TR(BR)-107 : Effect of orography on precipitation distribution in the Chenab basin

Abstract

Precipitation distribution with elevation has been studied in the Chenab basin located in the western Himalayan region. This basin comprises parts of outer, middle and greater Himalayan ranges and results could therefore be compared for different ranges of Himalayas. Because precipitation experienced in different seasons is derived from different weather systems which are influenced by different ranges of Himalayas differently. Grouping of stations with respect to the particular range and aspect has been done before making precipitation analysis. Analysis of the rainfall and snowfall distribution were carried out separately, for each range and season. Efforts have also been made to explain whether variation in precipitation is due to changes" in precipitation intensity or number of precipitation days or a combination of both. Rainfall analysis was made using rainfall data of 31 stations considering 17 years of daily rainfall data. To study snowfall distribution, snowfall data of 26 stations have been used for the same period as for rainfall.

Rainfall and snowfall exhibited different trends with elevation on the windward and leeward slopes of the three ranges of Himalayas. Seasonal characteristics of rainfall have shown a spill over effect on the leeward side during •winter, pre-monsoon and post-monsoon seasons in the outer Himalayas. For these three seasons rainfall increases linearly with elevation, but in the monsoon season rainfall on the windward side initially increases reaching to its maximum at about 600m and thereafter it decreases. Annual rainfall trends are guided by monsoon season rainfall and show similar pattern of distribution with elevation. Second order polynomials fitted well for rainfall distribution of monsoon and annual rainfall.

The role of orography in the middle Himalayas was found to be more pronounced for both rainfall and snowfall comparison to other ranges of Himalayas. Snowfall has shown more significant variation with elevation in comparison to rainfall. Rainfall follows similar type of distribution with elevation on both windward and leeward sides i.e. first it increases with elevation and then starts decreasing and distribution of rainfall fitted well with second order polynomials on both sides. The region of maximum rainfall on windward and leeward sides was found to be between 1600 and 2200 m. Snowfall distribution has shown a trend of linear increasing with elevation on the windward side whereas on the leeward side it followed the trend of rainfall in the middle Himalayas i.e. initially increases with elevation and then decreases. In this range maximum snowfall was found at about 2600m on the windward and at 1800 m on the leeward side. An increase in rainfall and snowfall intensities was found responsible for higher amount of rainfall and snowfall on the windward side in the middle Himalayan range.

In the greater Himalayan range it was found that rainfall decreases exponentially with elevation and snowfall increases linearly. Rainfall becomes negligible at elevations beyond 4000m on the windward side of great Himalayan range. Also, greater Himalayas experience lower snowfall in comparison to the middle Himalayas. Average number of snowfall days increases with elevation, but the intensity decreases.

Outer and middle Himalayas experience moderate rainfall whereas greater Himalayan range receives less rainfall. In the outer and middle Himalayas, average of all stations on a particular range indicates that monsoon rainfall exhibits maximum contribution, more than 40%, in annual rainfall followed by winter rainfall which is more than 26%. In the greater Himalayas, premonsoon rainfall was found to be relatively more prominent than the monsoon rainfall. Post-monsoon rainfall contribution was in general negligible over all Himalayan ranges. Generally, maximum rainfall is observed in the month of July over outer Himalayas, in March and July over middle Himalayas and in May over the greater Himalayas. Maximum snowfall is generally experienced in the months of January and February in the Middle and in March in the greater Himalayas.