Pan evaporation in Hong Kong

Transcription

1 Pan evaporation in Hong Kong John K. W. Chan Hong Kong Observatory, Kowloon, Hong Kong In 1957, the Hong Kong Observatory installed two evaporation pans at the King s Park Meteorological Station in Kowloon, Hong Kong, (Figure 1) and began taking pan evaporation measurements in Since then, the Hong Kong Observatory has operated the two evaporation pans continuously. The first comprehensive study of pan evaporation in Hong Kong was conducted in 1976 (Chen, 1976). In that study, meteorological data measured at the King s Park Meteorological Station from 1958 to 1975 were used to derive empirical formulae for estimating pan evaporation from meteorological parameters. Two approaches were taken to derive the formulae: (1) an aerodynamic or mass transfer approach; and (2) an energy balance or energy budget approach. With the accumulation of data since 1975, further studies were conducted using data obtained between 1958 and This article presents the results of the latest study to review the formulae used for estimating evaporation from meteorological measurements. It also discusses the observed trend in pan evaporation as well as attempts to identify contributing factors to that trend. Evaporation measurement The King s Park Meteorological Station is located at 22 19'N 'E at an elevation of 66 m above mean sea level. The two evaporation pans (Figure 1) installed at the station are commonly known as US Class A evaporation pans, which are widely used throughout the world as evaporimeters (WMO, 1996). The two evaporation pans were installed less than 10 m apart and each consists of a stainless steel circular pan, 1.20 m in diameter and 254 mm deep, supported 30 to 50 mm above ground on a wooden platform and filled with fresh Figure 1. One of the US Class A evaporation pans installed at the King s Park Meteorological Station in Kowloon, Hong Kong. water. The water level is measured using a hook-gauge, which consists of a moveable scale and vernier fitted with a hook, the point of which is set to a reference level at 5 cm below the rim of the pan. Each day at 1100 local time (LT), staff of the Observatory measure the evaporation by adding water to the pan so that the water level returns to the level indicated by the hook-gauge. The amount of water added represents the evaporation over the past 24 hours. During times of heavy or persistent rain, water has to be removed from the pan in order to return the water level to that indicated by the fixed-point gauge. The evaporation is calculated with adjustment for rainfall by: where: E o E o = W a + RF or E o = RF W r (1) W a W r is the evaporation over the past 24 hours (in mm) is the water added to the pan (in mm) is the water removed from the pan (in mm) RF is the 24-hour rainfall as measured by an ordinary 203 mm diameter raingauge installed close to the evaporation pan However, at times of particularly inclement weather, when it is not possible to take measurements, or of heavy rain that causes the evaporation pan to overfill rendering measurements impossible, the evaporation for those days was estimated by applying the empirical formulae obtained in the 1976 study (Chen, 1976) using relevant meteorological data. The data from one of the evaporation pans were used as the standard. Quality control was maintained by direct comparison of its data with those of the other evaporation pan. Both sets of data were highly consistent with one another. Pan evaporation estimation Evaporation occurs whenever the relative humidity of the air is below 100% and is 147

2 Pan evaporation in Hong Kong affected by other meteorological factors, such as wind and solar radiation. Estimation of pan evaporation has two approaches: (1) an aerodynamic or mass transfer approach; and (2) an energy balance or energy budget approach. Aerodynamic approach The empirical formula derived by Chen (1976) using an aerodynamic approach to estimate the monthly pan evaporation was: where: E aero E aero = ( U)(e w e d ) (2) U e w e d is the aerodynamic evaporation estimate in mm month 1 is the mean wind run of the month in km day 1 measured 152 mm above the rim of the evaporation pan (WMO, 1996) is the saturated vapour pressure in hpa corresponding to the mean evaporation pan water temperature of the month is the saturated vapour pressure in hpa corresponding to the mean dew point temperature of the month where: E p is the evaporation estimate in mm month 1 using the Penman approach A is the slope of the saturated vapour pressure versus temperature curve at the air temperature in hpa C 1 H is the evaporation equivalent of the net radiant energy in mm month 