Unsaturated zone modelling: The role of soil database classification

Abstract

Physical and hydraulic soil properties are essential input parameters for models from different sciences (e.g. hydrology, agriculture, water management, nature preservation). Generally texture composition, porosity and other easily measurable physical properties of soils are known. However, saturated hydraulic conductivity and characteristic values of the water retention curve are usually missing information. Therefore, based on the physical similarity of soils (classes), they are substituted by data derived from soil databases. The aim of this study was to assess the currently unknown uncertainties of such classified databases. To do so, a large variety of tests were carried out: (i) static and dynamic, (ii) 1D and 3D (iii) hydraulic and hydrologic applied tests, (iv) real and synthetic soils, parameterized accordingly, and (v) HUNSODA and/or HYPRES databases. The results were sorted with respect to FAO and USDA classification systems. Soil class overlapping was evaluated through the statistics of basic hydraulic parameters (retention curve, hydraulic conductivity). Indicators related to hydrologic extremities (excess water and drought) were used to quantify the uncertainties of soil texture based on parameter substitution. It was concluded that the two evaluated soil classification systems did not sort soils reliably from the hydrologic and hydraulic viewpoint: the test results of classes showed major overlaps. Moreover, in most cases class synthetic parameter combinations poorly represented real soils. As a general consequence the results based on classified soil databases should be accepted only with reservation.