The extent of soil erosion in some Spanish semiarid regions has caused great concern regarding the sustainability of soil
resources. Accelerated soil erosion, particularly in some Mediterranean areas, is likely to be one of the main environmental
problems associated with climate change. Fallout 137Cs has been shown to provide a reliable basis for assessing soil erosion rates in different environments around the world.
However, existing information concerning the spatial variation of 137Cs inventories at reference sites has identified a need for further investigation of the factors affecting their spatial variability
in semiarid environments, where stony and skeletal soils are predominant. Reference sites at three locations in the central
Ebro valley were selected to investigate the 137Cs content of several grain size fractions. Each site included both natural vegetated conditions and cultivated land and the
three sites were characterized by different values of mean annual rainfall. The results obtained demonstrate the influence
of lithology, land use and climate on the spatial variability of 137Cs inventories that increase from 1190, to 1500 and 1710 Bq·m−2 with increasing annual rainfall values from 300 to 500 mm at the study sites. The soils on marls at the Valareña site had
the highest proportion of 137Cs in the coarse fractions of cultivated soils (12%) in comparison with soils developed on limestones at Loma Negra (5%),
whereas no 137Cs content was found in the coarse fractions of soils on glacis-terrace materials at Peñaflor. The 137Cs reference inventories are higher in soils on marls and sands at cultivated locations at Valareña and Peñaflor, but have
similar values in soils at cultivated and uncultivated locations on limestones at Loma Negra. Therefore, in absence of level
undisturbed soils with natural vegetation cover, cultivated flat soils on hard rocks could provide reliable reference inventories.