22 Water Discharge of Imja Khola in Khumbu Himal

Water Discharge of Imja Khola in Khumbu Himal*

Keiji Higuchi**, Yutaka Ageta*** and Hideo Kodama**

Abstract

Observations of water discharge from a drainage area of 142.5 km2 including glacier zones were made at (4330 m) in east . The annual discharge of this drainage was

estimated as 1.7 •~ 108m3. The annual variation of the discharge showed a tendency to increase

in summer and decrease in winter. It is assumed that precipitation during the monsoon season

strongly contributes to high discharge in summer. On the basis of the data of ablation of glaciers

and precipitation in this region, preliminary estimates of water balance in the drainage during a

year were made. Since the variation of daily mean air temperature at Lhajung near Dingboche had a good correlation with the vatiation of daily discharge in summer but not in winter, the contribution of melt water from glaciers to the variation of the discharge was also discussed on

the basis of the air temperature over glaciers estimated from the monthly mean temperature at

Lhajung. Diurnal variations of the discharge in different months showed periodic tendencies

which might be due to the diurnal variations of discharge of melt water from glaciers. The seasonal change of this tendency can be explained by the change of air temperature. Monthly

discharge in depth of runoff from this drainage was compared with that at a point 460 m in

altitude, in the lower part of this drainage. The latter was nearly twice as much as the former during the monsoon season, since precipitation in the upper drainage during the monsson season

is less than below.

this figure. The area of this drainage is as 1. Introduction follows. Studies of water discharge in a river basin are Area of the whole drainage (A1) 142.5 km2 important for the assessment and a rational use Area of ice and snow zones free from debris of water resources. The water discharge in (A2) 42.9 km2 Nepal is believed to contain a large contribution of melt water from the glaciers in the . Therefore, observations of the water discharge of the Imja Khola (river) in Khumbu Himal were made at Dingboche near Lhajung station (4420 m) from June 1974 by the Glaciological Expedi- tion to Nepal, as will be described in this paper.

2. Observations of water discharge The drainage basin of the Imja Khola is shown in Fig. 1. Observations were made of the discharge from the 'upper Imja drainage' seen in

* Glaciological Expedition to Nepal , Contribution No. 8 ** Water Research Institute , Nagoya University, Fig. 1. Drainage basin of the Imja Khola. Nagoya 464 note: (1) Upper Imja Drainage. *** Water Research Institute , Nagoya University (2) Observation point of water discharge. ( present address, Faculty of Education, (3) glacier; debris free part. Yamaguchi University, Yamaguchi 753) (4) glacier; debris covered part. Seppyo, 1976 K. Higuchi, Y. Ageta and H. Kodama 23

use of a currentmenter were made on. January

24, 1975. The amount of water discharge was

calculated from the mean flow velocity and the

cross sectional area of the river.

Since the flow of the Imja Khola is turbulent,

the accuracy of the measurements is limited, and

the data are not available during some periods.

But some analyses of the observational results are

performed in order to obtain the outline of the hydrological regimen in this region as a basis for

further studies.

3. Annual variation of water discharge Fig. 2. Observation point of water level of Imja Khola. 3.1. Annual amount of water discharge

Daily values of the water discharge of the Imja

Area of ice and snow zones covered with debris Khola at Dingboche are shown by depth of (A3). 21.8 km2 runoff from the drainage in Fig. 3, together with

The altitude of the point where water discharge precipitation and air temperature data at Lhajung. was observed at Dingboche is 4330 m above sea The data from June 17 to December 31 were level, and a photograph of it is shown in Fig. 2. obtained in 1974, but those from January 24 to

Water level was measured by self-recording June 11 were obtained in 1975. The values gauge shown in Fig. 2. The maximum flow during the periods when the data were unavailable velocity of the water surface (Vs) was obtained were interpolated directly from two neighbouring by measurements of the speed of wooden chips results and shown by dashed lines in Fig. 3.

floating on the central part of the river. The When the annual variation of the discharge is mean flow velocity of the cross section (V,) was assumed to be as in Fig. 3, the annual volume calculated from V, by the use of the relation of the water discharge can be estimated to be

Vss=0.8 V,. Such measurements were made on about 1.7 •~ 106m3, which corresponds to the total June 13, July 11 and August, 5, 1974. In addi- runoff of 1200 mm. It can be seen in Fig. 3 tion, measurements of the flow velocity with the that the water discharge of the Imja Khola was

Fig. 3. Water discharge of the Imja Khola at Dingboche, precipitation and air temperature at Lhajung.

note: (1) Daily discharge shown by depth of runoff (mm) from 17 June 1974 to 11 June, 1975.

