HYDROLOGICAL PROCESSES Hydrol. Process. 19, 2419– 2435 (2005) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5893

The contribution of condensation to the water cycle under high-mountain conditions Carmen de Jong* B.E.R.G., Institut f¨ur Geographisches Wissenschaften, FU Berlin, Malkserstr. 74–100, 12249 Berlin, Germany

Abstract: Little is known about the interaction between condensation, precipitation and evaporation as an integral part of the water cycle under high-mountain conditions. This paper focuses on methods of identification and measurement of condensation under natural conditions in high alpine valleys by example of the Dischma in eastern Switzerland. The role of different vegetation zones in transferring water from and to the atmosphere is investigated above the treeline (1900–2600 m a.s.l.). Field measurements of condensation and evaporation at 10 min intervals show that condensation plays a more significant role in the water cycle than previously assumed. Both diurnal and nocturnal condensation are compared at different altitudes on slope and valley locations during the exceptionally hot and dry mid-summer of 1998. It is suggested that, in addition to standard hydrological components, water balance modelling in mountain zones should include more precise data on measured condensation. Copyright  2005 John Wiley & Sons, Ltd. KEY WORDS

condensation; hydrology; micro-meteorology; evaporation pans; alpine

INTRODUCTION Condensation can be described as the process whereby water changes from the vapour to liquid state, and this can occur on exposed vegetation surfaces as well as lakes, streams, rocks, scree surfaces, snow and glaciers. In terms of energy fluxes, condensation is traditionally described by a negative latent heat flux, indicating that, in general, positive latent heat flux describes evapotranspiration. Condensation is an important additional source of moisture to rainfall on mountain slopes, and the quantity of condensation is highly dependent on the foliage characteristics of the dominant vegetation (Stadtm¨uller, 1987). Micro-topography, wind characteristics and the role of advection (Zafra, 2002) are not normally included in the discussion and will be particularly emphasized in this study. Moreover, most investigations concentrate on snow–atmosphere interactions (Bengtsson, 1980; Lundberg et al., 1998; Hood et al., 1999). Until recently, few direct continuous measurements of condensation were available for snow-free highaltitude environments (de Jong et al., 2002). As a result, it has not been integrated into the calculation and modelling of mean and annual precipitation or the overall water cycle. Traditional methods for determining evaporation are based on the energy balance closure (Konzelmann et al., 1997). For example, the Bowen Ratio Energy Balance (BREB) method has been used extensively because of its robustness, since it allows automatic recordings of fluxes and is not as stringent as other methods (Angus and Watts, 1984). Therefore, BREB is easier to implement in long-term campaigns, in remote areas and on slopes (Nie et al., 1992). However, when water vapour gradients are minimal, hydrometers usually lie within the measurement error and the BREB method is not appropriate (P´erez et al., 1999). Under the extreme temperature gradients typical for highmountain zones (Urfer-Henneberger, 1979; Turner, 1982; Ulrich, 1987) other methods are required that are more sensitive to condensation and evaporation, as well as being cost effective and regionally representative. * Correspondence to: Carmen de Jong, Institute of Geography, University of Bonn. PO Box 1147, 53001 Bonn, Germany. E-mail: [email protected]

Copyright  2005 John Wiley & Sons, Ltd.

Received 24 April 2003 Accepted 4 January 2005

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Thus, in mountain terrain it is necessary to consider robust and automated systems (Sauter and McDonnell, 1994) that are also logistically practicable. Standard rain gauges do not commonly record condensation. So far, fog collectors are most commonly used in high-mountain environments to determine fog drip (Juvik and Ekern, 1978; Sobik and Migala, 1993; Schemenauer and Cereceda, 1994; Scholl et al., 2002). However, these are aimed at obtaining total sums of condensation for water budgets rather than exploring the high-resolution temporal dynamics of condensation processes. The lack of knowledge on the temporal and spatial characteristics of condensation is both the result of logistical problems associated with the determination of water flux components on steep terrain and of neglect of the topic’s dynamical significance. However, in terms of water management, it is important to quantify the amount of water loss and gain accurately during the active vegetation period especially in high-mountain environments. It is well accepted by now that, in most parts of the world, glacier retreat is enhancing the expansion of vegetated areas and may cause changes in the weighting of condensation relative to evaporation through altered mechanisms of energy exchange. In future, the additional supply of water from condensation in mountainous zones will play an important role within the local vegetation zone. For this reason, a shift in focus is expected towards the role of condensation in reserving water within different vegetation zones, as well as its interaction with evaporation and transpiration. The aims of this paper, therefore, are to investigate small-scale spatial and temporal interactions between measured condensation, evaporation and precipitation within the active vegetation zone above the treeline in a humid mountain basin, to relate them to discharge and to suggest methods suitable for measuring and modelling condensation. The fluctuations of condensation and evaporation will be predominantly interpreted in terms of temperature gradients and wind dynamics.

