrence of evaporation. The amount of the downward sensible heat flux (or evaporation as a result) was larger during rainfall than after rainfall.
Acknowledgements
We would like to thank Dr M. Tani of the Forestry and Forest Products Research Institute and Dr M. Suzuki of Tokyo University for their helpful suggestions. We also thank Dr R.C. Sidle of Netherlands Institute for Sea Research for reviewing the manuscript. The wind tunnel experiment was carried out with the assistance of MS R. Okushima of the National Research Institute of Agricultural Engineering.
References Calder, LR., Wright, I.R., 1986. Gamma-ray attenuation studies of interception from Sitka spruce - some evidence for an additional transport mechanism. Water Resour. Res. 22.409-417. Hattori, S., 1985. Explanation on derivation process of equation to estimate evapotranspiration and problem on the application. Bull. For. For. products Res. Inst. 332, 139-165 (in Japanese). Raimal, J.C., Wyngaard, J.C., Ixumi, Y., Cote, O.R., 1972. Spectral characteristics of surface-layer turbulence. Q.J.R. Meteorol. Sot. 98, 563-589. McMillen, R.T., 1988. An eddy correlation technique with extend applicability to non-simple terrain. Boundary-Layer Meteorol. 43, 231-245. Monteith, JL., 1965. Evaporation and environment. Symp. See. Exp. Biol. 19, 205-234.
K. Mizutani et al. /Agricultural
and Forest Meteorology 86 (1997) 193-203
Stewart, J.B., 1977. Evaporation from the wet canopy of a pine forest. Water Resour. Res. 13, 915-921. Suzuki, M., 1980. Evapotranspiration from a small catchment in hilly mountain (1). Seasonal variations in evapotranspiration, rainfall intercepted and transpiration. J. Jpn. For. Sot. 62, 46-53.
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Tsukamoto, Y., Tange, I., Minemura, T., 1988. Interception loss from forest canopies. Rolling Land Res. 6,60-82 (in Japanese, with English abstract).
Applicability of the eddy correlation method to meas,ure sensible heat transfer to forest under rainfall conditions K. Mizutani
a,*, K. Yamanoi a, T. Ikeda b, T. Watanabe a
a Forest Environment Division, Forestry and Forest Products Research Instimte, Tsukuba, Ibaraki 305, Japan, b Kansai Research Center, Forestry and Forest Products Research Institute, Kyoto 612, Japan Received 12 December 1995; accepted 20 January 1997
Abstract Evaporation of intercepted rainfall is maintained by sensible heat supplied from the atmosphere. However, no direct measurements have been reported for sensible heat during rainfall. Therefore, we first examined the accuracy of a superspnic anemometer thermometer during rainfall, and found that the sensible heat flux could be measured satisfactorily even under rainfall of low intensity (I 3 mm h-l). Second, we measured the sensible heat flux above a canopy during and af’ter rainfall with a supersonic anemometer thermometer, and estimated evaporation by the eddy correlation/energy balance method. These measurements co8nfirmed a downward flux of sensible heat used in evaporation of intercepted rainfall. Continuous evaporation even during rainfall and nighttime was estimated. Evaporation during rainfall was found to be larger than that after rainfall. 0 1997 Elkevier Science B.V. Keywords: Eddy correlation; Sensible heat; Rainfall; Evaporation; Energy balance
1.Introduction Evaporation of intercepted rainfall is known to be large in forests even if net radiation and vapor pressure deficit are small, because the aerodynamic resistance of a forest stand is low and the energy source for evaporation is a downward sensible heat flux from the atmosphere. Under the same level of radiation, the average rate of evaporation of intercepted rainfall has been found to be three times the average rate of transpiration from a dry canopy (Stewart, 1977). There are many reports in which the
* Corresponding author. E-mail: [email protected]
evaporation of intercepted rainfall increases in proportion to the amount of rainfall. Accordingly, it is likely that a large amount of intercepted rainfall evaporates during rainfall. Calder and Wright (1986) estimated canopy water storage by measuring the attenuation of a horizontal beam of gamma ray through the canopy. Their data indicated that evaporation of intercepted rainfall was higher during rainfall than after rainfall. Tsukamoto et al. (1988) made similar findings, including higher evaporation during higher rainfall intensities, by measuring the weight of a whole tree. In this study, we directly measured the sensible heat flux over a forest during rainfall using a supersonic anemometer thermometer, and estimated evaporation as a residual of energy budget.
