The experimental setup was designed to mimic natural environmental conditions and comprised two main components: (1) the fracture simulator (FS), an assembly designed to simulate natural field conditions for a host fractured rock, namely, rock saturation, pore water chemistry, and rock temperature; and (2) the climate control room (CCR), a large room that permits control of atmospheric environmental conditions, namely, temperature and wind. No-wind conditions were imposed on all experiments in order to exclusively explore the impact of atmospheric air temperature on ESEFs. In addition, ESEF rates were compared with PE by placing an evaporation pan in the CCR to measure both rates under the same conditions.
2.1.1. Fracture Simulator (FS)
 Two FSs were constructed. Rocks for the FS were obtained from the Avdat Group chalk formation, in the northern Negev desert, Israel. For this chalk formation, Dagan  reported an average porosity of 40%, and horizontal and vertical permeability of 1.1–2 and 0.7–2 mdarcy, respectively. Similar values have been reported in other studies [Nativ et al., 1995; Weisbrod et al., 1999]. Nativ and Adar  reported pore diameters of 0.009–0.296 μm with an average of 0.151 μm. Four FS rock fragments were cut to dimensions of 25 × 50 × 50 cm. Two quadrants of the (inner) fracture surface of each rock fragment were lightly scratched (Figure 1), creating a grid of grooves (about 1 mm wide × 1 mm deep) to investigate the effect of surface roughness on salt precipitation (not part of the study presented here).
Figure 1. Orientation of rocks in the fracture simulator (FS) assembly showing dimensions, labels for each block wall, and location of surface grooving. The distance between the rocks, which forms the fracture, is not drawn to scale in order to visualize the grooving in the inner surfaces.
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 For each rock fragment, all rock faces (Figure 1) except for the (inner) fracture surface and the (outer) surface opposite to it were sealed with epoxy cement (Duralite®, Bolidet, Netherlands). Previous work has shown excellent adhesion of this epoxy to chalk, with complete prevention of leaking [Weisbrod et al., 1998, 1999]. Inner and outer surfaces were left unsealed to allow evaporation from the fracture (inner) surface and solution inflow from the (outer) surface opposite to it. Solution containers were directly connected to the outer surfaces, providing the inflow solution source. A Mariotte bottle controlled liquid tension in the solution to −5 cm of gauge pressure with respect to the bottom of the rock. The containers were attached to the rocks with epoxy, sealing all possible gaps and providing a strong sealed cast (Figure 2). In addition, an aluminum heat shield covered the containers, providing thermal insulation and minimizing biofilm growth from light exposure.
Figure 2. The FS system. (a) Illustration of the FS (mirror image of Figure 2c, and not to scale). (b) Placement of instruments inside the rock. (c) Photograph of one FS. Numbers indicate 1, top surface with epoxy cement; 2, side surfaces with epoxy cement and polyurethane insulation; 3, solution containers with aluminum thermal cover; 4, small mass scales with Mariotte bottles; 5, large mass scale with the FS system; 6, thermocouples inside the rock; 7, spiral cylinder heaters inside the rock; and 8, second FS system.
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 The fracture aperture was oriented vertically between rock pairs separated by 10-mm spacers. Each rock provided a fracture surface of ∼2500 cm2. Apart from the top of the aperture, the fracture-aperture perimeter was sealed with silicon RTV (Tambour, Netania, Israel). To thermally insulate the rocks from the chamber, the entire system (except for the top opening of the fracture) was additionally coated with a thick layer of polyurethane fixing foam (diphenylmethane-4, 4-diisocyanate). The total weight of each FS system was approximately 350 kg.
 A suite of electronic instruments was installed within the FS, including thermocouples (TCs; Copper-Constant thermocouple wire, Omega Engineering Limited, Manchester, UK), relative humidity sensors (RH; Hygroclip SC04, Rotronic, Zurich, Switzerland; and CS-500, Campbell Scientific Inc., Utah), and cylindrical spiral heaters (Isaac Dery, Haifa, Israel). Ten TCs monitored the temperature in each rock fragment. Seven TCs were situated inside the rock, 10 cm from the fracture surface and distributed vertically, 7 cm apart, from 5 to 47 cm, as measured from the top surface. Three TCs were situated inside the rock 5 cm from the fracture surface and distributed vertically 14 cm apart, from 5 to 33 cm, as measured from the top surface. The cylindrical spiral heaters were installed along the bottom of each rock and used to maintain a constant temperature (23°C) at the bottom of the rock. The heaters were monitored and controlled by the lowest TC located in the rock at a depth of 47 cm (i.e., just above the heaters).
