Annual evaporation (E) is often determined using the water balance approach, E = P—Q, because precipitation (P) and runoff (Q) are easily obtainable for most parts of the world. From a regional perspective, P and Q are point observations, containing information pertaining both to the regional trend and to the local environment. Trend surface analysis provides a simple method to filter out the local element, permitting a regional estimation of evaporation. This technique was applied to northern Ontario in Canada where a limited P and Q data network covers a vast domain. The resulting estimated evaporation pattern, obtained as a difference between P and Q trend surfaces, is partially compatible with a manually constructed map published by the Government. Discrepancies arise where the data sources are different or inadequate. One major advantage of this method is that, given additional information, the macro-scale estimates of evaporation can be updated easily and objectively.