Pedotransfer functions for extended hydraulic models using high resolution soil hydraulic measurements

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An appropriate knowledge of water transport in the soil-plant atmosphere system plays a major role for many topics regarding environmental and agricultural questions. For modeling water transport with the Richards equation, the soil hydraulic functions, i.e. the water retention and hydraulic conductivity function must be known. These functions are usually derived by elaborate measurements with subsequent fitting of appropriate models to the data. Pedotransfer functions (PTF) are often used if such data are not available and cannot be taken. These PTF derive by means of regression approaches parameters of the hydraulic functions from basic soil properties (texture, organic carbon content or bulk density), which are often available. This is especially important for distributed modeling of environmental processes on large scales, like hydrological modeling at the catchment scale or modeling of the soil surface moisture in meteorological models. In the past decades, many PTF have been derived for such purposes. Yet, comparison of such PTF showed that general PTF are not precise for local applications and therefore a special calibration for local and regional applications was required. In our proposal (i) we hypothesize that the hitherto developed PTF neglect essential features of hydraulic properties, like water conductivity in films, and thus lead to systematic errors in the estimated parameters. Furthermore, (ii) these PTF are based on data with limited resolution and moisture range. Due to modern measurement techniques, there exists a multitude of highly resolved hydraulic data in an extended moisture range. New extended and consistent hydraulic models have been developed based on these data. The aim of the project is to combine the new models with the new data to derive new improved PTF. A comparison with classic PTF will show if, and how much, the newly developed PTF lead to better estimates of soil hydraulic properties.