Hydrology, Water Resources and Ecology in Headwaters (Proceedings of the HeadWater'98 Conference held at Meran/Merano, Italy, April 1998). IAHS Publ. no. 248, 1998. 79

Glacier runoff in Alpine headwaters in a changing climate

MICHAEL KUHN & NIKOLAUS BATLOGG Institute of Meteorology and Geophysics, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria

Abstract Glaciers may contribute a significant fraction of the runoff in Alpine headwaters. Their importance for the Alpine water cycle is illustrated with the annual and daily hydrographs from Austrian gauging stations at various altitudes and with various degrees of glacierization. Under present conditions, highly glacierized basins display low winter water and meltwater peaks of up to four times the annual mean in July and August. In a model study, the runoff from 20 Alpine basins was simulated for various temperature scenarios. From the whole spectrum of reactions found in this investigation, three basically different types were distinguished: (a) higher runoff throughout the year due to increased glacier melt, (b) an early runoff peak from the melting of seasonal snow and a secondary summer peak from glacier melt, and (c) a runoff peak in summer when precipitation becomes more important than snow and ice melt.

INTRODUCTION

The hydrographs of Alpine headwaters are dominated by the storage of snow and the seasonal and daily rhythm of meltwater. Within the Alps, regional differences arise on account of the varying total amounts and seasonal distribution of precipitation and of mean and range of elevation above sea level. In climatological terms these differences are due to the height-dependent temperature regime and the varying degree of screening from or exposure to northerly or southerly branches of the general circulation, all of which have an influence on the seasonal snow cover and the (quasi) permanent ice cover. Of the many facts of glacier runoff in Alpine headwaters, this paper emphasizes three: (a) the ratio of maximum to mean monthly runoff in the annual hydrograph, (b) the effect of the seasonally changing snow cover or bare glacier ice on the mean daily hydrograph of each month, at one station and (c) the effect of decreasing glacierization of a series of nested basins at one time of the year.

The present day situation is illustrated with records from 10 glacierized and two ice-free basins in the Austrian Alps. A conceptual model with one month time steps and 100 m height intervals was calibrated on nine of these basins and applied to scenarios with varying mean annual precipitation and temperature. The glaciological and hydrometeorological conditions of these basins and the model are described in detail in Kuhn & Batlogg (1998). Their ice cover and altitude range are given in Table 1. The physiographic and climatological characteristics of the basins cannot be safely attributed to separate categories. Nonetheless, there are some typical features in each of them: Verwall, Sulzau, Bucheben are exposed mainly to the north, Paznaun, Rofental, Stafflach, Neukaser are relatively dry basins situated at the

80 Michael Kuhn & Nikolaus Batlogg

Table 1 Mean and maximum monthly runoff 19960/61-1991/92 and physiographic data of basins used in the scenarios. Vernagt data are taken from Escher-Vetter & Reinwarth (1995) and are valid for 1973/74-1992/93.

Basin

Vernagt Rofental InnergschlôB Sulzau Neukaser Paznaun Verwall Bucheben Mallnitz Stafflach Schladming Kremsrminster

Ice cover %

80 42 38 20 12 10 6 5 2 2 0 0

Altitude (m)

2640-3630 1890-3740 1690-3670 880-3670

1820-3480 1900-3400 1820-3150 1050-3250 1170-3360 1100-3480 730-3000 340-1600

Monthly runoff (mm):

Mean

140 110 190 140 120 100 150 120 110 80 90 60

Max.

560 360 610 410 350 290 440 300 290 180 180 70

Max/mean

4.0 3.3 3.2 2.9 2.9 2.9 2.9 2.5 2.6 2.3 2.0 1.2

Month of maximum runoff

August August July July June August August June, July June June May March

central divide. So are InnergschlôB and Mallnitz. However, the former receives plenty of precipitation from both sides and is well glacierized, the latter has few glaciers and has predominantly southerly exposure.

