 |
| | | | | Rainfall Variability | |
| |
|
|
| Nepal
|
 |
Data
In order to apply the methodology used on the global scale with higher
spatial and temporal resolution in the period 1983 to 2002 dekad rainfall
grids at a spatial resolution of approx 0.1 degree are computed.
Concerning Burkina Faso, 133 rainfall stations have been available for
the selected time horizon. In this case interpolation techniques
can be applied directly on rainfall station data. The interpolation has
been performed with inverse distance and regression methods provided within
SEDI (AMS).
For Tanzania, Cambodia and Nepal available stata data was insufficient. So,
the generation of the rainfall grids has been achieved through a "downscaling"
procedure of global NOAA rainfall grids having a monthly resolution and a
spatial resolution of 0.5 degrees. Within this procedure for each of the
three countries a "virtual" grid of rainfall stations is constructed with a
densitiy and distribution of grid points comparable to the station data in
Burkina Faso. After the extraction of the rainfall data for each grid point
from the global NOAA rainfall grids, inverse distance and regression methods
(SEDI) are used again in order to generate the desired spatial resolution.
In coherence with Burkina Faso, most interpolation has been carried-out using
the inverse distance technique. Finally, the monthly rainfall grids are converted
to dekadal grids using a utility by R. Gommes.
The rainfall maps and the maps presenting the rainfall index for agroecological
zones and the first administrative level concerning Burkina Faso and Tanzania
are in Hammer-Aitoff projection and all maps concerning Cambodia and Nepal
are in Goodes Homolsine projection.
Climatological facts about Nepal
Nepals climate is a result of the Asian monsoon and the interaction with the
extreme topography of the country including an enormous range of altitude
within such a short south-north distance. The differential heating of land
and sea and the northward shift of the ITC to the Himalaya region provides
intense effects. From south to north, Nepal provides the Tarai, a low, flat
land, the forested foothills of the Mahabharat Lekh range, the midmountain
region with the valleys of Kathmandou and Pokhara and the Himalaya region
enclosing some of the worlds highest mountains. The strong indian summer
monsoon provides strong winds blowing from the southwest and heavy rainfalls
prevailing from June to September. But altitude also effects precipitation
patterns. Up to 3000 meters, annual rainfall totals increase as the altitude
increase, therafter annual totals diminish with increasing altitude. But there
are also higher rainfalls in the valleys (Pokhara Valley for example). Another
effect is caused by the horizonal extension of hill and mountain ranges with
resulting moist condition on southand westfacing slopes and major rain shadow
on the northern sides of the slopes.
In winter a reverse process occurs. The land surface cools faster than the
sea surface and the ITC moves southwards again. The northeast monsoon contains
only little moisture and is associated with dry winters.
Strong Indian summer monsoon years are normally associated with positive
tropospheric temperature anomalies, negative temperature anomalies over the
Indian Ocean and the Eastern Pacific and positive sea surface anomalies over
the Western Pacific. A failure in the monsoon can have extreme negative economic
and human effects as a result of crop failure. But a too strong monsoon can
also have extreme negative effects like crop distruction or severe flooding
and landslides. The amount of precipitation resulting from northeast trade
winds varies considerably, but increases with elevation. The secondary winter
precipitation in the form of snowfalls in the Himalaya is also important to
generating a sufficent volume of spring and summer meltwaters, which are
critical for irrigation in the lower hills and valleys where agriculture
predominates. But winter precipitation is also indispensible for the success
of winter crops.
|
|
|
