Evaporation and land surface energy budget at the Salar de Atacama, Northern Chile

Journal of Hydrology 310 (2005) 236–252 www.elsevier.com/locate/jhydrol

Evaporation and land surface energy budget at the Salar de Atacama, Northern Chile Stephanie K. Kampfa,*, Scott W. Tylerb,1, Cristia´n A. Ortizc,2, Jose´ F. Mun˜ozc,2, Paula L. Adkinsd a

Department of Civil and Environmental Engineering, University of Washington, P.O. Box 352700, Seattle, WA 98195-2700, USA b Graduate Program of Hydrologic Sciences, University of Nevada, Reno, NV, USA c Department of Hydraulic and Environmental Engineering, Pontificia Universidad Cato´lica de Chile, Chile d Desert Research Institute, Reno, NV, USA Received 25 September 2003; revised 22 December 2004; accepted 4 January 2005

Abstract Playa systems are driven by evaporation processes, yet the mechanisms by which evaporation occurs through playa salt crusts are still poorly understood. In this study we examine playa evaporation as it relates to land surface energy fluxes, salt crust characteristics, groundwater and climate at the Salar de Atacama, a 3000 km2 playa in northern Chile containing a uniquely broad range of salt crust types. Land surface energy budget measurements were taken at eight representative sites on this playa during winter (August 2001) and summer (January 2002) seasons. Measured values of net all-wave radiation were highest at vegetated and rough halite crust sites and lowest over smooth, highly reflective salt crusts. Over most of the Salar de Atacama, net radiation was dissipated by means of soil and sensible heat fluxes. Dry salt crusts tended to heat and cool very quickly, whereas soil heating and cooling occurred more gradually at wetter vegetated sites. Sensible heating was strongly linked to wind patterns, with highest sensible heat fluxes occurring on summer days with strong afternoon winds. Very little energy available at the land surface was used to evaporate water. Eddy covariance measurements could only constrain evaporation rates to within 0.1 mm dK1, and some measured evaporation rates were less than this margin of uncertainty. Evaporation rates ranged from 0.1 to 1.1 mm dK1 in smooth salt crusts around the margin of the salar and from 0.4 to 2.8 mm dK1 in vegetated areas. No evaporation was detected from the rugged halite salt crust that covers the interior of the salar, though the depth to groundwater is less than 1 m in this area. These crusts therefore represent a previously unrecorded end member condition in which the salt crusts form a practically impermeable barrier to evaporation. q 2005 Elsevier B.V. All rights reserved. Keywords: Evaporation; Energy budget; Atacama Desert; Playas

* Corresponding author. Fax: C1 206 685 3836. E-mail addresses: [email protected] (S.K. Kampf), [email protected] (S.W. Tyler), [email protected] (C.A. Ortiz), jfmunoz @ing.puc.cl (J.F. Mun˜oz), [email protected] (P.L. Adkins). 1 Fax: C1 775 784 6250. 2 Fax: C56 2 686 5876. 0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.01.005

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1. Introduction Playas, also known as salares in Chile, are dry salt lakes found in deserts throughout the world. A more specific definition given by Rosen (1994) describes playas as intracontinental basins with negative annual water balance and a capillary fringe close enough to the surface to allow water discharge via evaporation. As zones of groundwater discharge, these areas can be important sources of water in desert regions. For example, in the extremely arid Atacama Desert of northern Chile, playa groundwater and brines are a significant water resource for the mining industry, and surface water on playas and their margins is an important habitat for flamingoes and other fauna. Furthermore, playas often contain detailed paleoclimate records (Bobst et al., 2001; Lowenstein et al., 2003) and can be sensitive even to small climate fluctuations. A comprehensive understanding of how these unique systems respond to climatic and anthropogenic forcing must include a detailed characterization of all aspects of the water budget, including the elusive evaporation component. Previous studies of evaporation from playa environments (e.g. Allison and Barnes, 1985; Ullman, 1985; Jacobson and Jankowski, 1989; Malek et al., 1990; Tyler et al., 1997; Menking et al., 2000; Malek, 2003) have found that evaporation rates are quite low, due to the high salinity of groundwater and soil moisture and to the formation of salt crusts that either limit capillary rise or form impermeable barriers to liquid water. Although only a few studies have been conducted on the energy budget characteristics of playas (Malek et al., 1990; Tyler et al., 1997; Menking et al., 2000; Malek, 2003), those measurements that exist show that playa salt crusts are particularly suited to inhibiting evaporation, for they reflect the incoming solar radiation that provides the energy source for evaporation and often represent a cap that can be highly resistant to water vapor transport (Malek et al., 1990). Unfortunately, previous investigations have shown that accurate measurements of the low evaporation rates typical of playas can be difficult to obtain. At Owens Dry Lake in California, Tyler et al. (1997) tested several techniques for measuring evaporation. Of the techniques tested, they found that Bowen ratio and chloride profiling techniques could not adequately

