The role of fractures in transporting water in the unsaturated zone of the Chalk has been a source of controversy for many years. There is clearly a potential for fractures to transport water and contaminants rapidly to the saturated zone, although direct observations are rare. The evidence from well hydrographs, solute profiles and soil water measurements is ambiguous concerning the importance of by-pass flow. This paper examines drainage data from several years’ operation of a 5 m cube undisturbed monolith lysimeter at Fleam Dyke, Cambridgeshire, for evidence of by-pass flow and investigates the factors responsible for its onset. Evidence from measurements of unsaturated hydraulic conductivity of the Chalk at the site and in situ measurements of soil water are also presented. Both the lysimeter and soil water observations agree that flow through fractures accounts for approximately 30% of the total drainage from this site, significantly higher than has been inferred at other Chalk sites. By-pass flow was observed to occur above a threshold of approximately −50 cm H 2 O matric potential, or above a flow rate of approximately 1 mm day −1 . During periods of no soil water deficit, about 50% of drainage occurred via fractures. In general, there was a delay of three or four days between rainfally and drainage at the base of the lysimeter. On once occasion, intense summer rainfall satisfied the existing soil moisture deficit and initiated fracture flow, the peak in drainage through the lysimeter occurring within 24 hours. Drainage from the lysimeter was consistently higher than MORECS estimates of hydrologically effective precipitation during the period after the onset of soil water deficits.

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