STUDY AREA 2

The Dischma catchment, 51 km in total size, (and 43 km2 at the discharge station) is located in Graub¨unden, eastern Switzerland, near the Austrian boundary. It is a typical elongated, glaciated high-alpine valley with a central north north west–south south east axis harbouring the remnants of the Scaletta glacier at its southern boundaries (V¨ogele, 1984). Gneiss and other metamorphic rocks are the prevailing geology constituting steep slopes in most parts of the valley and altitudes ranging from 1500 to 3100 m a.s.l. (Cadisch, 1929). At the macro-scale, the climate in summertime is dominated by low-pressure systems flowing from the Weissfluhjoch in the northwest up the Dischma that can be interrupted by stable, alpine f¨ohn wind situations from the Engadin in the southeast. At the micro-scale, thermally induced local winds control the moisture and rainfall transfer within the valley. Measured at 2000 m altitude, mean July temperatures are approximately 12 ° C and cumulative rainfall varies between 400 and 600 mm during the summer months (mid-June to mid-September). Evaporation and transpiration, amounting to 300 mm in the summer months, is highest in the alpine shrub zone (1900–2200 m) and decreases towards the upper valley floor and on the higher slopes (de Jong et al., 2002). Water recharge comes from rainfall, snowfall and condensation within the vegetation and on rocky surfaces. Condensation has not been quantified in this area before. The Dischmabach, characterized by a glacier and rain-fed regime, drains the moraine-covered valley floor, together with several tributaries. Its average discharge at Kriegsmatte amounts to 800 mm during the summer months and 1200 mm per annum. Geomorphologically, the Dischma consists mainly of moraines, rock glaciers, paleo-landslides, scree cones and some debris flow deposits. Only about one-tenth of the valley is covered by forest, with the remaining area consisting mainly of alpine pasture and shrubs, as well as snow and ice fields and small lakes (Wildi and Ewald, 1986; Fischer, 1990). Strongly structured terrain and orography have an important influence on the hydrological and micrometeorological processes in the Dischma. Because of the small scale at which processes occur, approximations for extrapolation of weather variables are difficult. A mountain-valley wind (valley breeze) is developed only Copyright  2005 John Wiley & Sons, Ltd.

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for certain periods of the day, and slope winds only exist as long as there are strong temperature gradients between the ground surface and air (Hennemuth, 1986). On clear-sky days with metastable air layering, air temperatures increase at the same pressure level from the valley outlet towards the highest ridges at the valley head (by as much as 3 ° C) and develop a clear up-valley wind (Ulrich, 1987). In addition, air cools less rapidly there during the night than it warms during the early morning.

METHODS The interpretation of regional condensation in this paper focuses on measurements at five individual sites (Table I) in the upper Dischma valley during the mid-summer season of 1998 as part of a detailed hydrological field experiment (VERDI) carried out between 1995 and 1999. Condensation was determined between 1998 and 1999. Since the Dischma valley is heavily incised and the resulting insolation patterns are very diverse, sites were chosen at representative locations along the east- and west-oriented H¨ureli and Sch¨urli slopes and along the valley floor. Both rich (Jenatsch) and poor alpine pasture (H¨ureli Bowen Ratio and Sch¨urli Bowen Ratio) and alpenrose (H¨ureli Alpenrose and Sch¨urli Alpenrose) was instrumented in zones with different aspects ranging from 1960 to 2600 m in altitude. Evaporation pans were placed at two sites on the Sch¨urli slope, including Sch¨urli Bowen Ratio (6) and Sch¨urli Alpenrose (5), two sites on the H¨ureli slope, including H¨ureli Alpenrose (8) and H¨ureli Bowen Ratio (9), and at one site on the valley floor, i.e. Jenatsch (4) (Figures 1–3). Each experimental site consists of a basic meteorological profile station or a Bowen Ratio station (Table I) combined with a self-developed evaporation–condensation micro-measuring unit (Figure 4a). All measurements were continuously recorded in self-constructed dataloggers at 10 min intervals. At the CONRAD stations, air temperature and relative humidity were recorded at two levels above the ground (1Ð8 and 0Ð2 m), in addition to solar radiation, wind speed and wind direction. At the Bowen ratio stations the same variables were recorded in surplus to soil heat flux, whereby dew point and air temperature were recorded 2 and 1.5 m above the ground. As such, the variability of each meteorological variable is captured over short time periods even though there was occasional data loss. A small rain-o-matic collector with a tipping spoon was installed as an additional method for measuring rainfall. The results in this paper are based on condensation and evaporation measured from small, permanently installed water-filled evaporation pans placed on automatic weighing scales (Figures 2 and 4a; de Jong et al., 2002). Evaporation pans were lowered into the ground, ensuring that the water surface was level with the average ground surface in order to minimize wind impact and to maximize the dominance of the local microclimate of the surrounding vegetative cover (Figure 4a). Table I. Characteristics of the main sites in the upper Dischma valley, including location, elevation, gradient (slope inclination), aspect, exposure to wind, dominant vegetation type and average height

Location Elevation (m a.s.l.) Gradient (° ) Aspect Exposure Vegetation type Vegetation height (cm) Climate station

H¨ureli Bowen Ratio (HBR)

H¨ureli Alpenrose (HA)

