The energy and water balance of high-latitude wetlands: controls and extrapolation

This paper examines, with examples, controls on the energy and water balance of northern wetlands. Most wetlands have organic soils and are thus peatlands. High-latitude wetlands are underlain by ice-rich permafrost, which helps maintain wetland systems and also imparts special characteristics to their energy and water balances. In North America, components of the radiation balance decrease linearly poleward, whereas the poleward rate of decrease of temperature and precipitation lessens. During the four-month summer of a high subarctic wetland, net radiation is large and the latent heat flux dominates the energy cycle. The ground heat flux is substantial, especially in early summer, when the ice-rich ground is rapidly thawing. Winter begins in October and heat loss from the ground approximately balances negative net radiation. The summer energy and water balance differs among terrain units. Large shallow lakes exhibit larger net radiation and potential evaporation rates than surrounding wetland surfaces which, in turn, exhibit substantially larger magnitudes than dryland terrain. There is a variable withdrawal rate of soil moisture depending on soil types and plant rooting characteristics, which influences the actual evaporation from the surface. Synoptic weather systems play a major role in day-to-day energy and water responses to climate forcing. Long-term modelling of the water balance of a wetland shows year-to-year persistence in climatic patterns. Although net radiation, temperature and precipitation all influence the magnitudes of water deficit, the precipitation inputs are of paramount importance. Our ability to fully understand, model and extrapolate, in space and time, the major controls on the surface climate of wetlands, is evaluated. Spatial extrapolation is seen to be more readily achieved than temporal extrapolation.