Seasonal and Diurnal Changes in Soil and Ground Water at a Mixed Forest at Borden Ontario

English Abstract:

The Borden Forest Research Station, in Southern Ontario (44°19′N, 79°56′W), has been used to conduct research on forest–atmosphere interactions since 1985. This mixed, predominantly deciduous forest, located in the Great Lakes / St. Lawrence transition zone between temperate deciduous and boreal forest biomes, is a natural re-growth from farmland abandoned in the early 20^thcentury. The forest–atmosphere flux of water vapor has been measured above the forest, via the eddy covariance method since 1996; soil moisture (to depths of 1 m) and precipitation are also measured. In 2011, three wells, each located within 100 m of the flux tower, were instrumented to monitor the depth of the groundwater table throughout the growing season.

Here, we analyze the seasonal and diurnal patterns in the soil and ground water budgets of the forest, during three growing seasons (2011-2013), in light of the precipitation and evapotranspiration budgets. There was a decreasing trend in soil water and groundwater throughout each growing season, with recharge during winter and following heavier growing-season precipitation events. Shallower depths of the soil moisture profile show a strong diurnal pattern, with decreasing soil moisture during the day and little change at night. At two of the wells, there is little evidence of a diurnal pattern. However, at the third well, a strong diurnal pattern is seen in water table depth during the height of the growing season, with a daytime decrease in water level and nighttime increase. Unlike the other wells, the water table at this well is sufficiently close to the surface that the deepest roots of trees likely draw directly from the water table. The nocturnal increase indicates a recharge of the local water table around this well. The co-location of groundwater wells with the ecosystem–atmosphere flux measurements allows investigation of the relationships between evaporation, soil water storage, and the water table.