terrain is gently undulating and the soils are duplex soils with a sandy/silty A-horizon and a heavy clay B horizon ... scattered eucalypt trees and are referred to as open woodlands. Between 26 .... eliminated to reduce the effect of unresolved long waves. The airborne ..... breeze fronts in summer of 1988/89. Research Report ...
Exchange Processes at the Land Surface for a Range of Space and Time Scales (Proceedings of the Yokohama Symposium, July 1993). IAHS Publ. no. 212, 1993.
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Measurements and estimates of evaporation at a range of scales
COR HOFSTEE Department of Hydrology, Soil Physics and Hydraulics, Agricultural University, 6709 PA.Wageningen, Netherlands
JETSE D. KALMA CSIRO Division of Water Resources, Canberra City , Australian Capital Territory 2601, Australia
HELEN A. CLEUGH School of Earth Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia
JORG M. HACKER Flinders Institute for Atmospheric and Marine Sciences, The Flinders University of South Australia, Adelaide, South Australia 5001, Australia
Abstract Sensible and latent heat flux measurements using Bowen ratio and eddy correlation techniques and aircraft measurements are presented and compared with results obtained with a coupled slab-vegetation model for a 27 km2 grassland catchment in SE Australia. INTRODUCTION This note presents preliminary results of an intercomparison of local groundbased flux measurements using Bowen ratio and eddy correlation techniques, aircraft transect measurements and results obtained with a regional-scale coupled slab-vegetation model. The experimental work has been carried out in the 27 km2 Lockyersleigh catchment in S.E Australia. Elevations vary between 600 and 762 m + m.s.l. The terrain is gently undulating and the soils are duplex soils with a sandy/silty A-horizon and a heavy clay B horizon. The vegetation is a mixture of native and introduced grasses which are grazed. Most of the catchment (70%) has been cleared. However the native grasslands on higher ground in the eastern parts of the catchment have scattered eucalypt trees and are referred to as open woodlands. Between 26 February and 2 March 1992 evaporation measurements were made with two Bowen ratio systems and with two eddy correlation systems in the cleared pasture region in the southern half of the catchment. The four sites formed a square with sides of about 1 km. A research aircraft measured sensible and latent heat fluxes over the cleared pasture and the open woodland between 10 am and 3 pm on most days with eddy correlation equipment. Temperature and humidity profiles in the mixed layer were obtained with tethered balloon and radiosonde ascents/descents as well as with the aircraft. This report describes results for 1-2 March when weather conditions were favourable, despite some scattered cumulus cloud at the top of the ABL.
382
Cor Hofstee et al.
GROUNDBASED BOWEN RATIO AND EDDY CORRELATION MEASUREMENTS The Bowen ratio (b) is the ratio between sensible heat flux (H) and latent heat flux (AE) and is given by /? = H//Œ = (cp/l)(AT/ Aq). AT and Aq are the differences in air temperature and specific humidity over the same vertical height interval. cp is the specific heat at constant pressure and A is the latent heat of vaporisation of water. Substituting /?= H / AE in the energy balance equation yields AE = (Q*-G)/ (1 + b) and H= /?(Q*-G)/(l + b), where Q* is the net radiation and G the soil heat flux. Bowen ratio apparatus (see Kalma and Jupp, 1990) was operated at two sites. It uses reversing psychrometers between 1 m and 2.5 m above the ground. Net radiation was measured at 1.25 m above the ground and two soil heat flux plates, wired in series were buried at 5 cm below the surface. All sensors are sampled at 20 sec intervals and the data are integrated over 10 min periods centred on the reversal of the psychrometers. The eddy correlation method measures the turbulent vertical fluxes of atmospheric entities by sensing the properties of the eddies as they pass through a measurement level. H and AE are obtained from the time average of the instantaneous covariances of the vertical velocity and a volumetric measure of the sensible heat and water vapour. Thus H = p cp w' 0 ' and AE = p A w' q'. Here w' and q' are the fluctuations of vertical windspeed, potential temperature and specific humidity, respectively. Eddy correlation measurements were made at two grassland sites with measuring systems which consist of a sonic anemometer, fine wire thermocouple and krypton hygrometer. Windspeed, temperature and humidity fluctuations are sampled at a rate of 10Hz, processed and averaged over 15 min periods and stored. The "footprint" of these "local" flux measurements may be estimated with the method of Gash (1986). It shows that, with sensors at 2 m above the ground