Precipitation is known to exhibit a high spatial variability. For this reason, raingage measurements, which only provide a local information about rainfall, may not be appropriate to estimate areal rainfall. On the other hand, catchments have the ability to aggregate rainfall over their area and route it to a unique point - the outlet - where it can be easily measured. A catchment can thus be viewed as a large raingage, with the difference that what is measured at the outlet is a complex transformation of the rainfall.