Study of dew water collection in humid tropical islands

Journal of Hydrology (2008) 361, 159– 171

available at www.sciencedirect.com

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Study of dew water collection in humid tropical islands O. Clus

a,b

, P. Ortega

b,c

, M. Muselli

a,b

, I. Milimouk

b,d

, D. Beysens

b,e,f,*

a

´ de Corse and CNRS UMR 6134, Route des Sanguinaires, Ajaccio 20000, France Universite International Organization for Dew Utilization, OPUR, 60, rue Emeriau, Paris 75015, France c ´ de la Polyne ´sie Franc¸aise, B.P.6570, Faaa 98702, French Polynesia Universite d ´riaux et l’Espace, CNRS-ICMCB, Pessac 33608, France Equipe du Supercritique pour l’Environnement, les Mate e ´riaux et l’Espace, ESPCI-PMMH, 10, rue Vauquelin, Paris 75231, Equipe du Supercritique pour l’Environnement, les Mate Cedex 05, France f ´riaux et l’Espace, CEA-SBT, Grenoble 38054, France Equipe du Supercritique pour l’Environnement, les Mate b

Received 7 December 2007; received in revised form 23 June 2008; accepted 24 July 2008

KEYWORDS Tropical islands; Dew harvesting; Water resource; Sky radiation; Radiative cooling

An assessment of the potential for dew water to serve as a potable water source during a rainless season in a humid tropical climate was carried out in the Pacific islands of French Polynesia. The climate of these islands, in terms of diurnal and seasonal variations, wind and energy balance, is representative of the climate of the tropical Atlantic and Pacific oceans. Measurements were obtained at two characteristic sites of this region; a mountainous island (Punaauia, Tahiti Island) and an atoll (Tikehau, Tuamotu Archipelago). Dew was measured daily on a 30 tilted, 1 m2 plane collector equipped with a thermally insulated radiative foil. In addition, an electronic balance placed at 1 m above the ground with a horizontal 0.16 m2 condensing plate made of PolyTetraFluoroEthylene (Teflon) was used in Tahiti. Dew volume data, taken during the dry season from 16/5/2005 to 14/10/2005, were correlated with air temperature and relative humidity, wind speed, cloud cover and visible plus infrared radiometer measurements. The data were also fitted to a model. Dew formation in such a tropical climate is characterized by high absolute humidity, weak nocturnal temperature drop and strong Trade winds. These winds prevent dew from forming unless protected e.g. by natural vegetal windbreaks. In protected areas, dew can then form with winds as large as 7 m/s. Such strong winds also hamper at night the formation near the ground of a calm and cold air layer with high relative humidity. As the cooling power is lower than in the Mediterranean islands because of the high absolute humidity of the atmosphere, both effects combine to generate modest dew yields. However, dew events are frequent and provide accumulated amounts of water attractive for dew water harvesting. Slight modifiSummary

* Corresponding author. Address: Equipe du Supercritique pour l’Environnement, les Mate ´riaux et l’Espace, ESPCI-PMMH, 10, rue Vauquelin, Paris 75231, Cedex 05, France. Tel.: +33 140795806; fax: +33 140794523. E-mail address: [email protected] (D. Beysens). 0022-1694/$ - see front matter ª 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2008.07.038

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O. Clus et al. cations of existing rain collection devices on roofs can enhance dew formation and collection. Dew harvesting thus appears as an attractive possibility to provide the local population with a complementary – but on occasion, essential – water resource. ª 2008 Elsevier B.V. All rights reserved.

Introduction

Objective

Water in islands is mostly provided by the phreatic zones that are replenished by precipitation. Although water requirements of the local population are adequate in high mountainous islands, the situation is quite different in low islands (atolls). There, the available water is scarce and or saline, especially during the dry season. It is precisely during this season that tourism peaks, thus placing a new stress on the demand for fresh water. In the Pacific atolls, the ground water salinity has increased through overpumping and new wells have not followed the traditional rules requiring a minimal distance between wells, ocean, lagoon and waste water release (Dupon, 1987). As a consequence, rain water (and expensive imported bottled water) has become the exclusive source of potable water. The galvanized iron roofs of dwellings serve as rain collectors. Large water tanks provide about 4 months of water supply to inhabitants, with a rationed use in the dry season. However, droughts appear periodically and the situation can become hazardous.

