The Influence of Climate Change and Climatic Variability on the Hydrologie Regime and Water Resources (Proceedings of the Vancouver Symposium, August 1987). IAHSPubl. no. 168, 1987.

The glacial and hydrological regime under climatic influence in the Urumqi River, northwest China

Yao Tandong Lanzhou Institute of Glaciology and Geocryology, Academia Sinica, Lanzhou, China

ABSTRACT The glacial and hydrologie fluctuations and their distinct features under climate influence in the Urumqi River are analysed.

The glacial and hydrologie fluctuations in the Urumqi River have the same pattern in which high discharge and positive mass balance periods are mainly identical with cold periods, low discharge and negative mass balance periods identical with warm periods. A particular temperature-wetness pattern, which could be explained by prevailing general atmospheric circulation patterns is responsible for the pattern. Distinct features between glacier and discharge under climatic influence are distinguished. Glacial terminal fluctuates as a slow process with low frequency while discharge fluctuates as a rather fast process with high frequency in the basin, which results from the time lag difference between glacier and discharge responding to climatic fluctuation.

Variations glaciaires et hydrologiques sous l'influence climatique dans le bassin de la rivière d'Urumqi, dans le Nord-ouest de la Chine

RESUME Les variations glaciaires et hydrologiques ainsi que leurs caractéristiques distinctes on été analysées sous l'influence du climat dans le bassin de la rivière d'Urûmqui.

Les variations glaciaires et hydrologiques dans ce bassin ont la même tendance: les périodes de haut débit et de bilan de masse positif sont principalement identiques à la période froide, alors que les périodes de bas débit et de bilan de masse négatif sont identiques à la période chaude. Ceci qui dépend principalement du modèle particulier température humidité, qui peut être expliqué par la circulation atmosphérique régnant dans le Nord. Sous l'influence du climat, les variations de la glacière et du débit dans cette région ont des différences évidentes: la glacière terminale fluctue avec un dévelop­pement lent et une fréquence basse, tandis que le débit fluctue avec un développement rapide et une fréquence haute, ce qui resuite de la difference de retard de temps

367

368 Yao Tandong

entre la glacière et le débit à l'égard de la fluctuation climatique.

Introduction

Fluctuations of glacial terminal and discharge are generally related to climatic fluctuations. The mass budget process of glacier and discharge is dependent on heat balance and precipitation features which are related to climate. Some relationships between glacier, discharge and climate are discussed in the present paper by taking the Urumqi River as an example.

The Urumqi River originates from the eastern part of the Tianshan Mountain in the southern part of Urumqi. The river basin has an area of 924 km2 above Yingxiong Bridge Hydrologie Station which is the main station measuring the outlet discharge, and an area of 46 km2 of glaciers at the head area. The météorologie record started in 1940 at the Urumqi Station in the lower reaches of the basin, and in 1958 at the Da Xigou Station in the upper reaches. The hydrologie record started in 1950 but it can be extended to 1940 by interpolation. The observations and records of glaciers in the basin started in 1958. The Urumqi River basin is a comprehensive one in earth science studies in northwestern China, which is suitable for a comprehensive study of the relationship between glacier, discharge and climate.

The fluctuations of glacier, discharge and their relationship to climate and general atmospheric circulation in the Urumqi river

The study of fluctuations in glacier and discharge in the basin was limited to the period since the Little Ice Age during which more data are available. The Little Ice Age started in the late sixteenth century and ended in the early twentieth century in the basin according to Chen (personal communication). In Glacier No. 1, the change in glacial area, glacial thickness, glacial length and volume, and in glacial equilibrium line fluctuation were estimated based on detailed glacial maps and radar sounding data (Table 1). Glacier No. 1 has decreased by 21% in length, 33% in area and 38% in volume from the maximum of the Little Ice Age to present. A similar feature was found in the fluctutations of all the glaciers in the basin. It has been estimated that there was a decrease of 41% in glacial volume and 30% in glacial area since the Little Ice Age in the whole basin.