and an estimated surface roughness of z„ = 0.03 m, 90% of the measured flux originates from within 450 m upwind from the instrument. Differences in hourly values of sensible and latent heat fluxes between the four sites are less than 10%. These differences were of the same magnitude as the differences between the four systems when compared in the one location. The four daily values of AE obtained on 2 March are between 3% more and 8% less than the mean AE value. The four daily H values on that day are between 9% less and 8 % more than the mean H value. The small spatial variability in sensible and latent heat fluxes across the open pasture must be largely due to the wet conditions throughout the catchment: about 150 mm of rain fell from 4 February to 2 March 1992, including 5 mm rain between 20 and 26 February. Because of this small spatial variability, energy balance measurements at the four sites have been combined and regional energy balance values have been calculated for the whole pasture region for 1 and 2 March. AIRCRAFT MEASUREMENTS Measurements of H and AE were made over the grassland and open woodland regions with a Grob G109B aircraft. This aircraft is equipped with fast sensors for air
Measurements and estimates of evaporation at a range of scales
383
temperature, humidity and three-dimensional wind vector. Air temperature is measured with a thermocouple and a PtlOO sensor both located in direct flow housings similar to the NCAR k-probe. Humidity fluctuations are measured with a Lyman-a hygrometer which is continuously calibrated in flight against a dew point mirror. The wind vector is measured by combining a five-hole probe mounted at the tip of a wingpod and an Attitude and Heading Reference System which is continuously updated by a Global Positioning System (see Hacker et al., 1990). A data acquisition system with 20 channels is used which samples at a rate of 20 Hz. Time series of high-frequency deviation data are obtained for each transect by subtracting mean values. Thus, instantaneous covariances between vertical velocity and temperature (or humidity) are obtained which are averaged over time to generate fluxes of ÀB and H. The aircraft flew at 25-30 m above the ground for the pasture transects and at 3540 m for the open woodland transects. Flights took place after 10 am and it may be assumed that these flux measurements occurred in the surface layer. Each 4-5 km flight leg only took about 3 min so that flux measurements along each transect may be considered as near-instantaneous. The highest frequency which can be resolved is 10 Hz which corresponds to a wavelength of 3.5 m. Small eddies are therefore not resolved but it is unlikely that this results in an underestimation of turbulent fluxes greater than 5%. Wavelengths longer than 1300 m have been eliminated to reduce the effect of unresolved long waves. The airborne measurements were made at irregular times, whereas groundbased measurements were continuous. Average ÀE and H flux values obtained for each aircraft transect over pasture have therefore been compared with mean energy balance data obtained on the ground during the transect. Table 1 compares mean daily flux values obtained with the two types of measurements. The reasonable agreement between airborne and groundbased measurements is related to the good agreement between the four sites with groundbased measurements, the small spatial variability in wetness conditions in the catchment and the open, irregular nature of the open woodlands ( grasslands with scattered eucalypt trees). Table 1 Mean latent and sensible heat fluxes (W m"2) over pasture based on airborne and groundbased measurements.
Aircraft Ground-based Difference (%)
1 March XE 312 293 +6
1992 H 109 150 -27
2 March 1992 XE H 266 83 300 116 11 -28
Table 2 Mean latent and sensible heat fluxes (W m"2) over pasture and open woodland based on airborne measurements. 1 March 1992 XE H Q*-G
2 March 1992 XE H
Q*-G
Pasture
312
109
421
266
83
349
Open woodland
281
115
396
272
107
379
384
Cor Hofstee et al.
Mean flux values based on 23 (24) transects carried out with the aircraft over pasture and woodland between 10 am and 3 pm on both days are shown in Table 2. The H values on both days are somewhat larger over the open woodland than over the pasture. AE values over pasture on 1 March are larger than those over the open woodland, with a small opposite difference on 2 March. Values of Q*-G have been computed as AE+B., assuming energy balance closure. They show the same small (< 10%) differences between the two surfaces as AE. One possible explanation for the surprisingly small AE differences between the two regions is that evaporation from the grass covered ground in the open woodland is somewhat less than from the open pastures, but that this is largely offset by the evaporation from the trees.
