Technical Abstract: Preliminary soil and sub-soil hydraulic parameter estimates needed for SWAT simulations to determine sub-surface water movement were collected using downhole geophysical measurements. Gamma-ray logs are useful for distingishing sandstone from shales by measuring natural-gamma radiation emitted from rocks penetrated by a borehole. Downhole gamma-ray measurements can be related to a watershed soil and rock formation by analyzing corresponding drill-core and cuttings collected at the drill site. Correlation of gamma-ray measurements of sub-soil texture and density can be used to provide an estimate of textural data for different layers within the soil and sub-soil profile needed by two FORTRAN programs (WTCHARTETA and WTDRAINVR). These programs are used to determine parameters needed for new water table routine that recently has been incorporated in SWAT. Use of gamma-ray downhole logging methods can be in cased or uncased drill holes and can provide useful and reasonably inexpensive methods to obtain parameters needed to calculate the water table.