Figure 1 displays both basin precipitation and runoff of Paznaun, InnergschlôB and Mallnitz for the period 1960/61-1991/92. Paznaun has precipitation maxima in summer and spring and minima in May and October. While InnergschlôB has a narrow precipitation maximum in July and August, Mallnitz has a broad peak from April to August. Both show a secondary maximum in November that is typical of the mediterranean influence. The runoff records of the three stations have in common distinct summer maxima and very low baseflow in winter. InnergschlôB displays high runoff in July and August typical of a highly glacierized basin, while the other two stations have maxima of monthly runoff that are less than half that at InnergschlôB. In accordance with its altitude and glacierization, Paznaun experiences a late maximum, while the early maximum of Mallnitz is almost entirely due to the melting of its seasonal snow cover.

In mountain basins, glaciers are said to have a compensating effect on the runoff—in hot, dry-weather ice melts while in times of heavy precipitation melting is subdued and part of the precipitation is stored as snow in the higher part of the basin. This effect has been described by many authors, among them Kasser (1959), Fountain (1985), Ferguson (1985), Rôthlisberger & Lang (1987), Chen & Ohmura (1990), Kuhn (1994) and Krimmel & Tangbom (1994) and will not be further treated in this paper.

SEASONAL HYDROGRAPHS

Examples for the influence of glaciers and seasonal snow cover on the runoff regime of Alpine rivers have been given by Kresser (1965) for Austria and Aschwanden &

Glacier runoff in Alpine headwaters in a changing climate 81

Paznaun Paznaun 700

600

E 500 E ç 400

? 300 c 5 200

100

0

10 11 12 1 3 4 months

300

InnergschlôB InnergschlôB

2 3 4 months

Mallnitz Mallnitz

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

months

Fig. 1 Annual course of basin precipitation and runoff from three selected basins (see Table 1), mean values for 1960/61-1991/92.

Weingartner (1983) for Switzerland. They agree qualitatively in that the ratio of peak to mean annual flow (R) and the time of peak flow vary with the degree of glacierization (G) of the basin and with the amount and duration of snow cover. In the present study we have evaluated monthly means of runoff of 10 glacierized and two ice-free basins for the period 1960/61-1991/92 and have arranged values of R according to G in Table 1 and Fig. 2. Under present conditions it appears that for basins with glacierization G > 5% the ratio R = Qmax/Qmean is a linear function of G: R = 2.7 + 0.015 G with r2 = 0.96 if G is given in percent. At values of G = 5% the ratio R seems to depend mainly on altitude, i.e. on the duration and amount of seasonal snow cover. It is remarkable that the basins of Rofental and InnergschlôB have nearly identical peak to mean ratios although the absolute values of runoff differ by a factor of almost 2.

82 Michael Kuhn & Nikolaus Batlogg

0 50 100

% glacierized Fig. 2 The ratio of maximum monthly to mean monthly runoff vs degree of glacierization of the basin, using values from Table 1. Gauging stations with ratios in excess of 2.7 are situated above 1700 m a.m.s.l. and display a strong dependence on glacier cover, the regression line is valid for these stations only. Full squares are actual values, circles are model results for the +2CC scenario.

DAILY HYDROGRAPHS

Seasonal change at one station

The series of mean daily hydrographs of Vernagtbach published by Escher-Vetter & Reinwarth (1995) serves very well to demonstrate the change in amplitude and phase of runoff through the season in a basin with 80% glacierization. We have reproduced in Fig. 3 their curves for the year 1981 with near-zero mass balance for Vernagtferner, and for the year 1991 with extreme glacier melt. Although the difference is obvious from the peak flow values (6 m3 s"1 in August 1991, and 2.9 m3 s"1 in 1981), the two years have several features in common.

Appreciable streamflow starts in June and continues through September. In June the ratio of peak to mean flow is 1.2 in both years, in August it is 1.3 and 1.9 respectively. The late maximum and small amplitude of the mean June hydrographs reflect the long travel time of runoff trough the snowpack. With progressively less snow cover, increasing permeability of the snowpack and increasing energy supply the July hydrographs have a higher and earlier peak, and so on to August. While in September peak flow is reduced, it occurs at the same time, the rising limb being a bit steeper than in August. The associated travel times in the various parts of the drainage system have been investigated by Oerter et al. (1981) and Oerter & Moser (1982) who found a time constant of about 10 days for the drainage of the firn aquifer of Vernagtferner.

