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The mixed results concerning the ‘oasis effect’ in a rural settlement in the Negev Desert, Israel
ARTICLE IN PRESS Journal of Arid Environments Journal of Arid Environments 58 (2004) 235–248 www.elsevier.com/locate/jnlabr/yjare
The mixed results concerning the ‘oasis effect’ in a rural settlement in the Negev Desert, Israel Hadas Saaroni*, Arieh Bitan, Eyal Ben Dor, Noa Feller Department of Geography and the Human Environment, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel Received 22 July 2002; accepted 15 August 2003
Abstract The development of an ‘oasis effect’ in Kibbutz Mash’abei-Sadeh, a rural farm settlement within the Negev Desert was examined. Measurements took place during four summer days (July 30–August 2, 2000). Data indicated that whereas a slight ‘oasis effect’ was noted in the morning and afternoon hours (although the temperature differences of up to 2.3 C between the kibbutz and its surroundings were not significant), an opposite tendency was noted during noontime and nighttime with kibbutz temperatures being higher than the surrounding desert. The results indicate the complex thermal picture. The various mechanisms and causes for these results are discussed. r 2003 Elsevier Ltd. All rights reserved. Keywords: Oasis effect; Heat stress; Rural settlement
1. Introduction The ‘oasis effect’ is defined as a change in the microclimatological conditions in a vegetated (green) area as distinguished from an area without vegetation. The change is manifested in lower temperatures and higher relative humidity (Oke, 1987). According to Oke (1987), a green area is more humid and therefore cooler than its surroundings due to evapotranspiration processes that increase the proportion of latent heat in comparison with the sensible heat in accordance with Bowen ratio of energy balance. Also, shade from the vegetation prevents direct radiation from *Corresponding author. Tel.: +972-3-6406-470; fax: +972-3-6406-243. E-mail address: [email protected] (H. Saaroni). 0140-1963/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2003.08.010
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reaching the ground surface and warming it, thereby resulting in lower air temperatures above these surfaces. The oasis effect develops in any place where there is a source of humidity and therefore evapotranspiration is taking place. Kai et al. (1997), who studied the extent of the effect in the Gobi Desert in China, defined the phenomenon as the surface differences in the radiation balance of a dry and a green area (oasis). He defines the effect in terms of radiation (and not temperature as does Oke), which results in lower surface and aboveground temperatures in a desert oasis during daylight hours. Naot and Mahrer (1991) simulated and measured the effects of a 160-m-wide irrigated crop in the countryside in severely arid conditions and with maximum temperatures of over 45 C. At a height of 1.5 m (identical to the level of the canopy), they found that temperatures in the cultivated area were 7 C lower in the early afternoon hours and 5 C lower at midnight than in the surrounding uncultivated area. Studies of the influence of vegetation on microclimatic conditions in different types of regions (urban as opposed to non-urban, desert as opposed to non-desert) found, for the most part, a decrease in surface and aboveground temperatures due to the influence of vegetation. However, while temperature differences of more than 15 C were found at the surface, the differences in air temperatures fluctuated, for the most part, around 3 C, with the maximum difference reaching 7 C (Bernatzky, 1982; Parker, 1983, 1987, 1989; Sebba and Enis, 1984; Taha et al., 1989, 1991, 1992; Givoni, 1991; Huang et al., 1992; Sporken-Smith and Oke, 1998; Kai et al., 1997; Taha, 1997). However, other observational evidence supports this finding only in part. For instance, Jauregui (1990/91) found that Chapultepec Park in Mexico City was not cooler and even slightly warmer than its built-up surroundings during the daytime hours. Grimmond et al. (1996) studied the differences in energy balance between a ‘‘green’’ neighborhood and a neighborhood with a very little vegetation coverage in Los Angeles and found that the former was warmer by about 1 C in the daytime. This was explained as resulting from the lower albedo of the vegetation. The objectives of the present research were: (a) To determine whether Kibbutz Mash’abei-Sadeh, a rural and vegetated settlement located in a non-vegetated desert area (and therefore regarded as an oasis) creates an ‘oasis effect’. (b) To determine the extent, timing and specific location (regarding land use within the kibbutz) of this effect.
2. Methodology 2.1. The study area The study was carried out in Kibbutz Mash’abei-Sadeh (a rural farming settlement) and its nearby surroundings. Kibbutz Mash’abei-Sadeh is located in a
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Fig. 1. (a) The location of the kibbutz, (b) the general location of the fixed measuring stations (marked in numbers) and the traverse route measurements (solid line) on an aerial photograph of the region. Station no. 6 was located 1 km north of the kibbutz and is not shown in the figure, (c) a general panoramic view of the kibbutz.
flat area within the Negev Highlands at an altitude of 370 m a.s.l. (Fig. 1a), in a . region with an arid climate—BW according to Koppen. The kibbutz occupies an area of 3 km2 of which the ‘‘green’’ residential area is spread over about 1.5 km2, located in the western and central parts of the kibbutz. The ‘‘green’’ area consists of lawn fields and isolated trees (Fig. 1b and c). The dining room is located at the center of the lawn area. At the southwestern residential area there is a pine grove, whereas the hencoops, cowsheds and school, all with minimal vegetation, are located in the eastern and northeastern parts of the kibbutz.
