Spatial Variability Analysis of Soil Water Availability for Plants As a Tool for Precision Irrigation In Organic Soils.

Soils are known to have a ‘water capacity’ to supply plants growth which is generally designed as ‘soil water availability’. This soil water availability can be expressed by an ‘integral water capacity’ (IWC) as defined by Groenevelt et al. (2001) which replaced the so-called ‘plant available water’ (PAW) used since the mid-20^th century. This IWC is spatially variable as it depends on soil hydraulic conductivity, aeration, soil profile thickness, root penetrability; all parameters that could considerably vary at a local-field scale. In a perspective of precision irrigation, it is important to consider the spatial variability of IWC in a way to optimize water conservation and to improve crop yields. This study took place on organic soils (Histosols) of Quebec, Canada, under vegetables production. Intact soil cores were sampled in two different horizons, one in a compacted layer (15 to 25-cm depth) and one deeper (around 35-40 cm). Samples were therefore characterized in laboratory to get saturated hydraulic conductivities and water retention curves. Non-linear modeling was performed on each desorption curve in order to calculate the differential water capacity. Root penetrability, soil profile thickness and saturated hydraulic conductivity were considered in the calculation of IWC. Spatial variability of IWC was therefore analyzed using GS+ (LLC Gamma Design Software) and maps were produced in ArcGIS (ESRI) using GS+ semi-variograms and kriging results. The results clearly showed specific area where IWC is less important, thus irrigation should be carefully managed. Hydraulic conductivity of the compacted horizon also permitted to locate regions where IWC can be negatively affected by a problem of water infiltration deeper in the profile, causing ‘stuck’ water table and poor drainage, often associated to losses in yield. The IWC maps are regarded as an additional tool to optimize irrigation under a local-field management strategy.