237

capture low evaporation rates. Microlysimeters produced the most reliable results, and eddy covariance techniques for measuring evaporation compared most favorably with microlysimeters, though results from these two techniques could differ by up to 0.11 mm dK1. At the Salar de Atacama, the subject of this study, previous attempts to measure evaporation (Mardones, 1998; MINSAL, 1988) have found that evaporation rates over much of the basin may be within or below this range of uncertainty. Furthermore, previous evaporation studies in the Salar de Atacama (MINSAL, 1988) found that even lysimeters are difficult to use effectively in this environment, for they develop variable porosities and brine densities due to salt dissolution and precipitation. This causes them to imperfectly mimic natural conditions in the salt crust. Recognizing both the importance of characterizing evaporation rates for monitoring the water budget of the Salar de Atacama and the challenges in taking such measurements, this study used the eddy covariance technique to measure and analyze Salar de Atacama evaporation rates as they relate to the land surface energy budget. These energy budget measurements are used as ground truth for remote sensing energy flux models tested on the Salar de Atacama using data from the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER). The remote sensing models were developed with the objective of determining basin-wide evaporation rates and are described elsewhere (Kampf, 2002). Although other studies of playa evaporation (e.g. Allison and Barnes, 1985; Ullman, 1985; Jacobson and Jankowski, 1989; Malek et al., 1990; Tyler et al., 1997; Menking et al., 2000; Malek, 2003) have described the evaporation characteristics of particular playa crusts, none of these studies measured evaporation from a wide variety of salt crust types. Crusts examined in these previous studies have been subject to periodic inundation, which inhibits the growth of thick, rugged efflorescent salt crusts. The Salar de Atacama presents a particularly unique case study, for it contains an extensive range of salt crusts types ranging from smooth crusts formed in areas that are periodically inundated as well as very rough crusts in areas that have not been inundated in thousands of years. These rough crust areas present an ‘end member’ for salt crust development. At the Salar de

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Atacama, we therefore have the opportunity to put the end member condition into context with measurements from other salt crust types and begin to enhance our understanding of how playa systems respond to climate. This paper presents an integrated study in which we explore how land surface energy budget distributions and evaporation rates respond to salt crust morphology, climate and atmospheric forcing, and groundwater at the Salar de Atacama.

2. Location The Atacama Desert of northern Chile is possibly the driest region on the planet, with the extreme aridity driven by a combination of cool surface temperatures on the Pacific Ocean, latitude-driven descent of dry air, and the presence of the Andes Mountains, which block moisture originating in the Amazon basin. The Salar de Atacama is a large, highelevation playa within this desert (Fig. 1) at the latitude of the Tropic of Capricorn. Sitting in a regional graben at the base of the Andes Mountains, the Salar de Atacama covers a surface area close to 3000 km2 at an elevation of approximately 2300 m (Ramı´rez and Gardeweg, 1982). Rising above the Salar de Atacama are numerous strato-volcanoes on the Andean plateau (altiplano) to the east. Ongoing volcanic activity has left the Andean foothills covered with andesitic ignimbrites and pyroclastic deposits (Ramı´rez and Gardeweg, 1982). The salar is bounded to the southwest by the Cordillera de Domeyko and to the northwest by the Cordillera de la Sal, which contains evaporites and sedimentary deposits. The Salar de Atacama basin itself contains predominantly alluvial deposits along the southern, northern and eastern margins, and these areas are partially vegetated with shrubs and grasses (Fig. 2a). Further toward the basin interior, the surface consists of a series of ponds (lagunas) and marshes representing zones of groundwater discharge. This area also has salt crusts, which form in narrow bands along the salar margin (Fig. 2b). These ‘border’ crusts contain varying amounts of halite (NaCl) and are rich in sulfates, especially gypsum (CaSO4) (Ramı´rez and Gardeweg, 1982; Alonso and Risacher, 1996; Chapman et al., 1989).

Each of these margin crusts has a different surface morphology, reflecting the mechanisms of crust formation and the salt mineralogy. Border crusts with smooth surfaces are likely to have precipitated directly from surface water, whereas crusts with rougher surfaces tend to be efflorescent, precipitated as water travels up through the unsaturated zone. These latter crust types are more common at the salar, and they have rough, irregular surfaces with roughness heights ranging from around 2 to 15 cm in the border crust area. The center of the basin contains only rough efflorescent halite crust and is called the nucleus of the salar (Fig. 2c). The salt towers on the nucleus are cavity-rich, partially darkened by windborne sediments, and have roughness heights ranging from 15 cm to nearly 1 m. Such roughness can only form when the playa surface remains continually dry, a condition that is rare in playa systems, most of which are subject to occasional inundations that cause crusts to partially dissolve. The climate at the Salar de Atacama is extremely arid, with precipitation averaging only 20 mm yrK1 (Sa´nchez and Morales, 1990). Meteorological data collected at stations installed on the Salar de Atacama (operated by SQM S.A.) show that mean daily temperatures typically peak in February at approximately 22 8C and reach a minimum of around 8 8C in July. Diurnal temperature fluctuations match this seasonal variability, with maximum and minimum daily temperatures typically separated by around 14 8C. During spring and summer months (October– March), winds blow predominantly from the west, and during autumn and winter, the wind direction is more variable. Morning wind speeds throughout the year are generally !2 m sK1, and wind speeds typically increase in the afternoon, reaching up to 15 m sK1. These climate conditions give rise to a hydrologic system in which the primary inflow to the salar occurs via groundwater flow. Groundwater enters the basin from the north, south, and east and flows along a shallow gradient toward the southwestern corner of the basin, as described in Mun˜oz et al. (2004) and Tejeda et al. (2003). Most recharge occurs after the Bolivian Winter, which is a period from January to March during which the altiplano receives precipitation from the Amazon basin. Water entering the playa from the north and east is brackish, whereas water in the central part of the basin, the halite nucleus, is highly saline

S.K. Kampf et al. / Journal of Hydrology 310 (2005) 236–252

239

Bolivia

23 S

Argentina

Chile

70 W

68 W

Rough halite nucleus Border crusts Alluvial deposits Cordillera de la sal evaporites, sandstones, and siltstones

6

Sedimentary deposits

3

Ignimbrites

7,8

Altiplano strato-volcanoes

Salar de Atacama

Intrusives, sedimentary deposits, and ignimbrites

2 20 km

1

5 4

N

Andean plateau (altiplano) Cordillera de la Sal

Ignimbrite

Salar de Atacama Faults Fig. 1. Location and generalized geology of the Salar de Atacama. Numbers represent measurement locations. Adapted from Ramı´rez and Gardeweg (1982) and Mardones (1998).

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Fig. 2. Salar de Atacama surface features. (a) vegetated wetland; (b) halite–gypsum crust (Site 4); (c) halite nucleus (near Site 1). Photos (a) and (b) shown with eddy covariance stations.

brine. Very little surface water enters the basin, most coming from the San Pedro and Vilama Rivers to the north and some originating as runoff from the Andes after very infrequent high intensity storm events.

3. Measurement techniques Characterizations of the energy budget and evaporation from the Salar de Atacama were conducted using two eddy covariance/energy flux stations. These measurements are based on the land surface energy budget in which the net radiation available at the ground surface (sum of incoming and outgoing short and long-wave radiation) is dissipated by means of latent, sensible, and soil heat fluxes. If fluxes directed toward the ground surface are considered positive, this energy budget can be described simply as R n K H K G K Lv E Z 0

(1)

where Rn is the net all-wave radiation, H is the sensible heat flux, G is the ground heat flux, and LvE is the latent heat flux. In this study, all of these energy budget components were measured over 15 min averaging intervals. Net radiation was measured at each station using REBS Q7.1 net radiometers. Soil heat flux was measured using two Hukseflux HFP01SC self-calibrating soil heat flux plates on one station and two

Campbell Scientific, Inc. HFP3 soil heat flux plates on the other station. At both stations, Campbell Scientific, Inc. TCAV soil thermocouples were installed above the soil heat flux plates, and raw ground heat flux values were converted to ground surface heat flux based on heat storage above the plates determined using ground temperature, bulk density and moisture content (Campbell Scientific, Inc., 1990). Measurements of latent and sensible heat fluxes relied on the eddy covariance technique, commonly considered the most accurate method for measuring these fluxes. The eddy covariance technique assumes that the turbulent transport of constituents in the atmosphere can be characterized by high frequency measurements of vertical wind velocity and constituent concentration, such that Flux Z ðu C u 0 Þðs C s 0 Þ

(2)

where u represents the vertical wind speed, and s represents the constituent being transported. Terms with an overbar indicate mean values, and terms with a prime indicate fluctuations about the mean. Expanding Eq. (2) produces:  sÞ C ðu$s  0 Þ C ðu 0 $sÞ C ðu 0 $s 0 Þ Flux Z ðu$

(3)

Since the time average of vertical wind speed is 0 in most conditions, and the time average of fluctuations in the vertical wind speed is also 0, Eq. (3) can be

S.K. Kampf et al. / Journal of Hydrology 310 (2005) 236–252

reduced to: Flux Z ðu 0 s 0 Þ

(4)

In other words, the eddy flux is determined as the time-averaged covariance of the vertical wind speed and the constituent concentration. For latent heat and sensible heat fluxes, respectively, Eq. (4) becomes: Lv E Z Lv ðu 0 qv0 Þ

(5)

H Z rCp ðu 0 Ta0 Þ

(6)

where Lv is the latent heat of vaporization, qv is the water vapor density, r is the air density, Cp is the specific heat of air, and Ta is the air temperature. Eddy covariance measurements utilized a Campbell Scientific, Inc. CSAT3 three-dimensional sonic anemometer with a Li-COR LI7500 H2O–CO2 analyzer on one station and a CA27 one-dimensional sonic anemometer with a Campbell Scientific, Inc. KH20 krypton hygrometer on the other station. Latent heat fluxes measured using the KH20 krypton hygrometer were corrected for oxygen absorption according to the method outlined in Campbell

241

Scientific, Inc. (1989). Measured latent heat fluxes from both stations were also corrected for buoyancy effects using the method of Webb et al. (1980). Additional measurements of air temperature and relative humidity were taken using Campbell Scientific, Inc. HMP45C sensors. Barometric pressure was measured using a Campbell Scientific, Inc. CS105 sensor. Wind speed and direction were measured using a Campbell Scientific, Inc. RMY 03001 wind set, and at one station, incoming and outgoing shortwave radiation were measured using Li-COR 200SZ pyranometers. All data were recorded on Campbell Scientific, Inc. CR23X, CR21X and CR10 dataloggers. Additional data on wind speed, air temperature, incoming solar radiation, and relative humidity were obtained from two permanent meteorological stations located on the Salar de Atacama. Stations were installed on the Salar de Atacama during two field seasons at eight separate locations (Table 1, Fig. 1). The first field season was conducted during Chilean winter (August 16–25, 2001) and the second during Chilean summer (January 6–20, 2002). For most sites, complete energy budgets were

Table 1 Salar de Atacama field site characteristics Site

Site description

Period of measurement

Depth to ground-water (cm)

Salinity (g/cmK3)

Mean roughness height (cm)a

Mean albedo

1

Rough halite nucleus

18

0.18

0.0G0.0 K0.1G0.0 K0.1G0.0

August, 2001

82

15

0.49

0.0G0.1

August, 2001 January, 2002 January, 2002

16 30 31

10

0.46

2

0.65

0.4G0.1 0.6G0.0 1.1G0.1

January, 2002

30

Brine rcO1.2 Brine rO1.2 Brine rO1.2 Brackish 1.0!r!1.2 Brackish 1.0!r!1.2 Brine rO1.2

0.25

Western halite nucleus with detritus Northern transition from halite nucleus Halite-gypsum margin crust Carbonate margin crust Heterogeneous resolution and efflorescent halite and gypsum crusts Vegetated wetland

91 79 76

30

2

August, 2001 January, 2002 January, 2002

13

0.44

0.1G0.0

January, 2002

7

Partially vegetated wetland margin

August, 2001 January, 2002

70 64

3 4 5 6

7 8

Brackish 1.0!r!1.2 Fresh rZ1.0

Evaporation (mm dK1)b

50

2.8G0.2

13

0.4G0.0 1.1G0.1

The G indicates the standard deviation of daily evaporation measurements at each site. a Mean height of roughness elements above ground base level. b Mean daily evaporation calculated as the integrated sum of latent heat flux values for each 24-hour period. c r is groundwater density.

