Spatio-temporal soil moisture distribution in a Maize field

Beff, Laure [UCL] Couvreur, Valentin [UCL] Javaux, Mathieu [UCL]

Spatial distribution of soil water is driven amongst others by soil plant interactions. Preferential fluxes along roots, root water uptake, canopy interception are processes which affect how water distributed into soil, in combination with the agricultural practices. However, quantification of the impact of each of these processes on soil water dynamics is hard to characterize. We used geophysical monitoring and experiments to assess spatio-temporal soil moisture distribution in a Maize plot during one growing season. An experimental plot of 2m x 17m, situated in a Maize field in Louvain-la-Neuve (in the Loamy Beglian Region) was monitored between the 23 of July and the 21 of September 2009. The experimental plot was equipped with (i) 14 Time Domain Reflectometry (TDR) probes to observe the temporal evolution of soil water content; (ii) 132 electrodes (76 surface electrodes and 56 deep electrodes) for Electrical Resistivty Tomography (ERT), (iii) 7 temperature probes and 7 tensiometers. ERT was used to monitor the 3-D distribution of the apparent electrical conductivity (ECb). ERT measurements were made with a Syscal-pro equipement and inverted with the help of BERT 1.0 (developped by Günther and Carsten).The root colonization of soil in 2D was characterized by the trench profile method (for 2 rows and 2 inter-rows). A grid with a 5 cm mesh was used to count the number of roots present in each cell, resulting in a vertical 2D map of root impacts. The TDR results showed difference in soil water content between the Maize row and the inter-row (mainly observable at 10 cm depth and during dry periods). The soil water content dynamics is the most important at 10 cm depth and below the row. With ERT, we can see that the soil electrical conductivity at the upper layer is nearly the same for the three measurement time. However, ECb decrease in depth with the progress of the growing season, especially below the Maize row and under the plough pan level (-0.35 m). The triangular distribution of roots decrease with depth and at the plough pan level. The roots are more presents in the Maize row than in the inter-row but roots colonize the inter-row with time. Soil structure and vegetation have an important effect on soil water content distribution. The plough pan and the soil horizons affect the roots distribution and the soil electrical conductivity. These results demonstrated that the three dimensional ERT measurements are able to show the effect of root water uptake.