The neutron moisture meter and the dual-energy gamma-ray probe are routinely used in hydrological investigations to measure in situ the water content of soils and sediments, even though the geometric characteristics of the volumes of influence of these instruments and their spatial sensitivity are still very poorly understood. To reach a much better understanding of the physics of water content measurements in heterogeneous soils, and of how it impacts more complicated measurement processes (like the in situ determination of the unsaturated hydraulic conductivity) of which soil moisture measurement is only a component, we propose to have recourse to computer simulations on parallel/vector supercomputers. In particular, we propose to use a modified version of the Monte Carlo code MCNP, describing the transport of neutrons and photons in 3-dimensional geometries. The expected results of this research (which will also extend to other aspects of hydrology besides soil moisture measurement) will allow better use of existing instruments and, eventually (although it is not part of the present proposal), the design of improved ones.
