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

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The operation of Pueblo Reservoir, in particular the Winter Water Storage Program (WWSP), has a notable effect on the historic streamflow regime of the lower Arkansas River. In winter (November— March), prior to the implementation of the WWSP in 1975, irrigators in the lower Arkansas River Valley gener- ally diverted appropriated Arkansas River water onto fallow fields to increase soil moisture for later use by crops during the growing season. Alternatively, this water could have been stored during the winter and then released to the river for the downstream irrigators to use during times when streamflow was insufficient to meet irrigation needs. However, under Colorado water law, storage of water that is diverted with direct - flow water rights is not permitted. Therefore, the WWSP was created to allow several irrigation canal companies downstream from Pueblo Reservoir to store their direct -flow water in Pueblo Reservoir, John Martin Reservoir, and in several private off - channel reservoirs during the winter and to use this water during the crop- growing season. Under the WWSP, winter water storage is allowed from November 15 to March 15. Generally, WWSP water is released from storage at times when streamflow is not large enough to meet irrigation demands. This situa- tion usually occurs in early spring or late summer and autumn. Winter water was stored every year from 1975 to 1994, except during the 1977 -78 winter - storage season. During 1975 -94, the median annual volume of water that was stored in Pueblo Reservoir as part of the WWSP was about 42,200 acre -ft (Colorado Division of Water Resources, written commun., 1995). John Martin Reservoir John Martin Reservoir is a 608,200- acre -ft main -stem reservoir located 58 mi upstream from the Colorado- Kansas State line (fig. 1). The reservoir is used for flood control, irrigation -water storage, and recreation. Storage of irrigation water in John Martin Reservoir is by agreement between the States of Colorado and Kansas, under the terms of the Arkansas River Compact. The Arkansas River Compact is an agreement between Colorado and Kansas, signed in 1948, which ensures both States will receive their percentage share of Arkansas River flows. The Compact agreement dictated a winter- and a summer- storage period. During the winter - storage period from November 1 to March 31, all inflow to the reservoir was required to be stored, except that as much as 100 ft3 /s of water could be requested by Colorado water users downstream from the dam (Abbott, 1985). During the remainder of the year, river flow was stored, although Colorado could demand the release of as much as 500 ft3 /s of the water entering the reservoir and Kansas could demand releases of that part of the inflow between 500 and 750 ft3 /s. Provi- sions were made for the rate of release of stored water, without reference to the volume of stored water assigned to each State. To ensure that each State received its share of stored water, release demands had to be made concurrently. Although the Arkansas River Compact was developed to ensure that Colorado and Kansas irrigators received their legal shares of Arkansas River water, the plan had several problems and generally was unsatisfactory to both States (Abbott, 1985). Historically, following the winter - storage period, reservoir storage was usually drawn down to empty or almost empty very early in the irri- gation season, often by the middle of April. Because of the unsatisfactory nature of this operation, a resolu- tion was adopted by the Arkansas River Compact Administration in 1980. This resolution commonly is referred to as the 1980 operating plan (Arkansas River Compact Administration, 1980). Under the new plan, any water not immediately called for and released to downstream irrigators is stored in separate storage accounts for the States of Colorado and Kansas. Either State can call for the release of its stored water inde- pendently of the other. Two other recent changes have been made in the operation of John Martin Reservoir that affect reservoir storage and streamflow down- stream from John Martin Reservoir. A 10,000- acre -ft permanent recreation pool was established in 1976, and three irrigation canal companies have been allowed to store their approved WWSP water in John Martin Reservoir. Thirty -five percent of the winter water that the three canal companies store in John Martin Reservoir is shifted to Arkansas River Compact use and is subject to downstream release. These two changes, in conjunction with the 1980 oper- ating plan, have substantially increased the long -term storage of water in John Martin Reservoir and have altered the flow regime in the Arkansas River down- stream from the reservoir. WATER ADMINISTRATION AND RESERVOIR OPERATIONS 9