Spatial Distribution and Seasonal Variability of Rainfall in a Mountainous Basin in the Himalayan Region

Transcription

1 Water Resources Management (2006) 20: DOI: /s C Springer 2006 Spatial Distribution and Seasonal Variability of Rainfall in a Mountainous Basin in the Himalayan Region MANOHAR ARORA 1,, PRATAP SINGH 1, N. K. GOEL 2 and R. D. SINGH 1 1 National Institute of Hydrology, Roorkee, India; 2 Indian Institute of Technology, Roorkee, India ( author for correspondence, arora@nih.ernet.in) (Received: 17 August 2004; in final form: 8 June 2005) Abstract. The average distribution of precipitation provides essential input for understanding the hydrological process. The role of complex topography in mountainous basins makes the spatial distribution of precipitation different than the plain areas. Besides the rugged topography, the Himalayan basins also face the problem of limited physical accessibility and data availability. In this study, seasonal and annual distribution of rainfall with elevation and distance from the lower most station (Akhnoor) has been studied for the Chenab basin (western Himalayas). The study basin covers all the three ranges i.e. outer, middle and greater Himalayas. The rainfall stations are grouped into windward and leeward categories. The trends of spatial distribution of rainfall are discussed in detail. Attempts are also made to investigate the impact of reduced network on the mean annual rainfall of the Chenab basin. A reduction in rain gauges from 42 to 19 has resulted in an increase in the estimate of mean annual rainfall by 14% with respect to the estimate obtained using 42 stations network. Key words: rainfall distribution, Himalayas, elevation, distance, mean areal rainfall, kriging Introduction Information on precipitation distribution is needed for various hydrological applications such as realistic assessment of water resources, estimation of probable maximum precipitation and hydrological modelling of the basin. Some of the most significant data-related problems in mountainous basins are associated with the measurement of precipitation depth and its spatial distribution. WMO (1986) made a comparative study of various models and indicated that precipitation distribution assumptions and determination of the form of precipitation were the most important factors in producing accurate estimates of the runoff volume. Gan et al. (1997) reported that in order to simulate/forecast the streamflow from a basin, good precipitation input is more important than the choice of complexity of the hydrological model. The distribution of precipitation is different in the mountainous areas than the plain areas because of difference in their topography. In the mountainous basins, weather systems interact with topography and result in highly non-uniform precipitation. Uplift of moisture laden air currents striking against a mountain barrier

2 490 M. ARORA ET AL. provides a good precipitation on the windward side. Changes in rainfall with altitude make the rainfall distribution more complex in mountainous area. A number of studies have been carried out to understand the variation in precipitation with altitude in different mountainous areas of the world (Clayton, 1982; Loukas and Quick, 1994; Marquinez et al., 2003). Depending upon the relief of a mountain, there may be a continuous increase in precipitation with altitude, and it may begin to decrease above a particular altitude (Singh et al., 1995, Singh and Kumar, 1997). Thus, orography plays an important role in precipitation distribution, which varies significantly in space and time not only within a particular range, but also from one mountain range to another. A precise understanding of climatic conditions in the mountain regions is lacking because of poor observational network. Generally, a poor assessment of spatial precipitation is made in the mountainous basins, because of non availability of adequate network of precipitation gauges for recording the variability of precipitation with altitude. Statistical and geostatistical techniques have also been widely used for understanding the precipitation-elevation relationship in the mountainous basins (Hayward and Clarke, 1996; Sen and Zeyad, 2000; Martinez-Cob, 1995). Therefore, estimation of mean rainfall for the mountainous basin needs special attention (Wilk and Anderson, 2000). In Indian context, about 35% of the total geographical area of the country is mountainous and out of this about 58% is covered by the mighty Himalayas. Detailed studies to assess the orographic effect on precipitation in the Himalayan region are lacking due to various reasons. Singh et al. (1995) and Singh and Kumar (1997) studied the precipitation distribution in the mountainous basins located in different parts of Himalayan region. In these studies main emphasis was laid to study the effect of altitude on precipitation distribution. Dhar et al. (2000) reviewed the precipitation studies carried out for high altitude regions of Himalayas. In the present study, changes in rainfall distribution are studied with elevation and distance for the Chenab basin located in western Himalayan region. Study Area and Data Used The Chenab River is one of the five main tributaries of the great Indus River system. The river Chenab rises in two streams Chandra and Bhaga in the Himalayan canton of Lahaul in Himachal Pradesh. The catchment of Chenab is elongated and narrow in shape. The elevation of the catchment area varies widely from 305 to 7500 m showing very high relief of the basin. The mean elevation of the basin is 3600 m asl. The catchment area of the Chenab River up to Akhnoor, the lowermost rain gauge site in India is 22,200 km 2. Singh and Kumar (1997) have reported that on an average, about 70% of the total drainage area of Chenab basin up to Akhnoor is covered by snow during the month of March/April, which reduces to about 5400 km 2 (25%) during the month of September/October. This area (25%) can be considered to be covered by perpetual snow and glacier. The Chenab basin is a well-gauged basin. For this study, rainfall data of 42 stations for the period from

