There are some symbols that are not labeled, rotated 90 degrees.  Regardless, the raingages are more or less along a curved line, and could use more dispersion or perhaps nearby rain stations outside of area to improve triangulation.  Years ago, we plotted data and drew isohyetal lines of estimated equal rainfall. Due to lack of raingage dispersion, your estimates might better consider to correlate with elevation, since the angle of the isohyetal lines may be difficult to determine due to apparent dispersion is limited by their location.  Each storm may have some variance in how rainfall is distributed, but it is not unusual to have general trend of most storms due to air patterns and orographic lift..  However some storm types such as various types of tropical storms may vary from norms.  Probably most hydrology books will discuss this method to some degree, that has been or can be automated in GIS.  I am sorry I cannot answer that part of your question, but it probably is simlar to developing a DEM from elevation data, rasterizing the pixels with estimated rain levels, averaging the rain pixels in each drainage sub basin.

If your country has an analysis like TP40 (US Weather Bureau Technical Paper 40), you might be able to apply that, or at least see how the equal rsinfall contours fit your drainage areas 1, 2, and 3.  

If interested, I have several publications mentioning terms we use for hydrologic unit size or applying stream ordering).  Most of us hydrologists understand that these terms such as subbasin or watershed are applied generically, but it is sometimes helpful to understand that some classify hydrologic units by their size (USGS NRCS method).  

Theissen Polygon method can be applied to compute areal average rainfall. This method is well incroporated in ArcGIS software as a tool.

Dear Muhammad Shahid ;

The method of thiessen polygons is used to calculate the average rain in a watershed, for your case you have only 06 rain-gauge, distributed as following:

- One for the sub basin 1

- Five for sub basin 3 (are poorly distributed)

- Zero for the sub basin 2

spatial interpolation under ARC-MAP , little givea good results, and you can use the rain-gauge stations that are around your watershed to improve your estimate.

greeting

Hi Muhammad, .. When you are using the Thiessen polygon method the weighting factor for each polygon is  wi=Ai/A  , where Ai is area of the  polygon and A is the total area of interest. ArcMAP has already a builtin function for Thiessen polygon .. Once you have shp file for that you can get it from the attribute table as well.