Infiltration measurements in a small lowland catchment

Abstract

Infiltration rates into the glacial boulder clay of Northern Ireland, U.K., were determined from cylinder infiltrometers and from hydrograph analyses in a small instrumented catchment. Infiltrometer readings were made monthly at eleven sites in the catchment for a year. Organic matter appears to be the most important site factor controlling infiltration capacity with slope next in importance. Other factors investigated as site controls on infiltration capacity were soil texture and antecedent soil moisture. Infiltration capacity varied little from site to site at any time of year and was always very low, even in summer. Mean infiltration capacity determined from these site readings for the whole catchment varies from 0.6 mm hr. −1 in January to ∼ 9 mm hr −1 in June. Site variation was greatest in summer. The maximum site reading obtained was 19.7 mm hr. −1 in June. “Average infiltration” rates determined from conventional hydrograph analyses are very similar to the infiltration capacities determined for the whole catchment from the infiltrometer readings, especially in winter. The close similarity in winter between infiltration rates determined from the two independent methods suggests in fact that they each measure the same process (i.e. surface infiltration capacity) or that the process of infiltration is relatively uniform through the soil profile and proceeds at a rate close to the infiltration capacity. The possible occurrence of overland flow is discussed in the light of the results.