Runoff and subsurface drain response from mole and gravel mole drainage across episodic rainfall events

• We measure mole and gravel mole drain performance in a clay-loam soil.
• We examine surface runoff and subsurface drain discharge over 12 rainfall events.
• Response times were particularly reactive to changes in soil moisture status.
• Gravel mole drainage reacted faster to input precipitation than other treatments.
• Gravel mole drainage performed better than mole drainage in this soil type.

Shallow drainage techniques such as mole and gravel mole drainage are used in low permeability soils to siphon off infiltrating rainwater and mitigate the associated rise in watertable. Their purpose is to improve trafficability and agricultural production. In Ireland, long-term climate predictions envisage an increased level of short-term extreme rainfall events. Therefore, a key question is how these drainage techniques perform during episodic, high intensity rainfall events, specifically in terms of discharge hydrographs and associated parameters (principally flow start time, flow peak time, lag time, peak flow rate and flashiness index). We examined 12 rainfall events over a 1 year period on a clay-loam dominated grassland site of 1.4% slope in the south of Ireland. Four drainage treatments, namely; (A) an un-drained control, (B) Mole drainage installed in January 2011 (sub-optimal installation conditions), (C) Mole drainage installed in July 2011 (optimal installation conditions) and (D) Gravel mole drainage installed in July 2011 were examined. Results showed that gravel mole drainage exhibited shorter response times to rainfall events and ultimately drained greater volumes. Drain flow from mole drainage treatments B and C produced longer start, peak and lag times and lower peak and total flows relative to the other flow discharges. Variations in discharges from all treatments were closely correlated to soil moisture status, 30 days antecedent rainfall and rainfall event intensity. Drain flow response in all treatments was seen to deteriorate in time with the strongest responses evident in early events. Flow hydrographs showed strong variation in flow characteristics, within and across treatments and across events. If the predicted increase in short-term extreme rainfall events materialises then such systems will have to operate in increasingly adverse conditions. This will require changes in system design to improve the effectiveness of mole and gravel mole drainage.