Trough shoulder 2360 40 NE Convex Blueberry, grass, lichen (poor pasture) 12

Trough slope 2070 30 E-NE Concave Alpenrose

Bowen Ratio

Profile station Bowen Ratio

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Jenatsch (J) Valley bottom 1960 2 None Flat Grass (wet, rich pasture) 20

Sch¨urli Alpenrose (SA)

Sch¨urli Bowen Ratio (SBR)

Trough slope 2070 30 W Concave Alpenrose

Trough shoulder 2360 38 W-SW Convex Blueberry, grass, lichen (poor pasture) 12

35

Profile station Bowen Ratio

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1 = 2 = 3 = 4 = 5 = 6 = 7 = 8 = 9 = 10 = 11 = 12 = 13 =

Inner Hof Kriegsmatte Stillberg Jenatsch Schürli Alpenrose Schürli Bowen Ratio Schwarzhorn Hüreli Alpenrose Hüreli Bowen Ratio Hüreli Peak Dürrboden Gletschboden Oberer Schönbühl

= discharge station = pan / lysimeter = meteorological station Figure 1. Study area: Dischma valley in Graub¨unden, indicating the measuring sites during the study periods of the VERDI (Verdunstung Dischma) project

Since the scales underneath the evaporation pans had to be adapted in size to the steep slope gradients, their capacity was limited to 5 kg. All water losses by evaporation and water gain through condensation, as well as rainfall, were weighed accurately (with a resolution of š1 g and an error of 0Ð3% or equivalent of 0Ð015 mm). The units used were millimetres. Measurements were taken simultaneously at each site during the mid-summer season. At windy sites, a smoothening function based on a gliding average value over 40 min is applied to the time series: Y1 D

x10 C x20 C x30 C x40 x20 C x30 C x40 C x50 , Y2 D ,... 4 4

In 1999, a comparison was made of evaporative loss measured at 10 min intervals from the open-surface, water-filled evaporation pans and similarly sized lysimeters filled with vegetation and soil. Although a very Copyright  2005 John Wiley & Sons, Ltd.

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(a)

(b)

Figure 2. (a) View across east-facing H¨ureli slope and valley floor with both measuring locations (8) and (9); (b) example of evaporation/transpiration measuring station (8) combined with meteorological profile station in the alpenrose zone, 2070 m a.s.l. in 1998

good coefficient of determination (r2 ) above 0Ð98 was obtained for the 2-week calibration period (Figure 4b), average water losses from the lysimeters were approximately 5% higher than those from the evaporation pans. This difference is most probably due to the higher capacity of the plants to transpire in the lysimeters via their root system and leaf coverage. In the experiments, the temporal reaction of evaporation pans was very similar that of the lysimeters. Thus, it can be assumed that, for these particular conditions, transpiration by plants is comparable to the potential evaporation from the evaporation pans. Water-filled pans are also assumed to be representative for lakes and wet areas in the catchment.

RESULTS AND DISCUSSION The results confirm that condensation not only varies substantially with the general type and height of vegetation, but also with average hillslope gradient, exposure or shelter from wind as a function of convex or concave topography, and location along the valley axis (Table I). Sites situated along the longitudinal valley axis, such as Jenatsch, are strongly influenced by differences in temperature and humidity associated with regular adiabatic and cadiabatic winds concurring with the main valley axis, whereas sites along the slopes (Sch¨urli and H¨ureli) are influenced by smaller scale and shorter duration up- or down-slope winds (UrferHenneberger, 1979). Whereas condensation is on the valley floor and west-facing is lowest alpenrose-covered Copyright  2005 John Wiley & Sons, Ltd.

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(a)

(b)

Figure 3. (a) View across west-facing Sch¨urli slope with measuring locations (6) and (5) and valley floor; (b) example of evaporation/condensation measuring station combined with Bowen Ratio station (6) in the poor pasture zone, 2360 m a.s.l. in 1998

valley slopes above 2000 m it is highest amongst the dwarf shrub zone of 2200–2400 m for this exceptionally hot mid-summer. The highest overall condensation on the west-facing shrub zone can be explained in terms of its microclimate, in particular due to large differences in wind speed but small differences in temperature on the trough shoulder. In most cases, condensation not only constitutes a substantial part of the nocturnal water cycle, but also of the diurnal water cycle. The onset of condensation varies according to location within the valley, and its duration governs the onset of evaporation and transpiration. Condensation dynamics at 10 min intervals H¨ureli slope. At the H¨ureli site, as at Jenatsch, condensation begins during the night and lasts until sunrise (Figure 5a–c). Until approximately 08 : 00 a.m., there is little vertical change in the air temperature gradient (lower divided by upper arm) or wind speeds at H¨ureli Alpenrose and H¨ureli Bowen Ratio. However, after sunrise there is a particularly steep rise in absolute temperatures, as well as temperature gradients, on the H¨ureli side from negative (1). There is an increase in temperature gradient from 0Ð98 to 1Ð01 for H¨ureli Bowen Ratio and 0Ð96 to 1Ð05 for H¨ureli Alpenrose produced in only 10 min shortly after 08 : 00 on the 16 August, a typical clear-sky day. The sudden increase in air temperature gradient and wind speed (particularly at H¨ureli Bowen Ratio) is a consequence of the particular location of these sites with their easterly orientation, a slope inclined to absorb sunlight at an almost perpendicular angle and high surrounding slopes that allow a particularly abrupt sunrise. These are all favourable factors that induce high incoming solar radiation. Copyright  2005 John Wiley & Sons, Ltd.

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(a)

(b) 12 y = 0.9472x + 0.4884 r2 = 09888

evaporation pan (mm)

10 8 6 4 2 0

0

2

4 6 8 Iysimeter (mm)

10

12

Figure 4. (a) Sketch of typical combined condensation and evaporation micro-measuring station on an inclined slope with evaporation pan (EVA), weighing scales, small rain-gauge (pluviometer, PM), storage module (SM) and solar panel (SP) feeding a battery. Note integration of evaporation pan within vegetation (e.g. Vaccinium myrtillus). (b) Correlation between cumulative evapotranspiration from evaporation pan and lysimeter at 10 min intervals for 2 week calibration period at Jenatsch (1999). The variation around the trendline reflects cold (
) and warm (C) periods

Therefore, in the first hour after sunrise, the surface energy fluxes are rapidly reinforced on the east-exposed slope. Latent heat flux remains in equilibrium, and evaporation ultimately replaces condensation at 08 : 50 at H¨ureli Bowen Ratio and at 09 : 10 at H¨ureli Alpenrose. The measurements show that, 1Ð5 h after sunrise, latent and sensible heat fluxes become positive, indicating that the available net surface energy is positive. Net radiation exceeds 200 W m
2 by this time. Evaporation continues as wind speeds constantly increase in the late afternoon (up to 14 m s
1 maximum wind speed). At sunset, around 18 : 00, there is either a second phase of condensation (H¨ureli Bowen Ratio; Figure 5a) or evaporation ceases for approximately 20 min (H¨ureli Alpenrose; Figure 5b). In between, evaporation can be reinitiated for a short period. The occurrence Copyright  2005 John Wiley & Sons, Ltd.

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C. DE JONG

(c)

(d)

(e)

Figure 5. Influence of temperature gradient (lower divided by upper arm), Cve above 1 and
ve below 1 (missing for SA), radiation (missing for SBR) and mean wind speed (missing for SA) on condensation (negative values) and evaporation (positive values) at 10 min intervals for (a) H¨ureli Bowen Ratio (HBR), (b) H¨ureli Alpenrose (HA), (c) Jenatsch (J), (d) Sch¨urli Bowen Ratio (BR) and (e) Sch¨urli Alpenrose (SA) on 16 August 1998

of a similar second phase of evening transpiration was observed by Zweifel et al. (2001) in their transpiration experiments for Norway spruce at an alpine site near the Dischma. The explanation for these processes is as follows: at sunset, the net available surface energy is negligible and during a short period the surface layer remains at near-neutral conditions. Therefore, in some cases, negative latent heat flux may arise if advection diminishes due to sudden air cooling because of the high rate of evapotranspiration during the day. Advection seems to be negligible when wind speeds decay below approx. 0Ð5 m s
1 , since the total net available surface energy induces negative latent heat flux and condensation is reinitiated. In the Dischma, as for most alpine valleys, advection of sensible heat flux generally plays a key role in the energy balance inducing positive latent heat flux. Sch¨urli slope. Owing to its orientation, the Sch¨urli slope experiences delayed solar radiation (however, radiation data are missing for this particular day), such that the change from condensation to evaporation takes place slightly later, but much more abruptly, than on the H¨ureli slope. The local air temperature gradient on the Sch¨urli slope is close to zero before 08 : 00 and, as wind speeds decrease between 06 : 00 and 08 : 00, extremely high amounts of condensation are favoured (Figure 5d and e). Although the vertical temperature gradient becomes positive at 08 : 00, condensation continues until 09 : 00. Evaporation is only initiated when wind speeds rise above 2 m s
1 following the temperature change-over. During the day, between 08 : 00 and 19 : 00, wind speeds vary between 1 and 6 m s
1 and drop to 1 m s
1 by 21 : 00. After the sharp rise in evaporation in the morning, evaporation decreases more or less continually during the rest of the day. As a Copyright  2005 John Wiley & Sons, Ltd.