3 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 491 Figure 1. Chenab basin with location of rainfall stations to 1990 and 19 stations for the period from 1974 to 1998 have been used. Figure 1 shows the Chenab basin with raingauge network. Results and Discussion The rainfall received at a particular station depends upon its geographical location, orientation and elevation. Rain gauges located at same elevation may receive

4 492 M. ARORA ET AL. Figure 2. Distribution of annual rainfall with elevation in the study basin. different amount of rainfall due to orientation and location in different ranges. Therefore, for studying the rainfall distribution it becomes necessary to group the stations. In this study, grouping of stations has been considered on the basis of aspect of the mountain range. The study area covers all the three Himalayan ranges, i.e., outer, middle and greater. The grouping of stations for different mountain ranges is shown in Figure 2. It is notable that one station (Nandan) in the middle Himalayan range lies outside the circles. This station has different rainfall characteristics. For studying the rainfall distribution the data set of 19 stations with data length from are used. Subsequently, one station in outer Himalayas and five stations in the middle Himalayas with shorter data period are included, to ascertain the trend. Thus, the data of the twenty-five stations have been used for the study. The distribution of these stations for each range and aspect is given in Table I. The analysis is carried out on annual and seasonal time scales. For seasonal analysis, each year is divided into four principal seasons viz. post-monsoon (October December), winter (January March), pre-monsoon (April June) and monsoon (July September). Outer Himalayas The outer Himalayan range of this study basin includes six rainfall stations. The numbers of stations on the windward and leeward side of this range are 4 and 2 respectively. The elevation range of these stations varies from 305 m to 1000 m. On the windward side, two stations (Paoni, 600 m and Salal, 610 m) at about the same elevation, receive different intensity and magnitude of rainfall because of

5 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 493 Table I. Seasonal distribution of average rainfall in different ranges of the Himalayas Range Station Elevation in meters Distance in km. with respect Rain (mm) to Akhnoor Post-monsoon Winter Pre-monsoon Monsoon Annual Outer Himalaya (Windward) Akhnoor Paoni Salal Gainta Average Outer Himalaya (Leeward) Damni Dhamkund Average Average (Outer Himalaya) Middle Himalaya (Windward) Darabshala Doda Rot Nandan Gohala Thana Average Middle Himalaya (Leeward) Ohli Kishtwar Banihal Shirshi Bhadarwah (Continued on next page)

6 494 M. ARORA ET AL. Table I. (Continued) Range Station Elevation in meters Distance in km. with respect Rain (mm) to Akhnoor Post-monsoon Winter Pre-monsoon Monsoon Annual Chingaon Dusadudha Devigol Bunnencha Average Average ( Middle Himalaya) Greater Himalaya Sohal Yurod Udaipur Tandi Average (Greater Himalaya)

