Simulating soil-water movement under a hedgerow surrounding a bottomland reveals the importance of transpiration in water balance

Abstract : The objective of this study was to quantify components of the water balance related to root-water uptake in the soil below a hedgerow. At this local scale, a two-dimensional (2D) flow domain in the x-z plane 6 m long and 1*55 m deep was considered. An attempt was made to estimate transpiration using a simulation model. The SWMS-2D model was modified and used to simulate temporally and spatially heterogeneous boundary conditions. A function with a variable spatial distribution of root-water uptake was considered, and model calibration was performed by adjusting this root-water uptake distribution. Observed data from a previous field study were compared against model predictions. During the validation step, satisfactory agreement was obtained, as the difference between observed and modelled pressure head values was less than 50 cm for 80% of the study data. Hedge transpiration capacity is a significant component of soil-water balance in the summer, when predicted transpiration reaches about 5*6 mm day−1. One of the most important findings is that hedge transpiration is nearly twice that of a forest canopy. In addition, soil-water content is significantly different whether downslope or upslope depending on the root-water uptake. The high transpiration rate was mainly due to the presence of a shallow water table below the hedgerow trees. Soil-water content was not a limiting factor for transpiration in this context, as it could be in one with a much deeper water table. Hedgerow tree transpiration exerts a strong impact not only on water content within the vadose zone but also on the water-table profile along the transect. Results obtained at the local scale reveal that the global impact of hedges at the catchment scale has been underestimated in the past. Transpiration rate exerts a major influence on water balance at both the seasonal and annual scales for watersheds with a dense network of hedgerows