I have a rainfall map which covers large area. However, I'm interested in the total value of rainfall over a specific subcatchment. If I don't have the shapefile which defines the boundary of this subcatchment, is it possible to calculate the sum of the rainfall values upstream of a specific point in ArcGIS? or Do I have to delineate the catchment upstream of this point first?
You will need to create the catchment area. If you have the elevation raster for the same region as your rain fall data then you can do this. You need to read the hydrology section of the help file which is found under the spatial analyst section.