Surface heat turbulent fluxes: comparison of Bowen ratio and aerodynamic techniques A. de Miguel and J. Bilbao Department of Applied Physics I Sciences Faculty. University of Valladolid (Spain). E-mail:julia@cpd. uva. es Abstract The analysis of the two field experiment of campaign measurements in the years 1994 and 1995 has been carried out. The campaigns took place, one over bare soil and another over grass-land, in two different places and in two different times of the year, in order to take into account different meteorological situations. Different energy fluxes were evaluated from meteorological measured variables to know the relation between net radiation and soil heat flux, sensible heat flux and latent heat flux or evapotranspiration. 1 Introduction In this paper we have analysed the results of two field experiments between the year 1994 and 1995, in two different places, over bare soil and over gras-land. The first part of the work consisted on finishing off the measurement system, which was able to record the necessary meteorological variables to know the behaviour of the boundary layer next to the ground. In relation with the data treatment, the first aim of this paper has been the evaluation of the following energy fluxes which take part in the budget energetic equation in the LBS (Superficial Boundary Layer): net radiation, soil heat flux, sensible and latent heat fluxes and also to analyze the relation between them. Finally we have compared the two campaign
410 Measurements and Modelling in Environmental Pollution results in order to investigate the influence of the vegetation and the time of the year over energy fluxes. The way in which the energy is partitioning gives information about the physic processes which happen in the zone. The first field experiment was carried out in the C.I.B.A. Laboratory, in a rural area 45 km from Valladolid, in North direction. For the second experiment we looked for a zone with vegetation and among the possible places we selected a farm-land in the Centre of Investigation and Agricultural Studies, "Vinalta", in Palencia and the measurements were made over grass-land. In the last years, several authors have studied the energy fluxes in the superficial boundary layer: Idso [6] studied net radiation and soil heat flux, Fuchs [4], describe the relationship between the diurnal patterns of the radiant energy exchange. Hogstrom [5], analysed an extensive set of measurements of turbulent fluxes. Deheer-Amissah [3] analysed measurements of temperature, wind and moisture gradients. Berkowicz [1] compared experimental and modelled values of heat fluxes. Reinking [7], analyzed the effects of water vapour on heat fluxes. The purpose of this paper is to describe a method for estimating halfhour values of the surface energy fluxes from routine meteorological data and to compare the results obtained in two different places. 2 Theoretical Methods The Bowen ratio method is one of the most important methods to study surface energy fluxes. This method is based on the energy balance relation, Stull [8]: (1) where R* is the net radiation, G is the ground heat flux and LE is the latent heat flux (evaporation). With the assumption that the similarity functions for heat and humidity are identical, the ratio of the sensible and latent heat (the Bowen ratio) is proportional to the ratio of temperature and humidity gradient:
Measurements and Modelling in Environmental Pollution 411 where 7 is the psychometric constant; Sq is the humidity difference measured over the same height as the temperature difference is 68. From equations (1) and (2) we can obtain: (3) where the subscript BW means that H is estimated by the Bowen-ratio method. The profile method to estimate the turbulent flux is based on Monin- Obukhov similarity theory for the surface layer. Following the similarity theory, supossing stationary and horizontally homogeneous conditions, vertical gradients of any conservative quantity are functions of height z and (z/l). From wind speed and temperature at two levels, Cancillo [2], heat fluxes can be calculated from the equations: (4) where p is the density of air, Cp is the specific heat of air, u* is the friction velocity and 0* is the temperature scale. L =-Au*g* (5) where A is the water heat latent vaporization and q" is the humidity scale. The early equations and the equations obtained from de meteorological gradients, Cancillo [2], should be resolved by iterative methods. 3 Experimental Campaigns The first experiment was carried out in a flat bare soil field, 35 km from Valladolid, in C.I.B.A. site, 42 49'N, 4*56' W and 840 m above sea level; the instrumentation was installed on a spot with a daytime fetch of at least 400m. The output of all meteorological sensors was recorded digitally in a data-logger (Campbell Scientific, Model CR10). Sampling frequency was 0.1 Hz with an average taken every 15 min. Wind speed and direction.
