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18.1 INTRODUCTION

The hydrological importance of mountains has several aspects. The orographic effect and the resulting interaction with the atmosphere produce a highly enhanced precipitation input as compared to lowland areas. Moreover, winter precipitation is stored as snow cover and glacier ice and released, seasonally delayed, in spring and summer, when the demand for irrigation is highest. Not only the water benefits mountain regions themselves but also the rivers transport the surplus of water downstream and provide lowlands with this precious resource. Mountain areas can be regarded as water towers that yield and preserve water and distribute it over time and space.

The importance of such water towers for the lowlands and the strength of the remote impact are strongly influenced by the precipitation conditions of the lowlands. When surrounded by arid regions, mountains are the only relevant water supplier to support the existence of the lowland population. In humid lowlands, the mountainous runoff regime is gradually overlaid by a pluvial one, whereas in dry regions the nival or glacial runoff character is preserved further downstream.

This shows that hydrological processes and changes in mountain areas have a greater importance in drier parts of the continents, as in Central Asia, because here highlands are not only additional water suppliers like in Middle Europe but also the most important and sometimes even the only ones.

Against a background of globally increasing demand for water, climate change and conflicts that can arise from the unequal distribution of water on the earth's surface, one can imagine what great importance international water resources management will play in the future. Regions with great differences in water supply, for example, as a result of the above-described highland-lowland interaction, will have to solve even greater problems in water supply than they already have today.

The aim of this study is to estimate the effect of deglacierisation on river runoff in different climatic regions by applying a conceptual precipitation-runoff model in Central Asia and by comparing the results with similar investigations in the Alps.

18.2 CHARACTERISATION OF THE RESEARCH AREAS

In Central Asia, three test sites with different degrees of continentality were chosen: the Tuyuksu Glacier Region in Kazakhstan, which shows moderate maritime influence, Abramov Glacier in Kyrgyzstan and the highly continental Glacier No. 1 in China (Figure 18.1).

Climate and Hydrology in Mountain Areas. Edited by C. de Jong, D. Collins and R. Ranzi © 2005 John Wiley & Sons, Ltd

Figure 18.1 Location of the Central Asian research areas, schematic sketch after Aizen et al. (1995)

The Tuyuksu area is located on the northern slope of the Zailiskiy Range, which is the most humid part of the northwestern Tien Shan. The basin is drained by the Little Almatinka River, which meets the Ili River ending in Lake Balkash. The largest of the nine glaciers is Central Tuyuksu with an area of 2.5 km2 (Kommission fur Glaziologie 2001).

Abramov Glacier is located in the Pamiro-Alay, a transition zone between Tien Shan and Pamirs. The melt water feeds Koksu River, a third-degree tributary of Amu Darya, one of the main inflows of the Aral Sea. There are 10 small glaciers and Abramov glacier with an area of 26 km2 (WGMS 1999).

Glacier No. 1 is situated in the extremely continental eastern part of the Tien Shan. From the source area of Urumqi River, it supplies the city of Urumqi and large agricultural areas with water before the river dries up in the desert. Because of the strong influence of the Siberian High in winter, precipitation is concentrated in summer months. Therefore, accumulation and ablation take part simultaneously in different elevation belts, and both reach their maximum in summer (''summer accumulation glacier type'' after Ageta & Higuchi 1984).

In the Alps, the catchment Rofenache was chosen for comparison with the Central Asian results. Here, the HBV-ETH model has been applied previously (BayFORKLIM 1999), and additional model runs have been conducted for this study. The Rofenache is located in the upper Otztal in Austria and belongs to the Inn basin. A highly glacierised sub-basin of

Rofenache is Vernagtbach, where the Commission for Glaciology of the Bavarian Academy of Sciences carries out glaciological studies on Vernagtferner and hydrometeorological measurements.

The main characteristics of all test sites are shown in Table 18.1.