The present paper aims to assess the amount of dew water that could be collected in Pacific atolls during the dry season that lasts from May to October (Laurent et al., 2004). In addition, a secondary objective is to compare the yields with those of a Mediterranean island (Ajaccio) where data are available. Despite a slight difference of the oceanic currents and evapotranspiration, the climate of those atolls in terms of wind, rainfall or energy balance is very close to the tropical oceanic climate since they are devoid of relief and have a very small land surface. The atoll selected for the experiment is thus representative of a large number of islands spanning over millions of square kilometers in the oceanic tropical area (Pacific, Indian or Atlantic). For the Tuamotu Archipelago alone, there are more than 70 atolls. A Few studies have been concerned with dew formation in tropical continental areas (Salau and Lawson, 1986; Lhomme and Jimenez, 1992; Khedari et al., 2000; Weihong and Goudriaan, 2000; Barradas and Glez-Medellı´n, 2000). There are, to our knowledge, no studies concerning atolls or tropical islands, where dew forms under unusual conditions of strong (Trade) winds and high atmospheric humidity, the latter inducing a large natural green house effect. In order to characterize dew formation and collection in this environment, two typical sites were chosen: a low coral atoll in the Tuamotu islands (Tuherahera, Tikehau Island, French Polynesia) and a high, mountainous island (Punaauia, Tahiti Island, French Polynesia) 320 km apart. Data have been obtained on each site on inclined plane collectors and supplemented in Tahiti by continuous dew measurements on an electronic balance. Data are correlated with air temperature, air relative humidity, wind speed, wind direction and cloud cover plus infrared radiometer measurements. In addition, the model developed by Nikolayev et al. (2001) is used to interpret the data and the results.

Literature review In some regions of the world, dew water – if available – appears to be a simple solution to complement sources of potable water. Dew water is indeed used by plants and small animals where, in arid and semi-arid environment, it is significant to sustain their activity (Gindel, 1965; Steinberger et al., 1989). Dew comes from atmospheric humidity that is transformed into liquid water on a surface that is passively cooled by radiation (Monteith, 1957; Beysens, 2006). In theory, the maximum dew yield that can be expected is about 0.8 L/m2, being restricted by the available cooling power (25–100 W m2) with respect to the latent heat of condensation (2500 kJ kg1). The question whether dew can be a reliable source of water has been posed many times. Recently, systematic investigation were performed of adapted condensing architecture using high yield radiative materials with hydrophilic properties (complete wetting case) that enhance dew water nucleation (see Beysens, 2006) and dew drop gravity recovery. The influence of local meteorological parameters such as wind speed and relative humidity was systematically studied (Nilsson, 1996; Vargas et al., 1998; Muselli et al., 2002; Muselli et al., 2006a; Beysens et al., 2003, 2005; Jacobs et al., 2008; Sharan et al., 2007). Water quality was also investigated (Beysens et al., 2006; Muselli et al., 2006b). For a plane condensing structure an optimum tilt angle from horizontal was found to be 30 (Beysens et al., 2003). The maximum dew yield that the present authors are aware of is about 0.6 L m2 measured in Jerusalem (Berkowicz et al., 2004), not far from the expected maximum.

Experimental setup Measurement sites Tikehau The atoll of Tikehau is an island of the Tuamotu Archipelago. It consists of several small islands or ‘‘motus’’. Its total surface, including the lagoon, is 448 km2. Two dew sites were used at Tikehau Island on Motu Tuherahera. The first site (labeled # 1) is at the airport (lat 1407 0 S and long 14814 0 W). It is exposed to the dominant Trade winds. The second site (labeled # 2) is located 300 m west from site # 1 and is protected from the wind by coco trees. Both sites are 0.5 m above sea level (asl). The climate is humid tropical with two different seasons: the dry season between May and October and the humid sea-

Study of dew water collection in humid tropical islands son between November and April (Laurent et al., 2004). The wind regime is characterized by the permanent presence of the Trade winds, with a constant (day and night) dominant ENE direction. The average wind speed at 10 m was measured on the Takaroa atoll (300 km east of Tikehau, Tuamotu archipelago) from 1966 to 2002 and was 5–8 m/s during the night, (Laurent et al., 2004). Wind is stronger during the dry season. Tahiti The measurement site is located at Outumaoro on the west side of Tahiti (land area 1042 km2), at latitude 1734 0 S and longitude 14936 0 E at the University of French Polynesia. The elevation is 97 m asl and the face of the hill is oriented westwards. The climate is also humid tropical with wellcontrasted dry and humid seasons. The site, located on the west coast of Tahiti, is protected from direct ENE Trade winds. Nevertheless, during the dry season the wind sometimes turns to the SE direction, (‘‘Mara’amu wind’’) and becomes strong enough to prevent condensation on exposed surfaces.