 The air within the fracture aperture was monitored with TCs and relative RH sensors. Seven TCs were located within the aperture at the same elevation as the TCs inside the rock. Four RH sensors were used within the aperture: One was located at the top surface at the interface between the top of the fracture and the ambient air of the CCR; the other three RH sensors were semimobile and were used mostly at depths of 12, 26, and 40 cm.
 Artificial groundwater (AGW) [Arnon et al., 2005] was used for the feeding solution. The AGW contained 7738 mg L−1 total dissolved solids (TDS), with specific concentrations (mg L−1) of 345 Ca, 200 Mg, 2184 Na, 22 K, 3608 Cl, 1089 SO4, and 291 HCO3. Solution exiting the Mariotte bottles, representing evaporation from the fracture, was monitored by placing the Mariotte bottles on mass scales (BB 3100 ± 0.01 g, M.R.C., Tel-Aviv, Israel). The total weight of each FS, excluding the bottles, was also monitored (Rabbit 400 kg ± 25 g, Shekel Electronic Scales, Karmiel, Israel), such that changes in the overall rock water content could be distinguished from changes due to evaporation. In principle, if mass changes only occurred in the Mariotte bottles, this meant that all of the solution entering the rocks also left the rocks by evaporation from the fracture surface.
 In order to capture and mimic field conditions with the laboratory setup, and to minimize the disparity between field and laboratory situations, special attention was paid to thermal and fluid boundary conditions. It was assumed that the bottom of the rock, corresponding to a depth in the field of 50 cm, is not influenced by diurnal thermal variability and is at a constant temperature corresponding to the seasonal vadose zone temperature. A constant temperature of 23°C was selected and sustained by the cylindrical heaters. This was the average temperature measured in spring and fall at 50 cm depth at a field site in the Negev desert (same chalk rock that was used for the experimental work described here) [Pillersdorf, 2007]. With respect to pore-solution availability, it was assumed that under natural conditions the rock matrix provides an infinite source. In the laboratory, this was reproduced by the solution containers. Mimicking the natural fracture aperture required significant simplification as natural fractures vary in inclination, aperture, and connectivity with other fractures that can be sources and sinks for air [Dahan et al., 2000; Dragila and Weisbrod, 2004a, 2004b; Nativ et al., 1995]. These complexities were not simulated in the FS, which imposed ideal conditions consisting of a vertical fracture with a fixed aperture and limited cavity volume.
2.1.2. Climate Control Room (CCR)
 A CCR was used to test whether variability in atmospheric temperature influences evaporation, and to quantify its impact. Since the bottom boundary of the rock was kept at a constant temperature, changes in CCR ambient temperature resulted in temperature gradients between the atmospheric boundary and the rock bottom (ΔT = Trock − Tatm). The CCR was constructed inside an insulated shipping container, 6 m long, 2.5 m wide, and 2.5 m high. Cooling was obtained by flowing cool air (from an air conditioner) through aluminum pipes in a double-layered ceiling, which was sealed but noninsulated, thus cooling the room by radiation to impose no-wind conditions [Kamai, 2006]. Heating was controlled by six spiral heaters that were located around the CCR, 1.5 m above the floor. The temperature in the CCR was monitored with six TCs, and controlled by two of the TCs that were located at the same elevation as the FS aperture opening. Barometric pressure was monitored by a pressure transducer (MPX2100AP/GP, Motorola, Schaumburg, Illinois) located inside the CCR.