18.3 MODEL DESCRIPTION

The HBV-ETH model is a further development of the HBV model, a conceptual precipitation-runoff model that was developed in the 1970s in Sweden (Bergstrom 1976). The worldwide applied model was expanded at the Swiss Federal Institute of Technology (ETH) in Zurich for application in glacierised regions (Braun & Renner 1992). Further improvements and the programming for operational use on microcomputers were carried out in 1997 at the Commission for Glaciology of the Bavarian Academy of Sciences (Braun et al. 2000). Figure 18.2 shows the structure of the current version.

As input, the model needs the distribution of the basin area by altitude and topographic aspect, where the glaciated parts have to be treated separately. For running the model, the only required data are daily means of air temperature and precipitation. Daily runoff is needed for calibration.

The snow- and glacier-subroutine calculates terms of the snow- and ice-cover distributed for different elevation belts and aspect classes; the further steps of the model are performed on a lumped basis for the whole catchment area. The aggregational state (snow/rain) of precipitation

Table 18.1 Basin characteristics

Catchment 1

Catchment 2

Catchment 3

Catchment 4

Catchment 5

Name of the basin/area Mountain range Elevation range of entire catchment (m a.s.l.) Elevation range of glaciers (m a.s.l.) Latitude and longitude Area in km2 Geology

% glacierized Vegetation type

(dominant) % forested

Mean Q at catchment outlet (mm) Mean N (mm)

Tuyuksu Tien Shan 2450-4219

3415-4219

Alpine pasture

0 1012

1000

Abramov Pamiro-Alay 3580-4960

3625-4960

39°38' N/71°34'E 55.5 Complex (limestone, granodiorite) 51

No vegetation

0 1588

Glacier No. 1 Tien Shan 3695-4486

3700-4486

No vegetation

0 504

Vernagtbach

Alps 2635-3633

2765-3628

No vegetation

0 1801

1000

Rofental Alps 1893-3772

2400-3772

Para-gneiss, mica slate

Alpine pasture

0 1436

Air temperature Precipitation

Air temperature Precipitation

Snow- and icecover

Soil

Runoff formation

Figure 18.2 Schematic representation of the HBV-ETH model (based on Bergstrom 1976), (for explanations see Table 18.2)

Snow- and icecover

Soil

Runoff formation

Figure 18.2 Schematic representation of the HBV-ETH model (based on Bergstrom 1976), (for explanations see Table 18.2)

is determined with a threshold air temperature. The melting of snow and ice is calculated with the degree-day approach using a seasonally variable degree-day factor. In addition, glacier mass balance is determined for each elevation and aspect unit.

Table 18.2 Description of the free parameters in the HBV-ETH model

Parameter

Description

RCF

Rainfall correction factor

SCF

Snowfall correction factor

PGRAD

Precipitation gradient (%/100 m)

TGRAD

Temperature gradient (°C/100 m)

T0

Temperature divider (also general temperature

correction)

CMIN

Minimum degree-day factor on 21 December

(mm °C-1 day)

CMAX

Maximum degree-day factor on 21 June

(mm °C-1 day)

RMULT

Multiplicative factor to account for ice melt

REXP

Multiplicative factor to account for topographic

aspect

CWH

Water-holding capacity of snow

CRFR

Coefficient of refreezing

ETMAX

Maximum evapotranspiration on 1 August

(mm/day)

LP

Limit for potential evapotranspiration (mm)

FC

Field capacity (mm)

BETA

Coefficient to calculate outflow of soil moisture

storage

ko, ki, k2

Storage discharge constants

The sum of rainfall and meltwater output is then transferred to the soil moisture routine, a reservoir from which actual evapotranspiration is calculated as a function of potential evaporation and soil moisture storage.

In the last model component, the remaining water is transformed into the flow hydrograph. Three outflows with different response times are summed to yield total runoff at a daily time step. The calibration of the free parameters (Table 18.2) is done by a manual optimisation procedure, where the simulated hydrograph is compared with the measured discharge. To avoid a compensation of errors in the computation of basin precipitation by producing any desired glacier melt, it is helpful to compare calculated glacier mass balances with data observed in the field.

18.4 MODELLING RESULTS

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