Modelling the hydrological impacts of open ditch drainage

Open ditch drainage has been observed to have a significant effect on catchment runoff regimes, yet the processes generating the changes are not well understood. An approach of physically based modelling is used, with a fine grid resolution, on a very simple hill-slope model to analyse how ditches affect runoff. Results from the simulations show that the most important process is the change in behaviour of the sub-surface, under near-saturated conditions, when there is a much higher and more variable sub-surface exchange flow into the stream network. The speed of the surface runoff response is also affected. From the results of the analysis a method using effective parameterisation has been developed to recreate the effects of the drainage in a coarser resolution catchment model. The method involves calculating effective saturated hydraulic conductivities for the sub-surface flows and effective Strickler coefficients for the surface flows. The application of the method is demonstrated for a sub-catchment of the River South Tyne at Alston. Results from these simulations show that predictions of flow for the catchment can be improved by using the effective parameterisation to create a pseudo drainage representation of the sub-grid-scale channels within the model. Simulations of more intensive drainage scenarios show that the effects of open ditch drainage on hydrology are significant and that the direction of change is not intuitive.