1 C is a constant to maintain consistent units. Chen, using data collected at King s Park, empirically determined this to 1.7 hpa C 1 (Chen, 1976) E a is the evaporation rate in mm month 1 estimated by the aerodynamic approach. This is similar to Equation (2) except that the vapour pressure deficit is replaced by e a e d, where e a is the saturated vapour pressure corresponding to the mean air temperature, and takes the form: E a = ( U)(e a e d ) (4) Measured evaporation versus estimated evaporation Application of Equations (2), (3) and (4) in estimating the monthly pan evaporation using meteorological data from 1958 to 2004 shows a gradual overestimation by the formulae. Figure 2 and Table 1(a) summarize the estimated pan evaporation using Equations (2), (3) and (4) and clearly show that the estimates changed from a slight underestimation of around 5 mm month 1 in the 1960s to an overestimation by around 10 mm month 1 in the 1990s. Furthermore, the overestimation by the aerodynamic approach formula is much higher than that by the energy balance approach formula, by nearly 20 mm month 1 in recent years. To understand why both approaches significantly overestimated the pan evaporation in recent years, a closer examination of the aerodynamic influence has been made, as this is common in both approaches. The idea behind the aerodynamic approach is that air movement above the surface of an undisturbed body of water increases the rate of evaporation by Energy balance approach Penman (1950) derived a combination of equations to estimate the natural evaporation from an open body of water by simultaneously considering the theoretical concept of energy budget and empirical aerodynamic methodologies. The resultant formula to estimate the monthly pan evaporation was: E p AH + CE = A + C a (3) Figure 2. Evaporation estimated by the aerodynamic and energy balance approaches compared with measured pan evaporation between 1958 and Table 1 Pan evaporation estimates using the aerodynamic and energy balance approaches mm month 1 Measured mean monthly pan evaporation (a) Aerodynamic estimate: Mean Equation (2) Difference Energy balance estimate: Mean Equation (3) with (4) Difference (b) Aerodynamic estimate: Mean Equation (6a) Difference Energy balance estimate: Mean Equation (2) with (6b) Difference

3 transporting away moisture-laden air and bringing in unsaturated air to the water surface. The general form of the equation to estimate pan evaporation is: E aero = f(u) D (5) where: f (U) is a function of wind run D is the vapour pressure deficit at the surface of the water This postulates that the pan evaporation would be directly related to wind run and the difference in saturated vapour pressures corresponding to the water temperature at the water surface and the dew point of the air immediately above the water surface (vapour pressure deficit). The vapour pressure deficit, D, is taken as e s e d, where e s is the saturated vapour pressure corresponding to the temperature at the water surface and e d is the saturated vapour pressure corresponding to the dew point temperature just above the water surface. However, as the temperature at the surface of the water is difficult to determine, e s is normally replaced by e w, the saturated vapour pressure corresponding to the temperature of the water just beneath the surface, which is normally used to represent the temperature of the water at the surface. Chen s (1976) derivation of Equation (2), using data from 1958 to 1975, suggests that the ratio of evaporation to vapour pressure deficit would vary linearly with wind run. However, using observed mean monthly values for evaporation, vapour pressure deficit and wind run since 1970, Li et al. (2005) showed that the relationship between the ratio of evaporation to vapour pressure deficit with wind run deviates from the linear relationship described by Chen (1976) with low wind runs (Figure 3). A better representation would be obtained by a non-linear form resulting in the following equation to estimate the monthly pan evaporation by the aerodynamic approach: E aero = 3U 0.35 (e w e d ) (6a) Equation (6a) has the additional property that it would closely approximate to Equation (2) with wind runs greater than 100 km day 1 (Figure 3). In the energy balance approach, the aerodynamic contribution, Equation (4), can therefore be rewritten as: E a = 3U 0.35 (e a e d ) (6b) Table 1(b) summarizes the results of pan evaporation estimated using Equations (6a) and (6b), which show that the difference between the estimated and measured monthly pan evaporation was less than 5 mm month 1 from 1980 onwards, but the same degree of agreement