WATER STORAGE IN SOILS DURING THE FALLOW - PREDICTION OF THE EFFECTS OF RAINFALL PATTERN AND SOIL-CONDITIONS IN THE EBRO VALLEY OF SPAIN

Fallowing remains a feature of dryland cereal agriculture in some low rainfall areas of central and northern Spain. To complement the limite d number of measurements of water stored during the fallow, we develop ed a physically based simulation model to estimate the effects of vari ations in rainfall, soil characteristics and surface conditions on wat er storage in the profile. Mean annual rainfall in the locations inves tigated 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 ba sed on 25 simulations, varied from 11-39 mm. Rainfall in the last 3 mo nths of the fallow was the principal cause of this year to year variat ion in storage. Surface stoniness and crop residues decreased evaporat ion from the soil and increased storage: there was very little drainag e. These findings are consistent with measured water storage in soils in this part of Spain, and other areas of the world with similar clima tes and agricultural practices. Based on barley yield/rainfall regress ions for data from a dry area in the Ebro valley, we estimated that th e annual yields from a crop-fallow system would be 15% greater then th ose from annual cropping. For fallowing to be economic, yields per cro p would need to be about twice those obtained with annual cropping. Th ere may be yield benefits from fallowing apart from those resulting fr om extra water storage in the soil. Unless such benefits can be demons trated, fallowing would appear to be uneconomical in this area of Spai n. (C) 1998 Elsevier Science B.V.