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Recent fluctuation of glaciers in the eastern part of the Nepal Himalayas*

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ABSTRACT
   As a part of the Glaciological Expedition of Nepal (GEN), the fluctuations of the glacier termini of 14 glaciers were measured in the Dudh Kosi region, east Nepal, from 1970 to 1978. The results showed six retreating glaciers, four stationary glaciers, three advancing glaciers and one irregular glacier. Many glaciers havc been retreating in recent years. A glacier retreats when, at the terminus, the total annual ablation rate is larger than the total annual flux rate. The retreat of the snout of the Gyajo Glacier in 1976 is described in terms of the estimated mass balance of the terminus.

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Introduction

   A glacier inventory in the Dudk Kosi region, east Nepal, was made mainly on the basis of ground survey and aerial photogrammetry in 1974-1976, as a part of the Glaciological Expedition of Nepal (GEN) (Higuchi et dl., 1978) and it was found from comparison with the earlier inventory (~uller,1 970) that most glaciers in the upper part of this region were in retreat during the period 1960-1975 (Higuchi et al., 1980.

   Aerial observations were also made for mapping the present state of the glaciers and making the glacier inventory; the GEN had three flights, mainly to the Dudh Kosi region from 1974 to 1976 (Higuchi et al., 1976) and nine flights over most of the Nepal Himalayas in 1978 (Fushimi et al., 1980). A total of 5000 colour and 8000 black and white photographs were taken. These aerial photographs will be useful for checking the fluctuations of glaciers over a long period.

   Some results concerning the terminal fluctuations of the Gyajo Glacier (Glacier CB 480) in the Khumbu Himal, east Nepal, have already been reported (Fushimi, 1977a) and the data for the Gyajo Glacier are listed in the volume Fluctuations of Glaciers 1970- 1975 (~iiller, 1977).

   In the Karakoram and Pamir areas, surge related glacier advances have been reported (Desio, 1954; Suslov, 1960; Kick, 1962; Hewitt, 1969), while several glaciers in the western Himalayas have retreated in recent years (Ahmed, 1962; Jangpangi & Vohra, 1962; Kurien & Munshi, 1962; Tewari & Jangpangi, 1962). The present study shows the general trend of the recent glacier fluctuations in the Dudh Kosi region (Khumbu, Hongu & Hinku Himals), east Nepal (Fig. 1).


Glaciers surveyed and method of observations

   Moribayashi & Higuchi (1977) classified glaciers in this region into two types: debris-covered glaciers, namely large glaciers with the lower part covered by debris, which are called “D-type glaciers”, and debris-free or clean glaciers which are called “C-type glaciers”. It was shown from observations of glacier flow (Kodama & Mae, 1976) and structural studies (Fushimi, 1977b) that the lower part of a debris-covered glacier can be considered as an inactive ice body, having no direct relation to the present glacier flow. As the present active terminus of a D-type glacier is located in the middle part of the debris-covered ice body, it is very difficult to determine its exact position, and so the termini of C-type glaciers are more useful in studying the recent trends of glacier fluctuations.

   Higuchi et al. (1978) divided the glacierized area of the Dudh Kosi region into basins, designated A to H, and listed 664 glaciers. The present authors have made observations of the terminus fluctuations of 14 C-type glaciers: Glaciers CB 470 and CH 480 (Gyajo Glacier) in basin C, Glaciers EB 050, EB 060 (Dzonglha Glacierl, EC 020 (Lobuche Glacier), ED 010 (KOngma Tikpe Glacier), ED 020 (Kongma Glacier) and ED 580 (Chukhung Glacier) in basin E, Glacier GX 500 (a part of the Mera Glacier) in basin G, and Glaciers HX 200, HX 210. HX 220 (Hongu Nup Glacier), HX 340 and HX 350 in basin H (Figs 1 and 2).

   Most of these glaciers, for example the Gyajo Glacier, are small, having lengths of less than 4 km, and thicknesses of about 20-50 m. All except the Kongma Tikpe Glacier (a glacieret) are cirque type glaciers (Muller,1 977).

   Several base points are marked by paint on the bed rock near each glacier terminus. The distance between the base point and the glacier terminus was measured by tape along fixed directions at an interval of 1-3 years. The 19 conCrol points for the simple photogrammetry, have been set at both ends of the base line used for triangulation measurements of the glacier flow. Other observation points were set up in 1976 at five glaciers in basin H: HX 230, HX 240, HX 250, HX 280 and HX 050 (Naulekh Glacier) (Figs 1 and 2).

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Results and discussions

   Data of the characteristics of the 14 C-type glaciers (orientation, length, surface area, highest elevation, lowest elevation, average inclination and terminus fluctuation) are shown in Table 1. The details of the fluctuations of these glaciers have been previously described (Fushirni & Ohata, 1980). As seen from Table 1 and Fig. 1, six glaciers are retreating, four are stationary, three advancing and one irregular.

   Changes of the glacier terminus are caused by imbalance between the rate of glacier flow and ablation at the terminus. If the flow rate is larger than the ablation, the glacier terminus will advance. The glacier flow and ablation processes are influenced by climate and physical properties of the glacier ice. There are seasonal variations in both the flow rate and the ablation. The flow rate is greatest in May, at the beginning of the summer monsoon season, while ablation is highest in the middle of the summer monsoon season, especially in July (Muller,1968; Kodama & Mae, 1976). Since the changes of the termini reflect the effects of the seasonal variations of ablation and the glacier flow, it cannot be said whether a glacier is retreating or advancing unless the seasonal effects of these factors are eliminated by taking measurements at the same season after a long interval.

   A detailed study made of the relationship between the fluctuation of the terminus and the mass balance in the terminus area of the Gyajo Glacier, is described below.