with Equation (2) prior to 1980 was retained. Figure 3. Relationship of wind run with pan evaporation and vapour pressure deficit. Figures 4 and 5 show the improved estimation after applying the new aerodynamic formulae to data obtained between 2000 and Evaporation in Hong Kong Between 1958 and 2004, the measured monthly pan evaporation at the King s Park Meteorological Station decreased by 28% from an average of 146 mm month 1 in the 1960s to 105 mm month 1 in the 2000s (Table 2). However, looking at the evaporation trend more carefully, three distinct periods can be identified: a steady period prior to 1970, a declining period between 1970 and 1994 and a rising period then after (Figure 6). Over the period of decrease between 1970 and 1994, the greatest rate of change in the monthly evaporation occurred within the 1980s when it fell by 2.75 mm year 1. After 1994, the rate of change reversed and an increasing trend was observed. To identify possible causes of the variations in pan evaporation between 1970 and 2004, several key meteorological elements are analysed in this study: wind run, vapour pressure deficits and solar radiation. Variations of these meteorological elements are summarised in Table 2 and illustrated in Figures 7 to 9. This study adopts the commonly used regression and t-test techniques for longterm trend analysis (Leung et al., 2004). Linear regression lines were fitted to data by least squares and the trend inferred from the slopes of these fittings. A two-tailed t-test applied to test the statistical significance of the trends at 5% significance level is defined as: t = r n r where: r is the correlation coefficient n is the degree of freedom Figure 4. Estimation of evaporation by the aerodynamic method ( ). [devap is the absolute difference between the meaured and estimated pan evaporation.] (7) Pan evaporation in Hong Kong 149

4 Pan evaporation in Hong Kong Figure 5. Estimation of evaporation by the energy balance method ( ). [devap is the absolute difference between the measured and estimated pan evaporation.] Figure 6. Mean monthly evaporation at King s Park Meteorological Station ( ). From Table 2 and Figure 7, the average monthly vapour pressure deficits (e a e d ) and (e w e d ) show slight rising trends of 0.02 hpa year 1 and 0.03 hpa year 1 respectively between 1958 and The average daily wind run, U, (Figure 8) and the monthly total solar radiation, R s, (Figure 9) however, were found to decrease at a rate of 3.53 km day 1 year 1 and 3.00 MJ m 2 year 1 respectively. The t-test showed that all trends for the period were statistically significant at 5% significance level, even though linear monotonic trends may be too simple a measure in practice, especially for the wind run and global solar radiation (see sections below). Change in evaporation due to changes in solar radiation and aerodynamic mass transfer From Equation (3), any changes to evaporation will be partly due to changes in solar radiation and partly due to changes in aerodynamic mass transfer. It therefore follows that any changes in solar radiation, wind run and vapour pressure deficit would have a direct effect on evaporation. We can estimate the effects from changes in solar radiation, wind run and vapour pressure deficit on pan evaporation by examining the differential form of Equation (3), which can be expanded to: (8) H, in Equation (8), is the net solar radiation defined as the difference between the incoming short-wave solar radiation (R s ), adjusted for albedo, and the outgoing longwave back radiation (R b ): (9) where: r is the reflection coefficient and is taken to be 0.05 for open water (Chen, 1976) R s is the incoming short-wave solar radiation in MJ m 2 month 1 R b is the long-wave back radiation in MJ m 2 month 1 λ is the latent heat of vapourisation in MJ kg 1 The outgoing long-wave back radiation (R b ) was calculated using an empirical formula proposed by the British Ministry of Agriculture, Fisheries and Food, modified to correct for the units used in this report (Ministry of Agriculture, Fisheries and Food 1967). A trend plot of R b showed the longwave back radiation had been relatively constant (Figure 10), hence any changes in the net solar radiation (H) can only be due to changes in the incoming short-wave solar radiation. Table 2 Evaporation estimates using the aerodynamic and energy balance approaches. Trend* Measured mean monthly mm evaporation Rate of change mm year SVP deficit, (e a e d ) hpa SVP deficit, (e w e d ) hpa Wind run, U km day Monthly solar radiation, R s MJ m Note: * Trend values are determined with linear regression with significance level at 5% determined by t-test.

