Temporal stability of soil moisture in a large-field experiment in Spain

Here we analyze the temporal stability of the soil moisture content in an area of 1285 km(2) 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.