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A simple model for potential dewfall in an arid region
Atmospheric Research 64 (2002) 285 – 295 www.elsevier.com/locate/atmos
A simple model for potential dewfall in an arid region Adrie F.G. Jacobs a,*, Bert G. Heusinkveld a, Simon M. Berkowicz b a
Meteorology and Air Quality Group, Wageningen University, Duivendaal 2, 6701 AP Wageningen, The Netherlands b Minerva Arid Ecosystems Research Centre, Institute of Earth Sciences, Givat Ram Campus, Hebrew University of Jerusalem, Jerusalem 91904, Israel Received 7 November 2001; accepted 13 May 2002
Abstract It is not always easy to know, post-facto, whether both dewfall and fog may have occurred over a given evening period. Instrumentation limitations make it difficult to quantify dew deposition since they rely on artificial sensing surfaces that are either visually examined on a daily basis or recorded. In arid to Mediterranean regions, both dew and fog can play significant ecological roles as suppliers of moisture. Long-term observation records of dew and fog in such regions tend to be limited, however, due partly to a lack of interest and limited distribution of well-instrumented meteorological stations. Simple meteorological criteria are suggested here to calculate potential dewfall and to indicate whether fog was likely to have occurred over a given evening. A field campaign was carried out in the NW Negev desert, Israel, in September and October 1997, to collect meteorological data and carry out dewfall measurements. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Dew; Dewfall; Potential dew; Fog; Desert
1. Introduction Nocturnal dewfall and/or fog precipitation are processes whereby moisture from the atmospheric water reservoir is deposited on a given surface. Here, dewfall is atmospheric water vapour that diffuses to the earth’s surface and condenses in liquid phase on a given surface. Fog, however, consists of tiny water droplets floating in the atmosphere, with the *
Corresponding author. Tel.: +31-317-483-981; fax: +31-317-482-811. E-mail address: [email protected] (A.F.G. Jacobs).
0169-8095/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 0 9 5 ( 0 2 ) 0 0 0 9 9 - 6
286
A.F.G. Jacobs et al. / Atmospheric Research 64 (2002) 285–295
deposition of these droplets on a given surface mainly caused by settling and interception by objects. If the air is not too dry, and wind speeds low, the dewfall process can occur on most nights in many regions of the world, whereas fog occurs given particular atmospheric conditions only. It is not always easy to determine if both dew and fog deposition occurred, even though theoretical dew amounts can reach up to 1.0 mm per night (Jacobs et al., 1990), while fog water input through droplet interception can reach tens of millimeters per night in coastal ecosystems (Dawson, 1998). Although both fog and dew occurrences can easily be noted and recorded by observers, this is not always possible for remote or sparsely populated regions. In addition, fog and dew deposition measurements tend to rely on daily observations and/or early-morning water collection from artificial sensing or collecting surfaces. Both dew and fog have been relatively neglected topics in desert meteorology, even though measuring dew/fog formation, rate of accumulation, total deposition, and subsequent evaporation present interesting research questions (Jacobs et al., 1999, 2000a,b). Furthermore, there is an important ecological function of dew and fog in most desert ecosystems. Because of high temperatures and the paucity of rainfall, desert organisms have adapted to survive environmental stress (Acostav Baladon and Gioda, 1991; Evenari et al., 1982). Although fog can provide a significant moisture input in some deserts, such as the coastal desert of Namibia (Armstrong, 1990) and that of Chile (Munoz-Schick et al., 2001), dewfall can also contribute small amounts of water on an almost daily basis (Malek et al., 1999). In the NW Negev desert, long-term observations by Evenari et al. (1982) have shown that dew occurs about 200 times per year and can reach the equivalent of 30 mm of annual precipitation; in drought years, dewfall can exceed the annual rainfall. Such apparently insignificant but regular moisture contributions play an important role in a desert ecosystem. In a hot desert, dew and fog serve as sources of water for small animals, plants, and biological crusts. Desert arthropods, such as isopods, ants, and beetles, rely on dew as a significant moisture source (Broza, 1979; Moffett, 1985). Desert soil faunae, such as nematodes, are also sensitive to dew (and fog) deposition on a soil surface (Steinberger et al., 1989). Recent research on the effect of dew deposition on plant water relations and diurnal variations of photosynthesis in some Mediterranean shrubs and plants found that the leaves were able to absorb dew and thus restore plant water status (Munne-Bosch et al., 1999). A similar effect could be ascribed to fog since the vegetation canopy efficiently intercepts fog droplets. Arid and semi-arid sand dune ecosystems may contain biotic crusts, up to several millimeters thick, composed of cyanobacteria, mosses, lichens, and algae (Lange et al., 1992); such crusts can help to stabilize dunes (Danin et al., 1989). Since temperature, light, and moisture are essential elements for crust photosynthesis, moisture contributed by dew and fog promotes crust development. A major limitation in assessing the ecological and environmental role of dew/fog has been the very difficulty of measurement. A variety of direct approaches has been tried including moisture-absorbing material, dew-drop size calibrations, recording balances, electrical surface wetness circuits, and the interception by mesh of wind-driven fog droplets (von Ro¨nsch, 1990; Schemenauer and Cereceda, 1994). These approaches are greatly hindered in that they rely on artificial measuring surfaces and are thus unrepresentative of the dew/fog receiving surface (Berkowicz et al., 2001). In particular, fog
