Monitoring in situ soil moisture variations of expansive clay using neutron probes

The neutron probe has proven to be an effective means for long-tenn monitoring of in-situ soil moisture variations. However, correlation of neutron probe data (i.e. neutron counts) with moisture contents to give accurate absolute values is a very difficult task, particularly for expansive soils due to problems associated with soil volume changing and shrinkage cracks. In this study, a numerical model of neutron diffusion in soils is used to calibrate neutron-probe data collected in a field site over a period of 5 years. The envelopes of soil moisture content derived from the neutron probe, representing the extreme observation at various depths are reported. The effects of tress and covers on soil moisture changes are also examined.