Ecohydrology of a maritime-pine dominated landscape in Southern France

The Landes region of southern France is an important component of the European forest sector and also on the regional-scale carbon (C) and water dynamics. In general, the landscape is dominated by managed maritime pine stands (Pinus pinaster), with considerable acreage of irrigated crops. Vegetation in forested locations consists of a pine overstory and an understory that depends on the soil wetness. The terrain is flat and soil is predominantly sandy with an average rooting depth of 0.8-1.0 m. The local hydrological processes primarily govern the C-cycling in this ecosystem. We studied the ecohydrological processes in the Leyre watershed that cover ~45% of the Landes ecosystem using a spatially explicit ecohydrological model , STEPS (Simulator of Terrestrial Ecohydrological Processes and Systems) and high-frequency measurements of various hydrological components. We analysed the daily precipitation (P), evapotranspiration (ET), volumetric soil water content (VSMC) and water table depth (WTD) at Le Bray site, and runoff (R) at a near-by watershed (Bouron) for three years 2005, 2006 and 2007. From the measurements of P and R, we found that 2005 was a drier year whereas 2007 was a wetter year. The annual [ET/P] was ~ 0.61, 0.65 and 0.81 in 2005, 2006 and 2007, respectively. Analysis of VSMC (at 3 depths: 5cm, 34cm and 80cm) and WTD at Le Bray site suggested that the annual soil storage changes (ΔS) were +13.9, -53.5 and -25.5mm for 2005, 2006 and 2007 respectively. We speculate that the remainder of water was lost via subsurface flow (SSF) because Hortonian runoff mechanisms were rare. The SSF was lowest in 2007, although it was a wetter year because the water was mainly used to recharge the soil storage due to the previous year's deficits. At Le Bray, SSF was highest in 2006 owing to higher P and +ve ΔS from 2005. Our study revealed that ET was the main controller on the local water-balance which in-turn depended on ecophysiological processes. Model and remote sensing-based GPP and ET signals also showed consistent trends. We present a synthesis of the integrated use of the STEPS model, remotely sensed and ground based estimates to explain the ecohydrological complexity and related feedback mechanisms in this landscape.