The concept of virtual lysimeters to measure groundwater recharge and evapotranspiration

Virtual lysimeters are devices similar to traditional lysimeters to investigate the hydrological (or substantial) balance of a soil body but without separation from its environment. Improvements of measuring equipment facilitate the application of this simple concept to record the soil water balance in the field. The components of the hydrological cycle are determined by applying soil physical principles on soil water movement and storage and measured soil physical parameters like soil water content and soil matric potential. First experiences with this concept at field scale at the experimental field of the University of Agricultural Sciences Vienna proved the usefulness of this method but there are some suggestions for improvements. Virtual lysimeters have the main advantage to maintain the natural boundary conditions, to be cheap (no expansive constructions), not to obstruct root development and no side wall effect of water seepage may occur. There exist some problems to measure matric potential close to the soil surface. Moreover restricted usage of tensiometers and gypsum blocks are not solved satisfactory till today. It is recommended to calibrate TDR or FDR sensors in the field, soil temperature should be measured and all sensors should be installed from the soil surface. When soil conditions allow it, sensors should be installed at least to a depth of two meters. It is still a problem to measure deep seepage (groundwater recharge), but analyzing different flow conditions allows to estimate its amount from measured data by use of the gradient of the total soil water potential and changes of the soil water content.