Incorporating Peatland Surface Resistance into Evapotranspiration Models using the Cold Regions Hydrological Model (CRHM).

Abstract:

Evapotranspiration is a major hydrological flux in boreal peatlands, and is often assumed to occur at or close to potential rates in ecohydrological models at the watershed scale. This is due to the generally wet conditions found in peatlands, where the water table is often within 30 cm of the surface. However, peatland water table can fluctuate depending on the balance between precipitation and water loss, particularly during the growing season. A 5 cm drop in water table can lead to a reduction in VMC of approximately 40% in the top layers of the living moss. This living moss layer is also where evaporation occurs in the peat profile. This increases surface resistance to evaporation, lowering the actual evapotranspiration rate below potential evapotranspiration rate. By not accounting for this negative feedback, models can overestimate peatland water loss via evapotranspiration. Yet to date, peatlands are often modelled using Priestly-Taylor equation, which is a potential evapotranspiration estimation. Ecohydrological models such as CRHM, have the ability to model peatland ET using estimates that can incorporate surface resistance feedbacks (e.g. Penmann-Monteith), however they need to be modified to better physically represent evaporation from the living moss layer (0 - 5 cm).

Therefore, the objective of this research is to show results from a new modelling scheme that accounts for the relationship between surface resistance and peatland moss evapotranspiration using the Cold Regions Hydrological Model (CRHM). This work will investigate the assumptions that a peatland evaporates at potential rates, and build on existing evapotranspiration modules in CRHM. Our findings will contribute to broader peatland research by providing researchers with a cold regions ecohydrological model, that explicitly models peatland physical processes, and can be used by the peatland ecohydrological community.