Measurement procedure Dew volumes were measured on the same reference surfaces in Tikehau and Tahiti: condensers of 1 m · 1 m inclined at a 30 angle from horizontal. In Tikehau, the condenser is exposed W and remains shaded in the morning (until 08:00, local time UTC-10H). The condenser in Outumaoro is exposed NE and faces an open sky for radiative cooling. The condensers were coated with the same condensing foil, 0.35 mm thick, made of TiO2 and BaSO4 micro spheres embedded in low density polyethylene with a food surfactant (similar to Nilsson, 1996; made by OPUR, 2006). The foil was thermally insulated from the condenser frame by a 20 mm thick Styrofoam plate. Increasing the water yield also requires a properly designed condenser (Beysens et al., 2003) to prevent heating by the ambient air. This is especially important when a nocturnal wind is present, as is often the case on islands. Dew water was measured daily at 07:30 local time. The total condensed volume corresponds to water collected by gravity flow in a bottle and the residual dew drops scraped from the collecting surface (both amounts were recorded). Tikehau The following parameters are recorded every day on site at 07:30 local time (= UTC-10H): air temperature Ta (C), wind speed V (m/s) at 6 m elevation, wind direction (degree), cloud cover data N (octas). Rain was excluded from the dataset. Data were collected during the dry season, from 21/6/2005 to 7/10/2005. Recorded meteorological data were available every hour at the nearby automated station at the Rangiroa atoll (NE, 63 km): wind speed and wind direction Dir (degree), Ta (C), relative humidity RH (%) giving the dew point temperature Td (C). Due to the flatness of the atolls, the absence of topographical undulations and the close locations of these two places in the axis of the dominant wind, these data are expected to be also representative of the Tikehau conditions. Some deviations, however, occur (as the number of rainy day, see Fig. 2a) and the correlation of the dew yields with those data might then suffer of uncontrolled bias.

161 Tahiti In Tahiti, dew yield was also measured every morning on the same 1 m · 1 m foil collector as described above. Dew (and also to some extent, rain) mass m was also recorded every 15 min on a reference plane surface placed at 1 m above the ground. This surface is made of a 400 mm · 400 mm (collecting area Sc = 0.16 m2) and e = 1.05 mm thickness PolyTetraFluororoEthylene (PTFE, commercial name: Teflon) plate. The plate is placed on a 12.5 lm thick aluminum foil, using a 5 mm thick polystyrene foam for thermal insulation. It is placed on an electronic, temperature-compensated Mettler Toledo balance connected to a PC. The balance is protected from the wind up to the plate level. Zero is arbitrary. Note that the wind induces a force directed upwards due to the Bernoulli pressure. The following physical parameters were recorded every 15 min on a data logger connected to a computer: PTFE plate surface temperature Tc from a Type K thermocouple (±0.1 C), dew mass m (g), air relative humidity, air temperature and dew point temperature, foil temperature Tc. The wind speed was measured by a cup anemometer (stalling speed: 0.5 m/s) located within 1.50 m from the plate, 3 m from the foil collector and 1.6 m above the ground. Wind speed data have been extrapolated at z = 10 m height by using the classical logarithmic variation (see e.g. Pal Arya, 1988): VðzÞ ¼ V 10 lnðz=zc Þ=lnð10=zc Þ;

ð1Þ

where zc (taken here to be 0.1 m) is the roughness length. Rain (mm), wind direction (degree) and cloud cover data N (octas) are measured at the Faaa airport meteorological station situated about 2 km NNE. A typical recording is shown in Fig. 1. When necessary, the real mass is easily inferred from the events where wind is near

Figure 1 Data recorded during the night 6–7/9/2005 in Tahiti on PTFE (time t is UTC-10). Left ordinates: temperatures (C): PTFE surface temperature Tc, full line; air temperature Ta, small interrupted line; dew point temperature Td, dashed line. Right ordinates: dew mass m (g), crosses; cloud cover (octas), full line; wind speed V (m/s) at 10 m, interrupted line.

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zero. Typically, as soon as the condenser surface temperature Tc < Td in the evening, the balance detects a slow mass increase with a slope dm/dt < 8 g/h. When in the morning Tc > Td, evaporation takes place with a very large negative slope dm/dt. The time of dew production is determined by the time (dt) where Tc < Td (Fig. 1). This is a simplification whereby water is supposed to completely wet the substrate. The total dew mass is taken as the maximum condensed mass m0, when Tc reaches Td in the morning. From m, it is easy to deduce the equivalent dew water precipitation h hðmmÞ ¼ ½m or m0 ðgÞ=160:

ð2Þ

The numerical factor corresponds to the condenser surface area. There are no fog or frost occurrences. Rain events are detected by visual observation or the high value of m and/ or its rate dm/dt, which exceeds 5 · 102 mm/h. Rain events were excluded from the dataset. The dew data were obtained on the balance during the dry season, between 8/ 6/2005 and 14/9/2005. In order to evaluate the night-time radiative balance on the surface of the condenser, the global sky irradiance was measured by two radiometers installed at 5 m off the ground, at a distance of 20 m from the condenser and in an open area. The measurements were performed in the band 0.3–3 lm by a pyranometer (CM3 from Kipp & Zonen) and in the band 5–50 lm by a pyrgeometer (CG3 from Kipp & Zonen, with built-in heater to prevent condensation) with both sensors placed in the same setup. These data were collected during the dry season, from 14/7/2005 to 14/10/ 2005 (unfortunately, some data were lost between August 15 and September 24.)

