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Rainfall Variability Analysis of rainfall variability and drought in the 1961-2002 period
Nepal

Global analysis  Annual rainfall | National rainfall index | Global rainfall pattern
Case studies  Burkina Faso | Cambodia | Nepal | Tanzania
Downloads  Images (IDA) | Images (GeoTiff) | Movies (AVI) | Windisp | Quantum GIS


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.


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