Water storage in soils during the fallow: prediction of the effects of rainfall pattern and soil conditions in the Ebro valley of Spain

Abstract Fallowing remains a feature of dryland cereal agriculture in some low rainfall areas of central and northern Spain. To complement the limited number of measurements of water stored during the fallow, we developed a physically based simulation model to estimate the effects of variations in rainfall, soil characteristics and surface conditions on water storage in the profile. Mean annual rainfall in the locations investigated varied from ca. 300–500 mm and the mean amount of water stored during the last year of a bare fallow ranged from 1–48 mm, depending on soil and climate. The standard deviations of these amounts, each based on 25 simulations, varied from 11–39 mm. Rainfall in the last 3 months of the fallow was the principal cause of this year to year variation in storage. Surface stoniness and crop residues decreased evaporation from the soil and increased storage: there was very little drainage. These findings are consistent with measured water storage in soils in this part of Spain, and other areas of the world with similar climates and agricultural practices. Based on barley yield/rainfall regressions for data from a dry area in the Ebro valley, we estimated that the annual yields from a crop–fallow system would be 15% greater than those from annual cropping. For fallowing to be economic, yields per crop would need to be about twice those obtained with annual cropping. There may be yield benefits from fallowing apart from those resulting from extra water storage in the soil. Unless such benefits can be demonstrated, fallowing would appear to be uneconomical in this area of Spain.