5 Subsequently, E p can be approximated by: (10) where D is the mean vapour pressure deficit in hpa over the period. The three terms on the right-hand side of Equation (10) correspond respectively to the contributions to the changes in pan evaporation from changes in solar radiation E solar (term 1), wind run E wind (term 2) and vapour pressure deficit E vpd (term 3). Since the pan evaporation showed a decrease during the period and an increase from 1995 to 2004 (Figure 6), the estimated changes in pan evaporation during those two periods were calculated using data corresponding to those periods. The results are summarized in Table 3. Results in Table 3 show that over the 25 years between 1970 and 1994, the combined effect of decrease in solar radiation of 116 MJ m 2 month 1 and wind run of 118 km day 1 would cause an estimated decrease in monthly pan evaporation by around 50 mm month 1. This is comparable with the observed 53 mm month 1 decrease in the monthly pan evaporation for the period. However, the effects from solar radiation and wind run were slightly offset by the small increase in vapour pressure deficit of 0.4 hpa, which would reduce the net decrease in the monthly pan evaporation to 47 mm month 1. This still compares favourably with the observed decrease. In the subsequent years from 1995 to 2004, the increase in the monthly pan evaporation can be attributable to increases in both solar radiation of 34 MJ m 2 month 1 and vapour pressure deficit of 1.1 hpa, offset by the continued decrease of wind run of 11 km day 1. Their combined effect would bring a net estimated increase of 9 mm month 1 in the monthly pan evaporation. This can be compared with the observed 16 mm month 1 increase. Figure 7. Mean vapour pressure deficit ( ). [es(tw), es(ta), and es(td) are the saturated vapour pressure corresponding to the water temperature, air temperature and dew point temperature respectively.] Figure 8. Mean daily wind run at King s Park Meteorological Station ( ). Pan evaporation in Hong Kong Contributing factors to pan evaporation trends From the estimates in pan evaporation changes (Table 3) calculated for the period , the period of decrease in pan evaporation roughly coincided with significant decreases in solar radiation and wind run, offset by slight increases in vapour pressure deficit. The period of increased pan evaporation after 1994 roughly coincided with increases in solar radiation and vapour pressure deficit, offset by the continuous, though slower, decrease in wind run. Calculations presented in Table 3 show that the changes in global solar radiation and wind run in Hong Kong over recent decades could explain the decrease in pan evaporation. On Figure 9. Mean monthly global solar radiation at King s Park Meteorological Station ( ). 151

6 Table 3 Estimation of the change in pan evaporation due to change in global solar radiation, wind run and vapour pressure deficit (Equation 10). Pan evaporation in Hong Kong Change in evaporation due to: Change in Change in Change in Solar Wind run Vapour Total change Total solar wind run vapour radiation pressure in observed radiation pressure deficit evaporation change in radiation Rs U D E solar E wind E vpd E p E Obs (= E solar + E wind + E vpd ) MJ m 2 month 1 km day 1 hpa mm mm mm mm mm Figure 10. Equivalent evaporation rate estimated from the monthly net radiant and long-wave back radiant energies ( ). the other hand, increases in the vapour pressure deficit and solar radiation could only partly explain the increase in pan evaporation over the past decade. Rapid urbanization over the last few decades has seen an increasing number of high-rise buildings in and around the King s Park area (Figures 11(a) and (b)). This has altered the flow pattern and the wind strength, resulting in an overall reduction of wind run. Added to this is the increase of air temperature, especially over the past decade. Leung et al. (2004) have shown that air temperatures at the Hong Kong Observatory have risen more rapidly since 1989 at a rate of 0.61 degc per decade, more than three times the post-war average increase of 0.17 degc per decade for the period Since saturated vapour pressure increases monotonically with temperature, it follows that the saturated vapour pressure would also increase with the rising air temperature. Furthermore, the increasing trend in the vapour pressure deficit (Figure 7), especially the more rapid increase seen between 1995 and 2004, implies that the air temperature has been increasing faster than the dew point temperature. The periods of decreasing and increasing solar radiation in Hong Kong correspond (a) well with findings from recent studies on trends in solar radiation worldwide (Pinker