Comparison of soil water-contents as measured with a neutron probe and time domain reflectometry in a Mediterranean forest (Sierra de Gata, Central Western Spain)

The present work compares the results obtained with two indirect techniques (neutron probe and time domain reflectometry, TDR) for measuring soil water contents (theta) at different depths on an experimental plot in the Sierra de Gata (Central Western Spain). At the same time, the temporal and spatial evolution of theta was assessed in the zone studied. Measurements were made over three years (from September 1996 to August 1999). The results point to a good correlation (r = 0.98) between theta measured with both techniques, although TDR slightly (but significantly; P < 0.01) underestimated the θ values, but only during the drying periods and with low θ values. Non-significant differences between both methods were found for the estimation of soil water contents in the whole soil profile. Accordingly, both techniques are interchangeable and it is thus possible to avoid the potential risk of radioactivity. Considerable inter-annual variability was observed in the soil water content, governed by the annual rainfall distribution. A flowing-off effect was observed, followed by a progressive re-wetting of the soil profile from the bottom. Both the temporal and spatial variabilities of soil water content were found to decrease with depth.