Site description and environmental conditions

D. Amarakoon et al. Agricultural and Forest Meteorology 102 2000 113–124 115 The scheme of Priestley and Taylor 1972 is the best-known simplification of Penman 1948 scheme and is based on the concept of equilibrium evapora- tion Slatyer and McIlroy, 1961 from moist surfaces. Slatyer and McIlroy 1961 had presented the idea that over a very large homogeneous and thoroughly moist surface under well established steady conditions e tends to e. A consequence of this is the vanishing of the second term containing e−e from Eq. 1 and hence the first term in Eq. 1 may be considered to represent a lower limit to evaporation from moist surfaces which the authors referred to as the equilib- rium evaporation. In our work, equilibrium evapora- tion is denoted by E eq and the corresponding equi- librium latent heat flux which is the first term on the right hand side of Eq. 3 is denoted by l eq . Priestley and Taylor 1972 treated the equilibrium evaporation as the basis for an empirical relationship for evapora- tion from a wet surface under conditions of minimal advection. After analyzing data over ocean and satu- rated land surfaces they presented a scheme for wet surface evaporation, which is of the form E = αE eq 4 where the parameter α in Eq. 4 is known as the Priestley–Taylor parameter or coefficient. For large saturated land and advection-free water surfaces, Priestley and Taylor 1972 concluded that the best estimate for α is 1.26. As mentioned in the literature Brutsaert, 1982; Cargo and Brutsaert, 1992 there is enough experimental evidence to support the va- lidity of a value around 1.26 for α. Eichinger et al. 1996 have derived an analytical expression for α and their results have shown good agreement with the value 1.26. The equation for the latent heat flux l corresponding to Eq. 4 is l = αl eq 5 The scheme of De Bruin and Holtslag 1982 can be presented in the form l = α ′ 1 1 + γ R n − G + β 6 where α ′ and β are empirical constants and the first term on the right-hand side is the same as α ′ l eq . Ac- cording to the energy balance equation for the earth’s surface, R n , G, l and the sensible heat flux H can be linked by the equation, R n = l + H + G 7 Combining Eqs. 6 and 7, the sensible heat flux H can be given by H = [1 − α ′ 1 + γ ] 1 + γ R n − G − β 8 De Bruin and Holtslag 1982 have performed mea- surements over a 100 m×100 m field covered with short grass of about 8 cm in height. They have ana- lyzed data for two periods. During one period, which they call the normal period, the ground had been wet- ter than the other period to which they refer as the dry period. For the normal period, the analysis of hourly latent heat flux data has shown that α ′ can be assigned the value 0.95 and the β value 20 W m − 2 . For the dry period, the values are 0.65 and 20 W m − 2 , respectively. The results have shown that the hourly values of l es- timated using Eq. 6 and the above α ′ and β values compare well with experimentally determined l values and the values calculated using the Penman–Monteith approach Monteith, 1965; Brutsaert, 1982 which is a more complete scheme. The values of H estimated using Eq. 8 have also shown reasonably good agree- ment correlation coefficient of 0.92 and a root mean square error of 38 of average H with experimen- tally determined H values. Furthermore, their results have indicated that daily mean of α Priestley–Taylor coefficient is about 10 greater than its hourly value during daytime, and that α=1.26 yields good results for daily values of latent heat flux.

3. Site description and environmental conditions

The experimental sites were located at Mona and St. Catherine in Jamaica, West Indies. The latitude, longitude and altitude respectively for the stations are as follows: 17 ◦ 58 ′ N, 76 ◦ 45 ′ W, 150 m for Mona and 17 ◦ 58 ′ N, 77 ◦ 5 ′ W and 150 m for St. Catherine. The distance from Mona to St. Catherine is 35 km. The site at Mona was situated on an open rectangular lot of extent 120 m×80 m. The other site was located at the Agro 21 Horticultural Park, St. Catherine. It was situated on an open lot with a fetch of over 100 m 116 D. Amarakoon et al. Agricultural and Forest Meteorology 102 2000 113–124 Table 1 The number of data collection days and the daytime 07:00–18:00 hours average environmental conditions under which the experiments were performed during August to September of 1989, January to February of 1990 and January to February of 1994 Parameter a August–September 1989 January–February 1990 January–February 1994 N 26 31 32 R s W m − 2 170.0–890.0 90.0–750.0 73.5–629.6 R n W m − 2 74.2–564.2 37.1–473.0 6.7–394.0 θ ◦ C 25.3–32.1 20.7–30.0 22.9–28.3 u m s − 1 0.6–4.4 0.5–5.2 0.8–3.2 δ e mb 2.08–15.6 6.9–18.9 7.3–15.6 Average RH 81 63 79 L m − 0.014 to −275 0.004 − 4 to −348 2.7 − 6 to –151 0.04 a Definitions of the parameters R s , R n , θ , u, and RH are the same as in Table 2. N=number of data collection days and δe=vapor pressure deficit. The relative humidity shown is the ratio, vapor pressure measured at 1.5 m to saturation vapor pressure calculated using the result of Lowe 1977. The measured average relative humidity in 1994 was 73. L is the Monin–Obukhov length estimated by the method outlined in Appendix A. The positive values of L within parentheses are for stable situations in the morning at 07:00 hours or in the late afternoon at 17:00 hours. 75 of the L values were in the range −150L0 m. The values of all the quantities given are hourly average values, except N. Hourly average values refer to averages for each hour between 07:00 and 18:00 hours. upwind and 30 m downwind, followed by an embank- ment about 10 m high. The wind was primarily from the south. The surfaces at both sites were covered with grass of Bahamian variety of average height 10 cm and were irrigated regularly, three times a week, to ensure that there was enough water to evaporate. The average of the ratio, lmeasuredl eq , was 1.17 based on hourly values of the fluxes. The top soil surface at both sites was mainly loam. The data collection period at Mona was January to February of 1994, and the periods at St. Catherine were August to September of 1989 and January to February of 1990. The daytime environmental conditions during the three periods and the number of data collection days are given in Table 1. The atmosphere was predominantly unsta- Table 2 Parameters measured, instruments used and the height of measurements in this study, during the periods: August to September of 1989, January to February of 1990 and January to February of 1994 Parameter a Instrument Height of measurement m R n Fritschen Model 3032 and Swissteco type S-1 Model CH9463 net radiometers 2.0 R s Li-Cor Inc. Model LI-200SZC pyranometer 2.0 RH and θ b Campbell Scientific Inc. Model 207 temperature and relative humidity probe 2.0 e and θ c Campbell Scientific Inc. Dew-10 series Bowen ratio system 0.5 and 1.5 u MET 1 cup anemometer system 2.0 G s Radiation and Energy Balance Systems REBS, Inc. HFT-1 series heat flux plates − 0.10 θ s Thermocouples − 0.05 a R n = net radiation, R s = global short-wave radiation, RH=relative humidity, θ =air temperature, e=vapor pressure, u=wind speed, G s = soil heat flux at 0.10 m below the surface, θ s = soil temperature at 0.05 m below the surface. b RH measured only in January–February of 1994. c Bowen ratio system provided measurements of e and θ at 0.5 m and 1.5 m. ble, as indicated by the estimated values of Monin– Obukhov length L. A few early morning and late afternoon L values depicted stable L0 or neutral |L|150 m conditions. The method of estimation of L is given in Appendix A. The atmospheric instability observed in this work agrees with the other studies Hsu, 1982; Chen et al., 1990 on tropical atmospheric instability.

4. Measurements and methods