   The Gyajo Glacier (CB 4801, located in the upper part of the Gyajo Valley to the north of Namche Bazar, has an average surface slope of 17O and crevasses are not well developed. Measurements at one point of the glacier terminus showed a retreat of 2.5 m a-1 from 1970 to 1973, and an advance of 2.0 m a-1 from 1973 to 1975. Measurements at six observation points showed a terminus advance of 0.11 to 4.00 m a-1 from 1975 to 1976 and a retreat of 0.91 to 2.93 m a-1 from 1976 to 1978. The change of terminus shape for these years is shown in Fig. 3.

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   The ablation rate at a debris-free site, Ao (5320 m elevation), near the terminus of the Gyajo Glacier (Fig. 3) was measured from 8 August to 21 September 1970. The results(mm day-1 of ice) are shown below:
(8 Aug.)   (19 Aug.)  (20 Aug.)  (23 Aug.)  (5 Sept.)  (21 Sept.)
18            10            13             5             6
The ablation rate varied from 18 to 10 mm day-‘ of ice in the middle of August and from 5 to 6 rpm day-‘ at the end of August and in September.Ageta et. al. (1980) obtained the following empirical relationship between the mean daily ablation, a (mm water per day), and the daily mean air temperature, T (OC):
a = O.1O(T + 3.0)3.2  (T -3.0)
The air temperature was measured from August to September 1970, at the terminus of the Gyajo Glacier (5230 m), and the ablation rate was calculated for the terminus area of the Gyajo Glacier from this formula. The measured and calculated ablation rates are shown below:

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   Since the agreement is good, we can use this formula to estimate the terminus mass balance of the Gyajo Glacier. The lateral ablation rate is assumed to be the same as the horizontal rate.

The ice velocity was measured at seven observation points in the terminus area of the Gyajo Glacier between 23 July and 21 August 1970 (Table 2). Though the flow velocity was not measured in May when the maximum was expected to occur, the annual centreline velocity (V) was taken as the August value of 11 m a-1 (0.03 m day-1) (Table 2 ) .

If the terminal shape of the Gyajo Glacier is considered as a rectangular parallelpiped, it is possible to estimate the balance between the total ice flux as estimated from the empirical formula of Budd & Allison (1975), and the ablation. This balance is shown in Table 2 and Fig. 4. The value of a glacier cross section shape factor (S1) can be obtained from Budd & Jenssen (1975). For the Gyajo Glacier, with a half width of 250 m and an estimated thickness of about 40 m, Sl=0.9. As Fig. 3 shows, the percentage surface slope (α) near the terminus is 16%. Hence, when α= 16% and V = 11 m a-1 then S1. α = 14%, and a centerline ice mass flux rate per unit width (φO) of 0.6 x l03 m2 a-1 and an ice thickness (Z) of 50 m are obtained from the nomograph of Budd & Allison (1275). The total ice mass flux (F) is given by F =Y V Z where Y and V are mean cross section width and velocity. Since φ = V Z,  where V is the maximum velocity, then F = S2 S3 Y φ(Budd & Allison, 1975); S2 = Y/Y = 1 for the assumed rectangular cross section of the Gyajo Glacier, and S3=V/V = 0.5. Hence,F = 1 x 0.5 x 500 x 0.6 x l03 = 150 x l03 m3 a-1.

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   The total ablation rate (A) has been calculated using a mean air temperature estimated from meteorological data at nearby Lhajung station (4420 m) for the period 1973 to 1976 (GEN, 1976 and 1978) and assuming a moist adiabatic lapse rate (0.5’C per 100 m). The ice velocity of the Gyajo Glacier was not measured between 1973 and 1976 but the yearly variation of ice velocities in the terminus area of several glaciers in this region (Kongma, Khumbu and Nuptse) were small over this period (Kodama & Mae, 1976; GEN, 1977) and hence the annual flux rate (F) was assumed to be the same as in 1970. (F)+(A) in Fig. 4 indicates the annual mass balance between the ablation rate and flux rate at the terminus. The balances are slightly negative during the period from 1973 to 1975 when the glacier advanced, and it is not possible to explain the glacier fluctuation in terms of the estimated balance because of errors. Errors in the total ablation arise from (a) estimating ablation from air temperature,(b) estimating the Gyajo temperature from the Lhajung emperature, (c) assuming that lateral ablation is the same as the horizontal, and (dl taking a rectangular parallelpiped as the terminal shape of the Gyajo Glacier. Budd & Allison (1975) estimate an expected error of ±25% for the total mass flux estimated from their nomograph, and we have also assumed that this flux is constant for several years.

   However, as seen in Fig. 4, the terminus mass balance is significantly negative in 1976 when the Gyajo Glacier began to retreat. So, it can be said that the glacier retreated when the total annual ablation rate was larger than the total annual flux rate.

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ACKNOWLEDGEMENTS

   The 1970 observations were carried out as a part of the Japanese Skiing Expedition to Mt Sagarmatha (Chomo Lungma, Everesti, and the observations from 1973 to 1978 as activities of the Glaciological Expedition of Nepal. The present authors express their thanks to members of these expeditions. We thank Messrs Y. Ageta, K. Ikegami, K. Yokoyama and S. Iwata for their assistance wlth these observations and express our appreciation to many Sherpa people for their help in the field.

REFERENCES

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* Glaciological Expedition of Nepal, Contribution no. 78.
Sea Level, Ice and Climatic Change (Proceedings of the Canberra Symposium, Dec. 1979), IAHS Publ. no. 131.
Fushimi H., Ohata T. and Higuchi K.
Water Research Institute, Nagoya University,
Nagoya 464, Japan