7 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 495 their location and exposure. Salal is located between the ridges, whereas Paoni, like Akhnoor and Gainta, is located on the exposed side of the ridge and receive maximum moisture content. The variation in rainfall with elevation on seasonal and annual scale for the windward and leeward sides of this range is shown in Figure 3. Although the data for one station on windward side, namely, Gainta (1000 m) are available for a limited period from 1974 to However, these are included to ascertain the trend of rainfall distribution. It is observed that rainfall in the windward side of outer Himalayas increases linearly with elevation during winter season. While the trends in the post-monsoon, pre-monsoon and monsoon seasons show that there is an increase in rainfall up to certain elevation and then it starts decreasing. Salal and Paoni, located at about the same altitude receive significantly different rainfall during monsoon. The average annual rainfall observed at Salal is 1759 mm, whereas for Paoni it is 2423 mm. As the contribution of monsoon rainfall on the windward side is about 57% of the annual total, the same trend is reflected in annual rainfall. Seasonal and annual rainfall for different stations and average rainfall of different ranges are given in Table I. As the data of only 2 stations were available on the leeward side, the trend could not be established for the leeward side. However the total rainfall on the leeward side is found to be higher than windward side for all seasons except the monsoon season. This may be possible due to spill over effects of rainfall in the outer Himalayas, where altitude of mountain barrier is not very high. During winter, pre-monsoon and post-monsoon seasons, the moisture contents in the air are less in comparison with monsoon season, and the clouds precipitate at higher ranges after crossing the mountain barrier. It results in high rainfall at higher elevation stations on the leeward side. A significant drop in rainfall at lower stations on the leeward side is possible due to the fact that after precipitating at higher altitudes, a little moisture is left in the clouds to precipitate at lower altitudes. Also, winter rains, which make second largest contribution in annual rainfall and even largest contribution in some cases, follow western disturbances approaching from northwest. In contrast, during monsoon, the situation is entirely different, the moisture content in the air is much higher than other seasons and the clouds precipitate relatively at lower elevations on the windward side. The monsoon rainfall is the major contributor (45%) to the annual rainfall on the windward as well as leeward sides of the outer Himalayan range (Table I). Thus it influences the annual rainfall significantly and guides the distribution with elevation. Annual rainfall (1855 mm) on the windward stations is higher than that at the leeward stations (1601 mm), indicates that a reasonably high rainfall occurs on both sides of the mountain range with an average value of 1728 mm. Coefficient of variation (C v ) for seasonal and annual rainfall for all the stations in the different Himalayan ranges are computed and given in Table II. In outer Himalayas, the variability in seasonal rainfall is higher than annual rainfall. Post-monsoon season shows the maximum variability in rainfall. The windward stations exhibit higher variability than the leeward stations except for monsoon season.

8 496 M. ARORA ET AL. Figure 3. Variation in rainfall with elevation on the windward and leeward sides of the outer Himalayan range.

9 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 497 Table II. Coefficient of variation (C v ) for seasonal and annual rainfall of outer, middle and greater Himalayan stations Rainfall Elevation Post- Pre- Range Station (m) monsoon Winter monsoon Monsoon Annual Outer Himalayas Akhnoor (Windward) Paoni Salal Gainta Outer Himalayas Damni (Leeward) Dhamkund Middle Himalayas Rot (Windward) Nandan Doda Darabshala Middle Himalayas Banihal (Leeward) Ohli Kishtwar Shirshi Bhadarwah Chingaon Greater Himalayas Yurod Sohal Udaipur Tandi In order to investigate the influence of distance on rainfall distribution, attempts are made to study the changes in rainfall with distance. Distances are calculated with respect to the station at the outlet of the basin, i.e., Akhnoor. Figure 4 shows the variation in rainfall with distance for the outer Himalayan range. On the windward side, the variation shows linear trends for post-monsoon, winter and premonsoon seasons whereas, for annual and monsoon season the trends are linear with elevation. On the leeward side, annual and seasonal rainfall is decreased with distance. The respective governing equations with elevation and distance on both the windward and leeward sides of the Himalayan ranges are given in Table III. Middle Himalayas Rainfall records of 10 stations were available for the analysis in the middle Himalayan range. Four stations are located on the windward side and six are on the leeward side. The average annual rainfall in middle Himalayan range is about 938 mm. The value of C v is higher during post-monsoon season for both windward and leeward sides as compared to the other seasons. The annual rainfall values have lower C v than that of the seasonal rainfall indicating less variability in annual rainfall.

10 498 M. ARORA ET AL. Figure 4. Variation in rainfall with distance on the windward and leeward sides of the outer Himalayan range.

11 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 499 Table III. Governing equations describing the annual rainfall distribution in the outer, middle and greater Himalayan ranges of the study basin Elevation as variable x Distance as variable x Himalayan range Equation R 2 Equation R 2 Outer Himalayas Windward y = x 0.004x y = x x Leeward Only two stations available Only two stations available Middle Himalayas Windward y = x x y = 34.03x Leeward y = x x No clear trend observed Greater Himalayas Windward ln(y) = x ln(y) = x

12 500 M. ARORA ET AL. On the windward side of middle Himalayan range, rainfall at first increases with elevation and then decreases after a certain elevation (Figure 5). In other words, second order polynomial fits well to represent the distribution of rainfall with altitude. The variation of rainfall with elevation on the leeward side is similar to that observed on windward side except during the post-monsoon and winter seasons when rainfall decreases linearly with elevation. Although rainfall data for two stations on the windward side, namely, Thana and Gohala, and three stations on the leeward side, namely Bunnencha, Devigol and Dusadudha were available for a limited period ( ), their inclusion was necessary to find out the trend of rainfall variation in this Himalayan range. The variations in rainfall with distance are shown in Figure 6. On the windward side, a linear decreasing trend is observed for the post-monsoon, pre-monsoon, monsoon season on annual scale. No clear trend is noticed for the winter season. The seasonal and annual rainfall on the leeward stations is alike and did not exhibit much variability except for the winter season. Maximum rainfall is observed around m on both sides of this range. Figure 7 represents the relationship between the distances and elevations of the stations located in the three ranges, i.e. outer, middle and greater Himalayas. It is observed that the stations in the middle Himalayan range show wide variation in elevation at small distances. This may be the reason behind absence of clear trend of rainfall with distance in this range. The distribution of rainfall is explained better by elevation than the distance in this range. Greater Himalayas Data for four rainfall stations in the Greater Himalayas are available for the analysis and these stations are located on the windward side. The elevation of the stations varies between 2000 m and 3100 m. The stations are located in such a way that they covered major area of the basin and had more inter-station distance in comparison to that in the other ranges. Average annual rainfall in the Greater Himalayas is found to be 356 mm. Monsoon season contributed maximum (41%) to the annual rainfall. The value of C v in the Greater Himalayan range for one station namely, Tandi (3100 m) is higher during post-monsoon and winter seasons. In general the monsoon season shows higher variability in rainfall. The variations of rainfall with elevation are represented in Figure 8. Results show that the rainfall decreases exponentially with elevation for the post-monsoon, winter and annual rainfall. In the pre-monsoon season rainfall decreases with elevation and no clear trend is observed in monsoon season. This may be attributed to the fact that at higher elevations a major part of precipitation falls in the form of snowfall. The distributions of rainfall with distance are represented in Figure 9. These figures show the similar trends as observed for that of variation with elevation. In this range variation of rainfall with distance is explained better (R 2 = 0.98, where R 2 is coefficient of determination).

13 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 501 Figure 5. Variation in rainfall with elevation on the windward and leeward sides of middle Himalayan range.

14 502 M. ARORA ET AL. Figure 6. Variation in rainfall with distance on the windward and leeward sides of middle Himalayan range.

15 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 503 Figure 7. Distribution of elevation of stations with corresponding distances with respect to outer most station of the basin (Akhnoor) distance and elevation. Changes in Mean Annual Rainfall of Basin with Reduced Number of Rainfall Stations In this study basin the records of rainfall are available for 42 stations for a period of 17 years ( ). The numbers of stations were reduced from 42 to 19 in the basin from the year 1990 onwards. In the present analysis 19 rainfall stations having data for 25 years ( ) are considered. In order to study the impact of reduction in number of rain gauges on mean annual rainfall, it has been attempted to determine the increase or decrease in percent keeping the period of record fixed. Therefore, for the period the mean annual rainfall is computed using 42 rainfall stations and then it is computed using 19 rainfall stations. Tabios and Salas (1985) compared several Areal Average Rainfall (AAR) methods and concluded that a geostatistical method (ordinary and universal kriging) with spatial correlation structure is superior to Thiessen polygons, polynomial interpretation, and inverse-distance weighting. Assuming that the precipitation is a random field that possesses a given set of first and second order characteristics, ordinary KRIGING interpolation technique is used for the data set of 42 stations and value of mean areal rainfall is estimated to be 988 mm. The same technique is used for the data set of 19 stations for the same period and the mean areal rainfall is estimated to be 1127 mm. It is found that mean annual rainfall of the basin has increased by 14% with reduced network of rainfall stations. The result suggest that as data from the

16 504 M. ARORA ET AL. Figure 8. Variation in rainfall with elevation on the windward side of greater Himalayan range. reduced network will be available for the future hydrological studies in the basin, for the accurate mean annual rainfall estimates, the computed mean annual rainfall using reduced network should be reduced by 14%. The contour maps obtained after interpolation are shown in Figure 10.

17 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 505 Figure 9. Variation in rainfall with distance on the windward side of greater Himalayan range. Conclusion Precipitation distribution for the Himalayas is poorly known as compared with many other mountains of the world. In the present study, seasonal and annual distribution

18 506 M. ARORA ET AL. Figure 10. Computation of mean annual rainfall using kriging. rainfall for the Chenab basin is studied. In this study area, the precipitation is caused by different weather systems during different seasons of a year and varies from place to place because of highly rugged topography of the Himalayan mountains. Depending upon the availability of rainfall data, the rainfall stations are grouped with respect to ranges and aspect. The variability and trends exhibited by different ranges are given in the following table. Himalayan Seasonal maximum % of range rainfall contribution Total Trends Correlation factors Outer Windward Monsoon (1061 mm) 57 Second order polynomial in all seasons and annual scale The distribution and magnitude of annual rainfall is guided by monsoon season rainfall Leeward Winter (615 mm) 38 The average rainfall in seasons other than monsoon are more in comparison to windward stations (Continued on next page)