Change of daily hydrographs in nested basins

In a series of basins in the Oetz Valley, daily hydrographs change with the degree of glacierization, altitude above sea level and distance from the glaciers (Table 2). In

Glacier runoff in Alpine headwaters in a changing climate 83

|m'/s| 4 1981

^SSSEësâ

06:00 12:00 18:00 24:00 I h l

Fig. 3 Monthly mean diurnal hydrographs at Vernagt gauging station in an average year (1981) and an extreme year (1991). Note the progression of the time of the maximum runoff from June to September in both years. Reproduced from Escher-Vetter & Reinwarth (1995).

Fig. 4, daily hydrographs of these five basins have been drawn for fair weather periods in June and July. Both cases show a delay of the time of the peak flow and a decrease of the amplitude downstream. In June the Vernagt basin is still largely snow covered while the lower parts of Rofental have lost much of their snow cover. The larger supply of sensible heat at the lower elevation compensates for the difference in snow cover so that peak flow at Vernagt (2640 m a.m.s.l.) and Vent-Rofenache (1890 m) are similar. The difference, however, has markedly increased by mid-July. At that time the seasonal snow has practically disappeared, more glacierized basins have a larger daily amplitude and the ratio of peak runoff height (mm h"1) at Vernagt to that at Tumpen has increased from 2 to 4 approximately.

Table 2 The basins used in Fig. 4.

Basin Area (km ) Ice cover (%) Altitude of gauging station (m) Vernagt Rofental Huben Oberried Tumpen

11 98 517 623 785

80 42 22 21 17

2640 1890 1185 1160 930

Michael Kuhn & Nikolaus Batlogg

Fig. 4 Mean diurnal hydrographs for five sub-basins in the Oetz Valley that are progressively larger, lower and less glacier covered as indicated in Table 2. The two fair-weather periods display the down river damping and delay of the meltwater peak and the characteristic change in amplitude and phase from June to July. Raw data were supplied by W. Gattermayr, Hydrographischer Dienst Tirol, and O. Reinwarth, Bavarian Academy of Sciences.

SIMULATION OF RUNOFF IN A CHANGING CLIMATE

The model

We used a conceptual model with 100 m elevation zones and one-month time steps (Kuhn et al. 1982; Kuhn & Pellet, 1989; Kuhn & Batlogg, 1998). For given terrain elevation, forest and glacier areas, it simulates the build-up and decay of the snow cover including melting of glacier ice. Temperature determines the transition from rain to snowfall, and the degree-day melting. Evaporation depends on elevation, vegetation and snow, water retention in soil, and snow and baseflow are successively approximated. This model was calibrated for each basin to match the monthly mean runoff for the period 1960/61-1991/92.

The temperature and precipitation scenarios

Air temperature was varied by equal amounts in each month, using steps of - 1 , +1 , +2, +3°C with respect to the calibration period. All other parameters including glacier size were kept unchanged except water retention in snow and baseflow which were shifted with the snow-cover duration. Figure 5 shows the runoff simulation for

Glacier runoff in Alpine headwaters in a changing climate 85

precipitation scenario Paznaun

original P -30% P +30% |

temperature scenario Paznaun

-original T+1"C T+2°C T+3"C T-1°C|

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

months

precipitation scenario InnergschlôB

| • original P -30% P +30% |

temperature scenario InnergschlôB

[ original T+rC T+2°C T+3°C T-1°C|

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

months

precipitation scenario Mallnitz

original P-30% P+30%]

temperature scenario Mallnitz

-original T+1"C T+2°C T+3°C T-1°C|

Fig. 5 Left: Model response of runoff to changes in mean annual temperature for the three basins shown in Fig. 1. Note the different scales of the three diagrams. Right: Response to changes in mean annual precipitation.

three basins with varying glacierization (Table 1). InnergschlôB with 38% ice cover offers abundant ice to be melted at the time of maximum available energy so that here July remains the month of peak runoff in all scenarios. Peak runoff increases

86 Michael Kuhn & Nikolaus Batlogg

Table 3 Annual values of runoff Q, storage S, the ratio SIQ and the mean specific mass balance of glaciers in the basin, the ratio of maximum to mean monthly runoff and the month in which the maximum occurred. Mean values for 1960/61-1991/92 and for a scenario in which temperature was increased by 2°C in all months of the year.

