Overland flow initiation from a drumlin grassland hillslope

Abstract

AbstractAccurate prediction of overland flow requires an understanding of the hydrological processes controlling its occurrence over a range of spatial and temporal scales. This study investigated the factors controlling the initiation of overland flow from grassland on a drumlin. From January 2003 to December 2006, fine scale monitoring of soil moisture, rainfall and overland flow were carried out at a grazed grassland site in Northern Ireland. Logistic regression analyses were used to identify the factors controlling the initiation of overland flow. Relationships between the observed volumetric soil moisture (VSM) and soil moisture deficit (SMD) values predicted from regional meteorological data were also compared. Results demonstrated that although saturation excess overland flow occurs at this site, 59% of overland flow events occurred on days when soil moisture was below field capacity. The logistic regression analysis confirmed that when soils were below field capacity, average rainfall intensity was the most important variable in determining the probability of overland flow occurring followed by the SMD. Although VSM values at this site were correlated with two sets of modelled values of SMD, one based on the meteorological office rainfall and evaporation calculation system model of the UK Meteorological Office and the other based on a study from Ireland, neither model provided accurate indicators of the risk of overland flow. Each significantly overestimated the number of days when soils were at or above field capacity. This uncertainty and the predicted increase in high‐intensity rainfall events as a result of climate change pose challenges to the use of SMD as an indicator of the risk of overland flow.