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result of more intensive radiation, evaporation persists longer into the night on the west-facing slope. The moderate wind speeds and warm temperatures (around 10 ° C) suggest that advection plays an important role in inducing nightly evaporation. Nearly no evening condensation is recorded for those sites. In contrast to the eastfacing slope and valley floor, the vertical temperature gradient on the Sch¨urli slope does not decrease abruptly below zero at 20 : 00, remaining well above those recorded at the other sites with wind speeds of 1–3 m s
1 . Valley floor. The valley floor experiences solar radiation with nearly 1 h delay (at 09 : 00 as opposed to 08 : 00 at H¨ureli Bowen Ratio (Figure 5c)). Evaporation is common in the late morning after condensation has terminated. Since the temperature range is much higher on the valley floor, periods with uniform air temperatures over the upper and lower profiles are rare. This could explain why overall condensation is so low. On the saturated valley floor, condensation is most extensive after sunset and most probably as a result of advection with the change in the valley wind system. Unlike the valley slopes, the valley floor experiences steady wind directions and wind speed, with a very distinct change after sunrise (drop in wind speed from 3 to 1 m s
1 at 09 : 00, increasingly steadily thereafter) and sunset (drop from more than 4 m s
1 to nearly 0 m s
1 at 18 : 00). Nocturnal wind speeds are higher on the valley floor than on the slope sites (2 to 3 m s
1 ). A general outcome for all sites is that, as soon as solar radiation heats the surface and the surface layer becomes unstable, evaporation replaces condensation or equilibrium occurs. The results for all sites show that early morning condensation usually lasts only as long as a temperature inversion is recorded and wind speeds are low. The night-time temperature inversion is common for nearly all sites during the summer. Since the switchover from negative to positive temperature gradients, as well as low to higher wind speeds, occurs within a short period of time in the morning, condensation terminates within an hour at nearly all sites. With the pronounced maximum in morning evaporation, a normally distributed pattern of daytime evaporation is developed on the east-facing and valley slopes as opposed to a left-skewed pattern of evaporation on the west-facing slope. Evening condensation occurs preferentially during neutral conditions. Thus, the switch to condensation takes place between 18 : 00 and 19 : 00 on the east-facing slope but only at around 21 : 00 on the west-facing slope. In the late evening and during the night, evaporation reinitiates as vertical temperature gradients become positive again or experience an increase in gradient. Rainfall effects on condensation and evaporation In order to examine the effects of rainfall on condensation and evaporation patterns across different altitudes and vegetation zones along the same valley slope, two sites were selected on the H¨ureli slope and one on the valley floor at Jenatsch for the 12 and 13 August. In Figure 6, interactions between temperature gradient, wind speed, rainfall, condensation, evaporation and discharge are plotted. After midnight on 12 August, both H¨ureli Bowen Ratio and H¨ureli Alpenrose experience negative air temperature gradients (data are missing for Jenatsch on that day), indicating a negative latent heat flux. Until 08 : 00, wind speeds generally remain below 3 m s
1 . On the morning of 12 August, condensation at H¨ureli Bowen Ratio is negligible, but there is an extensive period of condensation between 06 : 00 and 09 : 00 at H¨ureli Alpenrose in association with large negative temperature gradients. There is also an intensive period of condensation at Jenatsch between 06 : 00 and 08 : 00. As temperatures start increasing with sunrise and the surface warms on the 12 August, a sharp positive temperature gradient is developed at H¨ureli Bowen Ratio and Alpenrose. The switching from negative to positive gradient at H¨ureli Bowen Ratio and at Alpenrose takes place at approximately the same time, but is accompanied by evaporation only at H¨ureli Bowen Ratio. Evaporation is delayed at H¨ureli Alpenrose until wind speeds approach 2 m s
1 . At Jenatsch, evaporation begins at the same time as the alpenrose site. Wind speeds increase to 5 m s
1 and 9 m s
1 at H¨ureli Bowen Ratio and Alpenrose respectively. This prominent advective effect maintains daytime evaporation until temperature gradients become level at H¨ureli Bowen Ratio and condensation is reinitiated at 18 : 00. Since the gradient remains positive at H¨ureli Alpenrose, no condensation can occur. Jenatsch also lacks condensation after a full day of evaporation. During the rainfall event from 21 : 00 to 00 : 00 (12 August), temperature gradients diminish and approximate each other at both sites. Wind speeds decrease to 3 m s
1 and less than 1 m s
1 at the two H¨ureli sites as Copyright  2005 John Wiley & Sons, Ltd.

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 6. (a) Air temperature gradient (lower divided by upper arm) and (b) wind speed for H¨ureli Bowen Ratio (HBR) and H¨ureli Alpenrose (HA). Temperature and wind data are missing for the entire period for Jenatsch and after 17 : 00 on the 13 August for HA. Evaporation, condensation and rainfall (mm/10 min) for (c) HBR (2360 m), (d) HA (2070 m), (e) J (1960 m) and (f) discharge at Kriegsmatte for the rainfall event of 12–13 August 1998