A.F.G. Jacobs et al. / Atmospheric Research 64 (2002) 285–295
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deposition is primarily a function of the interception of wind-driven fog droplets by objects, such as trees, bushes, and shrubs. Fog deposition on flat bare surfaces will be comparatively low because of poor droplet interception. Hence, there is a practical need to try to obtain such information for those needing to assess their influence. Although regular observations of dew and fog are seldom taken, basic meteorological data are more commonly available. Such data can be used to calculate dewfall potential, but can also be used to determine whether fog had occurred. This is an important piece of information since dewfall measurements can be ‘‘contaminated’’ by fog input. Thus, the goals of the present research are to: (1) develop a simple criterion for potential dewfall, and to compare theoretical results with measured dew amounts, and (2) determine theoretically if fog was present during the dewfall period.
2. Study site description An intensive micrometeorological measurement program was undertaken within the Nizzana sand dune experimental field station of the Hebrew University of Jerusalem Minerva Arid Ecosystems Research Centre. The field station is located in the NW Negev desert (Fig. 1) at an elevation of 190 m above mean sea level. General climatic data for Nizzana are contained in Sharon et al. (1997). Average annual rainfall is about 100 mm and occurs almost entirely between November and March. The mean annual minimum and maximum temperatures are 12.5 and 25.9 jC, respectively. The coldest month is January with a mean minimum temperature of 5.5 jC and the warmest month is August with a mean maximum temperature of 33.5 jC. The dunes in the study site are linear, trend west to east, and have a height ranging from 10 to 25 m. The mobile dune crests are largely unvegetated except for perennial grasses (Kadmon and Leschner, 1995). The slopes of the sand dunes are stable, more vegetated, and tend to have a biological crust forming on them.
3. Experimental setup The field campaign was carried out in September and October 1997. Data collection was designed, among other things, to study the complete surface energy budget. Special attention was paid to the nocturnal dewfall and the early morning (drying period) sensible and latent heat transports. With the eddy correlation technique, both convective transports were estimated in a direct way. The study period was chosen for a variety of reasons. First, the absence of rain between the spring and summer months permits the soil to dry out. This eliminates the influence of soil moisture (dew-rise) on the measurements and subsequent calculations. Second, both September and October experience a high frequency of dew events (Zangvil, 1996) and
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Fig. 1. Location map of Nizzana showing long-term rainfall isohyets (mm).
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289
can also have fog (Bitan and Rubin, 1991). Third, a wide range of dew conditions (i.e. from no dew to heavy dew) could be expected. The comprehensive set of meteorological measurements included: – a net radiometer (Schulze– Da¨ke), incoming and reflected shortwave radiation by solarimeters (Kipp CM5), and incoming long wave radiation by a pyrgeometer (Kipp CG1); – sensible and latent heat fluxes were measured by the eddy correlation technique (Solent sonic anemometer, custom-built rapid response thermometer, and a Krypton hygrometer); – soil heat flux by a heat plate (TNO, WS 31-Cp) buried at a depth of 0.5 cm; – temperature and moisture profiles were monitored at 1.0 and 2.5 m with aspirated psychrometers (custom-built); – wind speed was measured at three heights (1, 2.5, and 5 m) with small sensitive cup anemometers; – soil temperatures were measured at seven depths (1, 1.5, 3.5, 5, 10, 15, and 20 cm) with Pt100 resistance thermometers. Soil surface temperature was monitored with a pyrometer (Heimann KT15). In parallel with the meteorological measurements, small microlysimeters, with diameter d=60 mm and height h=35 mm (Boast and Robertson, 1982), were installed at 16 locations along a 200-m transect crossing two parallel dunes, in order to assess dewfall and the early-morning drying process (Jacobs et al., 1999). The microlysimeters were removed at hourly intervals, but at 30-min intervals just before and after sunrise, and weighed in the field using a portable sensitive (F1 mg) balance (Mettler PM1200) powered by batteries and a solar panel. To obtain an insight into changes in wetness of the very top layer of the soil, a leaf wetness sensor (Omnidata ES-460) was installed at a depth of about 1 mm. The fast-response data (sensible and latent heat) were sampled with a frequency of 10 Hz and stored on a data logger (Campbell 21X). The slow response data were sampled at 0.1 Hz, automatically averaged over 30 min and stored on a separate data logger. The instruments and data loggers were powered by solar panels. An optical remote dew wetness sensor was also used at the surface for continuous monitoring during the measurement period (Heusinkveld, 1998). The measurement site and some instruments are depicted in Fig. 2. More details can be found in Jacobs et al. (2000a).
