The performance and behavior of land drainage systems and their impact on field scale hydrology in an increasingly volatile climate

Escalations in rainfall intensity, both in terms of volume and frequency are increasing the volatility associated with grassland agriculture on poorly drained soils. The principal mechanism of reducing this volatility is by means of land drainage; however the efficacy of drainage systems is widely variable and has not been fully quantified. The excavation of soil test pits and a corresponding examination of the soil profile enables bespoke land drainage system design. Across heterogeneous soil-scapes this leads to variations to both groundwater and shallow drainage designs. In the present study we examine the performances of 9 site-specific drainage systems (5 groundwater and 4 shallow drainage designs), during a high rainfall period (01/10/2015-31/05/2016) in terms of response times (start, peak and lag times), discharge characteristics (peak flow rate, total discharge, flashiness index, discharge hydrographs) and water table control capacity. Response times were not affected by drainage system or drainage design type, showing similar responses despite variation in soil types where appropriate drainage systems are installed. Total discharge (1098.4 vs. 189.6 m3/ha) and peak flow rate (51.0 vs. 16.8 m3/ha/h) were significantly higher in groundwater designs relative to shallow alternatives. Groundwater drainage designs generally maintained a deeper mean water table depth (0.82 m) than shallow designs (0.53 m) during the study period. The functional capacity of each land drainage system was inherently different. The comparison of such systems highlights contrasting behaviors of individual drainage systems and drainage design types, which is dictated largely by the hydraulic capacity of the soil within their catchment and their connectivity to different water bodies (groundwater versus perched water). All systems reduced the overall period of waterlogging and improved the conditions for both the production and utilization of the grasslands they drain, although temporal variations in agronomic parameters are likely to be more pronounced in shallow designs.