Tuesday, 11 July 2006

Accuracy of Soil Water Balance Parameters Measured by Large Weighing Lysimeters.

Holger Rupp1, Ralph Meissner1, Juliane Seeger1, and Manfred Seyfarth2. (1) UFZ Centre for Environmental Research Leipzig-Halle, Department of Soil Science, Lysimeter Station, Dorfstrasse 55, Falkenberg, 39615, Germany, (2) UGT Environmental Measuring Techniques Ltd., Eberswalder Strasse 58, Muencheberg, 15374, Germany

A prerequisite to develop sustainable strategies for land use and protection of water resources in quantity and quality is the exact measurement of water and solute fluxes below the root zoone. The direct measuring of water flux densities with lysimeters has a long tradition. In the last years a new generation of lysimeter types with improved measuring techniques have been developed. The aim of the poster is to demonstrate the accuracy of the measuring technique (especially the electronic balancing system) and to present results from case studies regarding the high precision of water balance parameters. The measuring process of lysimeter weighing is individually adjustable on the used lysimeter type and allows a highly sophisticated spatial and temporal resolution. The installed computer software allows the presentation of all measured parameters in detail (e.g. average, minimum and maximum values of the measured data). An example for the high precision of the weighing technique will be demonstrated on a new constructed gravitation lysimeter (surface area 1 m2, depths 2 m). Here it is possible to ascertain experimentally, besides precipitation and seepage, the mass input of dew and fog. The accuracy of the used weighing system is high enough to measure hourly a weight increase of minimal 0.03 mm which can be for instance attributed to dew formation. Dew formation takes place, if the temperature of the surface it forms on falls below the dew point temperature. To check if the weight gain was indeed a result of dew formation, the minimum temperature observed and the dew point temperature obtained from data for air temperature and relative humidity are compared. The investigations have proven that the lysimeter weight increases of about 0.4 mm can indeed be attributed to dew. By another example it was demonstrated that a weight gain, which reached an equivalent of 0.24 mm precipitation was caused by fog. Therefore, this type of lysimeter allows a highly sophisticated spatial and temporal resolution of the soil hydrologic process and can be used as a basis to test and validate corresponding models. The described data are based on a 6 minute recording interval. A mean value was computed for the 10 observations per hour. Furthermore, first results of a newly developed weighable groundwater lysimeter are presented. In order to balance groundwater inflow and outflow, weight changes need to be recorded with a little delay as possible. Weight is recorded every minute. The measurements are condensed to 15-minute averages (rolling average) to reduce external effects, for example caused by wind (oscillation of the lysimeter weight) or short term weight increases (due to passing animals etc.). Estimates of the individual quantities of the soil water balance of the flood plain sites studied are presented. In fen soils (Histosols) lateral fluxes are very important. A special lysimeter was developed to measure these fluxes with high accuracy. This weighable special lysimeter (length 4m, width 1m, and 1.5 m depths) enables us to measure parameters of water and solute fluxes in a fen soil monolith. The load of the soil monolith is about 8,000 kg. First results of the fen lysimeter weighing are presented. The balancing system achieves a higher accuracy compared with a conventional rain gauge. Under consideration of the lysimeter surface area of 4 m2 the measured precipitation (by rain gauge) was reflected completely in the data of the lysimeter weighing. Therefore precipitation and evapotranspiration can be reliably measured with an accuracy of 0.1 mm.

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