This thesis aims to quantify variability in soil physical properties on arable land, uncertainties related to data sources, and effects of variability and uncertainty on selected functional criteria. The study was conducted in the Skuterud catchment in South-east Norway, representative of agricultural areas with cereal production on marine deposits. Data collected in other parts of Norway were used. Two soils for aggregate stability measurements were collected from a small catchment located in the municipality of Nes, Akershus county, to represent soils with assumed lower aggregate stability than found for the clay soils in the Skuterud catchment: an artificially leveled soil with clay loam texture, and a Gleyic Cambisol with silt texture. For pedotransfer function (PTF) performance evaluation, data from many parts in Norway were collected to form a database. Marine deposits, brackish flood sediment, fluvial deposits and glacial till are represented, all common parent material for arable soils in Norway. Textural compositions of the samples varied from sandy to heavy clay. Two point PTFs developed for soils in Norway, and six parameter PTFs developed for soils in Europe and USA, were evaluated using multiple statistical indicators. The point PTFs showed overall good performance. The parameter PTFs showed variable performance. The class PTFs showed poorer performance than the continuous PTFs, especially if organic matter was not an input to the PTF. Implications of variability, uncertainty and data source were investigated for selected functional criteria in the Skuterud catchment. This study also showed that differences related to choice of data source could be larger than differences as a result of different risk of runoff and erosion (crop covered situation versus "worst case" situation with reduced soil stability and without crop cover). The major conclusion of this work is that inadequate choice of input data sources can significantly underestimate or overestimate optimum workability (W opt), number of days until W opt is obtained, surface discharge and soil loss, and consequently the effect of e.g. climate change and measures.
