Seasonal soil moisture response time to rainfall in a rainfed organic vineyard with permanent ground cover under temperate climate

This study evaluates the soil moisture response time to rainfall of a small vineyard with two varieties of grapes (Blanco legítimo –BL–, and Agudelo –Ag–) and a permanent ground cover of resident vegetation with numerous plant species (n > 12). No tillage was done and weed control included three mowing passes. The soil –Umbrisol–, is shallow (35 cm depth), stony (36.1% weight of rocks), rich in organic matter (7.2%) –specially in the topsoil (10.0%)– and no difference was observed throughout the field. The study area is located in the municipality of Betanzos (43° 15' 56.20" N; 8° 12' 1.32" W), A Coruña, Spain; under a temperate oceanic climate. The time of response between each precipitation peak and its corresponding peak of soil moisture was calculated for a 242-day period (26^th February – 25^th October 2021), covering the whole crop cycle. The determined parameters were: (I) volumetric water content (ΔS, %), (II) the peak to peak time (TP2P, min), and (III) the initial response time (Ti, min). A weather station was installed in the field, along with 12 capacitance-based technology soil moisture sensors, 6 in the rows (R) and 6 in the inter-row areas (IR) for the two cultivars. Each probe measured at 5, 15 and 25 cm depth, every 15 minutes. During the study period, a total number of 118 rainfall events were recorded, observing a clear response in 96, 82 and 75 events at 5, 15 and 25 cm depth. No response was observed in 22 events of low rainfall. To refine the analysis, three hydro-thermic periods were identified: Drying and warming (spring), dry and warm (summer), and wetting and cooling (autumn). In the events of longer duration, no defined patterns were observed in responses to moisture between the three layers, but differences were observed in response to rainfall at the depth of 25 cm. In rainfall events of short duration (15-30 min), the pattern in response to soil moisture at 5 and 15 cm was similar in the 3 parameters (ΔS, P2P and Ti). With respect to the minimum values, there was a rise of moisture within the profile, specifically, in R and an interspersed pattern in IR. For the maximum values, there was a descending pattern within the profile regardless the zone or variety with the exception of Ag in IR. In Ag cultivars, both variety and zone were affected by depth, with the lowest correlation at 5 cm. However, in BL cultivars the correlations did not vary clearly between depths or zones. In response to rainfall, the highest correlations were observed at 25 cm and the lowest at 5 cm for variety and zone. On average, Ti was 67, 127 and 160 min at 5, 15 and 25 cm, and P2P was 228, 344 and 378 min at 5, 15 and 25 cm depth. The hydro-thermic periods as well as the intensity and duration of the precipitation events clearly modulated the moisture response to rainfall in the studied soil.