(2) Mean daily precipitation for every ten-day period (mm) in 1974.

(3) Daily mean temperature (•Ž) in 1974. (4) Monthly mean temperature (•Ž) in 1974. 24 Water Discharge of Imja Khola in Khumbu Himal

high in summer and low in winter. precipitation at Lhajung in 1974 was 0.53 m, the 3.2. Precipitation and water balance ratio between this value and that above is 1.2/

The mean daily precipitation at Lhajung for 0.53, namely, 2.3. Precipitation at 25 points in

every ten-day period from January to December this drainage was measured during the monsoon

1974 is shown in Fig. 3. It can be seen in this season in 1974. The ratio of precipitation at

figure that the monsoon precipitation contributes these points to that at Lhajung during the same

greatly to the increase of water discharge in period is 0.7 (Ageta, 1976). However, this value summer as mentioned above. is not the average in this drainage, since the

As described in the previous section, the annual locations of these observation points were at the

water discharge of the Imja Khola at Dingboche bottom of the valley and under 5400 m in

was 1.7 •~ 108m3, which can be divided into the altitude. The ratio of the mean annual precipita-

discharge of rain water and snow melt water tion in this drainage to that at Lhajung could

from the non-glaciated area and that of melt be much more than 0.7, since precipitation

water and rain water from the glaciers. The increased corresponding to increase of altitude in

annual amount of melt water from the glaciers the interior of the main range of the Himalayas

can be estimated to be 0.5 x 108m3. In this esti- as shown by Ageta (1976).

mation we assume that the ablation area of glaciers For the quantitative discussion on the water

in this area is approximately equal to A3 in the balance in the drainage it is necessary to measure

previous section, namely 21.8 km2, estimated by the precipitation at high altitude and the mass

Moribayashi (1974), and the mean annual abla- balance of glaciers more accurately. tion is 2.3 m which corresponds to the average 3.3. Air temperature and melt water from

of two values, namely, 459 cm as maximum in glaciers water equivalent and 0 cm as minimum, estimated Daily mean and monthly mean air temperature

by Inoue (1976) as the annual ablation in the at Lhajung in 1974 are shown in Fig. 3. As ablation area of in the drainage seen in this figure, the variation of daily mean neighbouring to upper Imja drainage. If the temperature in summer has a good correlation mass balance of glaciers is in equilibrium, this with the variation of daily discharge, but this amount of melt water would be nearly equal to relation disappears in winter. This means that the total value of annual accumulation in the melt water from glaciers made some contribution accumulation area of glaciers which can be to water discharge of the river in summer. assumed to be nearly equal to A2 in the previous Therefore, the area of glaciers where melting section, namely 42.9 km2. Under such an assump- occured in each month was estimated as follows. tion, the mean annual accumulation is estimated Air temperature at the lowest glacier terminus to be 0.5 •~ 108m3/42.9 •~ 106m2= 1.2 m. at 4800 m in the drainage can be estimated from

If the annual precipitation in the ablation area that at Lhajung by the use of the lapse rate of is also assumed to be 1.2 m, the annual discharge 0.6•Ž/100 m. From the monthly mean tem- of rain water from the glacier is estimated to be perature at Lhajung in Fig. 3, it was estimated 21.8 •~ 106m2 •~ 1.2 m= 0.3 •~ 108m3. Therefore, the that the air temperature at the lowest glacier annual discharge of rain water and snow melt terminus was above 0•Ž during the period from water in the non-glaciated area becomes (1.7 - June to October. In July and August when the

0.5 - 0.3) •~ 108m3 = 0.9 •~ 108m3. Since this area monthly mean temperature at Lhajung is 6.2•Ž , is A1- A2- A3= 77.8 •~ 106m2, the annual precipi- the area of glaciers above the melting point is tation in this area is estimated to be 0.9 •~ 108m3/ 20 km2 in the area covered with debris and 10 77.8 •~ 106m2= 1.2 m. km2 in the area free of debris, for a total of 30 km2

