SOP for Flood Forecasting & Early Warning - UNESCO Islamabad

Chapter 2
Flood Forecasting and Early Warning System
Indus basin encompasses all the five major rivers. Indus is the main river of the system,
while Jhelum, Chenab, Ravi and Sutlej are its main tributaries. Upper catchments of all
the rivers are mountainous with some of the highest peaks of the world located within the
Indus catchment.
2.1 Types of System
There are two systems, one is westerly wave & other is monsoon system.
i) Precipitation in winter (which is mainly in the form of snow) is on account of
eastward moving low pressure weather systems called westerly waves, which mostly
move above 35 degrees latitude and deposit snow over the mountains. Snow deposited
during winter becomes the major source of water supply into the rivers during summer.
ii) Rainfall in summer is on account of monsoon weather system. Unlike the westerly
waves the monsoon weather system is a low level weather system with heaviest rainfall
limited to below five thousand feet elevation. Approach of the monsonic winds as well as
that of the monsoon lows/depressions in from the east, south and south west directions.
Mountains along the west, north and north east constitute the physical barriers, beyond
which the monsonic winds cannot penetrate. Pir Punjal range along the east, the lesser
Himalayas range to the north and the Suleman range to the west constitute the limits of
monsoon activity and thus the areas of heavy rainfall.
During summer, even though monsoons are the dominating weather system, yet the
system of westerly waves also continues to affect the country along the Northern
latitudes. Intensification of the monsoon weather system and northwards recurving of the
monsoon depressions is due to the westerly waves.
c:\documents and settings\administrator\desktop\cd material nust\international training workshop\session 5\2_sop pmd_51 pages.doc