Dynamics of water movement on Chalkland

Abstract

The dynamics of water movement through the unsaturated zone of Upper Chalk in Southeast England were investigated during the winters of 1995–1996 and 1996–1997. Regular measurements of soil water content and soil matric potential were made weekly, by neutron probe (to 3.6m) and ordinary tensiometers (to 2.4m) and half-hourly by theta probe and pressure transducer tensiometers (both to 1.2m). Drainage was determined using the water balance method.Weekly measurements of water content showed changes of 5–8% in the upper 2.0m and about 2% at depths >2.0m during the season with only slight changes from week to week at 0.4m and 0.2m in response to rain showers. Matric potentials decreased with depth throughout the winter seasons with potentials in the soil ranging from −25 to −550cm water and those below 1m typically −100 to −950cm water. The mean daily rate of drainage for the two seasons was 1.6mmd−1 and 1.5mmd−1 at 1.2m depth and 1.45mmd−1 and 1.1mmd−1 at 3.6m. The mean maximum drainage rates calculated from weekly data were 4.6mmd−1 at 1.2m and 3.1mmd−1 at 3.6m. These weekly measurements suggested that flow was through the matrix because potentials were continuously low and the weekly rates of drainage were smaller than the hydraulic conductivity of the chalk matrix.However, half-hourly measurements showed rapid increases of water content and water potential at all depths to 1.0m under a range of conditions after rain giving short-term maximum rates of drainage at 1.2m of up to 26mmd−1. Typically, water took about 7–9h to travel from the surface to 1.2m depth depending on the rainfall and antecedent conditions. These results indicate that fissures conducted water in the upper 1.2m immediately after rainfall although the matric potential at 1.0m was >−50cm water on only one occasion in the two seasons and frequently <−300cm even after rain.