A comparison of water flux measurements: passive wick-samplers versus drainage lysimeters

Quantification of soil water flow is a prerequisite to accurate prediction of solute transfer within the unsaturated zone. The monitoring of these fluxes is challenging because the results are required to answer both scientific and practical questions regarding protection of groundwater, sustainable management of agricultural, forestry, mining or set-aside industrial areas, reducing leachate loss from landfills or explaining the fate of environmentally harmful substances. Both indirect and direct methods exist for estimating water-flux rates and have been used with varying success. In Europe, the use of direct lysimetry methods for measuring water and solute fluxes in soils has increased in recent years. This technique ensures reliable drainage data, but requires relatively large investment and maintenance expenses. Other research groups, especially in the USA, have developed alternative techniques. In this paper we compare the functioning of a passive-wick sampler, especially the deep-drainage meter type (DDM), with two different types of drainage lysimeters (weighing and non-weighing) under field conditions in Germany for the measurement period from May 2004 until April 2009. The study showed that under sandy soil conditions no significant differences occurred between the measurements from DDM and both drainage lysimeter types. Only in periods with increased precipitation was there a tendency of drainage over-estimation by the DDM in comparison with the lysimeters tested. For longer periods, no significant differences in the amount of drainage or the pattern of drainage formation were found between weighing and non-weighing gravitation lysimeters. The practical use of DDMs is restricted because the groundwater level must be >2 m from the soil surface. Suggestions are made for the technical improvement of the DDM as well as the testing of the device with more cohesive soils.