Basin

Rofental InnergschlôB Sulzau Neukaser Paznaun Verwall Bucheben Mallnitz Stafflach

Basin

Rofental InnergschlôB Sulzau Neukaser Paznaun Verwall Bucheben Mallnitz Stafflach

Mean 1960/61-1991/92

Q (mm)

1344 2271 1732 1470 1222 1751 1486 1306 940

T + 2°C

g (mm)

1882 2959 2113 1616 1346 1840 1546 1319 950

S (mm)

-46 -629 -363 -46

15 -14

51 -18 -35

scenario

5 (mm)

-598 -1332 -761 -214 -128 -126

-37 -51 -73

SIQ -0.03 -0.28 -0.21 -0.03 0.01

-0.01 0.03

-0.01 -0.04

SIQ

-0.32 -0.45 -0.36 -0.13 -0.10 -0.07 -0.02 -0.04 -0.08

b (mm)

-128 -1652 -1523 -388

137 -220 1038

-1178 -1393

b (mm)

-1421 -3331 -3746 -1770 -1223 -1964 -671

-3290 -3021

QmaxIQmean

3.3 3.2 2.9 2.9 2.9 2.9 2.5 2.6 2.3

QmaxIQmean

2.8 3.0 2.5 2.3 2.4 2.8 2.2 2.0 1.9

Month of Qmax

August July My June August August June, July June June

Month of Qmax

August July July July June May June July July

from 600 to 800 mm per month with a 3°C warming, the largest relative changes occur in spring. In the Paznaun basin, peak flow remains nearly the same in all scenarios but shifts from July to May and a secondary maximum develops in August. Compared to the precipitation input (Fig. 1) this late maximum must have a con­siderable contribution from glacier melt, while the early main maximum arises from snowmelt. Mallnitz has a fairly even input of precipitation from April to August (Fig. 1) and only 2% glacier cover so that changes in runoff in this basin reflect the response of the seasonal snow cover to the warming scenarios. In the -1°C scenario, peak flow occurs one month later than in the reference period. With the large fraction of low altitude terrain of this basin winter runoff increases appreciably in the warming scenarios. At the same time spring snowmelt loses importance compared to summer time convective rains so that with 2°C or 3°C warming the peak runoff returns to July.

In essence, Mallnitz is a headwater basin in which the seasonal snow cover cedes control over the runoff regime to summer rains in a scenario of stronger warming. In Paznaun the dominance partly shifts from snowmelt to glacier melt, while in InnergschlôB glacier melt dominates all scenarios. This is also true for the precipitation scenarios in Fig. 5. At InnergschlôB, July remains the month of peak flow regardless of decrease or increase of precipitation, likewise it is June in the Mallnitz basin. Only in Paznaun the maximum occurs earlier when precipitation is

Glacier runojf in Alpine headwaters in a changing climate 87

decreased. It is interesting, however, to compare the ratio of maximum runoff under increased precipitation to the actual maximum: it is 1.15 at InnergschlôB, 1.36 at Mallnitz and 1.45 in Paznaun. InnergschlôB is the highest of the three basins so that more precipitation has the least effect while in Mallnitz and Paznaun the storage and thus the summer melt become more important.

Basin storage and glacier mass balance

Table 3 compares the actual values of storage and maximum runoff to those of a +2°C scenario. The maximum to mean runoff ratios have been entered in Fig. 2 where they appear to converge with the actual values at high glacierization. This does not necessarily have to be at 100% ice cover, as both the maximum and the mean attain higher values in a warmer climate.