0168-1923/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PIZ SO168-1923(97)00012-9
194
K. Mizutani et al. /Agricultural
and Forest Meteorology 86 (1997) 193-203
Wind tunnel experiments were used to examine interference to anemometer performance during rain.
2. Laboratory tfsts 2.1. Method Noise might occur during and after rainfall because the probe of the supersonic anemometer thermometer gets wet in rain or a raindrop crosses the path of the supersonic wave. To investigate the accuracy of the measurement during and after rainfall, we examined the effects of water sprinkling and wetness of the probe on measurements of three components of wind speed (u, u, w) and air temperature (T) using a wind tunnel. 1. First, we used the supersonic anemometer thermometer (Kaijyo Denki, DAT-3OO/TR-61C) that is used at the experimental site in Section 3, and put water on the probe head of the w-axis by a spout and measured u, v, w and T in the wind
‘^‘E IV
“3 b
tunnel. The surface of the sensor window is circular and flat. The dimensions of the wind tunnel are 1.2 m (W) X 1.6 m (H) X 10.0 m (L). In this study, the mean wind speed in the tunnel was 6 m SK’, and the turbulence intensity was less than 1%. The depths of water on the head were 0.0, 2.5, 1.5, 1.0 and 0.5 mm. 2. Two kinds of supersonic anemometer thermometers (Kaijyo Denki, DAT-3OO/TR-61C; Kaijyo Denki, DA-600/m-61A) which have the same path length but different sensor arrays, were mounted in a larger wind tunnel with dimensions of 4.0 m (W) X 3.0 m (H) X 20.0 m (L). We sprinkled water over one of the anemometers to compare the data from dry and wet anemometers. The intensities of sprinkled water were 0, 2, 3, 6, 9, 24, 33, 36 and 60 mm h- ‘. The mean wind speed in the large tunnel was 5 m s-’ and the turbulence intensity was less than 0.5%. Each measurement lasted for 9 min. The power spectra and cospectra were computed using the Fast Fourier Transform (FFT) technique without detrending.
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1o-2 1o-3 : 1 O-44 1 o-3
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1 O-4_ 1 o-3
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10_4t’
n u-w Fig. 1. Normalized power spectra of fluctuations measured by the two anemometers ( sprinkling). The sprinkling ,intensity is 3 mm h-l. (A) u, (B) U, (C) w, (D) T.
n 0-m under the sprinkling, --- outside the
K. Mizutani et al. /Agricultuml
and Forest Meteorology 86 (1997) 193-203
2.2. Results 2.2.1. Effects of covering the probe head with water First, the standard deviation and power spectra of wind speed remained unchanged as a result of covering the probe head with water. Since the speed of sound is about 340 m s-l in air and about 1430 m s-r in water, a water cover of 1 mm thick on the bottom probe causes only 0.5% error in arrival time of sonic waves for the 200~mm span between probes. Measurements of wind speed were not greatly affected by water cover on the probe. Second, no changes were observed in the standard deviation and power spectra of air temperature, whereas apparent air temperature rose about 2°C mm-’ of water depth. This was caused by the above-mentioned overestimation in sonic velocity because the absolute value of apparent temperature is very sensitive to the estimation of sonic velocity, but the increase of 2°C causes an error of less than 0.7% compared with the absolute temperature (292 K). Since the sensible heat flux is calculated from instan-
195
taneous deviation from the mean air temperature, the water covering is responsible only for an error of less than 0.7% in the estimation of sensible heat flux. Hence, we conclude that there was little effect of covering the probe head with water on measurements of sensible heat flux. 2.2.2. Effects of sprinkling water We examined the effects of water sprinkling on measured wind speed and air temperature. First, T was calculated using the sonic velocity of the u-axis, since the water scarcely adhered to the head of the horizontal probes. When water sprinkling intensity was 3 mm h- ’, the level of the normalized power spectra of u, v, w and T were almost the same as those given by the dry instrument (Fig. 1). When the sprinkling intensity was more than 6 mm h- ‘, however, the difference between wet and dry measurements was serious for the low frequency range (Fig. 2), and it increased with water sprinkling intensity. The same tendency can be seen in Fig. 3 for standard deviation of w and T, exhibiting a consid-
, rJ4L1
1 o-3
1 o-2
1 o-1
100
10’
n w4
‘O’ 2
loo
21
o-1
s
(C)
_,-./
10-z
‘, I.
