The potential evapotranspiration ETP can be estimated by using Brochet and Gerbier's formula (1972), which is derived from Penman's equation

ETPmRg + n Ep (mm/day),

where m and n are tabulated coefficients depending on season, latitude and shelter type, and where the variables are the global solar radiation Rg and the Piche evaporation under shelter Ep.

In the small Ringelbach watershed (36 ha), in the Vosges Mountains (France), maps of daily ETP have been calculated with this formula, by mapping these two variables

- maps of daily global radiation have been computed for sunny days from horizontally measured Rg, by taking into account slopes, aspects and orographic masks.

- small and cheap Piche shelters, specially designed and calibrated, have been implanted over the whole watershed with Ep measured twice a day.

The results obtained for Rg, Ep and thus ETP reveal important differences within the watershed. Their spatial patterns depend on weather type, topographic structure, and soil surface humidity.

This mapping procedure, which gives a good estimation of ETP, enables us to better understand the hydrologic behaviour of any mountainous watershed.