4. Dew criterion Monteith (1981) and Garratt and Segal (1988) showed that for a natural surface, the maximum possible condensation rate by dewfall together with dew-rise during nighttime is: Df þ Dr ¼
s Q* sþc k
ð1Þ
where Df is rate of dewfall and Dr is rate of dew rise, both in kg m2 s1, s is slope saturation curve in Pa K1, Q* is net radiation in W m2, c is a psychrometric constant (66
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Fig. 2. View of the experimental area and selected instrumentation.
Pa K1), and k is latent energy for vaporization in J kg1. If it is assumed that no dew-rise is possible, such as in a hot arid environment after several rainless months, Eq. (1) can be rewritten as: Df þ Dr ¼
s Q* S sþc k
where S is soil heat flux in W m2.
ð2Þ
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Earlier studies have demonstrated (Jacobs et al., 1996) that an accurate application of the above so-called potential dew rates can be an accurate and reliable estimate for the actual total dew rate.
5. Fog criterion As evening descends, a stable atmospheric boundary layer (ABL) forms and grows increasingly deeper during the night. For a simplified case, when the terrain is more or less flat and the wind speed is calm, it can be shown that the temperature profile near the ground can be approximated (Stull, 1983) by: z DT ðzÞ ¼ DTs exp ð3Þ He where T is temperature in jC, Ts is surface temperature, D is difference with respect to the residual layer temperature TRL, z is height in m, and He is the depth of the stable boundary layer in m. Following Stull (1983), it can be shown that the depth, He, and strength, DTs, of the stable ABL depend on the nocturnal cooling. An estimate of these parameters can be given as: 3=4 pffi He ¼ aVRL t ð4Þ Z wVT Vdt DTs ¼
He
ð5Þ
where a is a constant (a=0.15 m0.25 s0.25), VRL is the mean wind speed of the residual layer, t is time in s, and the integral is the kinematic cumulative cooling in m s1 K, which starts from the moment the stable stratification develops. As long as the surface temperature, Ts, exceeds the surface dew point temperature, Td, no fog can develop. The time, to, when the dew point temperature at the surface is reached and the onset of fog can be defined as: 3=4
to ¼
a2 VRL ðTRL Td Þ2 ðwVTmVÞ2
ð6Þ
where the denominator is the averaged nocturnal cooling rate.
6. Results and discussions For all nights studied during the 1997 experimental campaign, the onset time, to, was calculated and compared with the actual length of the nocturnal period, tn. Here,
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Table 1 Cooling period in hours (h) needed for onset to fog, to, available nighttime period, tn (tn=measured time where Rs=0), after the residual layer temperature, TRL, has been reached, during 10 consecutive nights (258=September 15, 1997) Day of year
VRL
TRL (jC)
Td (jC)
to (h)
tn (h)
258 259 260 261 262 263 264 265 266 267
0.8 1.4 2.0 0.9 0.9 0.8 2.0 0.7 0.9 1.3
24.1 24.4 24.6 23.7 23.1 24.8 22.3 24.1 23.2 25.1
14.9 12.7 15.2 17.1 15.0 16.2 16.3 13.2 14.3 14.1
62 12 53 61 110 28 31 18 16 47
11.3 11.3 11.3 11.3 11.3 11.3 11.4 11.4 11.4 11.4
VRL is the mean wind speed of the residual layer and Td is the surface dew point temperature.
tn is taken as that period that the measured short wave radiation, Rs, is zero. If the onset time, to, is longer or much longer than the nighttime period, tn, fog should not occur during that particular evening, and consequently, cannot contribute to the water
Fig. 3. Course of daily potential dewfall amounts and measured dew observed between September 16 – 30, 1997. Error bars have been indicated in the bar diagram.
