Restoring hydrological barriers into hydrological models: enhancing realism and opportunities.

Field boundaries, such as hedgerows, dry-stone walls and fences are used on agricultural fields to control livestock and to separate one parcel of land from another; but these features could also have an important role in modifying hillslope hydrology. Here, we focus on boundary features that we would describe as barrier features, which include walls and hedge banks (i.e. hedges with an underlying earthen or stone bank). These features are effectively impervious barriers to overland flow, so are likely to have a substantial impact on hydrology at the field scale. Surprisingly, mention of these features  is mostly absent from the hydrological literature, particularly in relation to catchment-scale modelling, where it is common to use a topographic representation to model hydrological flow pathways, with little or no parameterisation of such man-made features.

In relevant catchments, including these features within the structure of our models could be beneficial, particularly when trying to better characterise hydrological response times. Furthermore, there are  potential opportunities with regards to semi-natural flood management. Most field boundaries contain gaps in order to provide access to the land via gateways and these access points are often located at the bottom of the field, allowing runoff to continue downslope relatively unimpeded. If these gaps are removed (e.g. by moving a gateway) then the majority of runoff is likely to infiltrate or pond on the surface, resulting in a delayed response. This type of activity could have implications for flood risk management at source, low-flow hydrology and water quality.

We used the semi-distributed hydrological model, Dynamic TOPMODEL for application to the Tamar catchment in South-West England. Using a combination of land use and high-resolution topographic data, we were able to map barrier features across the catchment; it was determined that the majority of agricultural fields contained these features. To account for these features within the model structure, for hydrological response units that were  completely blocked by a hydrological barrier, the overland flow velocity was reduced to ~0, resulting in infiltration and ponding. The model was then calibrated using this new model structure.

We then examined the potential impact of removing gaps in barrier features (i.e. relocating gates).  Rather than removing all gaps in barrier features, we focused on removing gaps that intersected major flow pathways in order to focus on modifications that provided the greatest hydrological impact. As such, we explored the impact of removing gaps that drained flow pathways with a drainage area of 1 and 10 ha, in comparison to the current state of the Tamar catchment. For all scenarios, model results indicate that removing gaps in barrier features leads to reductions in flood peaks but also significant increases in baseflow.