A classification of drainage and macropore flow in an agricultural catchment

Abstract

Abstract:This paper uses a variety of multivariate statistical techniques in order to improve current understanding of the antecedent and rainfall controls on drainage characteristics for an agricultural underdrained clay site. Using the dataset obtained from a two-year hillslope study at Wytham (Oxfordshire, UK) a number of patterns in the nature and style of drainage events were explored. First, using principal components analysis, a distinction was drawn between drainflow controlled by antecedent conditions and drainflow controlled by rainfall characteristics. Dimensional analysis then distinguished between two further types of drainflow event: antecedent limited events (ALE) and non-antecedent limited events (NALE). These were drainflow events requiring a minimum antecedent hydraulic head to occur (ALE) and events that occurred in response to rainfall irrespective of the antecedent conditions, because the rainfall was either of high enough intensity or duration to prompt a response in drainflow (NALE).The dataset also made possible a preliminary investigation into the controls on and types of macropore flow at the site. Principal components analysis identified that rainfall characteristics were more important than antecedent conditions in generating high proportions of macropore flow in drainflow. Of the rainfall characteristics studied, rainfall amount and intensity were the dominant controls on the amount of macropore flow, with duration as a secondary control. Two styles of macropore flow were identified: intensity-driven and duration-driven. Intensity-driven events are characterized by rainfall of high intensity and short duration. During such events the amount of macropore flow is proportional to the rainfall intensity and the interaction between macropore and matrix flow is kinetically limited. The second style of macropore flow is characterized by long-duration events. For these events the amount of macropore flow approaches a maximum value whatever the rainfall duration. This suggests that these events are characterized by an equilibrium interaction between macropores and matrix flow.

Long-term changes in hydrological pathways in an upland peat catchment—recovery from severe drought?

“…For example, Holden and Burt (2002) showed that there was no difference to flowpaths caused by drought when considering results of a bromide tracer experiment, but when considering moisture content an effect could be demonstrated. Therefore, chemical tracers should be considered as one line of evidence and that a study of changes in rainfall-runoff relationships and the event-based hydrology of catchment would be the next line of evidence to consider (Heppell et al, 2002). Secondly, that in such a long-term dataset, as used in this study, the flow pathways being represented are those of average conditions and not just storm runoff conditions.…”

Morphological characterization of solute flow in a brown earth grassland soil with cranefly larvae burrows (leatherjackets)

“…The greater organic matter content in the grassland soils will have reduced the swelling and shrinking (Hillel, 2004) and therefore reduced the likelihood of desiccation crack formation in the grassland soils. Many studies have examined preferential flow down desiccation cracks but most of these studies have been focussed on clay soils (Brown et al, 1999;Heppell et al, 2002) or peat soils (Holden, 2005;Holden et al, 2001) which are more susceptible to shrinkage and swelling and hence more prone to cracking. Field evidence suggested that in the arable sandy loam soils tested, the smaller cracks may have been destroyed by swelling during wetting and some of the larger ones were filled with debris which impeded flow.…”

Seasonal variation of DOC concentration and annual loss of DOC from an Atlantic blanket bog in South Western Ireland

“…In order to analyse the DOC data in terms of the relationship to changes in stream flow, the record was considered as a series of events (see approach by Heppell et al 2002;Worrall et al 2002). The following parameters were tabulated: absolute increase in discharge, absolute decrease or increase in DOC concentration and time between events.…”