 PE within the CCR was measured using an evaporation pan. Space limitations in the CCR prevented the use of a standard Class A evaporation pan; instead, we used an insulated metal pan, 20 cm in diameter and 40 cm deep, a heating plate, and a Mariotte bottle for keeping the solution at a fixed level in the pan. The Mariotte bottle was set on a mass scale to monitor the evaporation rate. The same AGW solution used in the FS was used for the pan. PE was quantified under two conditions: (1) with the evaporation pan bottom heated to the same temperature as the rock bottom; and (2) without heating the pan bottom, i.e., ambient temperature.
2.1.3. Experimental Procedure
 Before initiating the experiments, the rocks were saturated with solution under positive hydraulic head by raising the level of the Mariotte bottles to the top level of the FS. Mariotte bottle mass was monitored to determine the solution flux entering the rocks and calculate the sorptivity (S) for each rock [Jury et al., 1991]. Because all four rocks were exposed to identical conditions, outside without cover, for more than a year prior to their assembly, it was assumed that the initial water content and matric potential distribution within them were similar. Thus differences in S values were interpreted as differences in permeability between rocks. Rock saturation continued for 2–3 weeks until the imbibition rate dropped significantly and a water film appeared on the fracture surfaces. Note that previous studies have shown that due to the existence of very small pores, chalk cannot be completely saturated without using a vacuum; the maximum saturation without vacuum is about 70–75% of the porosity [Zvikelsky and Weisbrod, 2006]. Saturation by gravity satisfied the requirements of this study, as it more realistically mimics field conditions. Following the saturation stage, the Mariotte bottles were lowered to attain a pressure of −5 cm at the bottom of the container. About 250 mL (∼1% pore volume) returned to the bottles as they were lowered from the top to bottom position. The bottles remained in the same lowered position, namely, at constant head, for all subsequent experiments.
 Experiments were initiated once the FS systems stabilized at the applied constant head. As an initial stage for these measurements, the fracture opening was temporarily sealed with a nylon cover for 7 days to evaluate no-evaporation conditions and to ensure that the systems had no leaks. After no-evaporation conditions were demonstrated, the cover was removed and evaporation was measured continuously for 9 months. The two FS systems were denoted FS1 with rocks A and B, and FS2 with rocks C and D. Because of experimental problems in rock A, mainly having to do with erroneous evaporation rates, data from this particular rock are not reported, nor are they included in any of the analyses.
 Experiments consisted of measuring evaporation while maintaining a constant temperature of 23 ± 0.5°C at the bottom of all rocks, and varying ambient temperature in the CCR. Each time the CCR temperature was set to a new value, the fracture opening was sealed with a nylon cover to prevent vapor transport while the CCR temperature stabilized. This nylon cover served to reset and calibrate the system, and to verify that solution migration ceased when no vapor exited the fracture. For each temperature setting, evaporation rates were quantified only after steady state conditions had been reached.
 Within the 9-month experimental period, a 60-day experiment was performed to quantify the functional dependence of ESEF on thermal gradient. The 60-day period began 20 days after the FS systems were initiated, and consisted of varying the CCR air temperature as follows: 10°, 25°, 20°, 15°, and 10°C. This sequence was selected to achieve the following conditions and objectives: (1) identical conditions in the first and last stages for comparison, especially since salt precipitation accumulating during the experiment may also affect evaporation rates, and (2) a stepwise decrease in CCR temperature, which was expected to increase convective venting and subsequently ESEF rates. Following the 60-day experiment, CCR temperature was kept constant at 9°–10°C for the remainder of the observation period (a total of 270 days), imposing a maximum thermal gradient and consequently leading to maximum ESEF rates. This long-term period of constant temperature conditions enabled us to investigate the impact of salt precipitation on evaporation rate.
 It should be noted that while the intent of the first 60-day experiment was to evaluate the role of thermal gradient on evaporation, salt precipitation was likely occurring and potentially affecting the evaporation rate. To assess how valid it was to use this period of data to isolate the effect of thermal gradient only, the initial conditions (10°C in the CCR) were repeated at the end of the 60 days and the evaporation rate was monitored; it was found to be similar (±2 g d−1 m−2) to the initial rate. In addition, the specific temperature sequence was conducted to counteract the impact of possible salt precipitation. Thus the impact of temperature reported herein is conservative. If there was no salt precipitation during the 60-day experiment, evaporation rate changes due to the increasing temperature gradient could only be larger.