EFFECTS OF MOLING AND CULTIVATION ON SOIL-WATER AND RUNOFF FROM A DRAINED CLAY SOIL

Flow regimes of water draining from replicated mole drained and undrai ned plots under different cultivation systems were examined in a 10-ye ar study. In 9 out of 10 years, winter cereals were grown with all res idues removed by burning. One crop of oil-seed rape was sown-in 1985. A 2 year uniformity trial at the start of the experiment, when all plo ts were tine cultivated, showed that a cultivation pan exerted an impo rtant influence on soil-drainage and water movement. Once removed, eff ective subsurface drainage increased the depth to the water-table by a n average of 215 mm over the winter, with up to 90% of the flow occurr ing through the mole drains. Following the imposition of differential cultivations in 1980, no discernible change in runoff was observed on plots under ploughing compared to the previous tine cultivations. In c ontrast, direct drilling caused higher surface runoff than ploughing d ue to surface compaction, although better subsoil structure developmen t led to more rapid vertical movement of water, and especially in the years following mole drainage an increased peak drain-flow of up to 30 %. Although drainage decreased the overall flood risk by as much as 16 % in a 10 year return period event, cultivations were of considerable importance and direct drilling increased peak runoff by at least 70% f rom both drained and undrained plots.