Relations of Main-Stem Reservoir Operations and Specific Conductance in the Lower Arkansas River

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Statistical analyses were made of daily mean streamflow values. The daily mean streamflow is the mean streamflow for a given date and site. The median daily mean streamflow for a given month is the median value of all daily mean streamflows for that month during a specified period of time. WATER ADMINISTRATION AND RESERVOIR OPERATIONS In Colorado, water law is based on the doctrine of prior appropriation. The prior appropria- tion doctrine holds that the water in a State is the prop- erty of the public, which has a vested right to the use of this water. Specifically, the doctrine states that the first in time to use the water is first in right to receive that water in subsequent years. Prioritized direct -flow water rights for the Arkansas River were established as long ago as 1859. As irrigated agriculture spread throughout the basin, the list of prioritized water rights grew rapidly, and the Arkansas River and its tributaries were fully appropriated for normal or average years by the mid- 1880's (Abbott, 1985). In most areas, water rights with priorities dated after 1887 are little more than flood rights, which allow diversion of water only in periods of higher than average streamflow (Abbott, 1985). Flood rights do not provide a dependable supply of water because these flows generally occur at times inconve- nient to farming operations or at rates in excess of canal capacities. Water- storage rights were developed and reservoirs were constructed to take advantage of the flow not available to direct diversions, which includes streamflow in excess of direct -flow water rights (flood rights) and streamflow during the nonirrigation season (winter water) from November through March. During 1880 through 1910, storage rights were established that allowed for the yearly diversion and storage of almost 500,000 acre -ft of Arkansas River water in off - channel reservoirs. Addi- tionally, two large main -stem reservoirs were built in the lower Arkansas River Valley to manage Arkansas River streamflow. In 1948, the U.S. Army Corps of Engineers completed construction of John Martin Reservoir, a main -stem reservoir east of Las Animas (fig. 1). In 1975, the Bureau of Reclamation com- pleted construction of Pueblo Reservoir, a main -stem reservoir west of Pueblo (fig. 1). Pueblo Reservoir Pueblo Reservoir is used for the storage and regulation of water that is imported into the Arkansas River Basin from the Colorado River Basin as part of the Fryingpan- Arkansas Project (hereafter referred to as the Project). The Project is a multipurpose water development constructed by the Bureau of Reclamation. The main purpose of the Project is to divert unappropriated water from the western slope of Colorado for use on the more populated, water - limited eastern slope. The Project began importing water in 1972. From 1972 through 1994, the Project imported an annual median volume of about 47,300 acre -ft into the Arkansas River Basin (U.S. Department of the Interior, 1996). During summer, the Project diverts water from the western slope to the eastern slope, where the water is held in storage until municipal or irrigation water - supply demands need to be satisfied. Imported water may be stored in the upper basin or in Pueblo Reservoir, the farthest downstream facility of the Project. Imported water generally is stored in the upper basin as long as possible to minimize evaporative losses, which are lower in the upper basin than in Pueblo Reservoir. During winter, water stored in the upper basin may be released to the river for down- stream storage in Pueblo Reservoir in order to create upper basin storage space for the importation of western slope water during the upcoming snow- melt runoff. Storage began in Pueblo Reservoir in 1974, and the dam was completed in 1975. The reservoir had an initial storage capacity of 357,678 acre -ft (Lewis and Edelmann, 1994). Since water was first impounded in Pueblo Reservoir, reservoir storage has fluctuated because of variations in the inflow and in the demand for stored water. Most of the storage space in Pueblo Reservoir is reserved for Project water, although storage of some non - Project water is granted under a limited number of storage contracts. Most of the annual inflow to the reservoir usually occurs during May through July. Reservoir storage generally decreases substantially by the end of the growing season because of decreased inflow and large downstream demands for irrigation water. a Relations of Streamflow and Specific- Conductance Trends to Reservoir Operations in the Lower Arkansas River, Southeastern Colorado