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measured for at least two full days during each field season. At several sites, complete energy budgets were not measured continuously due to limitations in instrument availability or technical problems such as power shortages. Sensible and latent heat flux measurements were also not used for analysis if they were recorded during periods when wind blew from behind the eddy covariance sensors. To provide some frame of reference for comparing these sites, depth to groundwater and ground surface roughness measurements were also taken. Depth to groundwater was measured simply by driving a pipe into the ground until it intersected the water table or, if this was not possible, measuring from the nearest shallow groundwater monitoring well. Ground surface roughness was estimated by taking a series of random measurements of the height of roughness elements above the presumed ground base level.

4. Energy budget characteristics 4.1. Results Results of the field campaigns are grouped into three major zones within the Salar de Atacama, located in the nucleus (sites 1–3), border crust (sites 4–6), and vegetated alluvial zone (sites 7,8). General site characteristics are summarized in Table 1. Rough halite nucleus sites have the greatest depth to groundwater (79–91 cm) and highly saline waters, with water densities exceeding 1.2 g cmK3. Border crust areas have shallower water tables (16–31 cm), and most have more moderate salinity, with densities generally on the order of 1.1 g cmK3. The water table in the vegetated wetland area is within a few centimeters of the ground surface, whereas groundwater in the partially vegetated wetland margin is 64–70 cm below the surface. This latter site is the only site with fresh water. Net shortwave radiation albedo (0.4–1.2 mm wavelengths) was measured at several sites, as shown in Table 1. Salt crusts with the highest albedos (0.44– 0.66) tend to be the margin crusts that have low surface roughness (sites 3–6). More rugged crusts in the nucleus (sites 1,2) have lower albedos (0.18–0.25) due to accumulation of darker sediments on the surface and a high concentration of cavities.

Fig. 3 shows one-hour moving average energy budget measurements for sites characteristic of the vegetated, halite nucleus, and border crust regimes. The moving average is used to facilitate data analysis, for it smooths some of the sharp fluctuations in the 15min eddy flux data. Fig. 3a gives energy fluxes at the vegetated wetland site. At this site, latent (LvE), sensible (H), and ground (G) heat fluxes mimicked trends in net radiation (Rn), rising (becoming more negative) throughout the morning and early afternoon to reach maximum flux rates by 1:45–2:15 pm. All fluxes had comparable magnitudes, with LvE exceeding both H and G throughout the day. All energy fluxes decreased throughout the late afternoon and evening. At night, H and LvE dropped to 0, and the negative net radiation was accounted for by release of heat from the soil. The energy budget pattern in the rough halite nucleus (Fig. 3b) was quite distinct from that of the vegetated wetland. In this area, the ground surface heated very quickly in the morning (note that negative values of G indicate heat entering the ground, whereas positive values of G indicate heat being emitted from the ground). The ground heating then reached a maximum rate near K400 W mK2 at noon, an hour and a half before the peak in net radiation, and gradually declined (less negative values) throughout the afternoon. At or just before sunset, the ground heat flux dropped to about C200 W mK2 then gradually settled to a steady rate of heat loss from the ground during the night. Sensible heating lagged behind G and increased more gradually throughout the morning to reach a peak in mid-afternoon. Note that as a result of the sign convention used in this study, negative values of H indicate sensible heating of the air, whereas positive values of H indicate a loss of sensible heat from the air. Latent heat flux was not detectable at this site. The border crust sites (Fig. 3c and d) had much lower Rn throughout the day than the nucleus and vegetated sites. At the halite–gypsum site (Fig. 3c), both G and H increased gradually at similar rates and magnitudes until early afternoon, when sensible heat flux reached maximum rates close to K220 W m K2, whereas ground heat flux rates reached only K180 W mK2. As at the halite nucleus, G dropped precipitously in the late afternoon to C200 W mK2 then settled to a steady rate of heat

S.K. Kampf et al. / Journal of Hydrology 310 (2005) 236–252

(a)

800

(b)

243

800 600

600

Energy (W m-2)

Energy (W m-2)

400 400 200 0 -200 -400 15.0

(c)

-200

-600 15.5

16.0

16.5

8

17.0

(d)

600

8.5

9

9.5

10

16.0

16.5

17.0

800 600

Energy (W m-2)

Energy (W m-2)

0

-400

800

400 200 0 -200 -400 9.0

200

400 200 0 -200

9.5

10.0

10.5

11.0

Day of Year

-400 15.0

15.5

Day of Year

Fig. 3. One-hour moving average energy budget measurements from January 2002. (a) Vegetated wetland (site 8); (b) Halite nucleus (site 1); (c) Halite–gypsum crust (site 4); (d) Carbonate crust (site 5). XZRn; Open circles, G; Filled squares, H; Solid line, LvE. Note: due to poor field measurements, G values shown in (b) and (d) were calculated as GZRnCHCLvE.