Dew yields General dew data Fig. 2 and Table 1 contain rain and dewfall data for Tikehau and Tahiti. Note that the dry season was exceptionally humid during the study period, with more than 50% of rain events below 2 mm/night. Dew yields above 0.2 mm/night rarely occur at the Tikehau and Tahiti sites. It is less than the maxima that are collected elsewhere, e.g. in the Mediterranean basin where 0.42 mm/night was measured in Ajaccio, Corsica Island (France) (Muselli et al., 2002) and about 0.6 mm/night in Jerusalem (Israel) (Berkowicz et al., 2004). The Tahiti measurements show a high dew frequency, 53%, as compared to 33% in Ajaccio over one year (Beysens et al., 2005), but with a low mean yield equal to 0.068 mm. The dew events are relatively stable in amount and in frequency along the measurement period. The cumulated dew volume is high (5.58 mm in Tahiti and 3.5 mm in Ajaccio during the summer, dry season, see Beysens et al., 2005). Dew measurements in Tikehau at site # 1 during the first period (June–July) showed low dew collection. In contrast, site # 2 (used later) provided better results with more frequent dew events that also have good yields. The distribution of dew yields in Tikehau (Fig. 2c) and Tahiti (Fig. 2d) shows that the more frequent dew events do not correspond to the smallest yields.

Analysis of Tikehau data Radiation budget and condenser position Site # 1 at the airport was chosen because the area is exposed, giving a maximum radiative transfer to the sky. However, the presence of Trade winds was found unfavorable for representative data. Indeed, Fig. 2 shows only a few dew events and with small dew yields. During the 21/6/2005–8/8/2005 period, only four yields above 0.1 mm were recorded and these occurred on unusually calm nights. During these 4 days, the wind speed at 10 m elevation was below 2.5 m/s. In contrast, the nightly (20:00–7:00) mean wind speed at 10 m elevation measured in Rangiroa from 2001 to 2005 during the dry season was 5.48 m/s with a standard deviation of 3.2 m/s. The percentage of time with a wind speed below 2.0 m/s is only 9.8%. It is known that wind speeds (at 10 m elevation) greater than 3 m/s significantly decreases collected dew volumes (Muselli et al., 2002). Consequently, it was decided to move the condenser to a garden of a house enclosed by trees (6 and 8 m height). The dimensions of the open area are 44 m · 43 m, located at the end of the airport area, protected from direct dominant wind with 12 rows of coco trees. This situation is representative of most houses in the Tuamotu atolls. In order to evaluate the radiative budget corresponding to the new condenser position, a specific FORTRAN code was developed (Clus et al., 2006) for the conditions presented in Fig. 3a. The integration of the radiative budget on elementary solid angles of the hemispherical sky was performed in steps of 1 · 1 degrees. The incoming angular sky long wave radiation dIX (W/m2) in an elementary solid angle dX is modeled following Eqs. (3) and (4) (Berger and Bathiebo, 2003): dIX ¼ h  r  T 4a  dX=p:

ð3Þ

Here, h ¼ 1  ð1  total Þ1=ðb cos hÞ ;

ð4Þ

where r is the Stefan-Boltzmann constant, Ta is the ambient temperature measured at the ground level (fixed at 288 K for the simulations) and etotal is the relative sky emissivity, approximated here to 0.8, a reasonable value for an air with 80% RH (Nikolayev et al., 2001). The angular emissivity eh is given by the Eq. (4) where h is the angle relative to the zenith (vertical) direction and b = 1.66 (Berger and Bathiebo, 2003). The angular emissivity is assumed to be eh = 1 if an obstacle (tree, house, relief, etc.) is shading the sky for the considered dX. The condenser is taken as a gray body with emissivity 0.94 (Nikolayev et al., 2001) for the calculation of the angular long wave radiation emitted in the solid angle dX. Both incoming and dissipated power on the surface of the condenser are corrected by the tilt angle of the condenser when calculating the radiative budget for each dX. As shown in Fig. 3a, the condenser is placed in an open area bordered with 6 m high trees approximately along the S, W, N directions and with 8 m trees along the E direction. The house shades the same band of sky as the trees and can be ignored. The radiative budget is integrated for different positions (X) of the condenser along an axis situated 22 m

Study of dew water collection in humid tropical islands

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Figure 2 (a) Dew yield evolution (top half) collected at the Tikehau (Tuherahera) sites # 1 and # 2. The 55 rain data (bottom half) are from the Rangiroa airport. The 44 rain events were observed in Tikehau. (b) Dew yield evolution from the Tahiti site (Outumaoro). Rain data (bottom half) are from the Faaa airport. (c) Distribution of dew yields at Tikehau. Site # 1: black column; site # 2: gray column. (d) Distribution of dew yields at Tahiti.

within the S side (Fig. 3b). The position X = 13.5 m corresponds to the maximal heat dissipation, 77.8 W/m2. This value changes only slightly between 10 and 30 m. The same

integration carried out for an open field such as the airport area gives exactly the same value. There is thus no detectable energy loss due to the presence of trees.