$
IV
_.d
, o_3
C’
___/=
~;:si
l
/.---
/
O;40bm
10’ n 0-w
Ii;o& 1 o-2
1 o-1 n u-4
Fig. 2. Same as Fig. 1, except for higher sprinkling intensity (36 mm h-’ 1.
100
l(
196
K. Mizutani et al. /Agricultural
and Forest Meteorology 86 (1997) 193-203
sonic anemometer thermometer even in rainfall of low intensity. In addition, if T was calculated using the sonic velocity of the w-axis, T was hampered by noise caused by fluctuation in the depth of water cover and strike by water drops on the head (Fig. 4). Therefore, the use of the u-axis to calculate T is more advantageous for rainfall conditions.
3. Application to an actual forest
oi
’
’
1
1
’
b
t
I
I
I
0
2
3
6
9
24
33
36
40
60
’
P(mm/hr) Fig. 3.
Effects
of water
sprinkling
intensity
on the standard
deviation.
erable increase when the sprinkling intensity is higher than 3 mm h- ’. The sudden increase in a. observed for the sprinkling intensity of 36 mm h-’ was caused by water drops falling from the top probe. For lower intensities ( I 3 mm h- ’), however, standard deviations were relatively invariant. Judging from these measurements (Figs. l-31, the sensible heat flux can be measured satisfactorily with a super-
Fluctuation
in
the
depth
of
water
9 ;: Strike
on
the
head
0
60
30 Time
(set)
Fig. 4. Commonly observed noise in T, when T is calculated using tbe sonic velocity of the w-axis. The sprinkling intensity is 4Ommh-‘.
3.1. Site description and measurement The experimental site is an evergreen broad-leaved forest (Castanopsis cuspidata Schottky) located in the Mt Tatsuta experimental forest in Kumamoto, Japan (33”50’N, 130’45’E). The forest covers 125 ha and surrounds the instrument tower to a radius of 300 m. The dip of the study site is about 18” to the southwest. The mean height of predominant trees is 14.3 m, maximum tree height is 17.8 m and the density of predominant trees is 710 trees ha-‘. The subordinate trees are Castanopsis cuspia’ata Schottky, Mcrotropis japonica H. Hallier, Cleyera ochnacea DC., Gardenia jasminoides Ellis, Ilex oldhami Miq., Quercus glauca Thunb., Rhus silvestris Sieb. et Zucc and Gilibertia @da Makino. The mean height of subordinate trees is 2.3 m with a density of 9440 trees ha- ‘. A meteorological observation tower 21.5 m high was used. To measure sensible heat flux by the eddy correlation method, U, v, w and T were measured at 20 m every 0.1 s with a supersonic anemometer thermometer (Kaijyo Denki Inc., Tokyo, DAT300/TR-61C). Each measurement lasted for 9 min. The data which were not filtered, were recorded using a personal computer (NEC Inc., Tokyo, PC98OlIJV). Net radiation at 20.5 m, dry and wet bulb temperatures (resolution of O.l”C), wind speed at 18.5 m and 21.5 m and soil heat flux at a depth of 2 cm were measured every 10 min with a net radiometer (Eiko Seiki Inc., Tokyo, CN-111, ventilated psychrometers (Eiko Se&i, MH-020T), anemometers (Makino Oyosokki Inc., Tokyo) and a heat flow plate (Eiko Se&i, CN-811, respectively. A tipping bucket located about 380 m from the observation
K. Miwtani et al. /Agricultural and Forest Meteorology 86 (1997) 193-203
197
tower measured amount of rainfall with OS-mm accuracy. There was also a 4-m X l-m metal throughfall trough. Stemflow gauges were installed at breast height on three trees in the throughfall plots. In this paper we analyze data from 7 April 1991 and 23 April 1991.
specified. During and after rainfall, however, there will be unavoidable errors in measurements of net radiation due to wet net-radiometer dome. The estimated evaporation, therefore, may have some errors but will nonetheless demonstrate the occurrence of evaporation during rainfall.