19 SPATIAL DISTRIBUTION AND SEASONAL VARIABILITY OF RAINFALL 507 (Continued) Himalayan Seasonal maximum % of range rainfall contribution Total Trends Correlation factors Middle Windward Monsoon (407 mm) 38 Second order polynomial in all seasons and annual scale Leeward Monsoon (308 mm) 36 Linear in post-monsoon and winter seasons. Second order polynomial in other seasons and annual scale Greater Monsoon (145 mm) 41 Exponential decrease in post-monsoon, winter and annual. Linear decrease in pre-monsoon and no trend in monsoon The distribution and magnitude of annual rainfall is guided by monsoon season rainfall Except pre-monsoon, the average rainfall in other seasons are less in comparison to windward stations No clear trend is obtained in monsoon season possibly because higher elevations receive major part of precipitation in form of snowfall The average rainfall on windward side of outer and middle Himalayas is more comparison to leeward side. The values of Coefficient of variation (C v ) for each station suggest that there is not much variability in the seasonal and annual rain. The trends of variation of rainfall with distance for both the sides (windward and leeward) of the outer and middle Himalayas have been studied. Annual rainfall distribution on the windward side of the outer Himalayan range is better correlated with distance (R 2 = 0.87) than with elevation (R 2 = 0.51). In middle Himalayas, the annual rainfall on the windward and leeward side corresponds better with elevation (R 2 = 0.83 and R 2 = 0.42) rather than distance (R 2 = 0.46 and no trend). Both elevation and distance are equally important in explaining the variability in annual rainfall distribution (R 2 = 0.86 and R 2 = 0.98). An attempt is also made to study the impact of reduction in number of rain gauges on mean annual rainfall using two different sets of stations for the same period of time ( ). It is found that estimate of mean annual rainfall of the basin has been increased by 14% with reduced network of rainfall stations. Because the data from reduced network will be available for the future hydrological studies in the basin, therefore, for the accurate mean annual rainfall estimates, therefore, the computed mean annual rainfall using reduced network should be reduced by 14%.

20 508 M. ARORA ET AL. References Clayton, H. L., 1982, Distribution and stochastic generation of annual and monthly precipitation on a mountainous watershed in Southwest Idaho, Water Resources Bulletin 18(5), Dhar, O. N., Mandal, B. N., and Kulkarni, A. K., 2000, Review of precipitation studies carried out for high himalaya in recent years, High Altitudes of the Himalaya II (Biodiversity, Ecology & Environment), edited by: Y.P.S. Pangtey, 2: , Jan Gan, T. Y., Dlamini, E. M., and Biftu, G. F., 1997, Effects of model complexity and structure, data quality and objective functions on hydrologic modeling, Journal of Hydrology 192, Hayward, D. and Clarke, R. T., 1996, Relationship between rainfall, altitude and distance from the sea in the Freetown Peninsula, Sierra Leone, Hydrological Sciences Journal 41(3), Loukas, A. and Quick, M. C., 1994, Precipitation distribution in coastal British Columbia, Water Resources Bulletin 30(4), Martinez-Cob, A., 1995, Estimation of mean annual precipitation as affected by elevation using multivariate geostatistics, Water Resources Management 9, Marquinez, J., Lastra, J., and Garcia, P., 2003, Estimation models for precipitation in mountainous regions: The use of GIS and multivariate analysis, Journal of Hydrology 270(2003), Tabios III, G. Q. and Salas, J. D., 1985, A comparative analysis of techniques for spatial interpolation of precipitation, Water Resour. Bul. 21, Sen, Z. and Zeyad, H., 2000, Spatial precipitation assesment with elevation by using point cumulative semivariogram technique, Water Resources Management 14, Singh, P., Ramasastri, K. S., and Kumar, N., 1995, Topographical influence on precipitation distribution in different ranges of western Himalayas, Nordic Hydrology 26, Singh, P. and Kumar, N., 1997, Effect of orography on precipitation in the western Himalayan region, Journal of Hydrology 197, Wilk, J. and Anderson, L., 2000, GIS-supported modelling of areal rainfall in a mountainous river basin with monsoon climate in southern India, Hydrologic Sciences 45(2), WMO (1986), Intercomparision of models of snowmelt runoff. Operational Hydrology Report No. 23, WMO-No. 646, WMO, Geneva, Switzerland.