In mountain watersheds, the standard hydrological survey often underestimates components of the water balance. The aim of this study is a detailed water budget analysis of spruce forests (Norway spruce, Picea abies (L.) Karst.) in the Jizerka experimental catchment (the Jizera Mountains, Czech Republic). In the investigated mature spruce stand at the elevation of 975 m, the estimated annual evapotranspiration potential is 362 mm and the canopy interception by the absence of fog can reach 224 mm (i.e. 34% of the gross rainfall). But, in 2015-2017, the water budget of that forest stand has been significantly affected by fog/low clouds during 51 foggy days per a summer season. Based on canopy trough-fall observations, the interception loss was reduced to 106 mm (i.e. 16% of the gross precipitation). Thus, the summer fog drip reached 18% of the gross rainfall reducing the canopy interception by 54%. The Slinn model of fog drip was employed to extrapolate the data in both time and catchment scales. In the Jizerka catchment, the mean annual fog drip reached 81 mm (i.e. 7% of the gross precipitation and 11% of the mean annual runoff). Thus, the occurrence of fog/low clouds and the subsequent canopy fog drip represents an important income in the water budget of a forested mountain catchment.