|M.Sc Student||Ziv Moreno|
|Subject||Estimation of Aquifer Recharge below Agriculture Fields|
Using Resistivity Methods
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Furman Alex|
|Full Thesis text|
Water flow patterns below agricultural fields were investigated in this research by the use of electrical resistivity tomography in order to estimate local recharge. Four citrus orchards with different hydrogeological characteristics were investigated. At each site an electrode line was set perpendicularly to the irrigation lines. In addition, four time domain transmittance sensors were installed at different locations. Geophysical surveys were performed from April 2012 to September 2013, roughly monthly. The obtained data from the electrical measurements were inverted to produce two dimensional profile of the bulk electrical conductivity. Relations between the bulk electrical conductivity to the water content, porosity and the soil solution electrical conductivity were calibrated in a lab and at the field.
As the obtained results cannot be explained using the common approach that assumes uniform spatiotemporal distribution of the soil solution salinity, water flow and solute transport were modeled. The sites hydraulic parameters were calibrated based on the obtained bulk electrical conductivity profiles. By coupling solute transport to water flow, most of the obtained patterns of the bulk electrical conductivity could have been explained. Salts accumulation zones were noticed; roots distribution patterns were qualitatively described and preferential flow paths were detected.
The temporal recharge patterns have shown that the major contribution to the annual recharge occurs during sparse and intensive rain events. At the same time irrigation surpluses were noticed and were attributed to variations in the meteorological parameters.
Conventional methods for evaluating recharge correlate the water-table levels to the annual precipitation/irrigation amounts. In that approach, recharge evaluation at a short temporal scale (daily/monthly) is completely empirical and inherently biased. Further, recharge evaluations at a field scale may be significantly different compared to the regional, groundwater-level-based evaluations. Recharge evaluations at a field scale are mostly conducted by analyzing one dimensional depth profile. That approach treats the orchard as a homogeneous unit and can cause bias, since recharge evaluation depends upon the depth profile location. In that aspect the use of 2D profile and integrating/averaging recharge is superior.
The use of electrical resistivity tomography provides great amount of information without disturbing the soil structure and demanded little effort. Its major limitation is to separate the different parameters that are indirectly being measured. By incorporating different data sources or mechanistic process modeling into the hydrological analysis, it has a great potential to evaluate wetting patterns and fluxes in the vadose zone and set an alternative to conventional methods.