Use of farm survey returns from the Demonstration Test Catchments to update modelled predictions of sediment and total phosphorus loadings from subsurface drains across England and Wales

Both experimental and monitoring studies have underscored the importance of the agricultural subsurface drain delivery pathway for sediment and phosphorus in England and Wales. Modelling studies to date using the Phosphorus and Sediment Yield CHaracterisation In Catchments (PSYCHIC) model have included a default drain connectivity coefficient of 0.9. Recent telephone and field-based surveys in three Demonstration Test Catchments of farmer attitudes towards field drains and their maintenance have provided new data on the presence of permeable backfill, the frequency of subsoiling, mole draining and maintenance of the freeboard at drain outfalls. These survey returns have therefore been used to revise the default drain connectivity coefficient in PSYCHIC. Revised subsurface drain connectivity at the Water Management Catchment scale ranges from 0.52 to 0.89 with a median of 0.75. Relative to modelled baseline (2010) data using the default drain connectivity of 0.9, the revised estimates of connectivity resulted in a reduction in the prediction of relative contributions from the subsurface drain pathway at national scale of 8.1% for sediment and 6.8% for total phosphorus.