Surface Runoff and Stream Stage/Discharge

By Dr. Lynn Loomis

The watershed of Alamito Creek includes nearly 1 million acres. It extends 75 miles from its headwaters in the Davis Mountains (Jeff Davis County) to its confluence with the Rio Grande a few miles east of Presidio. Two impoundments (Marfa flood control dam and San Esteban Dam) capture surface runoff from 280,000 acres in the upper watershed. The area of the Alamito Creek watershed that can contribute surface runoff to the Alamito Ranch is about 195,000 acres. The area within the Perdiz Creek watershed is about 50,000 acres. The water flow in Alamito Creek and Perdiz Creek is ephemeral, water flowing in the channel only after precipitation. Standing water is retained for extended periods in pools that punctuate the channel.

Discharge of Alamito and Perdiz Creeks has been estimated/measured by three methods: On-site camera measurements, Onsite direct observation, and Computation.

On-site camera measurements

In July 2023, game cameras paired with stream staff gauges were installed at three locations along Alamito Creek: near the northern edge of the ranch, near the railroad trestle, with one between. One camera-staff gauge was placed along Perdiz Creek. Cameras were configured to capture photographs at 15-minute intervals. Downloaded every month or two, photos document the depth and duration of water when it flows down the creeks.

Juan Celaya, Joe Wade, and Levi Wade by the newly installed Alamito Creek Central staff gauge. Duration and depth of water during flows will be recorded by a game camera mounted across the channel. Photograph taken July 2023.

Leading edge of runoff water at Alamito Creek North game camera.

With a geographic information system (QGIS) software, elevations of stream cross sections were extracted from the USGS 1 m digital elevation model. Profiles of xy data pairs were loaded into NWS Normal Depth Calculator to calculate stream discharge from stage. Curves relating stream discharge to water depth have been created for each of the four stream gauges.

On-site direct observation

Discharge of Alamito Creek was observed on October 11, 2022, after a thunderstorm that dropped a 1-inch rain in one hour. Surface runoff in the watershed flowed to the channel of Alamito Creek. Placed in flowing water, a float passed through a 75-foot-long reach of Alamito Creek in 58 seconds, measured with an iPhone stopwatch. The QGIS program extracted elevations from the DEM of the channel floor along a cross section. The elevation of the high-water mark was also determined from the DEM. Cross-sectional area was determined with the Normal Depth Calculator program of the Alaska-Pacific River Forecast Center.

At 9:00 am on October 11, stream discharge was estimated at 55 cubic feet per second, or almost 24,700 gallons per minute. In eight hours after rain started falling on Alamito Ranch, stream discharge was almost 41 acre-feet, or about 13 million gallons. Assuming that runoff came entirely from Alamito Ranch (which it certainly did not), infiltration captured more than 85 percent of the storm’s precipitation.

A collection of floats has been placed near the railroad trestle. When flow is observed in the future, the velocity of floats placed in the water will be used to compute stream discharge.

Sediment-laden water flowing in the channel of Alamito Creek at the railroad trestle. Discharge was calculated at 55 cubic feet per second. Photograph taken October 2022.

Computation

Pairs of XY coordinates along cross section line were uploaded to the Normal Depth Calculator program of the Alaska-Pacific River Forecast Center. Along with cross section xy data, values of channel slope, Manning’s n number, and water surface elevation were furnished. The Normal Depth Calculator program computed streamflow parameters including cross sectional area, maximum depth, average velocity, top width, and stream discharge.