|M.Sc Student||Arnon Ali|
|Subject||Use of Electrical Resistivity Tomography for Study of|
Flow and Water Distribution in the Vadose Zone
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Alex Furman|
|Full Thesis text|
The flow patterns in the vadose zone above the Israel's Coastal Plain aquifer were investigated in this research by applying the electrical resistivity tomography (ERT) method. The main study was a mature grapefruit orchard near Ashdod. A second study site was an urban public park that consisted of 3 different land uses (grassy area, asphalt pathway, and undeveloped area).
ERT surveys were conducted throughout the year (between March 2008 and March 2009). In addition, a four days continuous survey was conducted around an irrigation event. The data obtained was inverted into 2D Resistivity profiles of 50 to 100 m long and about 10 m deep. Calibration of the petrophysical relations (relating the electrical conductivity and the water content, considering the salinity of the soil) was conducted at the laboratory scale, and at the field scale.
The results from the orchard revealed distinct distribution of water contents beneath the tree rows, and between the rows, which is related to the heterogeneity of the surface boundary conditions in both time and space (irrigation and rainfall) and to root uptake patterns. Analysis of water content profiles in consequent times showed seasonal dynamics that correlate with irrigation and precipitation. Excess irrigation water flows laterally in the subsurface from the tree rows towards the row gaps, with a clear delay of two months in the arrival of water to the row gaps with respect to the beginning of irrigation.
Some problems in the results are attributed to the inversion procedure and to local misfits of calibration, and should be further investigated. Possible solutions might be in the direction of incorporating different data sources into the hydrological analysis.
The use of the geophysical ERT method enabled us understanding of spatial patterns of water content distribution, fluxes and flow pathways in the vadose zone. This was achieved with hardly invading the subsurface. The distinct flow patterns revealed in the orchard indicate that an agricultural field should be treated as a heterogeneous domain, and the use of traditional, point measurements might be not satisfying for understanding such water distribution.