et al., 2005; Schiermeier, 2005; Wild et al., 2005). These findings highlight a general decrease in global solar radiation from the 1960s to the 1990s but an increase from the 1990s onwards. In these studies, possible causes for the variation of solar radiation have been linked to changes in cloud cover that would lead to changes in the diurnal temperature range, and in aerosols and particulates in the atmosphere that would cause changes in the atmospheric transparency. In the case of Hong Kong, there has been a slow increase in the overall cloud cover as well as increased incidents of reduced horizontal visibility due to haze over the past few decades (Leung et al., 2004). The increase in solar radiation seen in Hong Kong since the mid-1990s and its relation to the above observations warrant further investigation. Evaporation data in Hong Kong are essential for hydrological studies as well as for the development, planning, design and operations of lakes and reservoirs around the territory. In a recent application, the pan Figures 11(a) and 11(b). Environmental changes in and around the King's Park Meteorological Station in Hong Kong. Figure 11(a) (above left) shows the view looking west from the King s Park Meteorological Station around the 1950s. The view behind the white building was unobstructed and the height of the buildings in the background was about the ground level of the station. Figure 11(b) (above right) shows the same view looking west from the King's Park Meteorological Station as at The white building shown in Figure 11(a), and identified by an arrow in Figure 11(b), has been dwarfed by structures at the station, the tree line behind it and also the tall buildings in the background. (b)

7 evaporation data were used to estimate the loss of water from a man-made lake. The result showed that the water loss from the lake far exceeded what would normally occur through natural evaporation such that further engineering investigations should be called upon. Summary Formulae empirically derived from the first comprehensive study of pan evaporation in Hong Kong in 1976 overestimated pan evaporation over the past three decades. Using data obtained since the first study, a new set of empirical formulae has been derived. The result is that under the aerodynamic approach, a non-linear relationship proves to be better in representing the relationship between wind run and evaporation. It is reasoned that this non-linear relationship in the aerodynamic formula is related to the lower efficiency of wind in removing saturated air and bringing in relatively drier air to the surface of a water body during low wind conditions. Over the period , the mean monthly pan evaporation at King's Park had decreased by around 1.34 mm year 1 overall but three distinct periods could be identified: steady values prior to 1970, a decline from 1970 to the mid-1990s and an increase from the mid-1990s to Based on estimates in pan evaporation changes calculated for the period , the major contributing factors that led to a fall in pan evaporation were found to be the decrease in wind run coupled with reduction in solar radiation. However, the rise in pan evaporation between 1994 and 2004 can only be partly explained by the increase in solar radiation and the continued rise in the vapour pressure deficit, which is closely related to rising temperatures over the past few decades. Schiermeier Q Cleaner skies leave global warming forecasts uncertain. Nature, 435: 135. Wild M, Gilgen H, Roesch A, Ohmura A, Long CN, Dutton EG, Forgan B, Kallis A, Russak V and Tsvetkov A From dimming to brightening: decadal changes in solar radiation at Earth s surface. Science, 308: World Meteorological Organization (WMO) Guide to Meteorological Instruments and Methods of Observation, 6th edition. WMO-No 8, Part 10. Correspondence to: Mr John K.W.Chan, Hong Kong Observatory, 134A Nathan Road, Kowloon, Hong Kong. jkwchan@hko.gov.hk Royal Meteorological Society, 2007 DOI: /wea.29 Pan evaporation in Hong Kongn References British Ministry of Agriculture, Fisheries and Food Potential transpiration. Technical Bulletin No. 16, HMSO. Chen TY Evaporation and evapotranspiration in Hong Kong. Technical Note No. 42, Hong Kong Observatory. Leung YK, Yeung KH, Ginn EWL and Leung WM Climate change in Hong Kong. Technical Note No. 107, Hong Kong Observatory. Li SW and Chan JKW Analysis of evaporation and its long term trend in Hong Kong. 19th Guangdong-Hong Kong- Macau Seminar on Meteorological Science and Technology (in Chinese only). Penman HL Evaporation over the British Isles. Q. J. R. Meteorol. Soc. 76(330): Pinker RT, Zhang B and Dutton EG Do satellites detect trends in surface solar radiation? Science, 308: