Dischma Valley Grisons Switzerland

The Dischma catchment is located south of Davos in Graubuenden (Grisons), eastern Switzerland near the Austrian border (Figure 12.2). It is a typical elongated, glaciated high alpine valley (see Table 12.1 for details) with a central NNW-SSE axis and the remains of the Scaletta glacier at its southern boundary (Vogele 1984). The climate in summer is dominated by low-pressure systems moving from the Weissfluhjoch in the north-west up the Dischma valley and occasionally replaced by a stable, alpine fohn wind moving in from the Engadin in the south-east. At the local scale, thermally induced winds control the moisture and rainfall transfer within the valley. Evaporation and transpiration amounts to approx. 300 mm over the summer (June to

September) and is highest on the steep, lower valley slopes (1900-2200 m), low on the valley floor and decreases again on the highest slopes (de Jong et al. 2002). Water recharge comes from rainfall, snowfall and condensation within the vegetation and on rocky surfaces. Condensation was quantified for the first time in this area in 1998. The Dischmabach (river), characterized by a glacier and rain-fed regime in the summer, drains the moraine-covered valley floor together with several tributaries. As for the Giant Mountains, the catchment has steep slopes consisting mainly of metamorphic rocks (Cadisch 1929). Geomorphologically, the Dischma consists of corries, moraines, rock glaciers, paleo-landslides, glaciated trough slopes, scree cones and some debris flow deposits. Only about one-tenth of the valley is covered by forest, the remaining area consisting mainly of alpine pasture and shrubs as well as scree slopes, rock surfaces, snow and ice fields and small lakes (Fischer 1990, Wildi and Ewald 1986).

The strongly structured terrain and orography has an important influence on the hydrological and micro-meteorological processes in the Dischma. Because of the small scale at which processes occur, extrapolation of meteorological variables is difficult. A mountain-valley wind (valley breeze) develops 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 meta-stable air layering, air temperatures increase by as much as 3°C at the same pressure level between the valley outlet and the highest ridges at the valley head, thereby developing a clear upvalley wind (Ulrich 1987).

12.3 EXPERIMENTAL DESIGN

The interpretation of regional evaporation, transpiration and condensation in this paper is based on comparison of the two mountain catchments during the snow-free mid-summer season.

For the Szrenica test area, results are presented for the period between May 15 and July 7, 2001. The three main test stations correspond with the three main vegetation zones of the sub-catchment close to the upper tree line (Table 12.2(a) and Figure 12.1): a lower zone with blueberries and grass, a zone with shrubs and ferns and the highest zone with dwarf pine. Both an evaporation pan and lysimeter are installed at the lowest elevation Station 1. Stations 2 and 3 consist of an evaporation pan and a drop collector. Whereas station 1 lies in a less exposed region at the lower end of a nivation depression, station 2 lies at the transition between the steep walls of a small valley head and the flatter divide. Since it is

Figure 12.2 Dischma catchment with experimental sites, Orisons, Switzerland

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 12.2 Dischma catchment with experimental sites, Orisons, Switzerland located below a saddle structure, it is very exposed. While windward winds cause a jet effect, leeward winds create a leeside eddie with strong turbulence so that temperature differences are minimized under both conditions. Station 3 lies on the flatter divide and being the highest station it is most exposed.

In the Dischma, field data are presented from measurements made at seven individual sites (Table 12.2(b) and Figure 12.2(b)) in the upper valley from the midsummer season of 1999 as part of a detailed hydro-logical field experiment (VERDI) carried out between 1995 and 1999. Since the Dischma valley is heavily incised and resulting insolation patterns are very diverse, sites were chosen at representative locations along the east-oriented Hureli and west-oriented Schurli slopes and along the valley floor. Both rich and poor alpine pasture as well as alpenrose shrubs were instrumented at sites with different aspects ranging from 1960 to 2600 m in altitude. Evaporation pans and lysime-ters were placed at three sites on the Schurli slope, including Schurli Alpenrose (5), Schurli Bowen Ratio (6) and Schwarzhorn (7), two sites on the Hureli slope including Hureli Alpenrose (8) and Hureli Bowen Ratio (9) and at three sites on the valley floor, Inner Hof (1), Jenatsch (4) and Oletschboden (12) and one site at the valley head, Oberer Schonbuhl (13) (Figure 12.2(b)).

Table 12.2 (a) Giant Mountains. (b) Dischma. Combined évapotranspiration and condensation measuring test sites together with meteorological stations

Station 1

Station 2

Station 3

Location

Elevation (m a.s.l.) Gradient (0) Aspect Exposure Vegetation type Vegetation height Meteorological Station

Wind speed Ht. (m) Upper arm Ht. (m) Lower arm Ht. (m) ET micro-measuring station valley bottom 1030 14.7 NNE

Nearly flat Blueberry, moss 40 cm

Profile station/ Reinhardt

Evaporation pan/lysimeter concave slope 1190 23.5 NNE Concave Grass, fern 100-120cm (fern) Profile station/ Reinhardt/Drop Collector 2.0 1.50 0.10

Evaporation pan plateau-ridge

1285

11.5

Nearly flat Dwarf spruce 100 cm

Profile station/ Reinhardt/Drop Collector

Evaporation pan

Inner Hof

Hüreli Alpenrose

Jenatsch

Schürli Alpenrose (SA)

Gletschboden

Oberer Schonbühl

Schwarzhorn

Location

Elevation

(m a.s.l.) Gradient (0) Aspect Exposure Vegetation type Vegetation height

(cm) Meteorological

Station Wind speed Ht. (m) Upper arm Ht. (m) Lower arm Ht. (m) ET micro-measuring station

Valley bottom Trough slope

1610

15 SW

Convex Rich meadow 40

Profile station/

Reinhardt 2.15 1.60 1.20

Evapo. pan/ lysimeter

2070 30

E-NE Concave Alpenrose 35

Profile station/

Reinhardt 2.10 1.85 0.32

Evapo. pan/ lysimeter

Valley bottom

1960

None

Flat

Grass

Bowen Ratio

evapo. pan/ lysimeter

Troügh slope Valley bottom

2070

30 W

Concave

Alpenrose

Profile station/

Reinhardt 2.10 1.70 0.40

Evapo. pan/ lysimeter

2080 5

None Concave Alpenrose 35

Profile Station/

Reinhardt 2.25 1.75 0.45

Evapo. pan/ lysimeter

Upper valley head

2361

Flat

Poor pasture 8

Profile station/

Reinhardt 2.25 1.67 0.20

Evapo. pan/ lysimeter

Slope ridge below rock face 2600

West Convex Poor pasture 5

Profile station/

Reinhardt 2.10 1.56 0.24

Lysimeter

Data will not be presented from site 6 and 9 because of lack of overlap for the analysis period.

12.3.1 Evaporation pan and lysimeter

The evaporation pan is based on an automated micro-measuring system that was developed to determine evaporation losses and condensation gains on the steep mountains slopes of the Dischma valley (de Jong et al. 2002). Since then, it has been tested and implemented in Gut Frankenforst (Siebengebirge, Germany). The system consists of small, permanently installed water-filled evaporation pans placed on automatic weighing scales

(Figure 12.3(a) and de Jong etal. 2002). Evaporation pans are set into the ground, ensuring that the water surface is level with the average surrounding ground surface in order to minimize wind impact and to maximize the dominance of the local microclimate of the surrounding vegetative cover. Since the scales underneath the evaporation pans had to be adapted in size to the steep slope gradients, their capacity is limited to 5 kg. All water losses by evaporation (positive weight difference) and water gain through condensation (negative weight difference), as well as rainfall, are weighed accurately (with a resolution of ± 1gandanerror

EVA = Evaporation pan PM = Pluviometer SM = Storage module SP = Solar panel

50 cm

LYSI = Lysimeter PM = Pluviometer SM = Storage module SP = Solar panel

Figure 12.3 Micro-measuring system on steep slopes to determine (a) potential evaporation based on water-filled evaporation pan and (b) evapotranspiration based on soil and vegetation-filled lysimeters of 0.3% or an equivalent of 0.015 mm). The units used are millimetres. Measurements are taken simultaneously at each site during the mid-summer season. At windy sites, a smoothing function, based on a moving mean value over 40 minutes (X1 = (x10 + x20 + x30 + x40)/4, X2 = (x20 + x30 + x40 + x50)/4 ...) is applied to the time series, where x10 is a measurement taken at a 10-minute interval.

The lysimeter measuring system is based on the same automated principle as the evaporation pans (Figure 12.3(b)). A complete plant or several plants are removed with roots and soil, inserted in a container and then reinstalled in the same locality they were taken from. The different plants inserted in the lysimeter were well adapted to their new setting and did not suffer in texture or appearance during the experimental periods (Figure 12.4). This indicates their capability of surviving extreme climatic conditions.

In both environments, evapotranspiration measurements are higher than potential evaporation. The remarkable differences between the results of the lysimeter and evaporation pan demonstrate the sensitiveness of the measuring system. There are several reasons for the differences: firstly, the surface of the blueberry shrubs in the lysimeter is larger than the free water surface. Evaporation from the lysimeters is exclusively controlled by the process of transpiration, such that the stomatal control can greatly augment transpiration in relation to the free water surface. Secondly, the plants protrude approximately 20 cm above the ground while the water level in the pans is even with the surrounding soil surface amongst the shrubs. Since the evaporation pan forms a free surface to the air, it is unlikely to underestimate potential evaporation. Both systems are equally reliable under alpine conditions, but it seems that the

Figure 12.4 Example of optimal integration of lysimeter vegetation with its surroundings in the field (Station 1, Giant Mountains)

dynamics of evaporation are slightly better presented by lysimeters since their reaction is specific to plant type and allows more precise detection of nocturnal variations.

The uniqueness of this experimental study lies in the fact that potential evaporation and evapotranspiration can be measured and analyzed at 10-minute intervals, a time resolution that has not been presented for mountain environments before. If these time intervals were to be averaged or augmented, important phases of condensation would be leveled out completely and short, but important impacts such as morning or evening changes in wind direction and their effects on evaporation or condensation would be lost. Since other important dynamics or changeovers such as the onset of clouds, fog, sunrise or the termination of rainfall do not occur steadily over one hour but are a matter of minutes and may produce the largest dynamics of the day or night, it is very important to resolve evaporation and condensation processes at an appropriate resolution, that is, 10 minutes.

12.3.2 Drop collector

The drop collector is a pipe construction that consists of a circular plastic lid mounted on top of a vertical pipe. This construction was developed by the Meteorological Observatory of the University of Wroclaw. Horizontally and vertically accumulated moisture from the lid drains from the rim of the lid along a line into a collector. The diverted water caught in the collector is measured.

12.3.3 Meteorological profile and Bowen Ratio stations

Each experimental site consists of a basic meteorological profile station or a Bowen Ratio Station (Table 12.1) combined with a self-developed evaporation-condensation micro-measuring unit. All measurements were continuously recorded in self-constructed data loggers at 10-minute intervals. At the meteorological stations, air temperature and relative humidity were recorded at two levels above the ground in addition to solar radiation and wind direction (Conrad sensors) and wind speed (Reinhardt) (Table 12.1). The lower level of the Profile stations are located at the upper limit of the canopy. At the Bowen Ratio Stations, the same variables were recorded in addition to soil heat flux and dew point. Since dew point was not measured at all stations, the results will not be presented here. A small rain-o-matic collector with a tipping bucket was installed as an additional source for measuring rainfall at the evaporation and transpiration measuring sites.

