Interception evaporation processes and measurement techniques

Sammanfattning: In temperate regions, evaporation of water intercepted by the vegetation represents an important part of the evapotranspiration. A review of interception evaporation studies showed that knowledge about interception evaporation processes is limited and progress is hampered by technical difficulties. Existing snow and rain interception measurement methods were reviewed and evaluated with criteria that were established for the ideal method for interception evaporation process studies. No existing methods fulfil all criteria. Two new load-cell based devices for process studies are tested, one weighing-cut-tree technique for snow interception and a technique based on weighing of both net and gross precipitation (with new type of wind-shield) for rain interception. By comparing measured (weighed) and calculated (combination method with different ways to calculate the aerodynamic resistance) evaporation rates for wet intercepted snow it was shown that the most important factors for calculating the evaporation were the relative humidity, the aerodynamic resistance, the wind speed and the intercepted mass. Calculations of evaporation of intercepted snow with a water budget method showed an average evaporation of 0.24 mm/hour and a maximum evaporation of 3.9 mm/7 hours when snow canopy storage was measured with a gamma-ray attenuation system and throughfall with plastic-sheet net-rainfall gauges. Total snow interception evaporation was estimated to be of the order 200 mm/year in Scotland. Comparison with evaporation determined with a combination method (Penman) and two different aerodynamic resistances, the "standard" rain aerodynamic resistance ...