Forest floor water content dynamics in a Douglas fir stand

This paper considers the hydrology of the forest floor within a homogeneous Douglas fir forest. Time domain reflectometry measurements show that forest floor water contents have considerable spatial variabilities but similar temporal dynamics. Simple linear relations can be used to translate forest floor water content dynamics from one site in a forest to another. Forest floor evaporation rates were calculated using a previously developed forest floor evaporation model and a year of soil water and micrometeorological data. For a relatively wet site within the stand the calculated evaporation rate was 137 mm year −1, for a more representative site 112 mm year −1 and for a dry site 76 mm year −1. These amounts range between 7 and 13% of the total yearly forest evapotranspiration. Together with throughfall rates and transpiration rates, these forest floor evaporation rates served as boundary conditions to a soil water model with which we simulated forest floor and mineral soil water content dynamics. The simulations showed that throughfall and drainage dynamics determine the forest floor water content dynamics in wet conditions. In dry periods, forest floor evaporation and, to a lesser extent, root water uptake determine forest floor water content dynamics. The same simulations showed that 25% of the forest floor evaporation is replenished by capillary rise from the mineral soil.