loss from the ground throughout the night. Latent heat flux occurred at a relatively constant rate of K 40 W mK2 throughout the day, increasing slightly to a maximum of K50 W mK2 in late afternoon (6 pm). At the carbonate site (Fig. 3d), G increased rapidly throughout the morning to reach a maximum rate of K210 W mK2 just before 1 pm. Sensible heat flux remained quite low, increasing gradually in the morning and afternoon to a maximum of only K 67 W mK2 at 3:15 pm. LvE followed a similar pattern to H, reaching a maximum of K99 W mK2 at 3:30 pm. 4.2. Discussion The sample energy budgets shown in Fig. 3 illustrate the major characteristics of the land surface energy budget in the rough halite, border crust and vegetated areas on the Salar de Atacama. To compare these three regimes more quantitatively, Fig. 4 shows

the mean daily maximum fluxes for each site. Although these daily peak values cannot fully capture the diurnal energy flux dynamics, they are useful for comparing the relative magnitudes of energy fluxes at different sites and during different seasons. As Fig. 4 demonstrates, the magnitude of net radiation is both seasonally and spatially variable at the Salar de Atacama. Data from a continuously recording meteorological station on the salar nucleus show that incoming solar radiation peaks at 1600 W mK2 at midday in January (summer) and at 970 W mK2 in July (winter), leading to a more that 600 W mK2 difference from summer to winter. Since net radiation accounts for both longwave and shortwave (solar) radiation, the seasonal changes in peak Rn recorded at the Salar de Atacama ranged from only 90 to 190 W mK2 between August 2001 and January 2002 measurements. Spatial variability had a much greater effect on peak net radiation, with a range of

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Fig. 4. Daily maximum energy fluxes (absolute values). Daily maximum values were taken from 1-h moving average of H, G, and LvE and from 15-min measurements of Rn. Each bar in the graphs represents the mean of all recorded daily maximum fluxes, and error bars indicate the standard deviation of recorded daily maximum values. For those fluxes without error bars, only one daily maximum was recorded. Rn and G were not measured at Site 8 during August 2001 due to instrument limitations.

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peak Rn greater than 400 W mK2 between sites measured during the January 2002 field season. This spatial variability in Rn relates to differences in salt crust albedo. During the day, net radiation was highest at the low albedo rough halite sites and lowest at the more reflective border crust sites. Since this environment is so arid, net radiation was typically dissipated by means of ground and sensible heat fluxes rather than by latent heat flux. During all August 2001 (winter) measurements at all locations, peak ground heat flux exceeded the daily maximum sensible and latent heat fluxes. At all rough halite sites and two of the border crust sites (5,6) during both winter and summer, the peak ground heat flux reached 59–70% of the peak net radiation, and the salt crust tended to heat and cool very rapidly. Conversely, at the vegetated sites, peak ground heat flux reached only 30–33% of the peak net radiation, and the soil heated more gradually in the morning and remained more constant at night (Fig. 3a and d). Clearly, different thermal properties in each of these regimes affect the flux of heat in and out of the ground, but a comprehensive assessment of soil heat flux would demand more complete instrumentation and modeling of heat transport. Although always lower than peak ground heat flux in August, peak sensible heat fluxes exceeded G at some sites (2,4,8) during January 2002. This indicates that H responded to seasonal changes in radiative forcing and meteorological conditions. In particular, analyses of concurrent wind speed measurements show that sensible heat flux and wind conditions at the salar are strongly linked. In August, winds were generally calm, at !2 m sK1 throughout most days. Occasionally, though, winds would increase to more than 12 m sK1 in the afternoon just after maximum sensible heat flux and presumably in response to an increased pressure gradient caused by warm, low density air near the ground surface. These days correspond with peak sensible heat fluxes O20 W mK2 higher than on calmer days. In January, these afternoon increases in wind occurred every day after peak surface heating, leading to high sensible heat fluxes in all but the highly reflective salt crust sites. At most sites on the Salar de Atacama, latent heat flux comprised a small portion of the energy budget. At the nucleus sites in particular, recorded

245

latent heating never reached magnitudes greater than G10 W mK2 (see Fig. 3b). Responding to the fresher water, shallower water table, and different salt crust morphology, latent heat fluxes were higher at the border crust sites (Fig. 3c and d), reaching estimated daily peaks of K20 to K90 W mK2, values still significantly smaller than sensible and soil heat fluxes. Each of these border crusts displayed unique latent heat flux characteristics. For example, at the halite– gypsum site (Fig. 3c), latent heat flux did not come to a peak around solar noon but rather was relatively steady at around K40 W mK2 during much of the day, indicating that latent heat flux at that site was constrained by the rate at which moisture could travel upward through the unsaturated zone rather than by atmospheric conditions. At this site, latent heat flux rates were similar during winter and summer seasons, but the steady flux rate produced a greater total moisture loss in the summer, due to the longer period of daylight. In contrast, at the carbonate site (Fig. 3d), latent heat flux had a more pronounced diurnal pattern and increased gradually throughout the morning and early afternoon to peak at nearly K100 W mK2 just before 4 pm. Although both border crust sites had the same depth to groundwater (30 cm) during January 2002, they clearly have different moisture transport characteristics. The thicker, rougher halite–gypsum crust may present more resistance to water vapor than the thin carbonate crust at site 5, where latent heat fluxes mimicked diurnal patterns in net radiation. Latent heat fluxes were highest at the vegetated wetland site, which had a mean peak of K242 W mK2 in January 2002. A later section describes what this latent heat flux distribution indicates about evaporation from the Salar de Atacama basin.