Table 1 Changes in Glacier No.l since the Little Ice Age

Length Area Thickness Volume Equilibrium line (km) (km") (m) (10*m') (m)

Present 2.33 1.84 49 9000 4050

the Little Ice 2 9 8 2^3g 6 Q 1 4 œ 0 3 8 g Q

Age maximum

Glacial and hydrologie regime under climatic influence 369

There are three possibilities which could cause glacial advance in the Little Ice Age in the basin: (a) drop in temperature; (b) increase in precipitation; (c) combination of (a) and (b). There was certainly a drop in temperature on the whole hemisphere in the Little Ice Age, which must have influenced the basin. The temperature drop in the Northern Hemisphere in the Little Ice Age was about 1.0°C in annual average, 0.5°C in summer according to Flohn (1981), Lamb (1977) and Schuurman (1981). It was estimated by Chen (personal communication) that the temperature drop on Glacier No. 1 was about 0.6 C in the Little Ice Age. The annual temperature drop was no more than 1°C on Glacier No.l if the smaller amplitude in climatic change in the mountain area in the basin and the results in other regions are considered.

According to the calculation of the relationship between precipitation, temperature and equilibrium line in Glacier No. 1 in the basin, a rise of 80 m in equilibrium line could be caused either by an increase of 1°C in temperature or a decrease of 100 mm in precipitation. The equilibrium line in the Little Ice Age (3890 m) was about 160 m lower than that at present (4050 meter). It estimated by using the above relationship that the precipitation Glacier No. 1 during the maximum of the Little Ice Age is about

larger than that at present. Provided that the temperature

was on 100 is

1.5WC lower during the maximum of the Little Ice Age than at present, an increase of 50 mm in precipitation was estimated. Because the estimate of temperature drop in the Little Ice Age was based on proxy data from other areas, the above values are not the exact estimation of the precipitation in the Little Ice Age. But it at least revealed a trend: not only was there a temperature drop but also a precipitation increase, characterized by cold and wet conditions, during the Little Ice Age in the basin. To support the above proposed relationship, three sets of tree ring data are analysed from the basin and nearby regions (Table 2).

Table 2 The relationship between temperature and precipitation revealed by the tree rings in the basin and nearby regions

Place

UrttiKri River

A l t a i

Hami

Average

Total year in a tree

(years)

384

158

252

265

Lew temperature with high precipitation

(years)

152

32

58

81

High temperature-with low precipitation

(years)

142

68

118

109

High temperature with high precipitation

(years)

30

32

52

38

Low temjjerature with low precipitation

(years)

60

26

24

37

Percentage of high V' with low P" and low T with high P

(%)

77

63

70

70

Percentage of high T with high P and low T with low P

(%)

23

37

30

30

370 Yao Tandong

Two points should be kept in mind when estimating the discharge in the basin during the Little Ice Age. The first is that the temperature then was lower than at present; the second point is that it is wetter then than at present, based on the above analysis . Because it is warmer and drier at present than in the Little Ice Age, 1976 (a year with the lowest temperature and highest precipitation since meteorological data were recorded) was selected to estimate the discharge in the Little Ice Age. In 1976, the temperature was 0.6°C lower and the precipitation 46.2 mm higher than the secular average at Da Xigou Station in the upper reaches of the basin; the tempera­ture was 0.2°C lower and the precipitation 19.4 mm higher at Urumqi Station in lower reaches of the basin. The discharge at Ying Xiong Bridge Station that year was 280 670 000 m3 which was 40 000 000 m3

higher than the secular average. The discharge increased by 17% in the Little Ice Age than at present (or decreased by 14% at present than in the Little Ice Age) if taking the value of the discharge in 1976 as an estimation of the discharge in the Little Ice Age.