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Fig. 4. Scatter diagram of the daily potential dewfall and measured dewfall observed between September 16 – 30, 1997. Error bars have been indicated in the scatter diagram.
collected in the microlysimeters. Table 1 contains results of the onset time, to, the nighttime duration, tn, for 10 selected nights together with some important atmospheric variables. All other days during the experimental period showed the same pattern. From Table 1, it is clear that for the selected days, the time period, to, needed for onset of fog, is much longer than the available nighttime period, tn. This means that during the experimental period, fog was very unlikely to occur and could not contaminate the dew results. In Fig. 3, the daily calculated potential dew amounts are compared with dates when dew was measured using the microlysimeters. Accordingly, we can infer that the daily measured dew amounts agree reasonably well with the potential dewfall amounts. This agreement is to be expected given that the only water reservoir in a desert system is the atmosphere and not the soil. Only following rain, when there is enough water in the subsoil, can the soil itself function as a moisture reservoir. In our case, the experimental campaign took place at the end of the long dry summer period; hence, free soil water close to the surface has essentially been removed. In Fig. 4, the measured and potential daily dew amounts are plotted in a scatter diagram. The daily potential dewfall amount provides an acceptable estimate for the actual dew amount, even though the actual amounts are lower. Though the potential dewfall values are sometimes higher than the actual measured dewfall, this is a result of the simplicity of the model.
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7. Conclusions On the basis of the data analyses, we found that during the experimental period, fog did not play a role in the nocturnal wetting process of the soil surface and plants. Conditions for the formation of fog would have required unrealistic evening cooling periods. The calculated potential dewfall amounts gave reasonable estimates of the daily dew amounts measured using the microlysimeters. This approach could thus be very useful as a simple model to estimate dewfall in rainless seasons and as an indicator of whether fog may have occurred over a given evening. Furthermore, meteorological data archives could be harnessed to quickly prepare local or regional potential dewfall assessments. Dew and fog research remain wide-open subjects for investigation. Because of the importance of dew and fog to many scientific fields, including ecology, agriculture and plant diseases, air pollution, as well as sustainable development, it invites interdisciplinary approaches and cooperation.
Acknowledgements The authors are grateful to the Minerva Arid Ecosystem Research Centre of the Hebrew University of Jerusalem and the Dutch Organization for Scientific Research (NWO) for assistance in carrying out the field campaign. The authors thank Eyal Sachs for technical assistance and Shai Ron for field assistance.
References Acostav Baladon, A.N., Gioda, A., 1991. L’importance des pre´cipitations occultes sous les tropiques secs. Se´cheresse 2, 132 – 135. Armstrong, S., 1990. Fog, wind and heat: life in the Namib desert. New Sci., 46 – 50. Berkowicz, S.M., Heusinkveld, B.G., Jacobs, A.F.G., 2001. Dew in an arid ecosystem: ecological aspects and problems in dew measurement. Proceedings, 2nd International Conference on Fog and Fog Collection, 15 – 20 July 2001. St. John’s, Newfoundland, Canada, pp. 301 – 304. Bitan, A., Rubin, S., 1991. Climatic Atlas of Israel for Physical and Environmental Planning and Design, 3rd ed. Ramot Press, Tel Aviv. Boast, C.W., Robertson, T.M., 1982. A ‘‘micro-lysimeter’’ method for determining evaporation from bare soil: description and laboratory evaluation. Soil Sci. Soc. Am. J. 46, 689 – 696. Broza, M., 1979. Dew, fog and hygroscopic food as a source of water for desert arthropods. J. Arid Environ. 2, 43 – 49. Danin, A., Bar-Or, Y., Dor, I., Yisraeli, T., 1989. The role of cyanobacteria in stabilization of sand dunes in southern Israel. Ecol. Mediter. XV, 55 – 64. Dawson, T.E., 1998. Fog in the California redwood forest: ecosystem inputs and use by plants. Oecologia 117, 476 – 485. Evenari, M., Shanan, L., Tadmor, N., 1982. The Negev: Challenge of a Desert, 2nd ed. Harvard Univ. Press, Cambridge, MA. Garratt, J.R., Segal, M., 1988. On the contribution of atmospheric moisture to dew formation. Boundary-Layer Meteorol. 45, 209 – 236. Heusinkveld, B.G., 1998. New optical method to measure leaf wetness duration. 10th Symposium on Meteorological Observations and Instrumentation. American Meteorological Society, pp. 300 – 301.