5. Measurement uncertainties The energy budget data collected at the Salar de Atacama demonstrate persistent patterns in energy fluxes, but the accuracy of the recorded values remains uncertain. Typically, the accuracy of such measurements is assessed by means of energy budget closure, defined as c Z R n C G C H C Lv E

(7)

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Table 2 Energy budget closure, cZRnCGCHCLvE for sites with complete energy budget measurements and no apparent equipment failures Site

Description

Period of measurement

Mean c, daya (W mK2)

Mean c, nightb (W mK2)

Mean c, all data (W mK2)

1 2 4 7 8

Halite nucleus N. nucleus transition Halite–gypsum Vegetated wetland Partially vegetated wetland margin

August 2001 August 2001 January 2002 January 2002 January 2002

43 32 59 36 56

K9 11 11 28 17

13 17 34 32 36

a b

Day is from sunrise to sunset. Night is from sunset to sunrise.

with the assumption that values of c at or near 0 indicate good measurements. Closures for sites with complete energy budget measurements (good data for Rn, G, H, LvE; no significant gaps (O3 h) in record) are shown in Table 2, which gives the mean values of c for each entire data set compared with mean values of c during daytime (sunrise to sunset) and nighttime (sunset to sunrise) hours. At all sites, mean c values were positive, ranging from 13 to 36 W mK2, indicating that on average, measured non-radiative energy fluxes were not high enough to balance net radiation. Values of c in all cases were higher in magnitude during the day than during the night, though this is partly because the magnitude of net radiation was always much lower at night than during the day. Fig. 5 shows c values obtained for one of the vegetated sites (8) and the halite–gypsum site during January 2002 (these c values correspond to the energy budget measurements in Fig. 3a and c). At all sites, c values tended to be highest and vary most widely near solar noon at the time of peak net radiation. The abrupt shift from negative to positive closure apparent late in Day 9 at Site 4 (lower graph, Fig. 5) corresponds to the precipitous shift in ground heat flux that occurred at that site. In fact, when examined carefully, patterns in closure errors at most sites demonstrate that many of the peaks can be attributed to the abrupt shifts in G and thus may relate largely to challenges in measuring ground heat flux. Particularly in the rugged salt crusts, ground heat flux was virtually impossible to measure using the available instruments, as the plates and thermocouples could not be installed to achieve complete thermal contact with the crust or in a location representative of conditions throughout the crust. The large cavities

present at the surface of some salt crusts and especially at the nucleus may have significantly affected heat transport in the shallow zone of the crust. Furthermore, ground heat flux values are dependent not only on raw soil heat flux plate values but also on the corrections for heat storage above the plates, which were determined using ground temperature, bulk density and moisture content. Fig. 6 demonstrates the dramatic effect these corrections can have on ground heat flux values. Raw soil heat flux plate measurements from 8 cm depth are compared to values adjusted to represent flux at the ground surface. These corrections could change the value recorded by the heat flux plates substantially since a significant amount of heat may be stored in the 8 cm-thick soil or salt crust layer above the soil heat flux plates. Appropriate values for soil bulk density and moisture content required for these corrections were themselves difficult to obtain, and errors in these measurements can propagate and produce significant uncertainty in calculated ground surface heat flux. Some closure errors may also reflect inaccuracies in eddy flux measurements. Around the time of peak sensible heat flux, measurements at all sites tended to be highly variable, probably due to atmospheric instability caused by buoyancy. At times with low wind speeds (!2 m sK1), eddy covariance instruments could not as effectively measure latent and sensible heat fluxes. These times were more likely to have sharp fluctuations in recorded 15-min flux values or gaps in the data record due to winds blowing from behind the sensor. Measurement errors may also result from differences in measurement area for each energy budget component. Soil heat flux plates and net radiometers measure very small areas, whereas eddy covariance instruments integrate sensible and latent

S.K. Kampf et al. / Journal of Hydrology 310 (2005) 236–252

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Energy (W m-2)

300

0

-300 15.0

15.5

16.0

16.5

17.0

10.5

11.0

Day of Year

Energy (W m-2)

300

0

-300 9.0

9.5

10.0 Day of Year

Fig. 5. Energy budget closure, c, for Site 8 (Vegetated wetland, top) and Site 4 (Halite gypsum crust, bottom) from January 2002 measurements. Solid line represents 15-min measurements, and dotted line shows c calculated from one-hour moving average energy fluxes. Gaps in record for Site 4 represent periods during which the wind blew from behind the eddy covariance instruments.

heat fluxes occurring within several hundreds of meters of the instruments. Although mean closure errors for the Salar de Atacama measurements are high (13–36 W mK2 in Table 2), they are within the ranges of uncertainty recorded at co-located energy budget stations in other studies (e.g. Moran et al., 1994; Stannard et al., 1994). Moran et al. (1994) and Stannard et al. (1994) compared energy flux measurements at a variety of closely spaced stations and noted that net radiation measurements could differ by 12–13 W mK2, and G measurements could differ by 14–40 W mK2. They found that the mean absolute difference for H

measurements at different stations was 32.5 W mK2, and that for LvE was 20.6 W mK2. Stannard et al. (1994) noted that closure errors at any one energy flux measurement site may result from actual measurement errors, horizontal flux divergence, and/or the mismatched source areas for flux measurements. They point out that a near zero value of closure increases confidence in measurements but cannot necessarily verify them. Closure errors of up to 36 W mK2 yield significant difficulties when trying to use measurements of very low latent heat fluxes to resolve daily evaporation rates. Our data indicate that a measurement error of

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200

Energy (W m-2)

100 0 -100 -200 -300 -400 15.0

15.5

16.0

16.5

17.0

10.5

11.0

Day of Year

300

Energy (W m-2)

200 100 0 -100 -200 -300 -400 9.0

9.5

10.0 Day of Year

Fig. 6. Ground heat flux for Site 8 (Vegetated wetland, top) and Site 4 (Halite gypsum crust, bottom) from January 2002 measurements. Thick line indicates soil heat flux plate measurements at 8 cm depth; dots indicate ground surface heat flux calculated based on heat storage above the plates; thin line indicates ground heat flux evaluated as a residual term in the energy budget, e.g. GZRnKHKLvE.

