Ineed with the thiessen polygons and isohyetal contour methods..

the Thiessen polygon method is A method of assigning areal significance to point rainfall values. Perpendicular bisectors are constructed to the lines joining each measuring station with those immediately surrounding it. These bisectors form a series of polygons, each polygon containing one station.

The isohyetal method is used to estimate the mean precipitation across an area by drawing lines of equal precipitation. The method uses topographic and other data to yield reliable estimates. Isohyets are contours of equal precipitation analogous to contour lines on a topographic map. In the isohyetal method, precipitation values are plotted at their respective stations on a suitable base map, and isohyets are drawn to create an isohyetal map. Isohyetal lines are based on interpolation between rain gauge stations. While constructing isohyets, it is assumed that rainfall between two stations varies linearly, unless abrupt changes in topography indicate otherwise.