164 Table 1

O. Clus et al. Statistics of dew occurrence

Rainfall (cumulative, mm) Dewfall (cumulative, mm) Dew/rain (%) Minimum dew yield (mm) Maximum dew yield (mm) Average dew yield (mm/dew event) % Dew events (all days) % Dew events (excluding rainy days)

Tikehau sites 1 and 2 (21/6/2005–7/10/2005, 109 days)

Tahiti (16/5/2005–14/10/2005, 151 days)

247.8 2.64 1.09

176.2 5.58 3.17

0.013 0.23 0.102 23.9 38.8

0.004 0.22 0.068 53.5 69.8

Figure 3 (a) Site # 2 at Tikehau (schematic). C: 30 tilted condenser with the hollow back side facing the Trade winds. X (m): distance condenser – trees. Grey circles: 8 m high trees. Grey ellipses: 6 m high trees. (b) Simulation of the radiative budget as a function of X.

Wind speed Fig. 4 contains dew amounts collected in both sites with respect to wind speed. The mean wind speed in Rangiroa during the dry season nights is 10m = 5.5 m/s with a standard deviation of 3 m/s. Thus most of the wind speeds in Rangiroa are above 4 m/s, which is the limiting velocity for dew formation (Beysens et al., 2005). Fig. 4 stresses the difficulty to obtain significant dew yields in the free open area # 1, even with the condenser back side exposed to the incoming wind. In this configuration, only a few dew events correspond to V < 4 m/s. In contrast, dew is more frequent and occurs for V up to 7 m/s in the protected site # 2. Cloud cover Dew was observed at site # 1 for the lowest cloud cover amounts (N 6 3 octas, Fig. 5). During such strong wind conditions dew can form only when the cooling power is large; this condition is met only when the sky is clear or N small. Note that clear sky events are exceptional due to the presence of high absolute humidity in this tropical atmosphere.

Figure 4 Cumulated dew yields at Tikehau with respect to wind speed (0.5 m/s steps). Site # 1: black data; site # 2: gray data.

Study of dew water collection in humid tropical islands

165 tion time. The distribution of mean cloud cover (not shown), averaged between 20:00 and 8:00 during all nights of the same time period, is quite similar to Tikehau and shows many nights with N > 2. A detailed discussion using the radiometer data is given in Section ‘‘Sky radiation’’.

Figure 5 Cumulated dew yields collected at Tikehau with respect to cloud cover (octas) averaged during condensation time. Site # 1: black; Site # 2: gray.

Relative humidity and dew point temperature The relative humidity is a key parameter for dew formation. A high RH corresponds to a small Ta–Td and thus less cooling is needed for dew condensation. Fig. 8 shows the dew yield data correlated with Td–Ta, or alternatively with ln(RH) as these quantities are nearly proportional in the studied range. PTFE and foil dew data are included. Also shown for the sake of comparison are foil data obtained with the same 1 m2 condenser in a Mediterranean Island (Ajaccio) from 31/8/2003 to 8/7/2004. The maxima, selected within a temperature step of 1 K, are also reported in Fig. 8. Nearly all the data lie below a line that fits these maxima according to

Analysis of Tahiti data

h¼

Wind speed The dew yield per night on the foil is shown in Fig. 6 with respect to wind speed for the period 08/06/2005–14/10/ 2005. Here, the wind speeds have been averaged over the dew duration time (temperature PTFE Tc < Td). The situation of the experimental site, protected from the Trade winds, explains the low values of wind speeds (79% of the data have a mean wind speed < 1 m/s). The largest dew yields occur for wind speeds around 0.7 m/s, which also correspond to the most frequent wind speed. The high frequency of dew events (69.8% of the non-rainy days) as compared to those observed on Tikehau (38.8%, see Table 1) is due to the lower wind speeds as encountered in Tahiti. Cloud cover In Tahiti, the largest dew yields (Fig. 7) occur for N 6 2 octas. The cloud cover was averaged over the dew forma-

Figure 6 Cumulated dew yields collected at Tahiti on OPUR foil with respect to wind speed measured on the site (corrected for 10 m elevation) and averaged during the condensation period.

h0 ½DT 0  ðT d  T a Þ: DT 0

ð5Þ

DT0 is the maximum cooling temperature and h 0 the maximum dew yield. The data below this line simply means that Ta–Td (or RH) is the main parameter that limits the dew yield. Of course other factors such as wind speed and cloud cover also matter but they decrease the dew yield from this line of maxima. Tahiti data and Tikehau data (not shown) give nearly the same results when fitted to Eq. (5). Concerning DT0, the values for PTFE (4.3 K) and foil (4.7 K) correspond to a relative humidity RH  75%. The yield h 0 = 0.26 mm (PTFE) or h 0 = 0.30 mm (foil) are similar. In contrast, the Ajaccio results appear markedly different as DT0 = 10.3 K (RH  51%) and h 0 = 0.36 mm. Cooling power and mean dew rate are then larger in Ajaccio than in Tahiti. Note that no dew events were found in Tahiti for 0 < Ta–Td < 1.5 K or 100 > RH > 91%. (in contrast to Ajaccio). This observation is explained below.

Figure 7 Cumulated dew yields collected at Tahiti with respect to cloud cover averaged during the condensation time (08/06/2005–14/10/2005).

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Figure 8 Dew yield data with respect to Td–Ta (lower abscissa in K) or RH (upper abscissa, log scale, in %). (a) Tahiti PTFE data, (b) Tahiti foil data, (c) Ajaccio foil data. The maximum yields in Td–Ta steps of 1 K are bound by lines. The best fits of these data are shown by straight lines.

Discussion Two features of the humid tropical climate in the study region are strong Trade winds and high absolute humidity. These characteristics are at the origin of the particularities of dew formation and collection.

Wind speed and humidity The dew yield study at the two Tikehau sites demonstrates that protecting the condenser from direct wind is very effi-

cient for improving dew yield. However, wind has other effects on condensation. Firstly, wind enhances the heat exchange with air (the Nusselt number increases). At the first site (airport), a mean wind speed as large as 5.48 m/s was too high to permit sufficient cooling and frequent condensation (Fig. 4). However, the second site, protected by coco trees, allows higher dew yields to be observed. The influence of coco trees on the wind profile can be described as (i) a permeable windbreak with a highly porous base from 0 to 5 m and (ii) an impermeable dense leaf layer from 5 to 8 m. For a windbreak com-

Study of dew water collection in humid tropical islands posed of a permeable basis (e.g. tree trunk) with an impermeable superior level (e.g. tree foliage), Guyot (1999) noticed a decline of more than 50% in wind speed up to a distance of nine times the windbreak height, a result independent of the wind speed. Secondly, wind speed has an influence on the low atmospheric layers and on local relative humidity. Dew can form only when the condenser surface is cooled below the dew point temperature. What thus really matters is the gap Ta–Td  ln(RH). The tropical oceanic atmosphere is very humid with an average total atmospheric water vapor column close to 50 mm (American Geophysical Union, 2006). Strong wind (5.5 m/s on average) ensures that the surface atmospheric layer (close to the ground) is well-mixed and that the temperature profile close to the ground (in the first 15 m layer) is adiabatic (Guyot, 1999). Under such conditions, RH is quite independent of V. As a result, significant dew yields can be found even with high wind speed if the condenser is properly protected from convective heating. Fig. 9a displays RH data for years 2001–2005 with respect to mean wind speed during clear nights only (mean cloud cover N 6 2) at the Faaa (Tahiti) meteorological station. Selecting the clear nights excludes in a simple way rain and storm events. It shows that significant RH (>70%) can be maintained for V up to 7.5 m/s (10 m elevation). Such strong mixing of the surface atmospheric layer, however, smoothens the day and night RH variation and thus limits RH to values of the order of 90%, as outlined above in section Analysis of Tahiti data. For example, 5 years of measurements (2001–2005) at the Rangiroa atoll during the dry season (June–October, Laurent et al., 2004) shows a night (18:00–06:00) average RH = 77% and a day (07:00–19:00) RH = 70%. For the sake of comparison, Fig. 9b shows the RH variation with mean wind speed for a typical Mediterranean site (Ajaccio). Here, RH > 70% only for V < 4.5 m/s. This finding agrees with dew yields declining sharply for V > 3 m/s and the near absence of dew for V > 4 m/s (Muselli et al., 2002). In such climates (Mediterranean, semi-arid), dryer than the tropical climates, large differences in temperature are observed between day and night due to the cooling of

167 the planetary limit atmospheric layer in contact with the radiative cooling of the soil. The variations are important in a 500 m thick layer, with a thermal inversion in the surface layer at 15 m above the ground (Guyot, 1999). The inversion temperature profile can reach 5–10 C in the surface layer. In such climates, clear night temperature profile is not adiabatic. The important radiative cooling of soil induces high RH enhancement at the ground level that can reach nearly 100% when wind speed is low enough, as observed above in Fig. 8c. In contrast, increasing wind speed is an important cause of decreasing RH in mixing the surface layer and homogenizing the temperature profile and explains why dew hardly forms even in protected areas.

Sky radiation The absolute power Psky received on the condenser due to IR sky radiation cannot give precise elements for comparison with other sites as it depends strongly on local temperature and air absolute humidity. What really matters in dew formation is the cooling power Pr ¼ Pc  P sky ;

ð6Þ

that is, the difference between the condenser power Pc and the sky radiative power Psky. The latter is measured with the two radiometers (bands 0.3–3 lm and 5–50 lm). The condenser emission power can be estimated by the Stephan– Boltzmann law such as Pc ¼ Sc c rðT c þ 273Þ4 ;

ð7Þ

where Sc is the condenser surface, r is the Stephan–Boltzmann constant and ec is the emissivity of the condenser foil (=0.94 from Nikolayev et al., 2001). In Fig. 10a, the mean cooling power is compared with the dew yields in Tahiti and the mean relative humidity averaged during the condensation period (the period where Tc < Td). For comparison, the same quantities are plotted in Fig. 10b for a Mediterranean climate (Ajaccio). Strikingly, the individual dew yields are on average three times lower in Tahiti than in Ajaccio.

Figure 9 Mean relative humidity (%) correlated with mean wind speed (10 m elevation) during clear night conditions (from 20:00 to 8:00) and mean cloud cover N 6 2. (a) Faaa airport (Tahiti, French Polynesia) from 2001 to 2005. (b) Ajaccio (Corsica Island, France), 2004 data.

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Figure 10 Dew yield on foil (black dots, left ordinate) and mean relative humidity (open circles, right ordinate) with respect to the mean cooling power ( as measured during the condensation time. (a) Tahiti site, from 14/7/2005 to 14/10/ 2005. (b) Ajaccio site, Corsica Island (France)), from 20/9/2004 to 3/2/2005.

A first explanation can be found in the lower cooling power, which is 9% lower at Tahiti on average. This lower power is due to the larger content of humidity in the atmosphere. The total vapor equivalent water column is indeed 50 mm for the Tahiti area and 16 mm in the Ajaccio area (American Geophysical Union, 2006). However, the most important effect is due to RH, which, when dew forms, is lower on average in Tahiti. In addition, strong winds prevent large values to be reached at night as outlined in Section ‘‘Wind speed above’’. In contrast, the smaller amount of vapor in Ajaccio is distributed closer to the ground, permitting ground radiative cooling and larger RH in the lowest atmospheric layer (Guyot, 1999). As a result, RH is larger on average by 4% in Ajaccio. On the Mollier condensation diagram that relates Td to RH, this difference in RH corresponds to a Td lower by almost 1 K in Tahiti. This lower Td necessitates further cooling and the corresponding energy is lost for condensation (see Fig. 8 where the dew yield decreases very rapidly with increasing Ta–Td). This lower RH and the lowest cooling energy available due to enhanced green house effects explain why dew water yields are less in these tropical climates although the air, paradoxically, contains more humidity.

Modeling and comparison with other sites In order to further compare dew formation in tropical islands with other places (Mediterranean islands and European continental locations), data were fitted to the numerical model developed by Nikolayev et al. (2001). This model is based on the model by Pedro and Gillespie (1982) and Nikolayev et al. (1996) and is valid only during the night period (no evaporation). It is based on the heat balance equation dT c ðMcc þ mcw Þ ¼ Pr þ Phe þ Rcond : dt

ð8Þ

Here, M and m are the condenser and condensed water mass, respectively, cc and cw are the specific heat of the condenser material and of the water, and t represents time.

The variables in the right side of the equation represent the different thermal processes involved in the heat transfer at the condenser level: Pr for radiative transfer, Phe for heat convective exchange with the ambient air and Rcond for the energy gain due to the condensation latent heat Lc by unity of mass. Thus, Rcond ¼ Lc

dm dt

ð9Þ

and for Phe Phe ¼ Sc aðT a  T c Þ

ð10Þ

where a is a thermal transfer coefficient. The parameter a is correlated with the boundary layer thickness and depends on the wind speed in the case of laminar flows (the most current situation in this study) by: pffiffiffiffiffiffiffiffiffi a ¼ kf V=D: ð11Þ Here, the parameter f is empirical; fp=ffiffiffiffi4 W K1 m2 s1/2 for a flat plane with the dimension D ¼ Sc (Pedro and Gillepsie, 1982). A correction coefficient k, of order unity, has been introduced to account for the relative position of the condenser in relation to the anemometer. The equation representing the mass m is described by the condensation rate  if positive Sc b½psat ðT d Þ  pc ðT c Þ dm ð12Þ ¼ dt 0 if negative where psat(T) is the saturation pressure at temperature T, pc(Tc) is the vapor pressure on the condenser depending on the humidification state of the substrate before condensation, b is the mass transfer coefficient which is proportional to the parameter a b ¼ 0:656 ga=ðpca Þ:

ð13Þ

Here, p is the atmospheric pressure (considered as a constant during a simulation), ca is the air specific heat, g is an adjustable parameter of order unity, added for the simulation, which takes into account the particular conditions of the air flow around the condenser.

Study of dew water collection in humid tropical islands In the model, the sky irradiation energy (long-wave radiation term) is estimated by neglecting the effects on sky transmittance of aerosols, green house gases, etc. This approximation is justified by the fact that the more important green house gas is water, whose presence is already considered in the model by the cloud coverage N (see below Eq. (15)). Thus the equation is (Pedro and Gillespie, 1982 and Campbell, 1977) Rl ¼ Sc  c  s  rðT c þ 273Þ4 :

ð14Þ

Here, es is the emissivity of the sky that depends on the ambient temperature Ta and on the cloud cover N (in octas) es ¼ es0 þ N½1  es0  8=ðT a þ 273Þ;

