Quantifying Evapotranspiration in Seasonally Frozen Forests

Abstract

In seasonally frozen environments, hydrological processes are highly dynamic during and following the melt period in the spring, and this is when most of the runoff and groundwater re- charge happens. This is also when evapotranspiration (ET) fluxes start to increase in response to higher solar radiation, and a resumption of photosynthesis in evergreen species. This thesis applies the Canadian Land Surface Scheme (CLASS) to three Boreal Ecosystem Research and Monitoring Sites (BERMS) in the boreal forest in Saskatchewan; Old Jack Pine, Old Black Spruce and Old Aspen. CLASS was used to simulate the energy and water balance of the vegetation, soil and snowpack at the three sites. Consistent with previous studies, it was shown that ET is overestimated in the model during the melt/thaw period. A series of numerical experiments were undertaken to investigate in detail the controls on simulated fluxes within the CLASS model and explore the model behaviour. The phenomenon of freezing point depression, where water freezes below 0 °C in soils, is not represented in the CLASS model. Consequently, the model predicted a significant amount of transpiration to occur during the melt period while the soil was at 0 °C and ice was still present in the soil pores. Subtracting the transpiration that occurred from soil layers containing ice improved the simulated ET, compared with flux tower estimates. Therefore, it is suggested that implementing freezing point depression in the model and including a water stress function to shut down transpiration when the soil temperature is ≤ 0 °C would improve the simulated evapotran- spiration during the melt period. The study also showed that calibration of the model parameters improved the simulations but is unable to uniquely constrain the infiltration and soil drainage fluxes by either single objective (ET) or multi-objective (soil moisture and ET) calibration. Further research is needed to explore the hypothesis that root water uptake does not occur in soils where the soil temperature ≤ 0 °C would.