3.2. Method of analysis
3.2.2.1. The eddy correlation / energy balance method. Using Eqs. (1) and (21, the water vapor flux was calculated by the following equation:
3.2.1. Calculation of eddy correlation The sensible heat flux was calculated using the same method as McMillen (1988). To minimize possible errors caused by turbulent wind flow over the inclined experimental site, the coordinates were rotated such that W was equal to zero. When the inclination of the z-axis was detected as 8, the sensible heat flux in the vertical direction was then calculated by the following equation: H, = pCpm.
sin t9
3.2.2. Calculation of evaporation rate Evaporation rate was estimated by three different methods-the eddy correlation/energy balance method, the Bowen ratio/energy balance method and the Penman-Monteith equation. Hattori (1985) estimated that the ratio of advection and heat storage in gaseous phase of forest and tree to the net radiation is 8.6% and below 5%, respectively. The ratio of photosynthetic energy to solar radiation is commonly below 1%. The common basic equation of these three methods is the energy balance equation, i.e. (2)
where R, is the net radiation, A is the latent heat of vaporization, E is the water vapor flux, H is the sensible heat flux and G is the soil heat flux. In the following methods, R, and G must be
(3)
3.2.2.2. The Bowen ratio/energy balance method. Using the Bowen ratio ( /?I, the net available energy (R, - G) was divided into the latent and sensible heat fluxes as
(4)
(1)
where H, is the sensible heat flux by the eddy correlation method, p is the density of air, Cr is the specific heat of air at constant pressure, w’ is instantaneous wind speed in the direction of z-axis, T’ is the instantaneous deviation from the mean air temperature and the over bar denotes the mean value. One calculation was made for every 9-min sampling period.
R,=hE+H+G
hE,, = R, - H, - G
and R, - G Hb=P--l+P
(5)
respectively. The Bowen ratio was calculated from AT
(6)
P=Yz
where AT and he are differences in air temperature and vapor pressure, respectively, measured at two heights above the canopy, and y is the psychromettic constant. 3.2.2.3. Penman-Monteith equation. Evaporation was also calculated from the Penman-Monte&h equation (Monteith, 1965): hEP =
A(R, - G) +.&(e: - 4/r, A + Y( 1+ rc/rJ
(7)
where A is the slope of the saturation water vapor pressure curve at air temperature, ei is the saturated vapor pressure at air temperature, e, is vapor pressure of the air, r, is aerodynamic resistance and r, is canopy resistance. Since the value of r, depends on canopy wetness, an interception tank model (Suzuki, 1980) was adopted to determine canopy wetness. The intercepted model consists of two tanks of water
198
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and Forest Meteorology 86 (1997) 193-203
O.* v
I
c$O.Ol t
Fig. 5. The power spectra of w (A) and T (B), and the cospectra of w’T (C) before rainfall (23 April ll:OO-11:09x
Fig. 6. The power spectra of w (A) and T (B), and the cospectra of W’T’ (C) when rainfall intensity was 0.5 mm h-’ (23 April 14:30-14:39).
storage on foliage and on stems. We determined storage capacity of each tank from the measurements of rainfall, throughfall and stemflow, as 0.784 mm for foliage and 0.987 mm for stems. When rainfall
occurred, water is first stored in the foliage tank. If the foliage tank becomes full, then water overflows to the stem tank. During the period when tanks store
Fig. 7. Time courses of sensible heat flux and evaporation of intercepted rainfall (23 April 1991). (A) Vapor pressure deficit at 18.5 m. (B) Mean wind speed at 20 m. (C) Air temperaturqat 18.5 m. (D) The difference in air temperatare between hvo levels (21.5 rn, 18.5 m) above the canopy (T,s,S - ‘f,,,,). (E) Time courses of the evaporation estimated by the eddy correlation/energy balance method (A, E,), the Penman-Monteith equrrtion (m, I?,,)and the Bowen ratio/energy balance method (0, IT,,).(F) Time courses of the net radiation (0, R,), the sensible heat flux measm’ed by the eddy cormlation method (a, I&) and evaporation estimated by the eddy correlation/energy balance method (A, E, 1. (G) Rainfall intensity.
K. Mizutani et al. /Agriculhual
A
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and Forest Meteorology 86 (1997) 193-203
(W 1
??
.
