Here we analyze the temporal stability of the soil moisture content in an area of 1285 km2 located in the central sector of the Duero basin (Spain). The data from a network consisting of 23 soil moisture‐measuring stations (REMEDHUS) over a period of 36 mo (from June 1999 to May 2002) were used. At each station, soil moisture was measured fortnightly, using time domain reflectometry (TDR), at different depths to the bottom of the profile. From the hydrological point of view, the stations are independent from one another and were distributed in space using physiographic and pedological criteria. The results clearly show which stations are representative of wet and dry conditions and the persistence of the representative stations under even the most extreme situations. The temporal evolution of the stability patterns points to a considerable degree of persistence during the periods analyzed. The stations representative of dry conditions are seen to be much more stable at all the depths analyzed (5, 25, 50, and 100 cm) than those identified within the wet sector. Overall, the soil moisture content showed considerable temporal stability throughout of the investigation. However, a clear correlation was observed between mean soil moisture and variance (r = 0.85) across the range of measurements used. Accordingly, temporal stability is always higher when the soils are dry than when the water content is high. It was also observed that the periods with the lowest temporal stability coincided with situations involving the transition from dry to wet. The critical periods in terms of temporal stability are those in which the recharge of water in the profile occurs.

Abstract:The objective of this study was to validate the soil moisture data derived from coarse-resolution active microwave data (50 km) from the ERS scatterometer. The retrieval technique is based on a change detection method coupled with a data-based modelling approach to account for seasonal vegetation dynamics. The technique is able to derive information about the soil moisture content corresponding to the degree of saturation of the topmost soil layer (¾5 cm). To estimate profile soil moisture contents down to 100 cm depth from the scatterometer data, a simple two-layer water balance model is used, which generates a red noise-like soil moisture spectrum. The retrieval technique had been successfully applied in the Ukraine in a previous study. In this paper, the performance of the model in a semi-arid Mediterranean environment characterized by low annual precipitation (400 mm), hot dry summers and sandy soils is investigated. To this end, field measurements from the REMEDHUS soil moisture station network in the semi-arid parts of the Duero Basin (Spain) were used. The results reveal a significant coefficient of determination R 2 D 0Ð75 for the averaged 0-100 cm soil moisture profile and a root mean square error (RMSE) of 2Ð2 vol%. The spatial arrangement of the REMEDHUS soil moisture stations also allowed us to study the influence of the small-scale variability of soil moisture within the ERS scatterometer footprint. The results show that the small-scale variability in the study area is modest and can be explained in terms of texture fraction distribution in the soil profiles.

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Soil hydrology was investigated in the Guadelperalón experimental watershed in order to determine the influence of land use and vegetation cover on runoff and infiltration within the Dehesa land system. Five soil-vegetation units were selected:(1) tree cover, (2) sheep trials, (3) shrub cover, (4) hillslope grass and (5) bottom grass.The results of the simulated rainfall experiments performed at an intensity of 53·6 mm h −1 during one hour on plots of 0·25 m 2 , and the water drop penetration time test indicate the importance of water repellency in the Dehesa land system under drought conditions. Low infiltration rates (c. 9-44 mm h −1 ) were found everywhere except at shrub sites and in areas with low grazing pressure. Soil water repellency greatly reduced infiltration, especially beneath Quercus ilex canopies, where fast ponding and greater runoff rates were observed.The low vegetation cover as a consequence of a prolonged drought and grazing pressure, in conjunction with the soil water repellency, induces high runoff rates (15-70 per cent). In spite of this, macropore fluxes were found in different locations, beneath trees, on shrub-covered surfaces, as well as at sites with a dominance of herbaceous cover. Discontinuity of the runoff fluxes due to variations in hydrophobicity causes preferential flows and as a consequence deeper infiltration, especially where macropores are developed.

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