Soil moisture and groundwater drawdown in a dry grassland soil

A study of soil moisture is reported for a free-draining podzolic soil with a watertable at 4 m below ground level. This soil type, derived from limestone glacial deposits, is common in Ireland. Pore-water content and negative pore-water pressure were measured throughout the summer of 1992. Evapotranspiration and drainage to the groundwater were computed using both the zero flux plane and the water balance methods. Examples of soil hydraulic potential and soil moisture content profiles are presented. A mathematical model was used to model the soil moisture profile for a dry summer. Field results showed that a zero flux plane developed in early May and moved intermittently downwards to a maximum depth of 1.5 m as the summer progressed. The hydraulic potential and pore-water-content profiles responded to the continuously changing balance between evapotranspiration and rainfall. Pumping of the aquifer to lower the watertable by 3.5 m had no effect on the soil moisture above the watertable. There was reasonable agreement between measured evapotranspiration and the predicted values derived by Penman's equation (MAFF, 1967). The model study indicated that the zero flux plane reached a maximum depth of 1.75 m in a dry summer.