Modeling the interaction between fields and a surrounding hedgerow network and its impact on water and nitrogen flows of a small watershed

The rural landscape in Western Europe, as in many regions of the world, is structured by networks of woody hedgerows that surround agricultural fields. Although many studies at local scale have shown the strong impact of hedgerows on soil water and nitrogen balances, few have quantified the impact of hedgerows at the watershed scale. This study estimated via modeling the impact of hedgerow networks on soil water and nitrogen balances of a small watershed in a temperate climate. The spatially explicit model TNT2 (topography-based nitrogen transfer and transformation), to which a new sub-model of hedgerow functioning was added, was used to perform the study. The unique character of this sub-model is to consider “double-cover” cells (crop plus hedge) to take into account competition for light, water, and nitrogen. The effect of hedge management by branch pruning was also studied. The model was used to simulate a small experimental watershed in western Europe over 17 years with the hedgerow network that existed in 1999 (48mha−1), or without hedgerows. On average, predictions of hedge transpiration and hedge N uptake were consistent with observations from the bibliography. On double-cover cells, the hydro-chemical impact of the hedge resulted from the complex combination of processes, which can have opposite effects. At the watershed scale, the hedgerow network decreased predicted water flow at the outlet by 4.5% and nitrogen flow by 3.3%, respectively, compared to those flows when the watershed had no hedgerows. Finally, hedgerow pruning has a low effect on water and nitrogen flow at the outlet of watershed.