20 W mK2 in daily peak latent heat flux could result in evaporation errors exceeding 0.1 mm dK1 . For example, at one of the border crusts in this study (site 6), daily latent heat flux peaked at K18 W mK2, which corresponded with a mean evaporation rate of 0.1 mm dK1 at this site. This range of evaporation measurement uncertainty is sustained by data from one of the nucleus sites (site 2). At this site, when integrated to daily evaporation rates, the noise in latent heat flux recorded by the eddy covariance instruments indicated condensation at a rate of K0.1 mm dK1. Condensation has been observed at

night on playas with high surface salinity (Thorburn et al., 1992); however, we did not observe any condensation in the field, and the periods with measurements of positive latent heat flux occurred primarily during the day when condensation was unlikely to have occurred. The fact that the eddy covariance measurements suggested a small net condensation, therefore, gives us some uncertainty bounds for evaporation measurements using this technique. These examples suggest that during our field conditions, at best the eddy covariance technique can constrain evaporation rates to within 0.1 mm dK1.

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6. Basin evaporation Such uncertainties in evaporation rates make our analysis of evaporation at the Salar de Atacama a challenge, for many parts of the basin have reported evaporation rates less than 0.1 mm dK1 (Mardones, 1998). Table 1 shows evaporation rates for each site in mm dK1, as calculated by summing the 1-h moving average latent heat fluxes for each day. The G indicates the standard deviation of all daily evaporation measurements at each site. Except at the nucleus sites, these standard deviations are within 15% of the mean evaporation rate and range from 0.0 mm dK1 at low evaporation sites to 0.2 mm dK1 at the high evaporation wetland site (7), where we might expect more variability in daily evaporation due to minor fluctuations in the near-surface water level. Thus, the eddy covariance measurements appear capable of resolving evaporation rates to 15% accuracy. This level of accuracy allows us to examine broad evaporation trends throughout the Salar de Atacama. Since equipment and time limitations prevented us from comparing eddy covariance measurements of evaporation to other measurement techniques, discussion of evaporation will ensue based on the assumption that the eddy covariance measurements at the Salar de Atacama give a reasonable representation of actual evaporation rates. The reader should, however, bear in mind the significant uncertainties in these measurements, as described in Section 5. In a previous study using lysimeters to measure evaporation from the Salar de Atacama, Mardones (1998) concluded that evaporation rates in the nucleus were approximately 2 mm yrK1 (0.005 mm dK1), and those in the border crusts were approximately 0.1– 1.5 mm dK1. Evaporation rates found in the present study are within this range. Our results show highest evaporation rates in the vegetated wetland area (2.8 mm dK1), variable but low values in the border crusts (0.1–1.1 mm dK1), and no detectable evaporation in the halite nucleus (Table 1). Evaporation from soil or salt crusts may be influenced by a wide range of factors. If the water table is near the ground surface, evaporation may occur at or near the potential rate possible from radiation and meteorological forcing. Where the water table is further from the ground surface, evaporation becomes limited by the ability of soil to transmit

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water. Studies of evaporation from bare soils in arid regions have found that depth to groundwater can explain most of the variability in evaporation rates from these soils. As such, it is typically assumed that evaporation rates decay exponentially as depth to groundwater increases. Coudrain-Ribstein et al. (1998) examined a wide range of arid zone soils and found that in these soils, evaporation rates followed this exponential relationship with depth to groundwater. Furthermore, they found that for areas with deep water tables, this relationship was roughly the same for a wide range of soils. Salt crusts, however, have characteristics unique to those of most soils. Thorburn et al. (1992) examined a variety of existing data on evaporation from shallow water tables in bare soils and salt flats and found that for the same depth to groundwater, evaporation rates from salt crusts are lower than those from other bare soils. They attributed this finding to reduced relative humidity gradients, restricted water vapor movement, high albedo, and to the low hydraulic conductivity of salt crusts. Of these factors, they concluded that low hydraulic conductivity, perhaps caused by precipitation of salt in pores, is likely the factor that most decreases evaporation rates from salt crusts. The data assembled by Thorburn et al. (1992) are compared to Salar de Atacama evaporation measurements in Fig. 7. A general logarithmic trend relating evaporation rates to depth to groundwater is apparent for the compiled salt flat data. However, in some cases, for the same depth to groundwater, a wide range of evaporation rates are shown. Such variability likely reflects the influences on evaporation of factors other than depth to groundwater such as variable water salinity, salt crust morphology, or radiative forcing. For example, from our own measurements, the two border crust sites (4,5) with 30 cm water tables (January 2002) have evaporation rates that differ by more than 0.5 mm dK1, probably due to their distinct salt crust properties (described previously). Overall, the Salar de Atacama evaporation data from the vegetated and some border crust areas conform to the range of data from other salt flats with the exception of the two circled areas on Fig. 7. The dashed circle surrounds the evaporation value for the border crust Site 6, where for the same depth to groundwater, measured evaporation was much lower than at other salt flats. At this site, the water table was at

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S.K. Kampf et al. / Journal of Hydrology 310 (2005) 236–252 Evaporation (mm d-1)

Depth to groundwater (m)

0.001 0.1

0.01

0.1

1

10

y = -0.53Ln(x)-0.22 R2=0.84

1

10

Fig. 7. Evaporation vs. depth to groundwater for the Salar de Atacama compared with measurements at other salt flats and from bare soils. Diamonds represent playas or salt flats and lakes; squares represent vegetated areas, and x’s represent bare agricultural soils. Solid symbols represent data from the present study at the Salar de Atacama, and all other symbols represent data compiled by Thorburn et al. (1992) from Allison and Barnes (1985), Farrington et al. (1989, 1990), Jacobson and Jankowski (1989); Malek et al. (1990); Talsma (1963); Wind (1955); Woods (1990). The solid circle surrounds values from the Salar de Atacama nucleus, all of which should have evaporation rates of 0 but are shown at 0.001 mm dK1 to conform to the logarithmic scale. The dashed circle surrounds the value from Site 6, where the depth to groundwater was 0 though the point is shown at 0.1 m to conform to the log scale. The line represents a logarithmic fit to the playa data.

the ground surface, but a smooth, impermeable salt crust about 1 cm thick had formed over the water. This salt crust apparently presented a very effective barrier to evaporation, for only 0.1 mm dK1 of evaporation was measured at the site even under intense radiative forcing. The solid circle on Fig. 7 surrounds evaporation measurements for the Salar de Atacama rough halite nucleus sites. Since no evaporation was detected at these sites, they fall far below the range of evaporation measured at other salt flats with the same depth to groundwater. However, we note that many of the evaporation rates measured at other playas are less than the 0.1 mm dK1 level of measurement uncertainty that we estimate for the present study.

