Modelling of soil water regime in forested areas: potential benefits of seasonally variable soil hydraulic properties

Soil moisture plays a key role in the hydrological cycle by partitioning of precipitation between evapotranspiration and deep infiltration. The ongoing climate change is causing an increase in air temperatures, changes in precipitation patterns and decrease in winter snow cover. It simultaneously shifts spring snowmelt towards winter months. Both air temperature and precipitation patterns are suspected to be one of the influential factors affecting changes in soil hydraulic properties. Thus, the ongoing climate change can alter soil hydraulic properties, commonly considered time-invariant, and the prediction of future soil moisture regime can therefore be more uncertain than originally thought.

We measured a saturated hydraulic conductivity using an automatic single-ring infiltrometer thorough one entire year in a monthly time-step in the spruce covered site. Higher infiltration rates were regularly observed in the middle of a vegetation season compared to lower rates observed in a dormant season. Based on this finding we implemented a new function, enabling the seasonal variation of the saturated hydraulic conductivity, into the simple bucket-type soil moisture model. The root-mean square error of soil moisture prediction decreased by one-third and Nash-Sutcliffe efficiency increased significantly indicating possible benefits of a new concept. Main reasons behind the seasonal variability of soil hydraulic properties in uncultivated sites can be numerous (encompassing biological activity, changes in the root architecture, wetting/drying and freezing/thawing cycles altering the pore space) and deserve further investigation.

The major outcome is represented by the concept enabling a more efficient prediction of soil moisture regime outside the vegetation season, which is increasingly more important as the onset of soil drought can often be observed at the end of the dormant season. Furthermore, modelling of a climate change impact on the availability of water resources will also benefit from a better prediction of the soil moisture by considering regular structural changes of soil.