Assessment of Available Water Capacity for Estimating the Evapotranspiration of Agricultural Areas in Austria

The combination of different data bases and digital maps, such as soil texture classes, digital land use map, cadastral information, soil valuation and an analytic model makes it possible to estimate the available water capacity (AWC) on a grid for all arable land in Austria. This model has as input the soil horizons and their depths, grain size distribution (sand, silt and clay), humus content, calcium content and pH-value, coarse fraction, structure and compressibility of each soil horizon and depths of sampling. From structure and compressibility the effective density of soil can be estimated. The distribution of the grain sizes determines the type of soil. From soil type and density the air capacity, the wilting point and the total pore volume can be calculated. Field capacity minus wilting point results in AWC. Because the soil information are partly incomplete some assumptions and additional information are used. If compressibility is missing, density of soil is estimated with a certain algorithm. If there is a lack of coarse fraction the gravel content is estimated from the profile sketch. The soil types of the profile description are used if analysis of grain size is missing. All mineral soils are evaluated to a depth of 100 cm, organic soils are evaluated to a depth of 50 cm. The FAO soil map is combined by a GIS with the boundary map of the mapping units (MUs) and the agricultural area of the land use map. The results are ‘soil types after FAO’ for all agricultural areas of the MUs. The ‘typical soil units’ of the Austrian soil map of each cadastral or MU are assigned to the corresponding ‘FAO soil types’ (FST). If it is not possible to assign a ‘typical soil unit’ in one MU to a ‘FST’, the information of the ‘FST’ of the neighbour MU is used. Are there more than one ‘typical soil unit’ to be assigned to a ‘FST’, the information of the shared soil area is adopted. So one or more ‘typical soil units’ are assigned to each MU ‘FST’. If there is only one soil unit assigned to a FST its AWC is adopted, if there are more soil units assigned to one ‘FST’ the weighted average (corresponding to their areas in the MU) of the AWC is adopted. The result is a map of the averaged AWC of the major soil forms in each MU of the agricultural areas of Austria. The map is available as grid with a resolution of 1 km x 1 km. The AWC was classified after DIN 4220. Woodlands have been omitted in this procedure due to the here existing lack of area related data. The AWC map was finally applied in combination with the Penman-Monteith approach for the evaluation of the evapotranspiration for the agricultural used areas in Austria which then was adopted in the Hydrological Atlas of Austria.