While the storage term S in Table 3 applies to the entire basin, the mean specific balance b is restricted to the glacier only. Taking the difference of unweighted arithmetic means of the nine basins in the scenario and reference state and dividing by 2, results in indicators of the climate sensitivity of the basins and glaciers applying to present glacier sizes:

(a) average climate sensitivity of basin storage dS/âT = -125 mm per °C

(b) and of specific glacier mass balance db/dT = -840 mm per °C

Acknowledgement This study was supported by Forschungsinitiative Verbund, Vienna.

REFERENCES

Aschwanden, H. & Weingartner, R. (1983) Die Abflussregimes der Schweiz (The runoff regime in Switzerland.). Teil 1, Alpine Abflussregimes. Geographisches Institut, Universitât Bern.

Chen, J. & Ohmura, A. (1990) On the influence of Alpine glaciers on runoff. In: Hydrology in Mountainous Regions. I—Hydrological Measurements; The Water Cycle (ed. by H. Lang & A. Musy) (Proc. Lausanne Symp., August 1990), 117-125. IAHS Publ. no. 193.

Escher-Vetter, H. & Reinwarth, O. (1995) Two decades of runoff measurements (1974 to 1993) at Pegelstation Vernagtbach/Oetztal Alps. Z. Gletscherk. Glazialgeol. 30(1994), 53-98.

Ferguson, R. (1985) Runoff from glacierized mountains: a model for annual variation and its forecasting. Wat. Resour. Res. 21, 702-708.

Fountain, A. G. & Tangborn, W. V. (1985) The effect of glaciers on streamflow variations. Wat. Resour. Res. 21, 579-586.

Kasser, P. (1959) Der Einfluss von Gletscherriickgang und Gletschervorstoss auf den Wasserhaushalt (The influence of glacier retreat and advance on the water balance). Wasser- und Energiewirtschaft 51, 155-168.

Kresser, W. (1965) Ôsterreichs Wasserbilanz (Austria's water balance). Ôsterr. Wasserwirtschaft 17, 213-221. Krimmel, R. M. & Tangborn, W. V. (1994) South Cascade Glacier: the moderating effect of glaciers on runoff. In:

Proc. Western Snow Conference 42nd Annual Meeting, 9-13. Kuhn, M. (1994) Schnee und Eis im Wasserkreislauf Ôsterreichs (Snow and ice in the Austrian water cycle). Ôsterr.

Wasser- u. Abfallwirtschaft 46(3/4), 76-83. Kuhn, M. & Pellet, F. (1989) Basin precipitation as residual in Alpine water balances. In: Precipitation Measurements

(ed. by B. Sevruk) (Proc. WMO/IAHS/ETH Workshop, St Moritz, Switzerland, December 1989), 275-277. Kuhn, M. & Batlogg, N. (1998) Modellierung der Auswirkung von Klimaanderungen auf verschiedene Einzugsgebiete in

Ôsterreich (Modelling the effect of climatic change on various basins in Austria). Schriftenreihe Forschung im Verbund, Vienna, (in press)

88 Michael Kuhn & Nikolaus Batlogg

Kuhn, M., Nickus, U. & Pellet, F. (1982) Die Niederschlagsverhâltnisse im inneren Ôtztal (Precipitation in the innermost Oetz valley). In: 17. Intern. Tagung fur Alpine Météorologie, 235-237. Deutscher Wetterdienst, Offenbach.

Oerter, H. & Moser, H. (1982) Water storage and drainage within the firn of a temperate glacier (Vernagtferner, Oetztal Alps, Austria), In: Hydrological Aspects of Alpine and High-Mountain Areas (ed. by J. W. Glen) (Proc. Exeter Symp., July 1992), 71-81. IAHS Publ. no. 138.

Oerter, H., Baker, D., Moser, H. & Reinwarth, O. (1981) Glacial-hydrological investigations at the Vernagtferner Glacier as a basis for a discharge model. Nordic Hydrol. 12(4/5), 335-348.

Rôthlisberger, H. & Lang, H. (1987) Glacial hydrology. In: Glacio-Fluvial Sediment Transfer (ed. by A. M. Gurnell & M. J. Clark), 207-284. John Wiley, Chichester, UK.