PROFILE MEASUREMENTS IN THE LOWER ATMOSPHERE A tethered balloon system was used to measure meteorological parameters in the first 300 m of the boundary layer. The aerodynamic design of the balloon makes it turn into the wind enabling wind direction measurement. A sonde with thermistors, aneroid sensor, small 3-cup anemometer and a magnetic compass is suspended below the balloon. It contains the circuitry that samples every 9 s and transmits data to a data acquisition/storage system on the ground. Standard spin-type radiosondes were also used, attached to free-rising weather balloons. Pressure and dry/wet bulb temperature data are obtained every 9 sec and transmitted to the ADAS data acquisition system. PBL soundings with the tethered balloon and radiosonde systems on 2 March yielded the profiles of virtual potential temperature and specific humidity shown in Fig. 1. They show the evolution of a deep mixed layer. Virtual potential temperature increases slightly with height in the mixed layer, whereas specific humidity decreases with height on both days. This indicates that the mixing process is not vigorous enough to prevent the existence of gradients. The upper air was very dry and the mixed layer rather moist on both days. No major changes in virtual potential temperature occurred in the upper air during the two days, suggesting rather constant synoptic conditions and no frontal passages. Table 3 gives the height of the mixed layer as observed during both days.
APPLICATION OF COUPLED SLAB-VEGETATION MODEL The model Models for the day-time growth of the well-mixed (horizontally homogeneous) lower part of the boundary layer have been developed by Tennekes (1973) and Tennekes and Driedonks (1981), amongst others. In these models temperature and humidity of the mixed layer depend on the heat and moisture input from below and the heat gain (moisture loss) by entrainment across the interface at the top of the mixed layer. Combining a mixed layer or slab model with a model for evaporation from the vegetated land surface such as the Penman/Monteith combination approach provides the necessary lower boundary conditions. Such coupled models have been described by McNaughton and Spriggs (1986), Raupach (1991) and Cleugh (1991). These
Measurements
and estimates of evaporation at a range of scales
385
(b)
. 1\~~
320
0
10
Vidua! potential temperature (K)
12
Specific humidity (g kg"'
Fig. 1 Profiles of (a) virtual potential temperature and (b) specific humidity on 2 March 1992. Times shown are local time (EST).
models are one-dimensional: they ignore large-scale advection and most validations have been carried out over homogeneous terrain. The coupled slab-vegetation model of Cleugh (1991) has been modified and adapted for use in the prediction of evaporation at a regional scale. It comprises conservation equations for potential temperature and specific humidity; expressions for the vertical flux density at the inversion and the growth of mixed layer; the surface energy balance equation and the Penman/Monteith combination equation; and an expression for the potential saturation deficit .
Table 3 Height of the mixed layer determined with radiosondes.
Time 07.45 09.40 10.45 11.45 14.10 15.22
1 March 1992 m
280 450 700 700 1750 1680
Time 07.35 08.30 09.38 12.05 13.05 15.50
2 March 1992 m
400 550 700 900 1400 1600
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Cor Hofstee et al.
Simulation results Some 70% of the catchment is open pasture and 30% is occupied by native grasses with scattered eucalypt trees (very open woodland). Using the approach of Raupach (1991) it can be shown that the open woodland and pasture regions may be treated as heterogeneous at the microscale and that the mixed layer will develop as over homogeneous terrain with common mixed layer characteristics. The model requires aerodynamic resistance (rj and surface resistance (rc) values for both the pasture and open woodland regions. Hourly ra values over pasture were calculated with the method of Choudhury et al. (1986). Woodland values were compared by inverting the sensible heat flux gradient equation. Differences between the pasture and open woodland areas were surprisingly small and average ra values were used for the entire catchment. Hourly rc values were obtained from the PenmanMonteith combination equation using calculated ra values and measured evaporation values. Mid-day ra and rc values on 1-2 March were about 34 s m-1 and 55 s m"1The model is initialized with data obtained from the first sounding in the early morning of both days which reveals the existence of a (shallow) mixed layer. The model computes AE at each time step using the measured available energy, computed ra and rc values and the modelled potential saturation deficit of the previous time step. H is computed from (Q*-G)-/s •5
I
8 u 0
600 400
302
(b)
2 a> a. E 1
c
300
298
&
(C)
e io.o
11 13 Local time (EST)
Fig. 2 Simulated values of (a) height of mixed layer, (b) mean virtual potential temperature in mixed layer and (c) mean specific humidity in mixed layer (c) on 2 March 1992.
Simulations Symbols represent measurements in !he pasture
•""-—xcr-G XE
\
m^'
•/x
/
*\A •
•
y / *
H
>;
•
•\ 1
8
9
i
10
11 12 13 Local time (EST)
i
i
i
14
15
16
\
Fig. 3 Simulated and observed values of the regional energy balance on 2 March 1992. Hourly values shown with filled circles are based on measurements in pasture.
388
Cor Hofstee et al.
measurement and no marked shifts in wind and radiation . The eddy correlation method requires very accurate orientation and placement of the velocity sensors (Brutsaert, 1982). Comparison of the airborne eddy correlation data obtained along 4-5 km transects over the pasture and the energy balance values derived from groundbased measurements indicates that airborne sensible heat flux values are up to 30% less than groundbased values. This is consistent with figures quoted by Shuttleworth (1991). The agreement observed in the evaporation measurements is encouraging. Differences in airborne flux measurements between the open pastures and the native grasslands with the scattered trees are surprisingly small. This is also related to the wet conditions throughout the catchment. Under moisture-limiting conditions transpiration by the trees will become relatively more important due to their deeper rooting. The regional-scale coupled slab-vegetation model will perform best in clear sky and typically anticyclonic conditions, when convective conditions dominate in the planetary boundary layer. Despite some intermittent cloud on both days, predicted and observed temperature and humidity profiles agree well. The growth of the mixed layer is slightly underestimated. However the soundings do not provide time or space averages. Stull (1988) has also suggested that the thickness of the interface zone may be significant. The predicted regional sensible and latent heat fluxes agreed reasonably well with ground based and airborne flux measurements. Acknowledgements We thank Ian Moore, Larry Guerra, Haralds Alksnis, Paul Daniel, Caecilia Ewenz and Peter Briggs for their support during the field work. This research was supported by Project Grant 90/82 of the Land and Water Resources Research and Development Corporation. REFERENCES Brutsaert, W. H. (1982) Evaporation into the atmosphere. Reidel, Dordrecht, Netherlands. Choudhury, B. J. , Reginato, R. J. & Idso, S. B. (1986) An analysis of infrared temperature observations over wheat and calculations of latent heat flux. Agr. For. Met. 37, 75-88. Cleugh, H. A. (1991) Predicting evaporation at the catchment scale using a coupled canopy and mixed layer model. Vegetatio 91, 135-148. Gash, J. H. C. (1986) A note on estimating the effect of a limited fetch on micrometeorological evaporation measurements. Boundary-layer Met. 35, 409-413. Hacker, J. M., Hartman, J., Kraus, H. & Schwerdtfeger, P. (1990) Airborne measurements of the structure of sea breeze fronts in summer of 1988/89. Research Report No. 47, Flinders University of South Australia, Adelaide, South Australia. Kalma, J. D. & Jupp, D. L. B. (1990) Estimating evaporation from pasture using infrared thermometry: evaluation of a one-layer resistance model. Agr. For. Met. 51, 223-246. McNaughton, K. G. & Spriggs, T.W. (1989) A mixed layer model for regional evaporation. Boundary-LayerMet. 34, 243-262. Raupach, M. R. (1991) Vegetation-atmosphere interaction in homogeneous and heterogeneous terrain. Vegetatio 91,105-120. Shuttleworth, J. (1991) Insight from large-scale observational studies of land-atmosphere interactions. Rev. Geoph 29, 585-606. Stull, R. B. (1988) An introduction to boundary layer meteorology. Kluwer Academic Publishers, Dordrecht, Netherlands. Tennekes, H. (1973) A model for the dynamics of the inversion above a convective boundary layer. / . Atm. Sci. 30, 558-567. Tennekes, H. & Driedonks, A. G. M. (1982) Basic entrainment equations for the atmospheric boundary layer. Boundary-Layer Met. 20, 515-531.