412 Measurements and Modelling in Environmental Pollution air temperature and relative humidity were taken from three levels; soil heat flux, soil humidity and soil temperature were taken at three depths. The first experiment was carried out in 1994, from 25* September to 30* October. The heights for wind, temperature and humidity profiles were logarithmically distributed between 1 and 12 m. Additional weather data were available such as: precipitation, pressure, net and global solar radiation. The second experiment was carried out from 28* June 1995 to 31* July 1995, in a farm-land : 42<T N, 4 32' W, altitude 732 m. The place of measurements was situated 5 km from the city of Palencia in northwest direction. The measurements were taken over grass (polifita) of 25 cm in height. The fields surrounding the measurement station were covered with varied kinds of vegetation such as: sugar beet, lucerne, fodder, etc. 4 Results and Discussion The characteristics in CIBA site during the first campaign were: the maximum temperature oscillates between 27 C and 7 C and the minimum temperature oscillates between 11 C and 1 C; it rained for 9 days and the day 289 (16 October) the rain was 25 mm. Figure 1 shows the solar global radiation evolution, at the CIBA site. The cloudless days global radiation took the level of 300 W nr* and during cloudy days global radiation took the maximum of 50 W nr*. There were only seven days completely cloudless. Global radiation showed similar evolution to net radiation whose maximum levels were smaller than the global radiation ones. In Palencia: the average humidity diminishes during the campaign from 70% to 42%; the mean temperature, during the six first days, was over 22 C, after that the temperature increases over 32* and decreases the day 202, when wind velocity and humidity increase till 55% and 5.5 m s"* respectively. The average wind velocity was over 3 m s'% ranging from 2 m s~* to 5.5 m s~*. Soil heat was measured, at three different depths by means of HFT Campbell plates. In order to parameterize the soil heat flux from the net radiation and to know the net radiation percentage which is transformed in soil heat flux, the data were classified in: diurnal, nocturnal and all data and the following plots were made: i) diurnal hourly data of R^ and G. ii) nocturnal hourly data of R* and G. iii) All hourly data of R* and G. The results obtained at the CIBA site, taking into account all data, have been the following: the 20% and the 16% of the net radiation are converted in heat soil flux. In Figure 2 we can see net radiation, heat soil flux and the slope of the heat soil flux and net radiation lineal regression from the day 270 to 300, in autumn, at the C.I.B.A. site and in Figure 3, we show the same evolution but during summer in Palencia. In autumn average daily net
Measurements and Modelling in Environmental Pollution 413 8 12 16 8 12 16 8 12 16 8 12 16 8 12 16 272 273 274 275 276 8 12 16 8 12 16 8 12 16 8 12 16 8 12 16 277 278 279 280 281 8 i 3 o 800-600 - 400-200 - 8 12 16 8 12 16 8 12 16 8 12 16 8 12 16 282 283 284 285 286 n 11111111 ri M n i 11 i u 11 r 111 n 11 111 i M n i i n i 11 111 11 r 11 8 12 16 8 12 16 8 12 16 8 12 16 8 12 16 287 288 289 290 291 8 12 16 8 12 16 8 12 16 292 293 294 TIME (hour/juliane day) Fig. 1. 1994. Evolution of the solar radiation at the CIBA site; September-October,
414 Measurements and Modelling in Environmental Pollution 0.24 250 Slope Net radiation Soil heat flu 270 280 290 TIME (Juliane day) 300 Fig. 2. Average daily values of energy fluxes and slope at the CIBA site. 0.24 400 HI Q_ O CO 0.22-0.20-0.18-0.16 - ( 0.14-0.12-0.10-0.08 Slope Net radiation Soil heat flux t- 100 cc iu LU 0.06 I I I 1 1 1 1 198 199 200 201 202 203 204 205 206 TIME (Juliane day) Fig. 3. Average daily values of energy fluxes and slope in Palencia
Measurements and Modelling in Environmental Pollution 415 Net Radiation Sensible Heat UJ I ' 1 ^ I ' T 29-Sept. 30-Sept. 1-Oct. 2-Oct. 3-Oct. Fig. 4. Energy fluxes at the CIBA site during five days; September-October, 1994. 600 - Net radiation Sensible Heat Latent Heat LLJ 19-July 20-July 21-July 22-July 23-July Fig. 5. Energy fluxes in Palencia during five days; July, 1995
416 Measurements and Modelling in Environmental Pollution radiation ranges from 245 W nr* to 55 W nr*, G ranges from 40 to 10 W m"2 and the slope changes from 0.11 to 0.22, during the campaign. In Figure 3, in Palencia, daily average heat soil flux ranges from 50 to 10 W nr% daily average net radiation ranges from 210 to 350 W nr* and the slopes oscillate between 0.13 and 0.19. We have found that the minimum slope in summer is higher than the minimum in winter and the maximum in summer is smaller that the maximum in autumn and as it can be seen the slope increases when the soil was from wet to dry conditions. In Figure 4, we show the energy fluxes, at Ciba site, which have been calculated by means of the Bowen ratio method. The most important characteristic is that the sensible heat fluxes during the five days are bigger than the latent heat flux; net solar radiation fluctuated during the day reaching more than 400 W m~*. Figure 5 shows energy fluxes in Palencia, during the first four days, sensible heat is bigger than latent heat which means that the evapotranspiration is present. During the last two days sensible heat is bigger than the latent one which means lacking in water during those days. In both campaigns the energy fluxes have been calculated for two methods and the correlations between then have been studied. References 1. Berkowicz, R. and Prahm, L.P. Sensible heat flux estimated from routine meteorological data by the resistance method. Journal of Applied Meteorology, 1982, pp 1845-1864. 2. Cancillo, M.L. Estudio de los flujos de energia en la capa limite de Superflcie. Tesis Doctoral. Universidad de Extremadura, Facultad de Ciencias, 1991 3. Deheer-Amissah, A., H6gstrom,U. and Smedman-Hogstrom, A.S. Calculation of sensible and latent heat fluxes, and surface resistance from profile data. Boundary-Layer Meteorology, 1981, 20, pp 35-39. 4. Fuchs, M. and Hadas, A. The heat flux density in a non-homogeneous bare loessial soil. Boundary-Layer Meteorology, 1972, 3, pp 191-200. 5. Hogstrom, U. A field study of the turbulentfluxesof heat, water vapour and momentum at a 'typical' agricultural site. Quart. J. R. Met. Soc. 1974, 100, pp. 624-639. 6. Idso, S.B., Aase, J.K. and Jackson, R.D. Net Radiation - Soil heat Flux relations as influenced by soil water content variations. Boundary -Layer Meteorology, 1975, 9, 113-122
Measurements and Modelling in Environmental Pollution 417 7. Reinking,R.F. (1980). The respective effects of water vapour and temperature on the turbulent fluxes of sensible and latent heat. Boundary- Layer Meteorology, 1980, 19, 373-385. 8. Stull,R.B. An introduction to Boundary Layer Meteorology. Kluwer Academic Publishers, London, 1991.