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Jacobs, A.F.G., Van Pul, W.A.J., Van Dijken, A., 1990. Similarity moisture dew profiles within a corn canopy. J. Appl. Meteorol. 29, 1300 – 1306. Jacobs, A.F.G., Van Boxel, J.H., Nieveen, J., 1996. Nighttime exchange processes near the soil surface of a maize canopy. Agric. For. Meteorol. 82, 155 – 169. Jacobs, A.F.G., Heusinkveld, B.G., Berkowicz, S.M., 1999. Dew deposition and drying in a desert system: a simple simulation model. J. Arid Environ. 42, 211 – 222. Jacobs, A.F.G., Heusinkveld, B.G., Berkowicz, S.M., 2000a. Dew measurements along a longitudinal sand dune transect, Negev Desert, Israel. Int. J. Biometeorol. 43, 184 – 190. Jacobs, A.F.G., Heusinkveld, B.G., Berkowicz, S.M., 2000b. Force-restore technique for ground surface temperature and moisture content in a dry desert system. Water Resour. Res. 36, 1261 – 1268. Kadmon, R., Leschner, H., 1995. Ecology of linear dunes: effect of surface stability on the distribution and abundance of annual plants. Adv. GeoEcol. 28, 125 – 143. Lange, O.L., Kidron, G., Bu¨del, B., Meyer, A., Kilian, E., Abeliovich, A., 1992. Taxonomic composition and photosynthetic characteristics of the ‘biological crusts’ covering sand dunes in the western Negev Desert. Funct. Ecol. 6, 519 – 527. Malek, E., McCurdy, G., Giles, B., 1999. Dew contribution to the annual water balances in semi-arid desert valleys. J. Arid Environ. 42, 71 – 80. Moffett, M.W., 1985. An Indian ant’s novel method for obtaining water. Natl. Geogr. Res. 1, 146 – 149. Monteith, J.L., 1981. Evaporation and surface temperature. Q. J. R. Meteorol. Soc. 107, 1 – 27. Munne-Bosch, S., Nogues, S., Alegre, L., 1999. Diurnal variations of photosynthesis and dew absorption by leaves in two evergreen shrubs growing in Mediterranean field conditions. New Phytol. 144, 109 – 119. Munoz-Schick, M., Pinto, R., Mesa, A., et al., 2001. Fog oases during the El Nino Southern Oscillation 1997 – 1998, in the coastal hills south of Iquique, Tarapaca region, Chile. Rev. Chil. Hist. Nat. 74 (2), 389 – 405. Schemenauer, R.S., Cereceda, P., 1994. A proposed standard fog collector for use in high elevation regions. Am. Meteorol. Soc. 33, 1313 – 1322. Sharon, D., Margalit, A., Berkowicz, S., 1997. Local variations of rainfall across longitudinal dunes, nizzana, and the locally-perturbed windfield shaping them. Final Report to Arid Ecosystems Research Center, Hebrew University of Jerusalem. Steinberger, Y., Loboda, I., Garner, W., 1989. The influence of autumn dewfall on spatial and temporal distribution of nematodes in the desert ecosystem. J. Arid Environ. 16, 177 – 183. Stull, R.B., 1983. Integral scales for the nocturnal boundary layer. Part 1: empirical depth relationships. J. Clim. Appl. Meteorol. 22, 673 – 686. von Ro¨nsch, H., 1990. Tau und reif in Harzgerode. Z. Meteorol. 40, 197 – 204. Zangvil, A., 1996. Six years of dew observations in the Negev Desert, Israel. J. Arid Environ. 32, 361 – 371.
A simple model for potential dewfall in an arid region Adrie F.G. Jacobs a,*, Bert G. Heusinkveld a, Simon M. Berkowicz b a
Meteorology and Air Quality Group, Wageningen University, Duivendaal 2, 6701 AP Wageningen, The Netherlands b Minerva Arid Ecosystems Research Centre, Institute of Earth Sciences, Givat Ram Campus, Hebrew University of Jerusalem, Jerusalem 91904, Israel Received 7 November 2001; accepted 13 May 2002
Abstract It is not always easy to know, post-facto, whether both dewfall and fog may have occurred over a given evening period. Instrumentation limitations make it difficult to quantify dew deposition since they rely on artificial sensing surfaces that are either visually examined on a daily basis or recorded. In arid to Mediterranean regions, both dew and fog can play significant ecological roles as suppliers of moisture. Long-term observation records of dew and fog in such regions tend to be limited, however, due partly to a lack of interest and limited distribution of well-instrumented meteorological stations. Simple meteorological criteria are suggested here to calculate potential dewfall and to indicate whether fog was likely to have occurred over a given evening. A field campaign was carried out in the NW Negev desert, Israel, in September and October 1997, to collect meteorological data and carry out dewfall measurements. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Dew; Dewfall; Potential dew; Fog; Desert
1. Introduction Nocturnal dewfall and/or fog precipitation are processes whereby moisture from the atmospheric water reservoir is deposited on a given surface. Here, dewfall is atmospheric water vapour that diffuses to the earth’s surface and condenses in liquid phase on a given surface. Fog, however, consists of tiny water droplets floating in the atmosphere, with the *
Corresponding author. Tel.: +31-317-483-981; fax: +31-317-482-811. E-mail address: [email protected] (A.F.G. Jacobs).
0169-8095/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 0 9 5 ( 0 2 ) 0 0 0 9 9 - 6
286
A.F.G. Jacobs et al. / Atmospheric Research 64 (2002) 285–295
deposition of these droplets on a given surface mainly caused by settling and interception by objects. If the air is not too dry, and wind speeds low, the dewfall process can occur on most nights in many regions of the world, whereas fog occurs given particular atmospheric conditions only. It is not always easy to determine if both dew and fog deposition occurred, even though theoretical dew amounts can reach up to 1.0 mm per night (Jacobs et al., 1990), while fog water input through droplet interception can reach tens of millimeters per night in coastal ecosystems (Dawson, 1998). Although both fog and dew occurrences can easily be noted and recorded by observers, this is not always possible for remote or sparsely populated regions. In addition, fog and dew deposition measurements tend to rely on daily observations and/or early-morning water collection from artificial sensing or collecting surfaces. Both dew and fog have been relatively neglected topics in desert meteorology, even though measuring dew/fog formation, rate of accumulation, total deposition, and subsequent evaporation present interesting research questions (Jacobs et al., 1999, 2000a,b). Furthermore, there is an important ecological function of dew and fog in most desert ecosystems. Because of high temperatures and the paucity of rainfall, desert organisms have adapted to survive environmental stress (Acostav Baladon and Gioda, 1991; Evenari et al., 1982). Although fog can provide a significant moisture input in some deserts, such as the coastal desert of Namibia (Armstrong, 1990) and that of Chile (Munoz-Schick et al., 2001), dewfall can also contribute small amounts of water on an almost daily basis (Malek et al., 1999). In the NW Negev desert, long-term observations by Evenari et al. (1982) have shown that dew occurs about 200 times per year and can reach the equivalent of 30 mm of annual precipitation; in drought years, dewfall can exceed the annual rainfall. Such apparently insignificant but regular moisture contributions play an important role in a desert ecosystem. In a hot desert, dew and fog serve as sources of water for small animals, plants, and biological crusts. Desert arthropods, such as isopods, ants, and beetles, rely on dew as a significant moisture source (Broza, 1979; Moffett, 1985). Desert soil faunae, such as nematodes, are also sensitive to dew (and fog) deposition on a soil surface (Steinberger et al., 1989). Recent research on the effect of dew deposition on plant water relations and diurnal variations of photosynthesis in some Mediterranean shrubs and plants found that the leaves were able to absorb dew and thus restore plant water status (Munne-Bosch et al., 1999). A similar effect could be ascribed to fog since the vegetation canopy efficiently intercepts fog droplets. Arid and semi-arid sand dune ecosystems may contain biotic crusts, up to several millimeters thick, composed of cyanobacteria, mosses, lichens, and algae (Lange et al., 1992); such crusts can help to stabilize dunes (Danin et al., 1989). Since temperature, light, and moisture are essential elements for crust photosynthesis, moisture contributed by dew and fog promotes crust development. A major limitation in assessing the ecological and environmental role of dew/fog has been the very difficulty of measurement. A variety of direct approaches has been tried including moisture-absorbing material, dew-drop size calibrations, recording balances, electrical surface wetness circuits, and the interception by mesh of wind-driven fog droplets (von Ro¨nsch, 1990; Schemenauer and Cereceda, 1994). These approaches are greatly hindered in that they rely on artificial measuring surfaces and are thus unrepresentative of the dew/fog receiving surface (Berkowicz et al., 2001). In particular, fog
A.F.G. Jacobs et al. / Atmospheric Research 64 (2002) 285–295
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deposition is primarily a function of the interception of wind-driven fog droplets by objects, such as trees, bushes, and shrubs. Fog deposition on flat bare surfaces will be comparatively low because of poor droplet interception. Hence, there is a practical need to try to obtain such information for those needing to assess their influence. Although regular observations of dew and fog are seldom taken, basic meteorological data are more commonly available. Such data can be used to calculate dewfall potential, but can also be used to determine whether fog had occurred. This is an important piece of information since dewfall measurements can be ‘‘contaminated’’ by fog input. Thus, the goals of the present research are to: (1) develop a simple criterion for potential dewfall, and to compare theoretical results with measured dew amounts, and (2) determine theoretically if fog was present during the dewfall period.
2. Study site description An intensive micrometeorological measurement program was undertaken within the Nizzana sand dune experimental field station of the Hebrew University of Jerusalem Minerva Arid Ecosystems Research Centre. The field station is located in the NW Negev desert (Fig. 1) at an elevation of 190 m above mean sea level. General climatic data for Nizzana are contained in Sharon et al. (1997). Average annual rainfall is about 100 mm and occurs almost entirely between November and March. The mean annual minimum and maximum temperatures are 12.5 and 25.9 jC, respectively. The coldest month is January with a mean minimum temperature of 5.5 jC and the warmest month is August with a mean maximum temperature of 33.5 jC. The dunes in the study site are linear, trend west to east, and have a height ranging from 10 to 25 m. The mobile dune crests are largely unvegetated except for perennial grasses (Kadmon and Leschner, 1995). The slopes of the sand dunes are stable, more vegetated, and tend to have a biological crust forming on them.
3. Experimental setup The field campaign was carried out in September and October 1997. Data collection was designed, among other things, to study the complete surface energy budget. Special attention was paid to the nocturnal dewfall and the early morning (drying period) sensible and latent heat transports. With the eddy correlation technique, both convective transports were estimated in a direct way. The study period was chosen for a variety of reasons. First, the absence of rain between the spring and summer months permits the soil to dry out. This eliminates the influence of soil moisture (dew-rise) on the measurements and subsequent calculations. Second, both September and October experience a high frequency of dew events (Zangvil, 1996) and
288
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Fig. 1. Location map of Nizzana showing long-term rainfall isohyets (mm).
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can also have fog (Bitan and Rubin, 1991). Third, a wide range of dew conditions (i.e. from no dew to heavy dew) could be expected. The comprehensive set of meteorological measurements included: – a net radiometer (Schulze– Da¨ke), incoming and reflected shortwave radiation by solarimeters (Kipp CM5), and incoming long wave radiation by a pyrgeometer (Kipp CG1); – sensible and latent heat fluxes were measured by the eddy correlation technique (Solent sonic anemometer, custom-built rapid response thermometer, and a Krypton hygrometer); – soil heat flux by a heat plate (TNO, WS 31-Cp) buried at a depth of 0.5 cm; – temperature and moisture profiles were monitored at 1.0 and 2.5 m with aspirated psychrometers (custom-built); – wind speed was measured at three heights (1, 2.5, and 5 m) with small sensitive cup anemometers; – soil temperatures were measured at seven depths (1, 1.5, 3.5, 5, 10, 15, and 20 cm) with Pt100 resistance thermometers. Soil surface temperature was monitored with a pyrometer (Heimann KT15). In parallel with the meteorological measurements, small microlysimeters, with diameter d=60 mm and height h=35 mm (Boast and Robertson, 1982), were installed at 16 locations along a 200-m transect crossing two parallel dunes, in order to assess dewfall and the early-morning drying process (Jacobs et al., 1999). The microlysimeters were removed at hourly intervals, but at 30-min intervals just before and after sunrise, and weighed in the field using a portable sensitive (F1 mg) balance (Mettler PM1200) powered by batteries and a solar panel. To obtain an insight into changes in wetness of the very top layer of the soil, a leaf wetness sensor (Omnidata ES-460) was installed at a depth of about 1 mm. The fast-response data (sensible and latent heat) were sampled with a frequency of 10 Hz and stored on a data logger (Campbell 21X). The slow response data were sampled at 0.1 Hz, automatically averaged over 30 min and stored on a separate data logger. The instruments and data loggers were powered by solar panels. An optical remote dew wetness sensor was also used at the surface for continuous monitoring during the measurement period (Heusinkveld, 1998). The measurement site and some instruments are depicted in Fig. 2. More details can be found in Jacobs et al. (2000a).
4. Dew criterion Monteith (1981) and Garratt and Segal (1988) showed that for a natural surface, the maximum possible condensation rate by dewfall together with dew-rise during nighttime is: Df þ Dr ¼
s Q* sþc k
ð1Þ
where Df is rate of dewfall and Dr is rate of dew rise, both in kg m2 s1, s is slope saturation curve in Pa K1, Q* is net radiation in W m2, c is a psychrometric constant (66
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Fig. 2. View of the experimental area and selected instrumentation.
Pa K1), and k is latent energy for vaporization in J kg1. If it is assumed that no dew-rise is possible, such as in a hot arid environment after several rainless months, Eq. (1) can be rewritten as: Df þ Dr ¼
s Q* S sþc k
where S is soil heat flux in W m2.
ð2Þ
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Earlier studies have demonstrated (Jacobs et al., 1996) that an accurate application of the above so-called potential dew rates can be an accurate and reliable estimate for the actual total dew rate.
