Scientific Investigations Report
2008-5056 PDF (5.5 MB)

Abstract

Pueblo Reservoir is west of Pueblo, Colorado, and is an important water resource for southeastern Colorado. The reservoir provides irrigation, municipal, and industrial water to various entities throughout the region. In anticipation of increased population growth, the cities of Colorado Springs, Fountain, Security, and Pueblo West have proposed building a pipeline that would be capable of conveying 78 million gallons of raw water per day (240 acre-feet) from Pueblo Reservoir. The U.S. Geological Survey, in cooperation with Colorado Springs Utilities and the Bureau of Reclamation, developed, calibrated, and verified a hydrodynamic and water-quality model of Pueblo Reservoir to describe the hydrologic, chemical, and biological processes in Pueblo Reservoir that can be used to assess environmental effects in the reservoir.

Hydrodynamics and water-quality characteristics in Pueblo Reservoir were simulated using a laterally averaged, two-dimensional model that was calibrated using data collected from October 1985 through September 1987. The Pueblo Reservoir model was calibrated based on vertical profiles of water temperature and dissolved-oxygen concentration, and water-quality constituent concentrations collected in the epilimnion and hypolimnion at four sites in the reservoir. The calibrated model was verified with data from October 1999 through September 2002, which included a relatively wet year (water year 2000), an average year (water year 2001), and a dry year (water year 2002).

Simulated water temperatures compared well to measured water temperatures in Pueblo Reservoir from October 1985 through September 1987. Spatially, simulated water temperatures compared better to measured water temperatures in the downstream part of the reservoir than in the upstream part of the reservoir. Differences between simulated and measured water temperatures also varied through time. Simulated water temperatures were slightly less than measured water temperatures from March to May 1986 and 1987, and slightly greater than measured data in August and September 1987. Relative to the calibration period, simulated water temperatures during the verification period did not compare as well to measured water temperatures.

In general, simulated dissolved-oxygen concentrations for the calibration period compared well to measured concentrations in Pueblo Reservoir. Spatially, simulated concentrations deviated more from the measured values at the downstream part of the reservoir than at other locations in the reservoir. Overall, the absolute mean error ranged from 1.05 (site 1B) to 1.42 milligrams per liter (site 7B), and the root mean square error ranged from 1.12 (site 1B) to 1.67 milligrams per liter (site 7B). Simulated dissolved oxygen in the verification period compared better to the measured concentrations than in the calibration period. The absolute mean error ranged from 0.91 (site 5C) to 1.28 milligrams per liter (site 7B), and the root mean square error ranged from 1.03 (site 5C) to 1.46 milligrams per liter (site 7B).

Simulated total dissolved solids generally were less than measured total dissolved-solids concentrations in Pueblo Reservoir from October 1985 through September 1987. The largest differences between simulated and measured total dissolved solids were observed at the most downstream sites in Pueblo Reservoir during the second year of the calibration period. Total dissolved-solids data were not available from reservoir sites during the verification period, so in-reservoir specific-conductance data were compared to simulated total dissolved solids. Simulated total dissolved solids followed the same patterns through time as the measured specific conductance data during the verification period.

Simulated total nitrogen concentrations compared relatively well to measured concentrations in the Pueblo Reservoir model. The absolute mean error ranged from 0.21 (site 1B) to 0.27 milligram per liter as nitrogen (sites 3B and 7B) and the root mean square error ranged from 0.21 (site 1B) to 0.29 milligram per liter as nitrogen (sites 3B and 7B). The Pueblo Reservoir model generally simulated lower concentrations of nitrate and ammonia compared to measured concentrations from October 1985 through September 1987. Simulated ammonia compared better to measured concentrations during the verification period than during the calibration period, and simulated nitrate did not compare as well to measured concentrations as in the calibration period.

Simulated orthophosphorus concentrations in the Pueblo Reservoir model were similar to the measured concentrations for the calibration period. The absolute mean error for orthophosphorus ranged from 0.01 (sites 3B and 5C) to 0.02 milligram per liter (sites 1B and 7B) as phosphorus, and the root mean square error ranged from 0.01 (sites 3B and 5C) to 0.02 milligram per liter (sites 1B and 7B) as phosphorus. The absolute mean error for total phosphorus ranged from 0.02 (sites 5C and 7B) to 0.05 milligram per liter (site 1B) as phosphorus, and the root mean square error ranged from 0.02 (sites 5C and 7B) to 0.05 milligram per liter (sites 1B and 3B) as phosphorus. The greatest difference between simulated and measured values occurred in the hypolimnion at sites 1B and 3B in May through July 1987, where simulated concentrations were considerably less than the measured concentrations. Simulated orthophosphorus and total phosphorus compared better to measured concentrations during the verification period than in the calibration period.

The simulated distribution of algal populations was highly variable in Pueblo Reservoir during the calibration period. The highest algal biomass in Pueblo Reservoir generally occurred from May through September when blue-green and green algae were the dominant algal groups in the reservoir. The lowest algal biomass generally occurred from November through March when diatoms and flagellates were the dominant groups. The distribution of algae in Pueblo Reservoir during the verification period differed slightly from what was observed during the calibration period where diatoms and flagellates were the dominant algal groups in the upstream part of Pueblo Reservoir, and green and blue-green algae were the dominant groups in the downstream part of the reservoir.

Simulated chlorophyll a concentrations were similar to measured concentrations in Pueblo Reservoir during the calibration period. The highest chlorophyll a concentrations occurred at the two upstream reservoir sites, where the absolute mean error ranged from 3.0 (site 1B) to 3.7 micrograms per liter (site 3B), and the root mean square error ranged from 5.3 (site 1B) to 6.8 micrograms per liter (site 3B). Chlorophyll a concentrations generally were lower in the downstream part of the reservoir (sites 5C and 7B) where nutrients were less available for algal growth. The absolute mean error ranged from 1.6 (site 7B) to 2.2 micrograms per liter (site 5C), and the root mean square error ranged from 2.5 (site 7B) to 3.7 micrograms per liter (site 5C). Simulated chlorophyll a concentrations generally were less than the measured concentrations during the verification period.

Version 1.0

Posted May 2008

• Report PDF (5.5 MB)