Assessment of the evaporation rates of pools, strings and lawns of a boreal patterned fen

English Abstract:

In northern landscapes, the hydrological budget is largely influenced by evaporation (E), particularly in boreal watersheds where peatland is a predominant land cover. To improve the hydrological modelling of boreal watersheds, a better estimation of this term is required. The summer E variability of pools, strings and lawns of a patterned fen (3.6 ha) was assessed using 26 small lysimeters. To investigate the impact of water table depth on E, direct (weighing lysimeters) and indirect (automated lysimeter) methods were used during the snow-free period (June to October). Results indicated that E rates from pools, strings and lawns did not vary significantly throughout the fen. At the peatland scale, atmospheric conditions seemed to govern E more than the type of land cover. These results suggest that discriminating each land cover in the E budgets is not required and a mean value may be sufficient. A significant impact of water table depth on E was observed in Sphagnum lawn. Indeed, a rapid decline in E was monitored in the first 100 mm of peat. This decline was in all likelihood due to limited upward fluxes of water from saturated layers, which was limited by the low unsaturated hydraulic conductivity of the peat layers. The monitored E was consistent with previous values and a depth-dependent E relationship could be used in future hydrological modelling. The findings of this field work further validated the use of the classical Priestley-Taylor equation for patterned fens and demonstrated the potential of using an automated lysimeter in remote locations.