Temporal and spatial variation in shallow groundwater gradients in a boreal headwater catchment

In humid climates, shallow groundwater is often assumed to be a subdued replica of the surface topography. Nevertheless, the relation between the surface topography and groundwater table can change over time, especially when catchment wetness changes. To investigate the correlation between the surface topography and the groundwater table, we analyzed groundwater levels and gradients in a boreal headwater catchment using 1.5 years of continuous groundwater level data for 75 wells. As expected, groundwater gradients changed with catchment wetness. Gradient directions calculated over short distances (5 m) changed by up to 360 degrees; gradients calculated over larger distances (20 m) varied by up to 270 degrees. The groundwater gradient directions were generally most variable for flatter locations and locations where the local surface slope differed from the surrounding topography. Smoothed digital elevation models (DEMs) represented the groundwater surface better than high resolution DEMs. The optimal degree of smoothing varied over the year and was lowest for very wet periods, such as the snowmelt period, when groundwater tables were high.