Soil water-holding capacity assessment in terms of the average annual water balance in Southern Spain

Knowledge of the soil water-holding capacity, w(0), is essential to the evaluation of regional balance. In this paper, we produced a map of w(0) for the region Andalusia, southern Spain, using pedo-transfer functions (PTFs) and geostatistics. The information available of analytical and morphological data from 521 soil profiles the region, and the 1: 400 000 soil map of Andalusia. The w(0) values were calculated using 10 published PTFs. The soil map only slightly improved the spatial interpolation of the PTF-calculated w(0)' s. The PTF estimates for w(0) ranged from near 0 to 235 mm, with an average value of 110 mm and a SD of 48 mm. Since no independent field observations were available, the w(0) estimates were evaluated in terms of the average annual total runoff and actual evapotranspiration. Both components were calculated at 160 meteorological observatories using a simple bucket water balance model, driven by daily meteorological data. The spatial variability of w(0) had little effect on the calculated average annual balance of the region. Increasing w(0) to 150 to 200 mm produced better fit of the water balance predicted with Budyko's empirical functions. The difference could be partly explained by seasonality-related characteristics of the climate in the region. Comparison of the results with other studies suggests that the estimated w(0) values should be increased by 45%. These differences can be attributed entirely to an inconsistent definition of field capacity (FC).