How much is soil volumetric water content influenced by cover crops in an olive grove in central Spain?

Cover crops enhance soil infiltration, basically improving soil structure and reducing runoff speed. They are a key tool to reduce soil loss by water erosion in olive groves. In this work, soil volumetric water content at 15-and 30-cm depths in an olive grove under different managements was studied during five phenological phases: flowering, phase I (fruit growing), phase II (after pit hardening), phase III (oil accumulation) and rest. The management schemes were: i) annual cover crop of barley; ii) annual cover crop of a legume; iii) permanent cover crop of Brachypodium distachyon and iv) control (tillage once a year). The olive grove is situated in central Spain (south Madrid region). Trees are spaced at 6x7 m under rainfed conditions. The climate is semiarid, with highly irregular precipitation. The shallow gypsiferous soil is classified as Haplic Gypsisol. Water volumetric content was measured by employing EC-5 sensors plugged into a data-logger recording data every 15 min (from January 2010 to September 2014). Three repetitions per treatment were installed. In most cases, soil volumetric water content was greater at 15 than at 30 cm, except for the legume cover, which reversed the trend. The control treatment retained the highest soil volumetric water content at 15 cm during most phases, whilst legume cover was the treatment with the lowest volumetric water content in the soil. Nevertheless, at 30 cm in the key phases for oil production (flowering, phases I and III), the permanent cover crop was the treatment with the highest rates of volumetric water content in the soil, followed by barley. The worst treatment at 30 cm was the control, which raises doubts about the farmers' idea that tillage increases soil water content. Legume cover enhanced water infiltration in the soil at 30 cm, possibly because of its deep taproot system.