As seen in the estimation described above, it is or 20% of the drainage area. But in September reasonable to consider that the water balance in and October, the area of the glaciers where the the drainage of the Imja Khola is nearly in air temperature is above the melting point balance and the mass balance of glaciers is nearly decreases to less than 5% of the drainage area . in equilibrium, when the annual precipitation in Therefore, melt water from glaciers seems not to this drainage is 1.2 m and the bulk of it is made a contribution to the water discharge in discharged at Dingboche. Since the annual winter. Seppyo, 1976 K. Higuchi, Y. Ageta and H. Kodama 25

Glacier termini in the Imja Khola basin are on October 27. At Lhajung, snowfall of 2 mm, located at higher altitudes than those in many 1 mm and 7 mm occured on October 22, 23 and other regions in the Nepal Himalayas (Watanabe, 24 repectively after 14 days of no precipitation, 1976). Consequently, more contribution of melt and the daily mean air temperature decreased water from glacier to water discharge can be below 0°C after October 26. Therefore, the expected there than that described above. diurnal pattern of November is considered to be due to the melting of snow in the daytime in 4. Diurnal variation ofwater discharge the area near the point where the discharge was Averaged results of diurnal variations of water observed. discharge in different months are shown in Fig. 4. It can be seen in this figure that the discharge 5. Comparison with water discharge of the Dudh Kosi has the minimum at noon and the maximum at 21 hours during the period from June to As shown by depth of runoff in Fig. 5, the October, while the diurnal range is small in monthly discharge from the upper Imja drainage October. This tendency can be seen in almost was compared with that of the 6-year mean of all daily records of water level during these the Dudh Kosi (river), which is the trunk river months. Such a periodic tendency may be due of the Imja Khola. The discharge of Dudh Kosi to the diurnal variation of the discharge of melt was measured at Rabuwa Bazar by His Majesty's water from glaciers in the area distant from the Government of Nepal (1967-1973). The location observation point of discharge, since precipitation of Rabuwa Bazar is at an altitude of 460 m and did not show such a periodic diurnal variation. a distance of 70 km south of Dingboche. This Decrease of the range of diurnal variation of the drainage covers an area of 4100 km2 including a discharge in October may be due to the decrease large area free from snow and ice. The upper of melt water caused by lowering of the tem- Imja drainage is situated in the northeast corner perature in this season, as described before. of this drainage basin. As seen in Fig. 4, the pattern of diurnal varia- It can be seen in Fig. 5 that the discharge at tion in October with a minimum at noon changed Rabuwa Bazar is nearlly twice as much as that suddenly to the pattern with maximum discharge at Dingboche during the monsoon season. The at noon in November. Such a change occured reason for this tendency is as follows. In east Nepal, precipitation during the monsoon season

Fig. 5.Monthly discharge of the Dudh Kosi at Rabuwa Bazar and the Imja Khola at Dingboche, shown by depth of runoff. (Results at Rabuwa Bazar are the 6-year mean of 1964 and 1967-1971, Fig. 4. Averaged diurnal variations of water His Majesty's Government of Nepal, 1967- discharge in different months. 1973). 26 Water Discharge of Imja Khola in Khumbu Himal in the higher northern area is less than that in water from glaciers and explain the hydrological the lower southern area as reported by Muller regimen in the Nepal Himalayas. (1958/1959). Therefore, the discharge at low altitude such as at Rabuwa Bazar (460 m) is more References than that at high altitude such as at Dingboche Ageta, Y. (1976): Characteristics of precipitation (4330 m). during monsoon season in Khumbu Himal. in this issue. 6. Concluding remarks His Majesty's Government of Nepal (1967-1973): Surface water recorcs of Nepal. supplement No. As described at the end of section 3.3 and in 1-6, Kathmandu. section 5, it can be said that the characteristics Inoue, J. (1976): Mass budget of Khumbu glacier. of the water discharge are considerably different in this issue. between the upper Imja drainage and others. Moribayashi, S. (1974): Nepal Himalayas no hyoga Comparative studies of the water balance selecting ni tsuite. (On the characteristics of Nepal representative basins with different situations are Himalayan glaciers and their recent variation). necessary to clarify the contribution of melt Seppyo, 36, 11-21.