Water table is a relevant source for water uptake by a Scots pine (Pinus sylvestris L.) stand: Evidences from continuous evapotranspiration and water table monitoring

The objective of this study was to quantify the main terms of the water cycle in a Scots pine stand (Pinus sylvestris L.) growing on a sandy soil and to estimate the contribution of the shallow water table (0.80m deep in spring) to the forest water use. Continuous monitoring was organized in 2005 to measure climate, throughfall, soil moisture, tree transpiration and water table variations at a half-hourly basis. Leaf area index seasonal dynamic was measured and roots were counted down to the bottom of the soil profile. Forest floor evapotranspiration was modelled with Granier et al. [Granier, A., Bréda, N., Biron, P., Villette, S., 1999. A lumped water balance model to evaluate duration and intensity of drought constraints in forest stands. Ecol. Model. 116, 269-283]. From May to November, pine transpiration never exceeded 1.85mmd⁻¹ and reached a total of 176.4mm, which corresponded to 25% of potential evapotranspiration, whereas the understorey evapotranspiration was 130mm (i.e. 18-20% of the stand water use). The maximum soil water reserve measured over the soil rooted zone was 250mm, in which 145mm was extractable water. A 3.5-week period with no rain was observed in June, which induced a regulation of pine transpiration when the soil extractable water reached 0.25 of its maximum value. We applied the water table fluctuation (WTF) method [White, W., 1932. A method for estimating groundwater supplies based on discharge by plants and evaporation from soil. US Geol. Survey Water Supply Paper 659-A. United States Government Printing Office, Washington, DC] to estimate the water table daily loss of water. A relationship was established with potential evapotranspiration and the actual transpiration fluxes of the stand. Yet, it was not possible to extract from the WTF results the part that was effectively contributing to actual transpiration. We applied then the WTF methodology on longer time intervals, with a focus on periods with no rains. From May to November, the contribution of the water table to forest transpiration reached 61%. During the drought period in June, the water table contributed to 98.5% of the water uptake by vegetation, through its contribution to the capillary rise above the water table. The presence of a groundwater table with a floor down to 180-200cm allowed this stand to rely upon water that otherwise would have drained deeper.