During the period with observations and records in meteorology, glaciology and hydrology, the glacial mass balance, glacial equilib­rium line and discharge in the basin basically kept the same trend and experienced cycles. However, the glacial loss and discharge decrease trend is obvious during the same period. A comparison of two high discharge periods between the 1970's and the 1950's indicates that the total discharge in the 1970's was 148 million m3

smaller than that in the 1950's, demonstrating a decreasing trend in discharge. In Glacier No. 1, the average mass loss is 155 thousand m3

in water equivalence from 1959 to 1980 and the equilibrium line rise 10-12 m in the same period. The trends of the discharge and glacier decrease are related to climatic change in the basin. The average ten year temperature from the I960's to the 1970's has increased by 0.3°C in Urumqi, by 0.2°C in Xiao Quzi and by 0.1 C in Da Xigou. The average ten year precipitation at Da Xigou Station which is close to glacier No. 1 has decreased by 7.2 mm from the 1960's to the 1970's. During the period in which recorded data are available, the relation­ship between the fluctuations of glaciers and discharge and climatic change is identical with that in the Little Ice Age. Although there are different temperature-wetness patterns (warm-dry, warm-wet, cold-dry, cold-wet), the dominant patterns were warm-dry patterns and cold-wet patterns. The analyses of the moving curve and anomalies in spring and summer temperature and spring and summer precipitation indicate that the period of high temperature-low precipitation (warm-dry pattern) and low temperature-high precipitation (cold-wet pattern) is 76% of the whole period analysed in the upper reaches and 63% of the whole period analysed in the lower reaches. The period of high temperature-high precipitation (warm-wet pattern) and low temperature-low precipitation (cold-dry pattern) is 24% and 37% respectively in the upper reaches and in the lower reaches.

It seems that the fluctuation features of glacier and discharge are dependent on the temperature-wetness feature in the basin. Nevertheless, the temperature-wetness pattern in the basin could be explained by changes of the general atmospheric circulation. The general atmospheric circulation in the Eurasia continent was classified into three types by Jiersi (1974). According to studies, precipitation would decrease and temperature rise in most regions of

Glacial and hydrologie regime under climatic influence 371

middle latitude when W type prevails, there would be more opportuni­ties for high temperature and low precipitation in most regions of middle latitude when C type prevails, there would be low temperature and high precipitation in most regions of the Eurasia continent when E type prevails. In the Uriimqi River, the general trend of fluctua­tions of climate and discharge is basically identical with the general atmospheric circulation in the Eurasia continent (Table 3).

Table 3 Relationship between climate, discharge and general atmospheric circulation during different periods in the Uriimqi River

Temperature anomaly Discharge fluctuation

n . . Circulation Prec ipitat ion v. ,. v. ., , Period -, Xiao ,, „ . Da ïing Northern v. ..

type anomaly ,, . UrUmqi v. v. ? ,. v. .. Xtniian J Quzi ALgou Xiongqiao or Xinjiang

1941-1950 C -7.3

1951-1960 E +12.9 - 0 . 2 - 0 . 4

1961-1965 C -4.5 +0.3 +0.7 +0.1

Low discharge

High High High discharge discharge discharge

Low Low Low discharge discharge discharge

1966-1972 E +6.0 -0.4 -0.1 -0.2 ,. L ° W " ^ " ^ discharge discharge discharge

In the basin, E type generally corresponds to positive precipita­tion anomaly, negative temperature anomaly and a high discharge period; W and C type generally correspond to negative precipitation anomally, positive temperature anomaly and a low discharge period.

Distinct response between glacier and discharge under climatic influence in the Uriimqi River

According to what is discussed above, the glacial volume has decreased by 40% and the discharge has decreased by 14% from the Little Ice Age to present in the Uriimqi River basin. It means that the decrease rate of glacier is larger than that of discharge under the same background of climatic warming, which results from the distinctions in energy balance between glacier and discharge.

The input and output process in glacial system is expressed by the mass balance equation

B = P g - M - E g + L H : D + ; A (1)

(where Pg stands for annual precipitation on glacial the surface, M for glacial surface melting, Eg for glacial surface evaporation, L for glacial condensation, D for snow drifting and A for avalanche).

372 Yao Tandong

The input and output process in the discharge system is expressed by the water balance equation

P, = R + E, + AW + F d d —

R E, + AW d - F (2)

(where R stands for discharge, P-, for annual precipitation, EJ for evaporation, AW for underground water storage and F for seepage;. To reveal the essential features of the input and output process of mass, a simplification to equation (1) and (2) is necessary.

According to heat balance curves by Budyko (1974), the evapora­tion process in the basin should belong to the continental climate type in middle latitudes. The feature of the process at this latitude is that evaporation is maximum in summer, dramatically changeable in spring and autumn, and negative in winter. Summer evaporation could therefore approximately substitute for annual evaporation. The rain season is during the summer in the Urumqi River basin, the intense evaporation season is also an intense condensation season in the basin. From observations on some glaciers in Central Asia (Lvovich, 1975), evaporation and condensation are almost equal in summer. Therefore, E and L in equation (1) could be omitted. Avalanche influence in the studied area could also be omitted. As a secular process, the amplitude of annual change of snow drifting is not so

great and could be taken as a constant Equation (1) then could be simplified as

(D W_ for example).

B •g M + W„ (3)

There are only two variables P~ and M in the equation. As a secular process, F and in equation (2) are also relatively

stable and could approximately be taken as a constant (F + W = Wc, for example). Equation (2) could then be simplified as:

R pd " Ed + Wc (4)

There are also two variables, P-, and E, . in equation (4). Equations (3) and (4) demonstrate that the input of glacier and

discharge are of the same form, but the output of these two systems are essentially distinct: by melting in the glacial system and by evaporation in the discharge system.

The mass output of both glacier and discharge is, in physical essence, the result of the input of heat. The main components of heat input are radiation balance (R), latent (L) and sensible heat (S). The heat balance equation in the glacial system could be expressed as:

% m \ + h + (5)

The heat balance equation in the discharge system could be expressed as :

% = Rd + Ld + Sd (6)

From equation (i) and (4) the heat (Q) absorbed in the glacial system is mainly consumed in glacial surface melting, therefore

Glacial and hydrologie regime under climatic influence 373

Qg * M(Q) (7)

The heat (Q) absorbed in the discharge system is mainly consumed in evaporation, therefore

Qd - E(Q) (8)

Using latent heat of melting (80 cal.g ) and latent heat of evaporation (597 cal.g ), equations (7) and (8) could be expressed as :

M = Qg/80 (9)

E = Qd/597 (10)

Substituting equations (9) and (10) for the second item on the right side of equations (3) and (4) respectively, then

B = P - Qg/80 + W c (11)

R = P - Qd/597 + W c (12)

Equations (11) and (12) indicate that the response of the input and output process of mass to climate (actually to heat balance) is much more sensitive in glaciers than in discharge. Besides, evaporation in the discharge system only consumes part of the heat absorbed, the denominator in equation (12) is relatively enlarged. So, the response of water balance to climate is more sluggish than that expressed in equation (12).

The heat balance in the glacial system is, however, smaller than that in the discharge system, and the mentioned distinction has been weakened in some degree. But it is still evident according to the results from Glacier No. 1 and nearby area. Taking the average secular melting value (214 cm) at 3870 m a.s.l. on Glacier No. 1 and evaporation value (53 cm) at Da Xigou Station near Glacier No. 1, corresponding melting latent heat of 17 000 000 cal.cm- a- and evaporation latent heat of 32 000 000 cal.cm a were obtained. This is to say that although the heat consumed in discharge evaporation is two times that in glacial melting, mass loss in discharge is only 1/4 of that from the glacier. It can be deduced from the above discussion that the fluctuation amplitude of the glacier is larger than that of discharge in the past under the influence of climatic change in the Uriimqi River basin.