0
6
12
18
24
0
6
12
18
24
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K. Mizutani et al. /Agricultural
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and Forest Meteorology 86 (1997) 193-203
any water, rC is assumed to be zero even when the canopy is not completely wet. The stored water diminishes according to evaporation intensity. When both tanks are empty, the canopy is assumed to be completely dry and transpiration resumes. Aerodynamic resistance (r,) can be calculated by the following equation: 1 r=-lnz-dZ a
k2U( z) !
20 1
where z is the height of the anemometer, d is zero plane displacement, z. is roughness length, k is von Karman’s constant and U(z) is mean wind speed at height z. Parameters z. (1.42 m) and d (14.3 m) were determined from wind profile observation above the canopy using the following equations: ln(z-d)
=aU(z)
+b
(9)
z. = exp( b)
(10)
where a and b are constants. The value of d producing the highest correlation coefficient in Eq. (9) was taken as the correct value, and z. was determined from Eq. (10). 3.3. Time courses evaporation
of the sensible
heat jlux
and
First, we present the results of 23 April 1991 when there was 2 mm of rainfall. According to the theory of turbulent wind flow, the power spectra of both wind speed and air temperature show a -2/3 slope in the inertial subrange (Kaimal et al., 1972). For a period before the rainfall, the power spectra of w and T showed a -2/3 slope in the inertial subrange (Fig. 5(A) and (B)). The cospectra of w’T’ indicates upward sensible heat flux occurred (Fig. 5(C)). When rainfall intensity was 0.5 mm h- ‘, the power spectra of T deviated from a slope of -2/3 in the high frequency range over 3 Hz (Fig. 6(B)). The data in the high frequency range, however, have little effect on the sensible heat flux, because the cospectra in the same range is close to zero (Fig. 6(C)). The cospectra indicate that a downward sensible heat flux occurred. The time course of the sensible heat flux estimated by the eddy correlation method is shown in
Fig. 7(F). When it began to rain, the sensible heat flux was about - 200 W m-*, and remained negative during rainfall. After the rainfall, it increased to about - 100 W m-‘. Fig. 7(D) shows the difference in air temperature between two levels above the canopy. During and after rainfall, air temperature measured at the upper level was generally higher than that of the lower level. When evaporation of intercepted rainfall estimated by the eddy correlation/energy balance method peaked at 13: 10 and 19: 10 (Fig. 7(F)), air temperature above the canopy decreased (Fig. 7(C)). Energy consumption by the evaporation of intercepted rainfall made the temperature of the canopy lower than air temperature, which induced a downward flux of sensible heat. Evaporation estimated by the eddy correlation/energy balance method was larger during rainfall than aftcrwards. This agrees with the results of Tsukamoto et al. (1988) and Calder and Wright (1986). Fig. 7(E) shows time courses of evaporation estimated by the three methods. Since the gradient of air temperature above the canopy was very small as was accordingly the Bowen ratio, the estimated evaporation of intercepted rainfall was almost the same as the net radiation. Thus, the calculated latent heat flux by the Bowen ratio method was negative for nighttime. Evaporation estimated by the PenmanMonteith equation was larger than that estimated by the other methods when the mean wind speed (Fig. 7(B)) or the vapor pressure deficit (Fig. 7(A)) was large. In this case, the Penman-Monteith equation seemed to be too sensitive to the wind speed and the vapor pressure deficit. After the storage tanks had dried out at 19:40, the evaporation rate estimated by the Penman-Monteith method decreased to zero, while the eddy correlation/energy balance method showed continuous evaporation even during nighttime. Next, we show the results of 7 April 1991, when rainfall intensity was greater than the previous case. When rainfall intensity was higher than about 4 mm h-‘, the data were variable because rainwater dropped from the top probe of the anemometer. Thus, noise-free data for this day were only analyzed, and the sensible heat flux was estimated only for the period before 12:30. The average rainfall intensity between 0:OOand 12:00 was 1.625 mm h-’ (Fig. 8(G)). In this period, the vapor pressure deficit
K. Mizutani et al. /Agricultural
and Forest Meteorology 86 (1997) 193-203
Fig. S.-Same as Fig. 7, except for 7 April 1991.