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rainfall begins. However, during the course of the rainfall event, wind speeds increase to 14 and 7 m s
1 , respectively. Rainfall differs in intensity between the three sites. Rainfall begins at 20 : 15 at the H¨ureli Alpenrose site but 45 min later at the Jenatsch and H¨ureli Bowen Ratio sites, situated 300 m lower and 300 m higher respectively. Immediately after the rainfall event on 13 August there is a short and intensive peak in evaporation at all three sites (0Ð2 mm/10 min for H¨ureli Bowen Ratio and H¨ureli Alpenrose and nearly 0Ð1 mm/10 min at Jenatsch). At all three sites, evaporation continues for 1 h but at least 6 h at H¨ureli Alpenrose and Jenatsch. Total evaporation after the rainfall event is 0Ð85 mm at H¨ureli Alpenrose and 0Ð28 mm at Jenatsch after 5 h 20 min, and is 0Ð48 mm in only 40 min at H¨ureli Bowen Ratio. The ratio of evaporation to total rainfall is 0Ð07 for Jenatsch and H¨ureli Bowen Ratio, but 0Ð15 for H¨ureli Alpenrose. The high ratio at the alpenrose site is most probably caused by high interception evaporation from the twigs, branches and leaves of the alpenrose. As a result of the rainfall, a small discharge peak is created in the Dischmabach with 3 h delay on the next morning at Kriegsmatte 6 km farther downstream. Discharge decreases during the daytime parallel with evaporation. Between 00 : 00 and 02 : 00, after the rainfall event, there are both large positive and negative jumps in the temperature gradient at H¨ureli Bowen Ratio, explaining the multiple sequence of evaporation and condensation. Fluctuations are low at H¨ureli Alpenrose, but, as the gradient decreases, evaporation diminishes. Again, the valley site has a similar, but less accentuated, evaporation than the alpenrose site. During this period there are several jumps in the wind speed at both slope sites in the order of 4 m s
1 , indicating that condensation and evaporation are also linked to advective effects. Until the onset of condensation on the morning of 13 August, wind speeds decrease to a minimum. Condensation begins at H¨ureli Bowen Ratio in association with low wind speeds and negative temperature gradients. At H¨ureli Alpenrose, condensation dominates under the same conditions, although the onset is nearly 2 h earlier. After sunrise, H¨ureli Bowen Ratio regains a slightly positive gradient, begins evaporating, but is followed by another period of condensation. H¨ureli Alpenrose, however, experiences an extensive phase of condensation even though temperature gradients rise to positive values. Fluctuations in wind speeds are considerable during the daytime, but maximum wind speeds only reach about half the amount of the day before. The continued condensation at the alpenrose site may be caused by high amounts of humidity trapped within the twigs and branches of the alpenrose, as they are capable of creating their own microclimate after the rainfall event. It could also be the result of the position of the alpenrose site on the lower valley flanks, where moist air is moved away less rapidly during the night than on the higher slopes or windy valley floor. Full evaporation does not begin before 12 : 00 at either slope site. At Jenatsch, however, evaporation begins at the usual time, 09 : 00. All three sites produce lower evaporation than usual. In contrast to the other two sites, Jenatsch experiences continual evaporation between 09 : 00 and 17 : 00 and virtually no condensation. Evaporation is replaced by condensation at the uppermost site once the temperature gradient levels off again at around 19 : 00. There is practically no condensation at H¨ureli Alpenrose and Jenatsch in the evening. Since there are no temperature data available for these two sites at this time, it is postulated that the temperature gradient remains positive and higher than at H¨ureli Bowen Ratio. As expected, the sums of condensation, evaporation and rainfall varies between the different sites for the east-facing H¨ureli slope investigated. H¨ureli Alpenrose and Jenatsch have distinct phases of evaporation and condensation, whereas there are several small and erratic phases of condensation and evaporation at H¨ureli Bowen Ratio. After the rainfall event, condensation in the alpenrose on the H¨ureli slope substantially exceeds that of the blueberry and alpine pasture: its evaporation is slightly higher and rainfall is slightly lower. This stands in contrast to the Sch¨urli slope, where Sch¨urli Bowen Ratio has a much higher condensation than Sch¨urli Alpenrose. In summary, Figure 7 illustrates a conceptual model of the dynamics of condensation and evaporation for a typical vegetated alpine site at 10 min intervals. There are five clearly differentiated phases of nocturnal, sunrise, daytime, sunset and late-evening dynamics. During phase 1 there is continual interaction between Copyright  2005 John Wiley & Sons, Ltd.

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1

3

2

4

5

0.4 flow rate (mm / 10 min)

evaporation

condensation -0.4 0

3

6

9

12 time (h)

15

18

21

24

Figure 7. Conceptual model of condensation and evaporation at 10 min intervals for an ideal day at a vegetated site above 2000 m a.s.l. with optimal radiation. Phase 1: night-time; Phase 2: sunrise; Phase 3: main daylight period; Phase 4: sunset; Phase 5: night-time

