Assessment of effects of military maneuvers on the streamflow, water quality, and sediment yields at the U.S. Army Pinon Canyon Maneuver Site, Las Animas County, Colorado

Abstract

The U.S. Department of the Army began military maneuvers at the 381square-mile Pinon Canyon Maneuver Site in July 1985. The effects of military maneuvers on the surface water of the Maneuver Site and on the Purgatoire River are of concern to the military and to downstream water users. Streamflow, water-quality, and sediment-yield data are analyzed graphically and statistically to determine changes between preand postmaneuver periods. Temporal and spatial distribution of precipitation from August 1983 through September 1987 at the Maneuver Site is summarized. Mean monthly precipitation at the Maneuver Site for this period is compared to 40-year mean monthly precipitation for the Las Animas County Airport. Monthly precipitation at the Bent Canyon gage for May 1987 was 6.17 inches and was exceeded once at the airport during the period of record (1947-87). There was no trend in monthly precipitation during 1983-87 at the Maneuver Site. With the exception of Van Bremer Arroyo near Model, there was no significant difference in streamflow existed between preand postmaneuver periods for streams in and near the Maneuver Site. During the base streamflow period (November through March) and the summer streamflow period (July through October), a statistically significant increase in streamflow occurred at Van Bremer Arroyo near Model. The increase may be a result of irrigation return flows from farmland adjacent to the Maneuver Site. Changes in water quality, particularly increases in dissolved solids that could affect downstream agricultural water users, were investigated. Dissolved oxygen, flow-adjusted concentrations of selected water-quality constituents, and flow-adjusted monthly mean specific conductance at the streamflow-gaging stations Purgatoire River near Thatcher and Purgatoire River at Rock Crossing, near Timpas, were tested for changes between the preand postmaneuver periods. Except for total-recoverable zinc at the Purgatoire River near Thatcher, there was no significant difference (p>0.05) in dissolved oxygen, selected water-quality constituents, or monthly mean specific conductance between the preand postmaneuver periods. Monthly mean specific conductance for all streamflow-gaging stations at the Maneuver Site and Chacuaco Creek at mouth, near Timpas was analyzed for change between preand postmaneuver periods. No significant difference (p>0.05) in monthly mean specific conductance was detected at these streamflow-gaging stations. The effects of military maneuvers on sediment yields were evaluated by using a qualitative method for estimating mean annual sediment yields that was developed by the Pacific Southwest Inter-Agency Committee. Sediment yields were estimated for 1983-84 for 28 drainage basins in and near the Maneuver Site prior to the start of military maneuvers. Twenty-one of these drainage basins are located on the Maneuver Site and seven are located outside the Maneuver Site on adjacent rangeland. Drainage areas of these drainage basins ranged in size from 0.10 to 1.2 square miles. Generally, estimated sediment yields tended to decrease for the drainage basins on the Maneuver Site. During water years 1983 through 1987, improvement in ground cover and changes in the intensity and patterns of land use tended to offset and lessen the effects of military maneuvers on sediment yields. During November 1987, sediment yields were estimated for seven drainage basins located off the Maneuver Site. Estimated sediment yields for the postmaneuver period indicated a decrease for six drainage basins and no change for one. The Precipitation-Runoff Modeling System was used to simulate changes in streamflow from the August 9, 1987, storm by using hypothetical effects from military maneuvers. The hypothetical scenario increased the simulated peak streamflow about 1.2 percent; there was no change in the timing of the streamflow.