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2.2. Climatic conditions The weather conditions over the Eastern Mediterranean (EM) are highly consistent during the summer season, i.e., June–September, particularly in July– August, typified by permanent heat stress over Israel (Bitan and Rubin, 1994). The lower levels are dominated by the so-called ‘‘Persian Trough’’ (Bitan and Saaroni, 1992; Saaroni and Ziv, 2000), a surface low-pressure trough that extends from the Asian Monsoon low through the Persian Gulf and further, along southern Turkey, down to the Aegean Sea. As a result, the winds over the EM, i.e., the Etesian winds (e.g. Air Ministry, 1962; Prezerakos, 1984), flow from the northwest. It is worth noting that the system that dynamically dominates the southeastern part of the EM, including southern Israel, is the subtropical high, represented by a ridge that extends from Egypt northeastward (Alpert et al., 1990). The upper levels are characterized by persistent subsidence causing warming and drying, which counteracts the lower level advective cooling from the Mediterranean Sea (Saaroni and Ziv, 2000). Therefore, the standard deviation (STD) of the lower-level temperatures over the region attains its annual minimum, 2.8 C at the 850 hPa level, in July–August, expressing the high persistency of conditions during this season (Saaroni and Ziv, 2000). Table 1 presents the average climatic conditions in Kibbutz Mash’abei-Sadeh for the four studied days, July 30 through August 2, 2000, in comparison to those measured in Kibbutz Sde-Boker located 12 km south of Kibbutz Mash’abei-Sadeh at an altitude 470 m and in comparison to the long-term average data (1964–1979) for July–August in Kibbutz Sde-Boker. The data point to the discomfort conditions characterizing the region during mid-summer and the relatively warm and severe conditions during the research days. The research days were characterized by a shallow Persian Trough with northwest winds in the lower layers of the atmosphere and an upper level ridge that are typical for the summer season. The stronger intensity of the upper level subsidence and the weaker effect of the lower level cool advection due to the weakening of the Persian Trough during the research days resulted in relatively warm conditions and an
Table 1 Climatic conditions in Kibbutz Mash’abei-Sadeh for the four studied days, 30/7/2000–2/8/2000, in comparison with those measured in Kibbutz Sde-Boker and the long-term average data (1964–1979) for July–August in Kibbutz Sde-Boker
Daily average temperature ( C) Average daily max. temp. ( C) Average daily min. temp. ( C) Absolute maximum temp. ( C) Relative humidity (14:00) (%) Daily heat stress hours Daily moderate and severe heat stress hours
Mash’abei-Sadeh 30/7/00–2/8/00
Sde-Boker 30/7/00–2/8/00
Sde-Boker July–August avg.
28.6 36.8 22.5 40.5 30.5 24 14.5
27.5 35.4 22.3 39.2 33.5 24 12.5
24.8 32.2 17.3 41.0 31.0 12 7
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increase in heat stress. Nevertheless, the research days well represent the persisting mid summer climatic conditions. 2.3. Methods Measurements were carried out during a consecutive 4-day period at the height of the hot season, between July 30 and August 2, 2000 and compared to an ETM+(Landsat 7) thermal image, taken on August 7, 1999. The measurements were conducted in a set of six fixed meteorological stations set up in and around the kibbutz that made continuous readings of the temperature, relative humidity, and wind (direction and velocity). Their location is shown in Fig. 1b. Campbell HMP45C-type temperature and relative humidity sensors were used. The sensors were installed inside a Campbell URS1 unaspirated radiation shield. The temperature and relative humidity accuracy of the sensors is 70.2 C and 72%, respectively. Wind speed and direction were measured using Young 05103 at a height of 2.50 m. Sensor accuracy was 70.3 m/s for the speed and 3 for the direction. According to specifications of the screen manufacturer (Young, 1998), the screen may overestimate the air temperatures during the daytime hours (up to 1.5 C under calm conditions and less than 0.7 C and 0.4 C with winds of 2 and 3 m/s, respectively). In order to avoid erroneous conclusions stemming from the screen effect, we compared the temperatures of stations exposed to different wind speeds to those of a station sheltered from the wind. Temperature differences were less than 0.2 C, indicating that the effect of the reduced ventilation in the exposed screens in the kibbutz was negligible. Readings were taken every second with the resulting data averaged and stored every 5 min using Campbell 21X data logger. For wind direction the mode direction was stored. All the instruments were calibrated, checked and compared before and after the experiment under the same conditions. Four stations were exposed to direct sun (station no. 1 at the northern entrance of the kibbutz on a bare clay soil, station no. 3 on the center lawn area near the dining room, station no. 5 at the southeastern edge of the kibbutz and station no. 6 which was located 1 km north of the kibbutz). Both stations 5 and 6 were located on bare clay soil. Two additional stations were set up inside the kibbutz in an area shaded by trees (station no. 2, well shaded under a tree with dense foliage in the center lawn area near the dining room; and station no. 4, partly shaded, in a pine grove). In addition, the air temperature was measured at thirty points along a route inside and around the kibbutz (referred to hereinafter as the traverse route measurements; see Fig. 1b for the route) using an NTC-DT029 temperature sensor (accuracy of 70.1 C). Readings were taken after standing for 60 s at each measuring point, until the sensor stabilized, allowing a reliable reading. Following this, the measuring team would drive to the next point by car. Two sensors, each located on a separate car, were used so that the entire round of measurement took about 20 min, and no time corrections were needed. The data were gathered in Fourier MultiLog electronic data banks. These measurements were carried out six times a day (at 04:00, 07:00, 10:00, 13:00, 17:00, and 21:00 h according to local solar time, LST). The temperature
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data were plotted on the aerial photograph, and isotherm lines were derived through interpolation. Heat stress, which highly affects comfort, was calculated from the fixed stations data according to the Thom (1959) and Sohar (1980) index in which: Heat stress index ð CÞ ¼ ðTd þ Tw Þ=2;
ð1Þ
where Td and Tw are dry bulb temperature and wet bulb temperature, respectively. According to the heat stress index, no heat stress exists when the index is lower than 22 C. Values of 22–23.9 C are defined as mild heat stress, 24–27.9 C as moderate heat stress, and values over 28 C indicate severe heat stress. The thermal characteristics of the surface layer were studied using the thermal band (#6) of the ETM+ sensor as acquired from satellite orbit on August 7, 1999, at 10:00 (Figs. 2b and c). For comparison, we generated a natural color image using bands 3,2,1 encoded red, green, blue, respectively (each pixel representing a ground area of 30 m2, Fig. 2a). Due to the relatively low spatial resolution of the thermal band (each pixel representing a ground area of 90 m2), the images, shown in Figs. 2b and c, provide only a general picture of the thermal distribution pattern of the kibbutz and its surrounding arid areas. The raw thermal band was calibrated to a brightness temperature using emissivity value of unity. After applying standard classification methods, using the other six bands of the sensor (pixel representing 30 m2), the emissivity of each pixel was estimated and the absolute radiant temperature was calculated. The image was geo-referenced to a map, using a point-to-point registration.
3. Results 3.1. Thermal image Figs. 2b and c present the thermal characteristics of the surface layer for the summer season, for August 7, 1999, at 10:00. The thermal surface image indicates clearly that the surface level of the kibbutz was cooler than the surrounding arid area indicating a ‘‘cool island’’ at the surface level of the kibbutz. It is worth noting that the surface level of the kibbutz integrates various surfaces and materials, i.e., lawn areas, trees, buildings, asphalt surfaces, bare soil areas etc, and it includes different heights, i.e., top of the tree canopy, roofs, building walls and surface level. Thus, the image presents an average thermal picture of this heterogeneous surface. As expected, the coldest surfaces during daytime are the water reservoir north of the kibbutz, the irrigated fields west of the kibbutz, and the wet roof of the hencoop that are cooler by 20 C than the warmest surfaces. The warmest surfaces are the ploughed fields, B45 C, due to their relatively dark color that reduces the albedo and their dryness that increases the sensible heat flux. It is known that surface temperatures have a dominant effect on the air temperatures. However, the correlation between them is highly complicated, especially during daytime, and with strong turbulence (Arya, 2001; Ben Dor and
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Fig. 2. The ETM+ images of the area (August 7, 1999, 10:00): (a) natural color view using bands 3,2,1 encoded red, green, blue, respectively (pixel size of 30 m2), (b) the temperature image of the area surrounding the kibbutz (pixel size of 90 m2) after correcting to the area emissivity, (c) expansion of the kibbutz area only with relevant targets within the area: (1) residential area, (2) water bodies, (3) irrigated fields, (4) ploughed fields, (5) wet roof.
Saaroni, 1997). The thermal image, available only for 10:00 LST, indicates an average ‘‘cool island’’ at the surface level of the kibbutz, and it is compared to the spatial distribution of the air temperature analysed in the next section.
3.2. Spatial distribution of air temperature A decrease in air temperatures was noted during the four study days from 40.5 C (an average air temperature of the six fixed stations) on the first day to 34.5 C on the fourth day. Spatially, the maximal difference in temperature (both the six fixed stations and the 30 traverse route measurements) was 2.3 C. This was measured at 09:00 LST the second day: the high reading was found at the northern entrance of the kibbutz above a bare clay soil and the low one under a shaded tree in the center lawn area of the kibbutz. However, in 91% of the cases, the temperature differences were not more than 1.5 C.
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Fig. 3. The temperature distribution ( C) at: (a) 04:00, (b) 10:00, (c) 13:00, (d) 17:00, (e) 21:00, (f) 07:00.
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Fig. 3 (continued).
Figs. 3a–f show the isotherms that were derived from the average temperature over the 4 days of measurements in each of the locations according to the data from the traverse route measurements, at 04:00, 10:00, 13:00, 17:00, 21:00 and 07:00 h. At 04:00 h (Fig. 3a), spatial temperature differences were up to 1 C. The highest temperatures were registered in the green area inside the kibbutz, with a gradual fall in temperature toward the kibbutz’s perimeter. And thus, although slight, a ‘‘heat island’’ was found at 04:00 in the heart of the kibbutz. At 10:00 h (Fig. 3b), spatial temperature differences were up to 1.5 C. Lower temperatures were found in the greener area at the western part of the kibbutz indicating a moderate oasis effect. At 13:00 (Fig. 3c), when the highest temperatures were recorded, the temperature differences were up to 1.3 C, with the highest temperatures being found precisely in the green areas in the western part of the kibbutz. The kibbutz periphery and the open area around it were cooler than the central area. At 17:00 (Fig. 3d) temperature differences were up to 1.3 C, but as at 10:00 h, the western green area of the kibbutz was cooler than the kibbutz’s perimeter, indicating a moderate oasis effect. At 21:00 (Fig. 3e) there was almost no variation in temperature over the area studied and at 07:00 (Fig. 3f), while differences of up to 1.2 C were measured, no particular spatial tendency was found.
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3.3. Heat stress Heat stress was calculated from the continuous data measured at the six fixed stations (Fig. 4). During the 4 days of the study there was heat stress during almost 24 h of the day, of which 14.5 h were characterized by moderate or severe heat stress. Average maximal heat stress of the six stations was 31 C on the first day and 28 C on the fourth day. Fig. 5 presents the deviations of heat stress at each of four fixed stations from the average heat stress of the six fixed stations over the four days of the study. These four stations represent the most significant differences. In view of the low and non-significant variations in the conditions of temperature and relative humidity among the six measuring stations in the kibbutz, no significant differences were found in heat stress among the different areas of the kibbutz and its surroundings. However, around noontime, there was a slight increase in heat stress in the center of the kibbutz and in its northern perimeter, in the two stations exposed to the sun, one located on the lawn area and the other in an area without vegetation. A slightly lower heat stress was found in the two shaded stations inside the kibbutz, especially in the morning around 09:00 LST, a result attributable to the shade. In the evening and at night there was very little difference among the stations (for the most part, less than 0.5 ) although the shaded stations had somewhat higher heat stress, attributable to the greenhouse effect.
4. Discussion and conclusions The study area presents a complex picture. The daytime thermal image showed relatively lower surface temperatures in the kibbutz, by 10–20 C, in comparison
32 30
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Mild 22 No Heat Stress
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Fig. 4. Average heat stress ( C) for the fixed measuring stations during the 4 study days.
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Fig. 5. Heat stress deviations ( C) at each of four fixed measuring stations from the average heat stress of the six fixed stations during the 4 study days. The four stations are station no. 2 (shaded) and station no. 3 (exposed), both of these at the center of the kibbutz; station no. 1 in the northern part of the kibbutz; and station no. 6 located 1 km north of the kibbutz.
with the arid, non-vegetated surfaces surrounding the kibbutz and thermal differences of more than 20 C between the ploughed fields and the water bodies, but according to air temperature measurements, the differences are small and not consistent. Air temperature difference between the vegetated areas in the kibbutz and the arid non-vegetated areas was not more than 1.5 C in 91% of the measurements, and the maximum temperature difference recorded was 2.3 C. In 70% of the measurements, the difference between the relative humidity measurements was not more than 6% and the maximum difference was 12.5%. The maximum difference was recorded at 09:00 LST between the station located at the northern part of the kibbutz (station no. 1 in Fig. 1b) and the shaded station in the center of the kibbutz (station no. 2 in Fig. 1b). Similar results were found by the traverse measurements with differences of 1.5 C and 1.3 C at 10:00 and 17:00, respectively, between the greener area at the western and central part of the kibbutz and the non-vegetated surroundings at the eastern part of the kibbutz. Around noontime (13:00 h), an opposite tendency was observed, with the highest temperatures (no more than 1 C difference) being measured in the center of the kibbutz, the lawn area. Only the station located under the tree was cooler, by 1 C. From 21:00 until 07:00 higher temperatures characterized the green areas in the center and western part of the kibbutz (again, a difference of no more than 1 C). Two pilot studies, each about 36 h long, were conducted during the month of September 1999 and showed similar results. The night-time findings are in accordance with the hypothesis that nocturnal cooling is more efficient in exposed arid areas than in vegetated areas because the
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vegetation (due to both the larger amount of water vapor around it and the canopy) creates a greenhouse effect that impedes the cooling of the green area (Oke, 1987; Taha et al., 1991). The noontime air temperature findings contradict the study hypothesis that predicted a significant daytime cooling effect, an ‘oasis effect’, in the vegetated area of the kibbutz. A number of explanations may be offered to account for the current findings: (a) The lack of an ‘oasis effect’ during noontime may result from a cessation of transpiration (water being transported via the leaf pores out into the air). By halting transpiration the plant saves water (Raven, 1999), and thus, by not contributing humidity to the air, impedes the cooling process by evapotranspiration. This hypothesis is supported by the slight decrease in water vapor pressure that was observed during noontime (not shown). However, the shaded areas were cooler also during noontime indicating the importance of minimizing sun-exposed surfaces in a dry and warm region. (b) The lack of an ‘oasis effect’ during noontime may result from the kibbutz design and heat-producing activities within the kibbutz. A concentration of buildings and relatively large concrete and asphalt surfaces in the middle of the kibbutz may create a ‘‘heat island’’ (Oke, 1987). In addition, the various kibbutz buildings such as the kitchen, laundry and factory emit warm air. Hot air is also emitted from the air conditioners operating in all the kibbutz buildings, particularly during the hottest daytime hours. This intensive activity is the source of significant heat emission inside the kibbutz boundaries, a phenomenon not present in the classical desert oasis—either farmland or a ‘‘green’’ area without anthropogenic activity. This activity continues during the remaining hours as well, and may be responsible for blurring the ‘oasis effect’ during the morning and afternoon hours. The continued operation of the air conditioning units at night and less efficient long wave radiational cooling by the buildings and the green areas may also result in the development of a nocturnal ‘‘heat island’’. (c) The oasis effect may be moderated by the relatively strong wind during noontime. It is well known that microclimatic conditions, such as radiation frost and urban heat island are well developed under calm conditions, while an increase in wind speed weakens and even prevents their development (Oke, 1987). Average noontime wind speed of 7–14 ms1 was measured at the kibbutz periphery in comparison to only 3 ms1 in the center of the kibbutz (Fig. 6). This increase in wind speed during noontime may have reduced the extent of temperature variation between the different areas. All the above hypotheses may work in concert. Evapotranspiration as well as shading play a role in creating an ‘oasis effect’. Nevertheless, one or two factors may be found to be cardinal in explaining the research findings. Additional research is therefore needed. One should however note that if kibbutz design plays a role in determining the extent of the ‘oasis effect’, more careful planning, aiming to intensify the ‘oasis effect’, might be recommended. One of the most important implications is
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Fig. 6. Wind speed (m s1) at the fixed measuring stations during the 4 study days.
the need for more shaded areas and minimizing the sun-exposed lawn area that tends to warm up and also uses a lot of water that is in short supply in this dry region.
Acknowledgements This study is supported by the Israeli Science Foundation (ISF, Grant no 828/02). Special thanks are due to Yaron Yaakov for technical support in carrying out the measurements and to Orna Zafrir-Reuven for drawing Fig. 1a.
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The mixed results concerning the ‘oasis effect’ in a rural settlement in the Negev Desert, Israel Hadas Saaroni*, Arieh Bitan, Eyal Ben Dor, Noa Feller Department of Geography and the Human Environment, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel Received 22 July 2002; accepted 15 August 2003
Abstract The development of an ‘oasis effect’ in Kibbutz Mash’abei-Sadeh, a rural farm settlement within the Negev Desert was examined. Measurements took place during four summer days (July 30–August 2, 2000). Data indicated that whereas a slight ‘oasis effect’ was noted in the morning and afternoon hours (although the temperature differences of up to 2.3 C between the kibbutz and its surroundings were not significant), an opposite tendency was noted during noontime and nighttime with kibbutz temperatures being higher than the surrounding desert. The results indicate the complex thermal picture. The various mechanisms and causes for these results are discussed. r 2003 Elsevier Ltd. All rights reserved. Keywords: Oasis effect; Heat stress; Rural settlement
1. Introduction The ‘oasis effect’ is defined as a change in the microclimatological conditions in a vegetated (green) area as distinguished from an area without vegetation. The change is manifested in lower temperatures and higher relative humidity (Oke, 1987). According to Oke (1987), a green area is more humid and therefore cooler than its surroundings due to evapotranspiration processes that increase the proportion of latent heat in comparison with the sensible heat in accordance with Bowen ratio of energy balance. Also, shade from the vegetation prevents direct radiation from *Corresponding author. Tel.: +972-3-6406-470; fax: +972-3-6406-243. E-mail address: [email protected] (H. Saaroni). 0140-1963/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jaridenv.2003.08.010
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reaching the ground surface and warming it, thereby resulting in lower air temperatures above these surfaces. The oasis effect develops in any place where there is a source of humidity and therefore evapotranspiration is taking place. Kai et al. (1997), who studied the extent of the effect in the Gobi Desert in China, defined the phenomenon as the surface differences in the radiation balance of a dry and a green area (oasis). He defines the effect in terms of radiation (and not temperature as does Oke), which results in lower surface and aboveground temperatures in a desert oasis during daylight hours. Naot and Mahrer (1991) simulated and measured the effects of a 160-m-wide irrigated crop in the countryside in severely arid conditions and with maximum temperatures of over 45 C. At a height of 1.5 m (identical to the level of the canopy), they found that temperatures in the cultivated area were 7 C lower in the early afternoon hours and 5 C lower at midnight than in the surrounding uncultivated area. Studies of the influence of vegetation on microclimatic conditions in different types of regions (urban as opposed to non-urban, desert as opposed to non-desert) found, for the most part, a decrease in surface and aboveground temperatures due to the influence of vegetation. However, while temperature differences of more than 15 C were found at the surface, the differences in air temperatures fluctuated, for the most part, around 3 C, with the maximum difference reaching 7 C (Bernatzky, 1982; Parker, 1983, 1987, 1989; Sebba and Enis, 1984; Taha et al., 1989, 1991, 1992; Givoni, 1991; Huang et al., 1992; Sporken-Smith and Oke, 1998; Kai et al., 1997; Taha, 1997). However, other observational evidence supports this finding only in part. For instance, Jauregui (1990/91) found that Chapultepec Park in Mexico City was not cooler and even slightly warmer than its built-up surroundings during the daytime hours. Grimmond et al. (1996) studied the differences in energy balance between a ‘‘green’’ neighborhood and a neighborhood with a very little vegetation coverage in Los Angeles and found that the former was warmer by about 1 C in the daytime. This was explained as resulting from the lower albedo of the vegetation. The objectives of the present research were: (a) To determine whether Kibbutz Mash’abei-Sadeh, a rural and vegetated settlement located in a non-vegetated desert area (and therefore regarded as an oasis) creates an ‘oasis effect’. (b) To determine the extent, timing and specific location (regarding land use within the kibbutz) of this effect.
2. Methodology 2.1. The study area The study was carried out in Kibbutz Mash’abei-Sadeh (a rural farming settlement) and its nearby surroundings. Kibbutz Mash’abei-Sadeh is located in a
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Fig. 1. (a) The location of the kibbutz, (b) the general location of the fixed measuring stations (marked in numbers) and the traverse route measurements (solid line) on an aerial photograph of the region. Station no. 6 was located 1 km north of the kibbutz and is not shown in the figure, (c) a general panoramic view of the kibbutz.
flat area within the Negev Highlands at an altitude of 370 m a.s.l. (Fig. 1a), in a . region with an arid climate—BW according to Koppen. The kibbutz occupies an area of 3 km2 of which the ‘‘green’’ residential area is spread over about 1.5 km2, located in the western and central parts of the kibbutz. The ‘‘green’’ area consists of lawn fields and isolated trees (Fig. 1b and c). The dining room is located at the center of the lawn area. At the southwestern residential area there is a pine grove, whereas the hencoops, cowsheds and school, all with minimal vegetation, are located in the eastern and northeastern parts of the kibbutz.
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2.2. Climatic conditions The weather conditions over the Eastern Mediterranean (EM) are highly consistent during the summer season, i.e., June–September, particularly in July– August, typified by permanent heat stress over Israel (Bitan and Rubin, 1994). The lower levels are dominated by the so-called ‘‘Persian Trough’’ (Bitan and Saaroni, 1992; Saaroni and Ziv, 2000), a surface low-pressure trough that extends from the Asian Monsoon low through the Persian Gulf and further, along southern Turkey, down to the Aegean Sea. As a result, the winds over the EM, i.e., the Etesian winds (e.g. Air Ministry, 1962; Prezerakos, 1984), flow from the northwest. It is worth noting that the system that dynamically dominates the southeastern part of the EM, including southern Israel, is the subtropical high, represented by a ridge that extends from Egypt northeastward (Alpert et al., 1990). The upper levels are characterized by persistent subsidence causing warming and drying, which counteracts the lower level advective cooling from the Mediterranean Sea (Saaroni and Ziv, 2000). Therefore, the standard deviation (STD) of the lower-level temperatures over the region attains its annual minimum, 2.8 C at the 850 hPa level, in July–August, expressing the high persistency of conditions during this season (Saaroni and Ziv, 2000). Table 1 presents the average climatic conditions in Kibbutz Mash’abei-Sadeh for the four studied days, July 30 through August 2, 2000, in comparison to those measured in Kibbutz Sde-Boker located 12 km south of Kibbutz Mash’abei-Sadeh at an altitude 470 m and in comparison to the long-term average data (1964–1979) for July–August in Kibbutz Sde-Boker. The data point to the discomfort conditions characterizing the region during mid-summer and the relatively warm and severe conditions during the research days. The research days were characterized by a shallow Persian Trough with northwest winds in the lower layers of the atmosphere and an upper level ridge that are typical for the summer season. The stronger intensity of the upper level subsidence and the weaker effect of the lower level cool advection due to the weakening of the Persian Trough during the research days resulted in relatively warm conditions and an
Table 1 Climatic conditions in Kibbutz Mash’abei-Sadeh for the four studied days, 30/7/2000–2/8/2000, in comparison with those measured in Kibbutz Sde-Boker and the long-term average data (1964–1979) for July–August in Kibbutz Sde-Boker
Daily average temperature ( C) Average daily max. temp. ( C) Average daily min. temp. ( C) Absolute maximum temp. ( C) Relative humidity (14:00) (%) Daily heat stress hours Daily moderate and severe heat stress hours
Mash’abei-Sadeh 30/7/00–2/8/00
Sde-Boker 30/7/00–2/8/00
Sde-Boker July–August avg.
28.6 36.8 22.5 40.5 30.5 24 14.5
27.5 35.4 22.3 39.2 33.5 24 12.5
24.8 32.2 17.3 41.0 31.0 12 7
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increase in heat stress. Nevertheless, the research days well represent the persisting mid summer climatic conditions. 2.3. Methods Measurements were carried out during a consecutive 4-day period at the height of the hot season, between July 30 and August 2, 2000 and compared to an ETM+(Landsat 7) thermal image, taken on August 7, 1999. The measurements were conducted in a set of six fixed meteorological stations set up in and around the kibbutz that made continuous readings of the temperature, relative humidity, and wind (direction and velocity). Their location is shown in Fig. 1b. Campbell HMP45C-type temperature and relative humidity sensors were used. The sensors were installed inside a Campbell URS1 unaspirated radiation shield. The temperature and relative humidity accuracy of the sensors is 70.2 C and 72%, respectively. Wind speed and direction were measured using Young 05103 at a height of 2.50 m. Sensor accuracy was 70.3 m/s for the speed and 3 for the direction. According to specifications of the screen manufacturer (Young, 1998), the screen may overestimate the air temperatures during the daytime hours (up to 1.5 C under calm conditions and less than 0.7 C and 0.4 C with winds of 2 and 3 m/s, respectively). In order to avoid erroneous conclusions stemming from the screen effect, we compared the temperatures of stations exposed to different wind speeds to those of a station sheltered from the wind. Temperature differences were less than 0.2 C, indicating that the effect of the reduced ventilation in the exposed screens in the kibbutz was negligible. Readings were taken every second with the resulting data averaged and stored every 5 min using Campbell 21X data logger. For wind direction the mode direction was stored. All the instruments were calibrated, checked and compared before and after the experiment under the same conditions. Four stations were exposed to direct sun (station no. 1 at the northern entrance of the kibbutz on a bare clay soil, station no. 3 on the center lawn area near the dining room, station no. 5 at the southeastern edge of the kibbutz and station no. 6 which was located 1 km north of the kibbutz). Both stations 5 and 6 were located on bare clay soil. Two additional stations were set up inside the kibbutz in an area shaded by trees (station no. 2, well shaded under a tree with dense foliage in the center lawn area near the dining room; and station no. 4, partly shaded, in a pine grove). In addition, the air temperature was measured at thirty points along a route inside and around the kibbutz (referred to hereinafter as the traverse route measurements; see Fig. 1b for the route) using an NTC-DT029 temperature sensor (accuracy of 70.1 C). Readings were taken after standing for 60 s at each measuring point, until the sensor stabilized, allowing a reliable reading. Following this, the measuring team would drive to the next point by car. Two sensors, each located on a separate car, were used so that the entire round of measurement took about 20 min, and no time corrections were needed. The data were gathered in Fourier MultiLog electronic data banks. These measurements were carried out six times a day (at 04:00, 07:00, 10:00, 13:00, 17:00, and 21:00 h according to local solar time, LST). The temperature
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data were plotted on the aerial photograph, and isotherm lines were derived through interpolation. Heat stress, which highly affects comfort, was calculated from the fixed stations data according to the Thom (1959) and Sohar (1980) index in which: Heat stress index ð CÞ ¼ ðTd þ Tw Þ=2;
ð1Þ
where Td and Tw are dry bulb temperature and wet bulb temperature, respectively. According to the heat stress index, no heat stress exists when the index is lower than 22 C. Values of 22–23.9 C are defined as mild heat stress, 24–27.9 C as moderate heat stress, and values over 28 C indicate severe heat stress. The thermal characteristics of the surface layer were studied using the thermal band (#6) of the ETM+ sensor as acquired from satellite orbit on August 7, 1999, at 10:00 (Figs. 2b and c). For comparison, we generated a natural color image using bands 3,2,1 encoded red, green, blue, respectively (each pixel representing a ground area of 30 m2, Fig. 2a). Due to the relatively low spatial resolution of the thermal band (each pixel representing a ground area of 90 m2), the images, shown in Figs. 2b and c, provide only a general picture of the thermal distribution pattern of the kibbutz and its surrounding arid areas. The raw thermal band was calibrated to a brightness temperature using emissivity value of unity. After applying standard classification methods, using the other six bands of the sensor (pixel representing 30 m2), the emissivity of each pixel was estimated and the absolute radiant temperature was calculated. The image was geo-referenced to a map, using a point-to-point registration.
3. Results 3.1. Thermal image Figs. 2b and c present the thermal characteristics of the surface layer for the summer season, for August 7, 1999, at 10:00. The thermal surface image indicates clearly that the surface level of the kibbutz was cooler than the surrounding arid area indicating a ‘‘cool island’’ at the surface level of the kibbutz. It is worth noting that the surface level of the kibbutz integrates various surfaces and materials, i.e., lawn areas, trees, buildings, asphalt surfaces, bare soil areas etc, and it includes different heights, i.e., top of the tree canopy, roofs, building walls and surface level. Thus, the image presents an average thermal picture of this heterogeneous surface. As expected, the coldest surfaces during daytime are the water reservoir north of the kibbutz, the irrigated fields west of the kibbutz, and the wet roof of the hencoop that are cooler by 20 C than the warmest surfaces. The warmest surfaces are the ploughed fields, B45 C, due to their relatively dark color that reduces the albedo and their dryness that increases the sensible heat flux. It is known that surface temperatures have a dominant effect on the air temperatures. However, the correlation between them is highly complicated, especially during daytime, and with strong turbulence (Arya, 2001; Ben Dor and
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Fig. 2. The ETM+ images of the area (August 7, 1999, 10:00): (a) natural color view using bands 3,2,1 encoded red, green, blue, respectively (pixel size of 30 m2), (b) the temperature image of the area surrounding the kibbutz (pixel size of 90 m2) after correcting to the area emissivity, (c) expansion of the kibbutz area only with relevant targets within the area: (1) residential area, (2) water bodies, (3) irrigated fields, (4) ploughed fields, (5) wet roof.
Saaroni, 1997). The thermal image, available only for 10:00 LST, indicates an average ‘‘cool island’’ at the surface level of the kibbutz, and it is compared to the spatial distribution of the air temperature analysed in the next section.
3.2. Spatial distribution of air temperature A decrease in air temperatures was noted during the four study days from 40.5 C (an average air temperature of the six fixed stations) on the first day to 34.5 C on the fourth day. Spatially, the maximal difference in temperature (both the six fixed stations and the 30 traverse route measurements) was 2.3 C. This was measured at 09:00 LST the second day: the high reading was found at the northern entrance of the kibbutz above a bare clay soil and the low one under a shaded tree in the center lawn area of the kibbutz. However, in 91% of the cases, the temperature differences were not more than 1.5 C.
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Fig. 3. The temperature distribution ( C) at: (a) 04:00, (b) 10:00, (c) 13:00, (d) 17:00, (e) 21:00, (f) 07:00.
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Fig. 3 (continued).
Figs. 3a–f show the isotherms that were derived from the average temperature over the 4 days of measurements in each of the locations according to the data from the traverse route measurements, at 04:00, 10:00, 13:00, 17:00, 21:00 and 07:00 h. At 04:00 h (Fig. 3a), spatial temperature differences were up to 1 C. The highest temperatures were registered in the green area inside the kibbutz, with a gradual fall in temperature toward the kibbutz’s perimeter. And thus, although slight, a ‘‘heat island’’ was found at 04:00 in the heart of the kibbutz. At 10:00 h (Fig. 3b), spatial temperature differences were up to 1.5 C. Lower temperatures were found in the greener area at the western part of the kibbutz indicating a moderate oasis effect. At 13:00 (Fig. 3c), when the highest temperatures were recorded, the temperature differences were up to 1.3 C, with the highest temperatures being found precisely in the green areas in the western part of the kibbutz. The kibbutz periphery and the open area around it were cooler than the central area. At 17:00 (Fig. 3d) temperature differences were up to 1.3 C, but as at 10:00 h, the western green area of the kibbutz was cooler than the kibbutz’s perimeter, indicating a moderate oasis effect. At 21:00 (Fig. 3e) there was almost no variation in temperature over the area studied and at 07:00 (Fig. 3f), while differences of up to 1.2 C were measured, no particular spatial tendency was found.
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3.3. Heat stress Heat stress was calculated from the continuous data measured at the six fixed stations (Fig. 4). During the 4 days of the study there was heat stress during almost 24 h of the day, of which 14.5 h were characterized by moderate or severe heat stress. Average maximal heat stress of the six stations was 31 C on the first day and 28 C on the fourth day. Fig. 5 presents the deviations of heat stress at each of four fixed stations from the average heat stress of the six fixed stations over the four days of the study. These four stations represent the most significant differences. In view of the low and non-significant variations in the conditions of temperature and relative humidity among the six measuring stations in the kibbutz, no significant differences were found in heat stress among the different areas of the kibbutz and its surroundings. However, around noontime, there was a slight increase in heat stress in the center of the kibbutz and in its northern perimeter, in the two stations exposed to the sun, one located on the lawn area and the other in an area without vegetation. A slightly lower heat stress was found in the two shaded stations inside the kibbutz, especially in the morning around 09:00 LST, a result attributable to the shade. In the evening and at night there was very little difference among the stations (for the most part, less than 0.5 ) although the shaded stations had somewhat higher heat stress, attributable to the greenhouse effect.
4. Discussion and conclusions The study area presents a complex picture. The daytime thermal image showed relatively lower surface temperatures in the kibbutz, by 10–20 C, in comparison
32 30
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Mild 22 No Heat Stress
Time (LST) 30.7.00
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Fig. 4. Average heat stress ( C) for the fixed measuring stations during the 4 study days.
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Fig. 5. Heat stress deviations ( C) at each of four fixed measuring stations from the average heat stress of the six fixed stations during the 4 study days. The four stations are station no. 2 (shaded) and station no. 3 (exposed), both of these at the center of the kibbutz; station no. 1 in the northern part of the kibbutz; and station no. 6 located 1 km north of the kibbutz.
with the arid, non-vegetated surfaces surrounding the kibbutz and thermal differences of more than 20 C between the ploughed fields and the water bodies, but according to air temperature measurements, the differences are small and not consistent. Air temperature difference between the vegetated areas in the kibbutz and the arid non-vegetated areas was not more than 1.5 C in 91% of the measurements, and the maximum temperature difference recorded was 2.3 C. In 70% of the measurements, the difference between the relative humidity measurements was not more than 6% and the maximum difference was 12.5%. The maximum difference was recorded at 09:00 LST between the station located at the northern part of the kibbutz (station no. 1 in Fig. 1b) and the shaded station in the center of the kibbutz (station no. 2 in Fig. 1b). Similar results were found by the traverse measurements with differences of 1.5 C and 1.3 C at 10:00 and 17:00, respectively, between the greener area at the western and central part of the kibbutz and the non-vegetated surroundings at the eastern part of the kibbutz. Around noontime (13:00 h), an opposite tendency was observed, with the highest temperatures (no more than 1 C difference) being measured in the center of the kibbutz, the lawn area. Only the station located under the tree was cooler, by 1 C. From 21:00 until 07:00 higher temperatures characterized the green areas in the center and western part of the kibbutz (again, a difference of no more than 1 C). Two pilot studies, each about 36 h long, were conducted during the month of September 1999 and showed similar results. The night-time findings are in accordance with the hypothesis that nocturnal cooling is more efficient in exposed arid areas than in vegetated areas because the
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vegetation (due to both the larger amount of water vapor around it and the canopy) creates a greenhouse effect that impedes the cooling of the green area (Oke, 1987; Taha et al., 1991). The noontime air temperature findings contradict the study hypothesis that predicted a significant daytime cooling effect, an ‘oasis effect’, in the vegetated area of the kibbutz. A number of explanations may be offered to account for the current findings: (a) The lack of an ‘oasis effect’ during noontime may result from a cessation of transpiration (water being transported via the leaf pores out into the air). By halting transpiration the plant saves water (Raven, 1999), and thus, by not contributing humidity to the air, impedes the cooling process by evapotranspiration. This hypothesis is supported by the slight decrease in water vapor pressure that was observed during noontime (not shown). However, the shaded areas were cooler also during noontime indicating the importance of minimizing sun-exposed surfaces in a dry and warm region. (b) The lack of an ‘oasis effect’ during noontime may result from the kibbutz design and heat-producing activities within the kibbutz. A concentration of buildings and relatively large concrete and asphalt surfaces in the middle of the kibbutz may create a ‘‘heat island’’ (Oke, 1987). In addition, the various kibbutz buildings such as the kitchen, laundry and factory emit warm air. Hot air is also emitted from the air conditioners operating in all the kibbutz buildings, particularly during the hottest daytime hours. This intensive activity is the source of significant heat emission inside the kibbutz boundaries, a phenomenon not present in the classical desert oasis—either farmland or a ‘‘green’’ area without anthropogenic activity. This activity continues during the remaining hours as well, and may be responsible for blurring the ‘oasis effect’ during the morning and afternoon hours. The continued operation of the air conditioning units at night and less efficient long wave radiational cooling by the buildings and the green areas may also result in the development of a nocturnal ‘‘heat island’’. (c) The oasis effect may be moderated by the relatively strong wind during noontime. It is well known that microclimatic conditions, such as radiation frost and urban heat island are well developed under calm conditions, while an increase in wind speed weakens and even prevents their development (Oke, 1987). Average noontime wind speed of 7–14 ms1 was measured at the kibbutz periphery in comparison to only 3 ms1 in the center of the kibbutz (Fig. 6). This increase in wind speed during noontime may have reduced the extent of temperature variation between the different areas. All the above hypotheses may work in concert. Evapotranspiration as well as shading play a role in creating an ‘oasis effect’. Nevertheless, one or two factors may be found to be cardinal in explaining the research findings. Additional research is therefore needed. One should however note that if kibbutz design plays a role in determining the extent of the ‘oasis effect’, more careful planning, aiming to intensify the ‘oasis effect’, might be recommended. One of the most important implications is
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Fig. 6. Wind speed (m s1) at the fixed measuring stations during the 4 study days.
the need for more shaded areas and minimizing the sun-exposed lawn area that tends to warm up and also uses a lot of water that is in short supply in this dry region.
Acknowledgements This study is supported by the Israeli Science Foundation (ISF, Grant no 828/02). Special thanks are due to Yaron Yaakov for technical support in carrying out the measurements and to Orna Zafrir-Reuven for drawing Fig. 1a.
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