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K. Mizutani et al. /Agricultural
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was not equal to zero (Fig. 8(A)). The downward flux of sensible heat measured by the eddy correlation method between 1:00 and 7:30 was about 100 W rnw2, after which it increased to about 200 W rnp2 (Fig. 8(F)). Th e time course of the downward flux of sensible heat corresponds well to those of the mean wind speed (Fig. 8(B)), indicating that evaporation from wet forest canopies is largely dependent on wind speed and vapor pressure deficit. Evaporation of intercepted rainfall estimated by the eddy correlation/energy balance method began to increase at 8:00 and approached 400 W me2 at 1O:lO. During the period between 8:00 and 12:00, the mean rainfall intensity was 1.75 mm h- ’ and the mean evaporation intensity was 254 W rne2. The data of Calder and Wright (1986) showed that when the mean rainfall intensity was about 2.4 mm h- ‘, the mean evaporation intensity was about 280 W rnp2 for 2 h. Our observations indicated that evaporation of intercepted rainfall under high rainfall intensity was higher than that under low rainfall intensity. Evaporation estimated by the Penman-Monteith equation was approximately the same as that by the eddy correlation/energy balance method, but the Bowen ratio method gave smaller values than the other methods. It is really difficult to estimate the evaporation during and after rainfall using the Bowen ratio/energy balance method, because it requires quite accurate measurements of the temperature and humidity gradients above the canopy which are often very small under the conditions discussed here. The Penman-Monteith equation needs values of ra and rc, which are also difficult to quantify accurately. An error of r, directly results in an incorrect estimation of evaporation from a fully wet canopy. The assumption that r, is zero in spite of a partially wet canopy is not suitable for a rainfall of low intensity, and leads to an overestimation of evaporation when the wind speed and the vapor pressure deficit are large. More studies should be done on modeling the evaporation from a partially wet canopy. The eddy correlation/energy balance method will provide useful dam. 4. Conclusions We examined the effects of water sprinkling and wetness of the probe of a supersonic anemometer
thermometer using a wind tunnel to investigate the accuracy during and after rainfall. This investigation showed that the sensible heat flux could be measured satisfactorily with a supersonic anemometer thermometer even under rainfall of low intensity ( < 3 mm h- ’>. During rainfall intensity of more than 4 mm h- ‘, however, the data contained much noise because rainwater continuously dropped from the probe head of the anemometer. The sensible heat flux over a forest was measured with the supersonic anemometer thermometer during and after rainfall, and the evaporation of intercepted rainfall was estimated by the eddy correlation/energy balance method. It was observed during rainfall that a large amount of sensible heat was transferred to the forest, supporting the occurrence of evaporation. The amount of the downward sensible heat flux (or evaporation as a result) was larger during rainfall than after rainfall.
Acknowledgements
We would like to thank Dr M. Tani of the Forestry and Forest Products Research Institute and Dr M. Suzuki of Tokyo University for their helpful suggestions. We also thank Dr R.C. Sidle of Netherlands Institute for Sea Research for reviewing the manuscript. The wind tunnel experiment was carried out with the assistance of MS R. Okushima of the National Research Institute of Agricultural Engineering.
References Calder, LR., Wright, I.R., 1986. Gamma-ray attenuation studies of interception from Sitka spruce - some evidence for an additional transport mechanism. Water Resour. Res. 22.409-417. Hattori, S., 1985. Explanation on derivation process of equation to estimate evapotranspiration and problem on the application. Bull. For. For. products Res. Inst. 332, 139-165 (in Japanese). Raimal, J.C., Wyngaard, J.C., Ixumi, Y., Cote, O.R., 1972. Spectral characteristics of surface-layer turbulence. Q.J.R. Meteorol. Sot. 98, 563-589. McMillen, R.T., 1988. An eddy correlation technique with extend applicability to non-simple terrain. Boundary-Layer Meteorol. 43, 231-245. Monteith, JL., 1965. Evaporation and environment. Symp. See. Exp. Biol. 19, 205-234.
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and Forest Meteorology 86 (1997) 193-203
Stewart, J.B., 1977. Evaporation from the wet canopy of a pine forest. Water Resour. Res. 13, 915-921. Suzuki, M., 1980. Evapotranspiration from a small catchment in hilly mountain (1). Seasonal variations in evapotranspiration, rainfall intercepted and transpiration. J. Jpn. For. Sot. 62, 46-53.
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Tsukamoto, Y., Tange, I., Minemura, T., 1988. Interception loss from forest canopies. Rolling Land Res. 6,60-82 (in Japanese, with English abstract).