Soil moisture distribution predicted from topography and gamma radiation
Abstract: In this study, GIS methods were used in order to evaluate different wetness indicators — topographic indices and airborne gamma radiation measurements — from their ability to distinguish wet areas from other areas. As a surrogate measure of extreme wetness, the occurrence of mires from land use maps were used. The evaluation of each wetness indicator was made in two ways. First, mire and non mire values for each measure was tested for similarity. Then, predicted mire maps were produced by defining mire area class limits (threshold values), which gave the same fraction of mire area in the catchments as in the land use maps. The predicted mire maps were then compared to the map showing mires from land use surveys. The best predictions were made by the drainage efficiency index (46.8% correct mires of all mire cells), which was proposed as an alternative to the ln(α/tanβ) index (the TOPMODEL index) and slope. This index quantified the downslope drainage ability for any point in the catchment, which is likely to be important for the wetness status. Mire predictions from K-40 gamma radiation were correlated to the different geological regions in the area and did not reproduce an accurate overall mire pattern. The TOPMODEL index class limits were strongly correlated to the fraction of mire area, which prevented it from being a good indicator of mires. It did not reproduce accurate amounts of area in the subbasins from a global threshold value. However, the general pattern of the predicted mires agreed relatively well with the pattern of actual mires from the land use map.
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