There are obvious distinctions between the glacier and discharge in the basin under modern climatic influence. One of the distinctions is that a glacial terminal responds to climatic change as a slow process with low frequency while discharge responds to climatic change as a fast process with high frequency. Taking Glacier No. 1 as an example, the glacial terminal maintains its retreating trend since observations in the late 1950's. But the discharge in the basin experienced high and low discharge cycles during the same period, which is basically identical with the climatic fluctuation in short period and therefore rather sensitive to climatic change. Because temperature in the basin shows a warming

374 Yao Tandong

trend in this century, it seems that the retreating trend of the glacial terminal is identical with the warming trend in climate and the glacial terminal is a rather stable indicator for the secular climatic trend. The reason for the distinctions could be explained by time lag differences between glacier and discharge in their response to climate.

The time lag of the glacial terminal ranges from several years (small mountain glaciers) to decades (larger mountain glaciers) (Nye, 1958, 1965) or even thousands of years (Budd and Smith, 1979). The time lag in the discharge process is much shorter, ranging from several hours to several days or 10's of days. This demonstrates that the discharge system could reflect climatic influence immediately in the lower reaches of a river, but the glacial terminal could require a rather longer period to reflect it. Many factors affect glacial time lag. The main factors are mass balance, which mainly depends on precipitation and temperature, glacial size and slope, glacial velocity and glacial temperature, if surging glaciers are neglected. There is no pattern to follow governing the importance of these factors. It is, therefore, difficult to find a model including all these factors and suitable for all types of glaciers. Glacial length is, however, one of the most important factors responsible for glacial time lag and is easy to obtain. A model could be established by using glacial length to estimate approximately glacial time lag in different sizes. Based on the statistical analysis of 35 "normal" (ie. non-surging) glaciers in the Northern Hemisphere, a simple model was established as

T - 13L°-375

(where T stands for the glacial time lag, and L for glacial length). The relationship factor for the model has a significance of 95%. Because only glacial length L was introduced in the model, the time lag estimations are approximate values. It was estimated from the model that the time lag for Glacier No. 1 may be 18+5 years.

The time lag of the discharge in the basin is different in different seasons, but depends mainly on the character of high water season of a year. The high water season in the Urùmqi River could be classified as spring and summer. Summer high water season is characterized by precipitation discharge with a short time lag. It was calculated from the recorded data at an experimental discharge station in the Urumqi River, that the peak flood is 7-8 hours after peak precipitation from Hou Xia to Ying Xiongqiao. According to this, the time lag from the head to the lower reaches of the river is about one day. Spring high water season is characterized by snow melting discharge and is rather complicated. Generally, the amount of spring high water reflects the amount of precipitation during the last winter or even previous autumn plus the heat absorbed in the mountain area in spring. The time lag could therefore be several months. Although this is the result of interruption by other factors, it is still a feature in the discharge forming process. In addition, time lag of discharge also changes with climate at different altitudes. Usually, summer precipitation in the middle and lower parts of the mountain is in the form of rainfall and could be reflected immediately in the lower reaches, while the summer

Glacial and hydrologie regime under climatic influence 375

precipitation in the high mountain area is mainly snow and needs a longer time to be reflected in the lower reaches. In conclusion, the time lag of discharge in the basin ranges from several hours to several months from the lower reaches in summer to the upper reaches in spring.

If 10 years represents the order of magnitude of the time lag of the glacial terminals of most glaciers in the basin and 1-2 days represents the time lag of discharge from the head to the lower reaches of the basin, the time lag of the former is several orders of magnitude longer than that of the latter. Even taking several months as the time lag of the discharge in the basin, the time lag of the glacial terminal is still one order of magnitude longer than that of discharge. This partly explains why the glacial terminal responds to climate in low frequency while discharge responds to climate in high frequency.