ð15Þ

where es0 = 0.72+0.005Ta. As N is usually obtained on a 3hourly observation basis, a linear interpolation is performed to calculate it for smaller (15 min) time intervals. Input data of the model are t, V, N, Ta, RH, Tc and mexp, the experimental mass of the condensed water. The model determines the simulated temperature of the substrate Tc,fit with the adjustable parameter, k, by minimizing the difference between simulated and experimental data. Firstly, the parameter g is fixed as it has a negligible influence on the temperature Tc and k is determined. Then, with the given value of k, the difference between mexp and mfit is minimized by adjusting the parameter g. Here, the fit is con-

169 cerned with the 1 m2 condenser whose nightly cumulated yield is measured in the morning (one data of dew water volume available mexp_total). The final simulated dew mass is modified by the following equation that enables the value of g to be determined: mfit ðg; time ¼ timefinal Þ ¼ mexp

total

ð16Þ

The results of the fits in terms of both parameters k and g are reported in Fig. 11 with respect to date. The values keep roughly constant, as expected. The mean values are listed in Table 2 and compared with values obtained with sites at nearly the same latitude in Europe (42–45 N) for Ajaccio and two continental locations: Bordeaux (Atlantic coastal area, France) and Grenoble (Alpine valley, France). The values of the coefficients k and g in Tahiti are in reasonably good agreement with those from Ajaccio in spite of the large differences in dew yields and climate. This means that the radiative power and the wind effects have been properly taken into account in the model. The heat and mass transfer parameters for both islands are, however, systematically larger than for the continental locations. The reason can be found in the high wind speed in islands, in contrast to weak wind speed currently found in continental locations that correspond to free convection. This situation, which is characterized by weak, turbulent air convection, is indeed not completely satisfactorily taken into account in Eq. (11) of the model (Beysens et al., 2005). Its complete description is out of the scope of the present study.

Concluding remarks

Figure 11 The heat (k) and mass (g) transfer coefficients obtained from fitting the dew data according to the model of Section ‘‘Modelling and comparison with other sites’’.

This study was motivated by the evaluation of dew water as a supplementary water resource in water-scarce low islands (atolls). In such islands, the need for fresh water is high, especially during the dry season. For the 2005 dry season in French Polynesia, the dew water resource was found to be around 5.6 mm. This amount (3% of rain water) remains low when compared to precipitations during the particularly rainy season that was studied but is nearly twice what is currently found in Mediterranean Islands during the summer dry season. It corresponds to a significant amount of water (560 L on a modest 100 m2 roof). Dew formation in a tropical humid climate shows interesting and paradoxical features when compared to other climates. The presence of strong Trade winds and the high absolute humidity of the atmosphere do not allow dew rates in tropical climates as large as those from some other regimes (e.g. the Mediterranean climate). In low islands or

Table 2 Dimensionless heat and mass exchange coefficients as obtained from the model of Section ‘‘Modeling and comparison with other sites’’ Site

Date

Heat exchange coefficient (k)

Mass exchange coefficient (g)

Tahiti Ajaccio Bordeaux Grenoble

20/06/2005–2/10/2005 10/09/1999–28/11/2002 14/10/2001–14/01/2003 4/25/2000–6/7/2001

3.9 ± 0.2 3.57 ± 0.03 2.54 ± 0.06 2.52 ± 0.13

0.23 ± 0.02 0.37 ± 0.025 0.12 ± 0.023 0.13 ± 0.04

The uncertainty corresponds to one standard deviation.

170 wind-facing sides of high islands, the condensing surface should be protected from the strong direct Trade winds, e.g. by a natural vegetal windbreak. The mean yield per dew event is about 0.1 mm in Tahiti (tropical climate), to be compared with a mean of about 0.17 mm in Ajaccio (Mediterranean climate). In contrast to Mediterranean climate, the most important yields occur when the air is the driest such as to provide the highest cooling power. Even though the mean yields are modest, dew events are very frequent because of the continuous high relative humidity level. This makes the cumulated volume important and dew harvesting by radiative cooling becomes useful as a complementary water resource. This is especially true for the periodic drought conditions where no significant rain occurs from April to September and where the population is 100% dependant on external water supplies. Simple adjustments of rain collection areas (roofs) could be made to harvest dew in addition to rain as already been carried out in Croatia (Beysens et al., 2007) or India (Sharan et al., 2007). Radiative and hydrophilic coatings or paints (see OPUR, 2006) have been used there, with the additional benefit of providing efficient cooling under sunny conditions. Dew harvesting thus appears as an attractive possibility to provide a complementary water resource and could become quite essential for extreme events such as droughts, cyclones and tsunamis.

Acknowledgments Funding support is gratefully acknowledged from the ‘‘Ministe ¸ais de l’Outre-Mer’’ and ‘‘Collectivite `re Franc ´ Territoriale de Corse’’. We are indebted to S. Berkowicz for his critical reading, kind remarks and relevant suggestions. We are also thankful to the Natua family for having kindly welcomed us. We also thank R. Matehau and D. Teiva for their help in the data collection. We express gratitude to the French Polynesia branch of Meteo France for providing the meteorological data.

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