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. . Water supplies in the Purgatoire River Basin are over- appropriated (Colorado Department of Natural Resources, 1975), and shortages of water occur when irrigation demand exceeds available streamflow. The U.S. Geological Survey (USGS) conducted a field study from July 2000 through June 2004, in cooperation with the PRWCD, the Colorado Water Conservation Board (CWCB), and the BOR, to charac- terize and quantify streamflow losses or gains in eight unlined irrigation canals that divert streamflow from the Purgatoire River between Trinidad Dam and the city of Hoehne. Colorado (fig. I). Results from this study will provide area water managers with additional hydrologic infonnation needed to more effectively manage water resources within the Purgatoire River Basin. Specifically, results from the canal loss and gain measurements will assist PRWCD and others in identifying areas within canal subrcaches or entire canals where modifica- tions or maintenance may be needed to reduce canal losses or , improve irrigation water delivery efficiency. Purpose and Scope The purpose of this report is to present findings of a loss and gain study conducted for eight unlined canals (Picketwire, Baca, EI Moro, Chilili, Enlarged Souths ide, Model, John Flood, and Hoehne canals) within the Purgatoire River Basin downstream from Trinidad Dam from July 2000 through June 2004. The major objectives of the study were to charac- terize losses or gains along the eight canals and to compare flume-rated and measured discharge at the canal headgates. The study area is in southeastern Colorado near Trinidad in Las Animas County (fig. I). Description of Study Area Trinidad Dam and Trinidad Lake (hereinalier referred to as "Trinidad Dam") were completed in 1976 on the upper Purgatoire River about 2.75 miles upstream from the city of Trinidad, Colorado (fig. I). Trinidad Dam serves as a multi- purpose reservoir authorized for flood control, irrigation, sedi- ment retention, and recreation (U.S. Anny Corps of Engineers, 1994). The Corps is responsible for Ihe operation and main- tenance of Trinidad Dam. The BOR administers the contract (which includes repayment of the costs of construction, opera- tion, and maintenance allocated to the irrigation portion of Trinidad Dam) with PRWCD (U.S. Army Corps of Engineers, 1978). PRWCD is responsible for the regulation of irrigation water within the PRWCD that is consistent with administra- tion of water rights by the Water Division 2. State Engineer's Office, Colorado Division of Water Resources (U.S. Army Corps of E~ginecrs, 1994). The Purgatoire River originates in [he Sangre de Cristo Mountains west of the study area. It tlows in a gcncrally northeasterly direction through parts of Las Animas, Otero, Introduction 3 and Bent Counties to its contluence with the Arkansas River near Las Animas, Colorado. Trinidad Dam controls flow from 671 square miles of the 749-square-mile Purgatoire River Basin upstream from the city of Trinidad (U.S. Army Corps of Engineers, 1994). Trinidad Dam is in a narrow river valley bordered by mountainous foothills. Downstream from Trinidad Dam, the foothills transition to rolling, grassy plains, and the river valley widens. The study area has a semiarid continental climate. Mcan annual precipitation at the Trinidad Airport from 1948 through 2005 was 15.1 inches. Most precipitation falls during the spring and summer months. The average maximum daily temperature was 67.2 degrees Fahrenheit CF), and the average minimum daily temperature was 37.4"F for the period January 1948 through December 2005 (http://www.wrcc.dri.edulcgi.bin/ cliMAIN.pl?cotrin, accessed August 7, 2006). The highest temperatures generally arc recorded in July, and the lowest temperatures in January. Flow in the Purgatoire River, downstream from Trinidad Dam, is controlled by releases from the lake, inflow from downstream tributaries, and irrigation diversions. Little or no water is released from Trinidad Dam from about October 15 through April 15 because water is stored for irrigation pur- poses (U.S. Army Corps of Engineers, 1994). To protect downstream water rights, the original Model Reservoir (fig. I) right to store 20,000 acre-feet of water from flows in the Purgatoire River was transferred to Trinidad Dam. During the irrigation season (as determined by PRWCD but falling between Aprill and October 15), downstream releases from Trinidad Dam range from about 100 to 300 cubic feet per second (ft3/s). Irrigation canals divert streamt10w from the Purgatoire River downstream from Trinidad Dam, usually from April through the middle of November; diversions generally are largest from June through August. At times, little surface water flows in some river reaches downstream from the study area because the irrigation canals divert the entire flow of the river. Parshall t1umes, ranging from small structures (9-li)/s free-flow capacity) to moderately large structures (300-ft3/s free-flow capacity), are used to mcasure stream- flow diverted into the irrigation canals. Daily divcrsions are recorded at the canal headgates. Diversion records and record- ing devices are maintained by Colorado Division of Watcr Resources (CDWR) (Steve Kastner, Colorado Department of Natural Resources, oral common., 2000). Pieketwire, Model, Hoehne. Chilili, and Enlarged Southside canals divert water directly from the Purgatoire River (Iig I). The Baca Canal and EI Moro Canal waters arc diverted at the Picketwire (Baea Joint) headgate and are car- ried to their respectivc lands as a part of the Picketwirc Canal. Baca Canal water is dc\iverl,';d to Baea lands through a series of lateral turnouts from the main Picketwire Canal beginning about 1.5 miles downstream from the Picketwire head gate. The Baca Canal lateral turnouts arc distributed along about a