12.4 RESULTS AND DISCUSSION

The results for the two study sites are presented at three different scales: hourly, daily and weekly. Measurements are taken at a very high temporal resolution to ensure detailed comparison for single days or weeks. Although the two test days chosen are subject to different local conditions, they have similar weather conditions, that is, no rain, fine weather in the morning and only some clouds in the afternoon and evening. Considering the season from June to mid-September observed at the Stillberg station over the past 30 years (WSL, Birmensdorf, Turner 1988), summers in the Dischma are typically wetter than winters, and sequences with precipitation exceeding 5 mm/day are common. It is for this reason that the weekly period selected has a succession of wet days. At the Szrenica site, summers are typically drier than winters with some evening fog at the higher sites (according to the 100 year Meteorological Observatory, Pereyma etal. 1997).

12.4.1 Hourly variations

In order to compare the two catchments, the results of a full test day are presented for May 27 2001 for Szrenica (Figure 12.5) and for August 5 1999 in the Dischma (Figure 12.6) for all selected stations.

Szrenica

The meteorological variables show that May 27 was a mostly fine day with some clouds passing between 11-16:00 and a thin cloud layer settling in after 18:00 (Figure 12.5(a)). Maximum wind speeds fluctuate around 15m/s between 03 and 04:00, 4m/s from 10:00 to 14:00, 15m/s at 17:00 and 13m/s around 23:00 (Figure 12.5(b)). Wind speeds are lowest during periods of maximum radiation.

Evapotranspiration and condensation are higher for the lysimeter at station 1 than for all other pan stations (Figure 12.5(d)). Evapotranspiration forms a well-developed bimodal distribution with a first phase between 07 and 12:00 and a second phase from 12 to 19:00. Between sunrise and sunset, there are two peaks of transpiration (0.21 mm/10 min. at 09:20 and 0.14 mm/10 min. at 14:30 with a break at noon). In contrast, evaporation starts 30 minutes later and rises slowly until noon with 0.07 mm/10 min. During the afternoon, there are three successive evaporation peaks just above 0.1 mm/10 min., but evaporation stops suddenly shortly after 16:00. In the late afternoon and night, condensation dominates (with approximately 0.1mm/10min. for both evaporation pan and lysimeter) alternating at roughly hourly intervals with evapotranspiration. Pronounced differences remain between lysimeter and evaporation pan. Whereas pan evaporation takes hours to gain momentum in the morning at station 1 because of low wind speeds, the pans at station 2 and 3 already reach maximum values of 0.2 mm/10 min. between 09:00 and 10:00. Potential evaporation follows similar patterns at station 2 and 3 (Figure 12.5(f) and (g)). During the afternoon, growing wind speeds and changing properties of incoming air masses (e.g. fog) are responsible for a rapid switch between evaporation and condensation at all evaporation stations. For station 2, evaporation declines much more slowly and only terminates at 20:00. At station 3, it already terminates at 17:00. Low-level condensation is pronounced throughout the day and night and may be explained by the influence of altitude on the formation of clouds and deposition of moist air through fog.

Most of the evapotranspiration dynamics can be explained in terms of radiation, wind speed and temperature gradients (Figure 12.5(a)-(c)). At station 1, the temperature gradient is highest at 11:00, decreasing rapidly thereafter. At station 2, the maximum temperature gradient is not well developed and generally lower than at station 1 (Figure 12.5(c)). It increases between 06 and 08:00 parallel with station 1 but fluctuates at a ratio of 1.1 because of significant exposure to wind. There is only a weak decrease in the temperature gradient after 14:00. The strong increase between 07:00 and 09:00 at station 2 and 07:00 and 11:00 at station 1 coincides with the steep increase in evaporation at the stations. High wind speeds encourage evaporation and evapotranspiration in the afternoon at station 1 and 3 but less so at station 2 where the temperature gradient is already decreasing as a result of strong topographical differences. Northeasterly to south-easterly winds set in the afternoon and mobilize warm, upslope air from the valley floor that reinitialize another short phase of evaporation. Neither evapotranspiration from the lysimeter nor from the evaporation pan reacts in parallel to radiation. Evapotranspiration from the lysimeter is initiated with radiation onset but terminates before sunset.

The striking contrast between the patterns of evapotranspiration from the lysimeter and evaporation from the evaporation pan at station 1 cannot be explained by meteorological data alone. The inverse patterns are not exceptional since this contrast is measured for all days (Figure 12.7(d)-(g)). All other evaporation pans have very similar regimes and are comparable to the lysimeter.

In total, there were approximately 5 mm of evapotranspiration, 3 mm of evaporation and 1 mm of condensation during the daytime at station 1. At night, the

-0.1
-0.2

Figure 12.5 Typical daily cycles of (a) incoming radiation (station 1 and station 2), (b) maximum wind speed (station 1), (c) temperature gradient (Tiow/Tup) (station 1), (d) evapotranspiration and (e) potential evaporation for station 1 and (f) potential evaporation at station 2 and (g) at station 3 at 10-minute intervals on May 27, 2001 for Szrenica, Poland lysimeter measured 1 mm of water loss, whereas the pan gained 1.6 mm of water. In other words, approximately 9 litres of water are mobilized over 1 m2 of vegetation, whereas little more than 4 litres are mobilized over

1 m2 of free water surface, a difference close to 50% (Figure 12.8).

In summary, evapotranspiration behaves in accordance with the physical setting of the three sites, especially topography and vegetation. The lowest station (1) is situated in a rather moist location at the lower end of a nivation depression close to a small stream surrounded by blueberries and moss-covered rocks. Station 2 lies on a small hummock next to a steep channel, and is wind-exposed with low vegetation and at the lower cloud boundary. The last station is surrounded at its upper edge by dwarf pines and forms a less wind-exposed stand of dwarf spruce with substantial influence of fog. Under these conditions, maximum evaporation occurs at station 2 in contrast to the wetter lower station 1 and the flatter station 3. A general gradient of evaporation only exists between station 1 and 3 but not with station

2 since it is influenced by wind as a result of its special topographical setting.

Dischma

Figure 12.6(a)-(c) illustrates the influence of dominant meteorological variables on evapotranspiration and evaporation on August 5 in the Dischma. As in the Polish example, the day was not quite cloud-free. The daily average wind speeds experiences a maximum between 14:00 and 18:00, and radiation decreases in this phase with incoming cloud fields. The temperature gradient in the upper valley starts increasing between 04:00 and 08:00 and flattens between 08:00 and 16:00, with an abrupt decrease between 15:00 and 16:00 due to upcoming wind and clouds. It decreases rapidly thereafter until 20:00, increasing rapidly once more between 20:00 and 21:00, then fluctuating at a constant level until midnight. An asymmetry in meteorological variables has developed between morning and evening because of incoming clouds and air masses.

With the exception of Schwarzhorn, all sites experience a short, intensive rainfall event between 22:00 and 23:00, reaching a maximum of 1.7 mm/10 min. At Schwarzhorn, rainfall persists from 19:00 to 24:00. At Inner Hof and Jenatsch, rainfall peaks at 22:00, at Schonbuhl toward 23:00, at Gletschboden at 23:00 and at Schurli and Hiireli at 22:00. The rain moves in fast from the north. In the lower valley, it begins and ends earlier, therefore evapotranspiration begins sooner after rainfall. In the higher regions, evapotranspiration can only set on at the end of rainfall, which is after midnight.

Evapotranspiration takes place between 07:00 and 21:00 in the whole valley with the exception of Schwarzhorn, which does not start until 09:00 and already ends at 16:00 (Figure 12.6(d)). On the valley floor at Inner Hof (Figure 12.6(i)) and less so at Jenatsch (Figure 12.6(g)), there is a short spell of condensation around 16:00 followed again by evapotranspiration. At Hureli Alpenrose, condensation takes place over a short period at 13:00. At Gletschboden (Figure 12.6(m)), there is no condensation and in contrast to Jenatsch and Inner Hof, the lysimeter records the strongest evapotranspiration period immediately after 10:00 in the morning. The Schwarzhorn site (Figure 12.6(d)) does not conform to the other sites. Here, evapotranspiration is uniformly high and is initiated late (after 08:00), experiences an early peak at 14:00 and terminates early (16:00). Rates of evapotranspiration reach 0.12 mm/10 min. for prolonged periods. Some sites experience late peak evapotranspiration, for example, at 15:00 at Schonbuhl and Inner Hof (Figure 12.6(e) and (i)) and 17:00 at Jenatsch.

The lowest evaporation rates are measured at Schonbuhl (<0.05 mm/10 min) and the highest at Inner Hof (0.06 mm/10 min). This can be explained by the differences in locations: whereas Inner Hof is situated more on a slope, it experiences less air saturation and is surrounded by a very diverse wet meadow, Schonbuhl is close to the glacier with stagnating wet areas, higher humidity and very short grass. Evapotranspiration and evaporation rates are very high after the short rainfall event (i.e. 0.13 mm/10 min. maximum at Inner Hof (Figure 12.6(i) and (j))). Overall, it can be stated that with the exception of Schonbuhl all sites have the same local pattern of evapotranspiration and evaporation, although the pattern is less evident for smaller fluctuations in the afternoon.

The Schwarzhorn site has a completely different regime. Incoming cold air switches off evapotranspiration far earlier. Inner Hof and Jenatsch, both located in the rich pasture zone, are also locally affected by this phenomenon. The expected decrease of evaporation with increasing elevation, increasing air humidity, decreasing temperature and poorer vegetation only counts for the evaporation pans situated along the longitudinal valley axis at Schonbuhl (Figure 12.6(f)), Jenatch (Figure 12.6(h)) and Inner Hof (Figure 12.6(j)). However, the three lysimeters respond quite uniformly, and Schonbuhl, in particular, transpires substantially more than Jenatsch. A weak elevational gradient of evapotranspiration exists, but regional differences must be considered in terms of botany, that is, plant type and density.

800 700 2 600 J 500

800 700 2 600 J 500

5

— Jenatsch up (1930 m)

)

Mean wind speed (m/

-»•2 3 4

(b)

0 0

2 0

4 0

6 0

8 0

0

2

4

68 11

02 22

1.8 1.7 ~ -I.6

1 1.5

o) 1.3

£ 1.0

M

A

K

W

M

A

V™

t

\

/

V A

0.8

V

V,

/

(c)

0 0

24 00

6 0

80 01

^ -t CD

80 12

2 2

Schwarzhorn (2600 m) ivsimeter

A,

A

i

m

J

J

L

'rw

" 1

r

J

Evapotranspiration M Condensation r: Rain

-1.Í 8020 T CM CM O

Schónbühl (2361 eva. pan

ti

u

C Evaporation 9 Condensation Rain

Jenatch (1960 m) eva. pan

ï

I

J

1

»

r

Evaporation Condensation :: Rain

Figure 12.6 Comparison of (a) incoming radiation (Jenatsch), (b) average wind speed (Schiireli Alpenrose and Jenatsch) and (c) temperature gradient (Gletschboden) at 10-minute intervals; evapotranspiration, potential evaporation, condensation and rainfall for (d) Schwarzhorn, (e) and (f) SchonbUhl, (g) and (h) Jenatch, (i) and (j) Inner Hof, (k) and (l) Schiireli Alpenrose, (m) and (n) Gletschboden and (o) and (p) Hiireli Alpenrose at 10-minute intervals for August 5, 1999 for Dischma, Switzerland. Periods with no data indicate data loss. Nearly no rainfall was recorded at (f) and (h) since the evaporation pan was already full

Gletschboden ( eva. pan

2080 m)

i

M

Evaporation

Condensation

Rain

0.10 0.05 0.00 -0.05 -0.10

Schürli eva. pa

Alpenrose (2075) n

:

j

hi

L

i

1

r

Evaporation

Condensation

Rain

"03

0.20

antr 0.05

ot p

Hüreli Alpenrose lysimeter

(2075 m)

ii<

f

A 1,,

Evapotranspiration

Condensation

Rain

0.10 0.05 0.00 -0.05 -0.10

Hüreli Alpenrose (2075 m eva. pan

)

1!

1

i

i* J

*

J

J

u

M

A

?

Condensation

Rain

Szrenica, Poland

Szrenica, Poland

Station 3 (1285

m)

■ Condensation □ Evaporation

111

■ jjJ

-

Figure 12.7 Comparison of meteorological variables at Szernica, Poland (22-28.5.01): (a) radiation per 10 min. at station 1, (b) maximum wind speed per 10 min. at station 1 (c) temperature gradient at station 1, (d) evapotranspiration and condensation at station 1, (e) potential evaporation and condensation at station 1, (f) potential evaporation and condensation at station 2, (g) potential evaporation and condensation at station 2

Dischma, Switzerland

BQG 7QG ) 6QG 5QG 4QG SQG 2QG 1QG G

BQG 7QG ) 6QG 5QG 4QG SQG 2QG 1QG G

-----------\

V

u

w 1

tí 1

Schwarzhorn (26QQ m) lysimeter m

!

ï

JL

A

m)

Schönbühl (2S6 lysimeter

Condensation

2.G 1.5 1.G G.5 G.G -G.5

1

ï

i

j"

Vr^

Jii

f

Evaporation

eva. pan

Condensation

Rain

Gletschboden lysimeter

2GBG m)

Condensation

P

-A

jJ«

't

r* .

Gletschboden (2QBQ m) n Evaporation eva pan 1 Condensation eva. pan : Rain

Evaporation Condensation

Rain

Figure 12.7 (continued) and at Dischma (5-11.8.1999) (h) radiation per 10min. at Jenatsch (i) maximum wind speed per 10min. at Jenatsch (j) temperature gradient at Gletschboden, (k) evapotranspiration and condensation Schwarzhorn (l) evapotranspiration and condensation SchonbUhl, (m) potential evaporation and condensation SchonbUhl (n) evapotranspiration and condensation at Gletschboden, (o) potential evaporation and condensation Gletschboden. Blank spaces indicate missing data due to battery failure

Station 1 (1030 m) Day

Station 1 (1030 m) Day

Station 1 ( Night

103C

m)

8 Transpiration O Evaporation ■ Condens. lysimeter

s Condens.

eva pan

I

Schwarzhorn (2600 m) Day

Evapotranspiration Evaporation Condens. lysimeter Condens. eva pan

Schwarzhorn (2600 m) Night

Evapotranspiration r] Evaporation m Condens. lysimeter H Condens. eva pan

Station 2 (1190 m) Day

1

M

i

1

1

i

i

Station 2 (1190 Night

)

□ Evaporation

,_,

rn

H

is

Figure 12.8 Comparison of transpiration, evaporation and condensation in lysimeters and evaporation pans at Szernica, station 1, by (a) day and (b) night, station 2 by (c) day and (d) night and station 3 (e) day and (f) night and in the Dischma, Schwarzhorn by (g) day and (h) night, at SchonbUhl by (i) day and (j) night and at Gletschboden by (k) day and (l) night. Zero data indicates data loss through battery failure

Station 3 (1285 m)

Day

Pi

m

$

m

Station 3 (1185) Night

Station 3 (1185) Night

Gletschboden (2080 m) Day

□ Evapotranspiration " Evaporation g Condens. lysimeter Condens. eva pan

Gletschboden (2080 m) Day

□ Evapotranspiration " Evaporation g Condens. lysimeter Condens. eva pan

Gletschboden (2080 m) Night

Evapotranspiration Evaporation Condens. lysimeter Condens. eva pan

Comparison of the three alpenrose sites (Figure 12.6(k) and (l); (m) and (n); (o) and (p)) shows that evaporation and evapotranspiration dominate at HUreli and Gletsch-boden at night with little condensation. Considerable amounts of nocturnal CO2 release due to alpine plant respiration was also observed by Korner (1999). Evapotranspiration persists longest on the valley floor at Gletschboden, followed by Schurli then Hiireli. Gletsch-boden and Hureli reach their maximum at 12:00, but Schurli follows later at 14:00-15:00. Schureli has both the highest transpiration and condensation rates.

The pattern of daily evapotranspiration and evaporation is complicated, but indicates some coherence over time. However, over space there is no single general rule of thumb (such as one gradient concept) that may apply to the interpretation of evapotranspiration, and the impact of plant distribution on evapotranspiration dynamics is substantial. If water loss and gain are balanced for each other, the highest lysimeter station Schwarzhorn loses the highest amount of water (2.8 l/m2), Gletschboden loses only 2.5 l/m2 and Schonbuhl loses 1.5 l/m2 on the same day. Neither evapotranspiration theory nor functions would allow this case to arise under linear assumptions. However, under natural conditions the effects of extremely high local wind speeds on evapotranspiration may outweigh all other factors (Figure 12.8(g) and (h), (i) and (j) and (k) and (l)).

12.4.2 Daily and weekly variations

The daily variations of evapotranspiration and condensation in the Dischma (Figures 12.7 and 12.8) are influenced by a sequence of wet days that frequently caused data loss at the more remote sites Schwarzhorn and Schonbuhl. In general, rainfall occurs in the evening with three days of rainfall in the morning and no rain on the first day. In the Dischma, rainfall decreases the dynamics of evapotranspiration only marginally compared to Szrenica. In contrast, for the week in Poland (Figure 12.7), there is an absence of rain with long, fine weather periods but a more dominant influence of fog and some clouds at midday and in early afternoon. As such, the dynamics of condensation are far more pronounced at this site.

Szrenica

In contrast to the Dischma, the weather in the Szrenica catchment was continuously fine (Figure 12.7(a)-(c)). Patterns of radiation were very regular with a strong increase in radiation in the morning, clouds and fog in the afternoon and secondary peaks in radiation due to cloud break up in the late afternoon and early evening.

Maximum wind speeds behave inversely to radiation. The higher the radiation, the weaker the wind speed. Similarly, wind speeds are very high at night. The local temperature gradient between the lower and upper arm of the meteorological station increases strongly in the early morning and reaches a maximum toward 10:00, decreasing up to 22:00. Minimum values are obtained by surface cooling during the nighttime.

In this catchment, evapotranspiration and condensation from the lysimeter especially at station 1 is very regular on a day-to-day basis (Figure 12.7(d)). On May 26 evapotranspiration reaches more than 1.5 mm/hour. Parallel with radiation, there is frequent bimodality in the water exchange patterns, with rapidly increasing evapotranspiration between 10:00 and 11:00 and a secondary maximum in the late afternoon. Maximum condensation occurs at 22-23:00. The bimodality in evapotranspiration can be explained by the daily cloud cover. Clouds appear in the early afternoon but in the late afternoon the sun often reappears for a short time. The temperature gradient (Figure 12.7(c)) increases in parallel with the strongest increase in evapotranspiration. As already observed (Figure 12.5(e)), the distribution of evaporation at station 1 (Figure 12.7(e)) is asymmetrical and peaks in the afternoon. Evaporation at the higher station 2 follows a different pattern but at a slightly higher level. Particularly strong

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