5. Fog criterion As evening descends, a stable atmospheric boundary layer (ABL) forms and grows increasingly deeper during the night. For a simplified case, when the terrain is more or less flat and the wind speed is calm, it can be shown that the temperature profile near the ground can be approximated (Stull, 1983) by: z DT ðzÞ ¼ DTs exp ð3Þ He where T is temperature in jC, Ts is surface temperature, D is difference with respect to the residual layer temperature TRL, z is height in m, and He is the depth of the stable boundary layer in m. Following Stull (1983), it can be shown that the depth, He, and strength, DTs, of the stable ABL depend on the nocturnal cooling. An estimate of these parameters can be given as: 3=4 pffi He ¼ aVRL t ð4Þ Z wVT Vdt DTs ¼
He
ð5Þ
where a is a constant (a=0.15 m0.25 s0.25), VRL is the mean wind speed of the residual layer, t is time in s, and the integral is the kinematic cumulative cooling in m s1 K, which starts from the moment the stable stratification develops. As long as the surface temperature, Ts, exceeds the surface dew point temperature, Td, no fog can develop. The time, to, when the dew point temperature at the surface is reached and the onset of fog can be defined as: 3=4
to ¼
a2 VRL ðTRL Td Þ2 ðwVTmVÞ2
ð6Þ
where the denominator is the averaged nocturnal cooling rate.
6. Results and discussions For all nights studied during the 1997 experimental campaign, the onset time, to, was calculated and compared with the actual length of the nocturnal period, tn. Here,
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Table 1 Cooling period in hours (h) needed for onset to fog, to, available nighttime period, tn (tn=measured time where Rs=0), after the residual layer temperature, TRL, has been reached, during 10 consecutive nights (258=September 15, 1997) Day of year
VRL
TRL (jC)
Td (jC)
to (h)
tn (h)
258 259 260 261 262 263 264 265 266 267
0.8 1.4 2.0 0.9 0.9 0.8 2.0 0.7 0.9 1.3
24.1 24.4 24.6 23.7 23.1 24.8 22.3 24.1 23.2 25.1
14.9 12.7 15.2 17.1 15.0 16.2 16.3 13.2 14.3 14.1
62 12 53 61 110 28 31 18 16 47
11.3 11.3 11.3 11.3 11.3 11.3 11.4 11.4 11.4 11.4
VRL is the mean wind speed of the residual layer and Td is the surface dew point temperature.
tn is taken as that period that the measured short wave radiation, Rs, is zero. If the onset time, to, is longer or much longer than the nighttime period, tn, fog should not occur during that particular evening, and consequently, cannot contribute to the water
Fig. 3. Course of daily potential dewfall amounts and measured dew observed between September 16 – 30, 1997. Error bars have been indicated in the bar diagram.
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Fig. 4. Scatter diagram of the daily potential dewfall and measured dewfall observed between September 16 – 30, 1997. Error bars have been indicated in the scatter diagram.
collected in the microlysimeters. Table 1 contains results of the onset time, to, the nighttime duration, tn, for 10 selected nights together with some important atmospheric variables. All other days during the experimental period showed the same pattern. From Table 1, it is clear that for the selected days, the time period, to, needed for onset of fog, is much longer than the available nighttime period, tn. This means that during the experimental period, fog was very unlikely to occur and could not contaminate the dew results. In Fig. 3, the daily calculated potential dew amounts are compared with dates when dew was measured using the microlysimeters. Accordingly, we can infer that the daily measured dew amounts agree reasonably well with the potential dewfall amounts. This agreement is to be expected given that the only water reservoir in a desert system is the atmosphere and not the soil. Only following rain, when there is enough water in the subsoil, can the soil itself function as a moisture reservoir. In our case, the experimental campaign took place at the end of the long dry summer period; hence, free soil water close to the surface has essentially been removed. In Fig. 4, the measured and potential daily dew amounts are plotted in a scatter diagram. The daily potential dewfall amount provides an acceptable estimate for the actual dew amount, even though the actual amounts are lower. Though the potential dewfall values are sometimes higher than the actual measured dewfall, this is a result of the simplicity of the model.
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7. Conclusions On the basis of the data analyses, we found that during the experimental period, fog did not play a role in the nocturnal wetting process of the soil surface and plants. Conditions for the formation of fog would have required unrealistic evening cooling periods. The calculated potential dewfall amounts gave reasonable estimates of the daily dew amounts measured using the microlysimeters. This approach could thus be very useful as a simple model to estimate dewfall in rainless seasons and as an indicator of whether fog may have occurred over a given evening. Furthermore, meteorological data archives could be harnessed to quickly prepare local or regional potential dewfall assessments. Dew and fog research remain wide-open subjects for investigation. Because of the importance of dew and fog to many scientific fields, including ecology, agriculture and plant diseases, air pollution, as well as sustainable development, it invites interdisciplinary approaches and cooperation.
Acknowledgements The authors are grateful to the Minerva Arid Ecosystem Research Centre of the Hebrew University of Jerusalem and the Dutch Organization for Scientific Research (NWO) for assistance in carrying out the field campaign. The authors thank Eyal Sachs for technical assistance and Shai Ron for field assistance.
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