Rainfall all of the canal commands. The Monsoons originate

Rainfall in
Pakistan is markedly variable in magnitude, time of occurrence and its aerial
distribution. However, almost two-thirds of the rainfall is concentrated in the
three summer months of July – September. The mean annual precipitation ranges
from less than 100 mm in parts of the Lower Indus Plain to over 750 mm near the
foothills in the Upper Indus Plain.

There are two
major sources of rainfall in Pakistan: the Monsoons and the Western
Disturbances. The relative contribution of rainfall in most of the canal
commands is low when compared with the two other sources of irrigation water
i.e., canal water and groundwater. More than 60% of the kharif season rainfall
is concentrated in the month of July for almost all of the canal commands.

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The Monsoons
originate in the Bay of Bengal and usually reach Pakistan, after passing over
India, in early July. They continue till September. The Indus Plains receive
most of their rainfall from the Monsoons. There are two periods of
thunderstorms in Pakistan: (1) April-June (2) October-November. These periods
are the driest parts of the year, particularly October and November. During
this time, thunderstorms caused by convection bring sporadic and localized
rainfall.

Pakistan lies in
an arid and semi-arid climate zone. The entire Indus Plains (canal command
areas) receive an average seasonal rainfall of 212 mm (95% confidence interval
± 28) and 53 mm (95% confidence interval ± 8) in the kharif and rabi seasons,
respectively.

The rainfall
varies as we move from the north and northeast to the south of the country. It
is only the canal command areas in the North-West Frontier Province (NWFP) and
the northern-most canal commands of the Punjab Province that receive some
appreciable amount of rainfall during the summer as well as the winter season.
The canal commands upstream of the rim stations (i.e., in the NWFP) receive
almost 55% of their annual rainfall during the kharif season. The canal
commands in the Upper and Lower Indus Plains receive 75% and 85- 90% of the
annual rainfall respectively, during the kharif season. The annual variability
of rainfall increases as one moves south. The canal command areas of Guddu and
Sukkur Barrages fall in an area where variability is the highest.

Based on 10-year average (1990-1999), data from the
Pakistan Meteorological Department of annual rainfall in some of the major
cities is as follows: