Completed Study
Field Studies to Estimate Groundwater Recharge beneath Irrigated
and Nonirrigated Regions in the Southern High Plains, Texas
Bridget Scanlon, principal investigator; Robert C. Reedy
Field studies were conducted in collaboration with the U.S. Geological Survey (USGS) National Water Quality Assessment (NAWQA) program to estimate recharge beneath irrigated and nonirrigated regions in the Southern High Plains. Two boreholes were drilled in areas that had been irrigated since 1958 (Roberts and Maple sites) and one borehole was drilled in a non-irrigated site in the Muleshoe National Wildlife Refuge (MNWR) for comparison with irrigated sites. The drilling, sampling, and analyses were conducted by the USGS. as part of the NAWQA program. Post-bomb tritium was generally restricted to the root zone at the non-irrigated site and indicates negligible recharge whereas postbomb tritium was found throughout the unsaturated zone in the irrigated sites indicating much higher recharge.
Heat dissipation sensors were installed to monitor the negative pressures in the unsaturated zone to determine the direction of water movement and to evaluate drainage beneath the irrigated plots. Heat dissipation sensors were installed in shallow boreholes beneath the pivot irrigation system and in the deep boreholes drilled by the USGS. The instruments are logged daily and data are telemetered to the Bureau using a cell phone system.
Matric potential profiles in the non-irrigated site were generally much lower (more negative) than those in the irrigated sites in the upper 10 ft in the spring and summer indicating generally drier conditions in the non-irrigated site. The vertical matric potential profile in the non-irrigated site indicates matric potentials as low as -20 to -25 bars in the shallow subsurface and increasing to close to zero at a depth of ~ 38 ft. The increase in matric potentials with depth indicates an upward driving force for water movement and suggests upward flow. The vertical matric potential profiles in the irrigated plots are close to zero throughout the profile indicating fairly wet conditions as a result of irrigation. The time series plots of matric potentials provide information on infiltration of water as a result of precipitation and irrigation and indicate very little water movement over time beneath the irrigated and nonirrigated sites. These data suggest that recharge from irrigation return flow is negligible beneath the current very efficient center pivot irrigation systems but may have been much higher in the past when flood irrigation was used.
Reedy, R. C., Scanlon, B. R., Bruce, B. W., McMahon, P. B., Dennehy, K. F., and Ellett, K., 2003, Groundwater recharge in the southern high plains, in T. N. Blanford, D. J. Blazer, A. R. Dutton, and B. R. Scanlon, editors, Groundwater Availability of the Soutehrn Ogallala Aqufier in Texas and New Mexico Numerical Simulations through 2050. Contract Report Submitted to Texas Water Development Board.
Reedy, R. C., and Scanlon, B. R., 2002, Comparison of different approaches for estimating recharge in the High Plains Aquifer, Texas (abs.), in Eos, v. 83, no. 47, Fall Meeting Supplement, American Geophysical Union, Abstract H61B-0777. [PDF]

August 2005