Observations and modelling of soil water content changes around trees

Abstract

This paper presents the results of a 5-year study of water content variations in a clay soil due to tree roots and evapotranspiration. The site of the study program is Maisonneuve Park which is located in the eastern area of Montreal, in the heart of a zone known for settlement problems experienced by lightweight buildings supported on shallow foundations. The clay deposit around several trees (both isolated and in a row) has been monitored by means of several arrays of 3–5 m long thin aluminium tubes, spaced from 0.1H to about 3H from the trees, where H represents the height of the trees. The volumetric water contents around the tubes have been measured by means of a neutron probe. The study shows that while for some trees, the effects of the roots are confined to a volume of soil located at a distance less than the height of the tree, in other cases, the zone affected by the trees is of much greater extent, reaching as far as 1.5H. In addition, for some of the trees studied, their effect extends to depths exceeding 4 m. Water content changes have also been estimated using the Versatile Soil Moisture Budget model, which was developed at Agriculture Canada. Comparisons between soil moisture readings in the field with estimates from the model show the feasibility of estimating soil moisture changes from standard meteorological data. Key words: water content changes, clay deposit, tree roots, evapotranspiration, observations, modelling.