night-time condensation and evaporation. Phase 2 indicates a prolonged and intensive period of condensation after sunrise. Phase 3 extends over the main period of daytime evaporation. During phase 4 there is a short period of condensation after sunset. Phase 5 experiences a final short, but intensive, peak of night-time evaporation, followed by a prolonged phase of constantly decreasing evaporation. Condensation at daily intervals in 1998 The pattern of condensation and evaporation during the last 3 weeks of August is a reflection of the local weather (Figure 8). Beginning with the relatively hot, first half of August, both evaporation and condensation are very high. This is followed by a wet period between 16 and 22 August with nearly no evaporation, yet high amounts of rainfall and condensation. In the final warm week, evaporation remains only average, whereas condensation exceeds the values of the previous weeks. On the whole, all five sites follow this pattern, so that days with high amounts of condensation are as common as days with low condensation. Daytime. The sums of condensation and evaporation during the month of August indicate considerable regional variability according to their location on the valley floor, lower and upper valley slopes (Figure 8, Table I). Most absolute diurnal condensation is accumulated at the high shrub site of Sch¨urli Bowen Ratio (2360 m, 1Ð3 mm) and is least on the wet valley floor at Jenatsch (1960 m, 0Ð4 mm). The large amounts of condensation at the highest altitude, west-facing site can be explained by the late sunrise. Since it takes the sun approximately 2 h longer to reach the Sch¨urli slope, condensation at these high-altitude sites can continue longer than on the opposite slope and shift the beginning of evaporation. The lowest proportion of daytime condensation to evaporation is recorded within the wet alpine pasture at Jenatsch, whereas the highest proportion is recorded within the shrubs and grasses of the poor alpine pasture zone at Sch¨urli Bowen Ratio (Table II). On average, daytime condensation makes up 20% of the total evaporation. This is a high amount considering that evaporation should dominate during the daylight hours. However, condensation dominates for quite some length of time during the morning hours, and it can easily reinitiate with a change in temperature gradient or wind speed. Nightime. The dynamics of condensation and evaporation are significant during the night in the absence of shortwave radiation (Figure 8). It is surprising that nocturnal condensation does not exceed evaporation (Table II). This is typical for the hot and dry summer weather of 1998. Whereas the lowest absolute nocturnal condensation is recorded at the alpenrose sites, the alpine pasture site at H¨ureli Bowen Ratio records the highest Copyright  2005 John Wiley & Sons, Ltd.

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(b)

(a)

(c)

(e)

(d)

Figure 8. Evaporation (C) and condensation (
) measured during daylight hours (upper graph) and nightly (lower graph) at (a) Sch¨urli Bowen Ratio, (b) H¨ureli Bowen Ratio (c) Sch¨urli Alpenrose, (d) Jenatsch and (e) H¨ureli Alpenrose for the month of August 1998

condensation (up to 0Ð9 mm per night). Apart from the H¨ureli Bowen Ratio site, nocturnal condensation does not vary as much from site to site as diurnal condensation. However, nocturnal condensation varies substantially with evaporation (35% on average), with the highest proportion of the water exchange being reached on the valley floor and the lowest in the alpenrose on the lower valley slopes. Copyright  2005 John Wiley & Sons, Ltd.

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Table II. Daytime (daylight hours) and night-time condensation and evaporation during the period 7–30 August 1998. Percentage condensation is calculated as part of total evaporation

Day Night

Evaporation (mm) Condensation (mm) Condensation (%) Evaporation (mm) Condensation (mm) Condensation (%)

H¨ureli Bowen Ratio (2360 m)

H¨ureli Alpenrose (2070 m)

Jenatsch (1960 m)

Sch¨urli Alpenrose (2070 m)

Sch¨urli Bowen Ratio (2360 m)

55Ð5 7Ð7 13Ð9 12Ð5 5Ð9 47Ð2

45Ð1 13Ð5 29Ð9 8Ð2 1Ð8 22

54Ð2 4Ð7 8Ð7 4Ð2 2Ð9 69

40Ð8 6Ð3 15Ð4 6Ð9 1Ð2 17Ð4

50Ð1 15Ð8 31Ð5 7Ð8 1Ð9 24Ð3

The highest overall condensation can be found in the higher altitude dwarf shrubs on the west-facing slopes (such as Sch¨urli Bowen Ratio) and the low-altitude east-facing slopes (such as Hureli Alpenrose). Conversely, the lowest overall condensation is recorded in the west-facing alpenrose and on the valley floor. Lower wind speeds cause more water to be retained during the daytime and, thus, over the summer months in the alpenrose on the east-facing slope than on the windier opposite slope (Figures 8 and 9). Alpenrose at the same altitude on the west-facing slope is only capable of condensing half the amount of water. Indeed, H¨ureli Alpenrose is much more sheltered, especially at night, than the opposite valley side. This is due to the shape and exposition of the slope and its location according to the valley long axis. The lower flanks of the H¨ureli slope are covered by alpenrose, oriented towards the northeast and face into the Rhinert¨ali. By facing away from the main valley axis and its main, diurnal valley winds they remain quite sheltered. Owing to low wind speeds, on average less than 2Ð5 m s
1 , condensed water can be retained and is not evaporated immediately. Similarly, at the Sch¨urli Bowen Ratio site, wind speeds are also very low for the given altitude and hardly exceed those for the alpenrose sites. The site is located on a ridge turned slightly towards the northwest, again sheltered from most down-valley winds. The lowest daytime condensation is on the valley floor, where evaporation is high yet wind speeds are low. However, on the valley floor, temperatures rise rapidly early in the morning so that there is insufficient time at the beginning of the day for high amounts of condensation. Also, optimal condensation can only proceed in locations where high daytime temperatures allow for continued warm temperatures during the night as a result of heat storage on the surface. Yet, after sunset, wind speeds increase steadily at Jenatsch so that again the physical conditions do not facilitate condensation. At H¨ureli Bowen Ratio, evaporation is optimal due to very high wind speeds (with maxima of up to 12 m s
1 ) and warm temperatures (Figure 9). Even with decreasing wind speeds during the night, evaporation rates remain high but there is continual exchange of episodically high condensation. Wind speeds diminish considerably towards the morning, so that warm temperatures facilitate condensation on the one hand and an exchange with evaporation on the other hand. It has been shown that vegetation and temperature differences alone cannot explain increased evaporation or condensation for different time periods and locations in the valley. Rather, wind speed accounts for major differences (Figure 9). Evaporation increases constantly with wind speed up to a maximum of nearly 1 mm h
1 , and condensation decreases constantly with wind speed from a maximum of 0Ð6 mm h
1 . Sites such as H¨ureli Bowen Ratio have a large range in condensation, but optimal condensation rates at wind speeds below 4 m s
1 . In general, evaporation reaches maximum values at wind speeds above 3 m s
1 . Condensation is very low or is absent for very high wind speeds. CONCLUSIONS The role of condensation under high-alpine environments has largely been ignored. Condensation contributes a significant proportion to the daily and seasonal water cycle and should be measured and integrated into Copyright  2005 John Wiley & Sons, Ltd.