The Salar de Atacama data show that in an endmember condition, salt crusts can so severely decrease hydraulic conductivity that little to no evaporation can occur, even with relatively shallow water table depths. To our knowledge, such an extreme resistance to evaporation has not previously been observed for other playa environments. Although the groundwater in the halite nucleus is extremely saline, the near-surface halite is both fractured and cavity-rich, and open pore structures of these subaerially deposited salts have been reported by Bobst et al. (2001). Observations of core samples from the upper 20 m and those reported by Bobst et al. (2001) suggested that pores in the near surface salts may be as large as 1 cm, with even larger pores at the land surface. Such large pores, combined with the lack of clay fillings results in a material with essentially no capillary rise or capillary fringe. As a consequence, the salt crust immediately above the water table is very dry, and the unsaturated hydraulic conductivity is negligible. Transport of water from the water table to the atmosphere is therefore limited to vapor transport trough the thick (80–100 cm) porous and dry salt curst. The negligible evaporation rates measured in this study and Mardones (1998) in the nucleus suggests that the rough surface of the salt crust further limits the ability of high winds to exchange significantly with water vapor and air in the pore spaces. While the eddy covariance method was not capable of resolving the very small evaporative flux from the halite nucleus crust, it is possible to confirm that this flux is indeed very small by considering the rate of salt accumulation from nucleus evaporation. Mardones (1998) reports the halite nucleus evaporation to be w2 mm yrK1 (w0.005 mm dK1). Assuming a ground water salinity of 300 kg mK3 (essentially saturated sodium chloride), a halite density of 2200 kg mK3, and a halite crust porosity ranging from 40 to 60%, the rate of salt crust accumulation for a 2 mm yrK1 evaporation is 0.45 m kyrK1. Lowenstein et al. (2003) report the subaerial halite deposition rate over 10,000 year periods in the Salar de Atacama to be 0.5– 2.4 m kyrK1. This independent dating of halite accumulation confirms that evaporation from the halite nucleus of the Salar de Atacama is much smaller than any previously reported evaporation rates from other playas or salars.

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7. Summary and conclusions In this study, we take a comprehensive approach to examining playa evaporation through measuring complete land surface energy budget distributions during winter and summer seasons over sites with very distinct surface characteristics. Energy budget measurements at the Salar de Atacama have shown that in this extreme environment, available energy (net radiation) at the ground surface can vary significantly in response to differing albedos of salt crusts. The nature of the surface salt crust can dramatically alter the amount of energy available for evaporation, for salt crusts measured at the salar reflect from 18 to 65% of incoming solar radiation, with smooth crusts having much higher reflectance than the rugged, cavity-rich halite crusts in the center of the salar. Throughout the Salar de Atacama, very little available energy is used to evaporate water. Rather, most of the net radiation is dissipated by means of sensible and soil heat fluxes, with sensible heat flux responding to seasonal and spatial variations in surface heating. Latent heat fluxes are quite low throughout the Salar de Atacama, and this study highlights the limitations of the LvE measurements with eddy covariance at these very low rates. Results of the study indicate that the eddy covariance technique can only resolve evaporation rates to G0.1 mm dK1, a level of uncertainty greater than the evaporation rates from some rough, dry salt crusts on the salar. Nevertheless, although evaporation rates are low from many salt crusts, significant moisture loss from the basin still occurs via evaporation. Evaporation rates measured at the Salar de Atacama ranged from 0 to 2.8 mm dK1. When examined from a basin-wide perspective, measurements of evaporation from the present study concur with the conclusions of Mardones (1998) and Mun˜oz et al. (2004) that moisture loss from the Salar de Atacama occurs principally along the basin margins. These margin areas are therefore important to monitor when attempting to model the basin water budget and would be ideal settings in which to examine in more detail how evaporation rates respond to variability in meteorological and radiative forcing, depth to groundwater, and salt crust morphology. Results of this study have shown that no simple indicator such as depth to groundwater or available

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energy provides sufficient information to predict evaporation rates from playas. The morphology of the salt crust can dramatically change the nature of the evaporative response to the same climatic conditions. Thicker rugged salt crusts can exert significant resistance to water vapor transport. In fact, evaporation recorded on some parts of the Salar de Atacama is much lower than rates measured at other salt flats with the same depth to groundwater. Although previous studies have suggested that evaporation may fit on a continuum (Cnoise) depending on depth to groundwater, this study shows that in fact certain salt crusts effectively block both liquid and vapor transport, no matter how deep the water table. Thus, in extreme conditions such as those in the Salar de Atacama, salt crusts can appear practically impermeable to evaporation.

Acknowledgements This work was supported by NASA Headquarters under the Earth System Science Fellowship Grant NGT5-30385 and by the University of Nevada, Reno International Affairs Committee. Pauline De Vidts and personnel of the Sociedad Quı´mica y Minera de Chile (SQM) including Valentin Letelier, Julio Mondaca, and Dixon Toroco generously provided both accommodations and logistical support in the field. Gayle Dana of the Desert Research Institute provided some of the equipment used in this study, and the authors appreciate the helpful discussions with T. Lowenstein.

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