Improvement of Peatland Hydrolocal Processes for the Canadain Land Surface Scheme

摘要

Most wetlands, especially peatlands, are dominated by organic soils with much bigger pore size than mineral soils. Water table can rise sharply just 0-0.2m beneath or above peatland surface in spring due to large amount of water supply from snow pack melting and decline quickly to 0.50-0.1m depth in summer because of strong evapotranspiration. High water-temperature variety near peatland surface at daily - seasonal scales is the most significant difference of peat from mineral soils. The Canadian Land Surface Scheme (CLASS) is a physically-based LSPs model of energy and water exchanges between three soil layers, the vegetation canopy and the atmosphere. For wetland modeling, an organic soil parameterization and water table diagnostic algorithm have been included into CLASS using the concepts of specific yield and specific retention. Although the model made good performance in a variety of wetland types with shallow water table during growing season when water table, it could not make good simulation for a long term in peatlands with dry surface and normally deep but highly fluctuating water table. In order to simulate the high variability of water-temperature throughout all seasons in the whole peat soil profile, an improvement of hydrological processes especially the algorithm for water table depth is made as follows: 1. Equivalent soil layer depths: locations of the three soil layers are adjusted based on the limit of normal minimum and maximum depths of water table variety in three peat soils (Fibric, Hemic and Sapric) as well as the heat-moisture regimes in CLASS.2. Lateral subsurface flow: a parameterized relationship between lateral subsurface flow and water table depth is added according to the field measurement so that CLASS can be capable of shedding groundwater laterally with the constraint of water conservation.3. Conditions for water table existence are modified according to the concepts of specific yield and specific retention in order to keep water table varying continuously especially when soil freezes and thaws. Validation of modeled water table depth and peatland soil temperature was performed against the field measurement at an ombrotrophic bog near Ottawa, Canada during 1998-2002. With the new scheme, modeled peatland water table varied within a range of real one and captured almost all phases as the measurement. The statistic comparison of simulated-observed water table reached 0.90 agreement.