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CONTRIBUTION OF CONDENSATION TO THE WATER CYCLE

(b)

2

HBR

1

0 −1 −2

evaporation condensation

0

2

4

6

8

10

condensation (−) evaporation (+)

condensation (−) evaporation (+)

(a)

12

2

HA

1

0 −1 −2

evaporation condensation

0

2

wind speed (ms−1) 2

J

1

0

−2

evaporation condensation

0

2

4

6

8

wind speed (ms−1)

8

10

12

wind speed (ms ) (d)

−1

6

−1

10

12

condensation (−) evaporation (+)

condensation (−) evaporation (+)

(c)

4

2

SA

1 0 −1 −2

evaporation condensation

0

2

4

6

8

10

12

wind speed (ms−1)

Figure 9. Relation between average wind speed and total evaporation (C) or condensation (
) for hourly intervals between 6 and 31 August 1998 for (a) H¨ureli Bowen Ratio (b) H¨ureli Alpenrose, (c) Jenatsch and (d) Sch¨urli Bowen Ratio. Wind data are missing for Sch¨urli Alpenrose

models more accurately in future. Relative to the location in a high-alpine valley, the amount of daytime condensation can exceed that accumulated by night. Also, the absolute amounts of condensation over several weeks for 1998 are highest on the west-facing upper slopes in the poor alpine pasture and can exceed the sums measured on the valley floor by a factor of two. At an annual scale, approximately 100 mm of average condensation is estimated from this paper in relation to 1100 mm of rainfall (Spreafico et al., 1999) and 4–600 mm of evaporation (measured) for a typical high-alpine valley. Condensation is a function of firstly temperature gradient, secondly wind speed related to topography and thirdly vegetation type. A dominant control for condensation is the temperature gradient: the higher and more positive the temperature gradient, the more evaporation is to be expected; and the lower, and more neutral or negative the temperature gradient, the more condensation is to be expected. Even so, evaporation will only initiate under positive temperature gradients and as soon as a minimum wind speed of ¾2 m s
1 has been exceeded. In terms of topography, the dominant control is advection through variable wind speeds, so that high-altitude sites can still achieve high condensation rates in sheltered locations. The higher the wind speed, the less likely it is for condensation to occur and the more likely high evaporation rates are, as long as the temperature lies above a certain threshold. Under very hot, dry summer conditions, vegetation is less likely to control condensation unless related to rainfall events. Under moister conditions, and after rainfall events, shrubs and dense vegetation with a high leaf area index are better adapted physically than short grass to retain water. In order to understand and interpret these highly variable processes it is important to adjust to observation intervals of 10 min or less. Copyright  2005 John Wiley & Sons, Ltd.

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Contrary to assumptions made by ‘linear altitudinal gradient’ concepts, evaporation and its closely related condensation do not follow a linear trend with height. Most evaporation is achieved from within the dwarf shrubs at the H¨ureli Bowen Ratio site (2360 m a.s.l.), complemented by the lowest values between the alpenrose at the Sch¨urli Alpenrose site (2070 m a.s.l.). The highest condensation occurs between the shrubs at the Sch¨urli Bowen Ratio site, and least condensation occurs amongst the alpenrose on the Sch¨urli slope. At night, the highest evaporation and condensation occur Hureli slope, whereas the lowest evaporation is recorded on the wet valley floor and the lowest condensation is in the alpenrose sites. This behaviour may be considered as typical for the extremely warm weather period in the Dischma for the month of August 1998. It underlines the importance of considering zones above 2300 m within and above the alpine dwarf shrubs for field studies and the danger of simple estimations and extrapolations. What is considered as normal (i.e. locations on the valley floor) is always least representative for a steep, alpine valley since it is mostly too wet or too cool. The significance of field work in the alpenrose and pasture zone above 2000 m a.s.l. cannot be emphasized enough. Without a good knowledge of the terrain, daily weather conditions and specific site-to-site reactions during different conditions, no satisfactory explanations and suggestions can be provided for inadequate measuring set-ups and modelling approaches.

ACKNOWLEDGEMENTS

This study was financed by a habilitation fellowship of the Deutsche Bundesstiftung Umwelt. I am grateful to the FU Berlin for supplementary financing of student assistants. I also thank Peter Ergenzinger and Francesc Castellvi for many fruitful discussions, and two anonymous reviewers.

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