The future trend of the glacier and discharge in the Uriimqi River

Based on the discussion above, the glacial and discharge fluctuations in the basin are controlled by climatic change. The forecasting of the glacial and discharge fluctuation is only possible if the trend of the climatic change is forecast. But climatic change is still a subject which is beyond the forecasting ability of man. Most climatic forecasts only propose possibilities. This is also the case in the Uriimqi River. Based on tree ring analysis and forecasting of the general atmospheric circulation by Jiersi (1974) and the relationship between the climatic change in the basin and general atmospheric circulation, a possibility of the future climatic trend in the basin was estimated. It was forecast by Jiersi (1974) that W+E circulation would prevail from 1979 to 1986, and W+C prevail from 1986 to 1996. A possible climatic trend accompanied with the circulation is that the precipitation from 1979 to 1986 would fluctuate near its secular average and that the precipitation from 1986 or so would decrease and last to the late 1990's. The decrease in precipitation may have started in 1985. Two conclusions could be made based on the tree ring data: (a) the present is a dryer period than the Little Ice Age; (b) this trend would continue until the 2020's-2030's. A wet period of several years may appear between the late 1990's and the early 2000's. The temperature rise in the next 10-15 years is probably 0.1-0.6°C if the temperature in the basin keeps the rising trend at present.

In recent years, the C02 greenhouse effect has been much discussed by Manabe & Stouffer (1980) and Manabe and Wetherald (1981). It was estimated by them that the average global temperature would increase by 2-3°C because of the increase of C02 and other gases in the atmosphere in the next 50 years. A temperature increase of 1.5-4.5 C in the next 50 years was announced at the Villach Conference in 1985. There was not much discussion about the potential temperature increase in the next 10-15 years. It may be still small before the twenty-first century and an estimation of 0.5°C might be suitable because there is a time lag from C0„ content increase in the atmosphere to actual temperature rise. The time lag would be even longer if the influence of oceans is considered.

376 Yao Tandong

A rise of 0.5-1.0°C in temperature would be possible in the next 15 years if taking 0.5°C as the temperature increase caused by CO2 and provided that the natural climate keeps its present warm trend. Assuming climatic warming in the next 15 years occurs with a range from 0.5 to 1.0°C, the forecast for glaciers and discharge around the year 2000 could be made (Table 4).

Table 4 Estimation of changes in glacier and discharge under the condition of temperature rise of 0.5-1.0°C in the Urumqi River around 2000

Temperature Precipitation Altitude of Glacial Discharge Discharge State in Glacial Per iod inc rease dec rease glacial equi i i - mass l)alaiice of the Glacier glacial tJûjinirig

( aC) (mm) briun 1 inu (mm) River No. 1 terminal (m) (m) (106m3) (106ni3)

1985-2000 0 . 5 - 1 . 0 1 0 - 3 0 4 0 7 0 - 4 1 0 0 -100 300 22 .1-23 .7 1 .5-2.1 retreating 2 - 8

Conclusions

The following conclusions were made from the above discussion: (a) The glacial and discharge fluctuations in the Urumqi River

basin are related to climatic change from the Little Ice Age to present, which were demonstrated to be glacial mass balance and discharge decrease when the climate becomes warmer; glacial mass balance and discharge increase when the climate becomes colder.

(b) The glacier and discharge fluctuations in the basin are related to general atmospheric circulation through climate. It is advantageous to glaciers and discharge in the basin when an E type of circulation prevails, but it is not advantageous to them when W+C types of circulation prevail.

(c) Under the influence of a warming climatic trend since the Little Ice Age, glacier and discharge in the basin have both decreased, 41% in glacial volume and 14% in annual discharge. The rate of glacial decrease is larger than that of discharge decrease, which is the result of the distinction in energy balances between glacier and discharge.

(d) Glacial and discharge fluctuations possess different climatic significance because of their distinctions in time lag and other processes. Glacial terminal fluctuation (a slow process with low frequency) could be taken as a relatively stable indicator for secular climatic change, while discharge fluctuation (a rather fast process with high frequency) could be taken as a relatively sensitive indicator for climatic fluctuation in a short period.

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