Infiltration from surface and buried point sources: The average wetting water content

The assumption in analytical solutions for flow from surface and buried point sources of an average water content, (theta) over bar, behind the wetting front is examined. Some recent work has shown that this assumption fitted some field data well. Here we calculated (theta) over bar using a steady state solution based on the work by Raats [1971] and an exponential dependence of the diffusivity upon the water content. This is compared with a constant value of (theta) over bar calculated from an assumption of a hydraulic conductivity at the wetting front of 1 mm day(-1) and the water content at saturation. This comparison was made for a wide range of soils. The constant (theta) over bar generally underestimated (theta) over bar at small wetted radii and overestimated (theta) over bar at large radii. The crossover point between under and overestimation changed with both soil properties and flow rate. The largest variance occurred for coarser texture soils at low-flow rates. At high-flow rates in finer-textured soils the use of a constant (theta) over bar results in underestimation of the time for the wetting front to reach a particular radius. The value of (theta) over bar is related to the time at which the wetting front reaches a given radius. In coarse-textured soils the use of a constant value of (theta) over bar can result in an error of the time when the wetting front reaches a particular radius, as large as 80% at low-flow rates and large radii.