Measurements and methods Directory UMM :Data Elmu:jurnal:A:Agricultural & Forest Meterology:Vol102Issue2-3May2000:

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

The parameters measured, instruments used and the height of measurements are given in Table 2. The total number of data collection days was 89 as shown D. Amarakoon et al. Agricultural and Forest Meteorology 102 2000 113–124 117 in Table 1. The data were stored on-site as 20 min av- erages in a datalogger Campbell Scientific Inc. 21X micrologger and downloaded to a computer for anal- ysis. The surface fluxes l, H and G were determined using the procedures outlined in Sections 4.1, 4.2 and 4.3 later. The values of l and E, H and G determined in this manner are referred to as the measured values of these quantities. 4.1. Determination of l and H Bowen ratio energy balance method was used to determine the latent heat flux l and the sensible heat flux H. Bowen ratio, B o , is equal to Hl Brutsaert, 1982. Combination of this result with Eq. 7 provides the expression for l given in Eq. 9. l = R n − G 1 + B o 9 The values of l can be calculated using Eq. 9 know- ing R n , B o and G. R n was measured in this study. The values of B o and G were determined using the meth- ods given in Sections 4.2 and 4.3, respectively. The sensible heat flux, H, was determined using Eq. 7 after calculating l. 4.2. Determination of Bowen ratio B o Bowen ratio was determined using a Campbell Sci- entific Inc. Bowen ratio system to measure the vapor pressure and temperature at heights of 0.5 and 1.5 m above the surface and employing the result for B o Campbell Scientific Inc., 1987; Chen, 1992 given in Eq. 10. B o = pc p θ 1 − θ 2 λεe 1 − e 2 10 In Eq. 10, ε is the ratio of the molecular weight of water vapor to the molecular weight of dry air 0.622, θ i i=1, 2 and e i i=1, 2 are the temperatures and the vapor pressures at 0.5 and 1.5 m, respectively. Other letters and symbols in Eq. 10 have the same definitions as before. A quality control of Bowen ratio values obtained from Eq. 10 was performed, before using them in final analysis, according to the objective criteria formulated by Ohmura 1982. This process removed the values of B o that were very close to −1 and those corresponding to flux direction the same as that of the gradient. The criteria Ohmura, 1982 were effective mostly to the early morning and late afternoon values. 4.3. Determination of the heat flux into the surface, G The heat flux into the surface G was determined by employing the method based on the soil calorimetry approach Brutsaert, 1982; Campbell Scientific Inc., 1987. On the basis of this method the heat flux G into the surface and the flux G s at a depth d below the surface can be related by the following: G = G s + g s 11 where g s is the heat flux stored in the layer of thickness d which can be estimated from the product of the soil heat capacity and the change in the temperature of the soil layer. In this study d was equal to 0.10 m and G s was de- termined as the average of two heat flux plates buried at 0.10 m. The plates were 1.0 m apart spatially. The heat flux storage term g s was determined using an ap- proach Campbell Scientific Inc., 1987 which can be cast in the form: g s = C s [θ si − θ si−1 ] d t 12 where t=1200 s averaging time for sampled data=20 min, θ si is the average temperature at a depth of 0.05 m over the last 2 min of a 20 min time interval and θ si−1 is the average temperature at the same depth over the last 2 min of the previous 20 smin time interval. The temperature measurements were accom- plished by means of thermocouples buried at 0.05 m below the surface and directly above the heat flux plates. The term C s in Eq. 12 is the volumetric soil heat capacity in J m − 3 K − 1 and was determined using the result Brutsaert, 1982 in Eq. 13. C s = 1.948 m + 2.508 c + 4.19810 6 13 where 8 m is the volume fraction of mineral soil, 8 c the volume fraction of organic matter and 8 is the volume fraction of water. The average soil heat capac- ity was 1.96×10 6 J m − 3 K − 1 with a deviation of about 7. 118 D. Amarakoon et al. Agricultural and Forest Meteorology 102 2000 113–124

5. Data analysis