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r-'\ OS1\J The river has a relatively steep slope in a narrow basin upstream from Trinidad Dam, which is about 1.3 mi upstream from the city of Trinidad. Since the 1970's, the dam has protected the city and the agricultural valley downstream from damaging floods, while water storage in Trinidad Reservoir has provided a more timely water supply for agricultural use on downstream farms and ranches. Before the reservoir was operational, streamflow needed for crop use late in the growing season usually was not available to many users in the study area. The agricultural water supply has historically been erratic (Bureau of Reclamation, 1964a, 1988) with considerable seasonal and annual variations. Maximum water storage in Trinidad Reservoir from 1984 through 1992 ranged from 8,230 to 46,900 acre-ft. The Purgatoire River traverses porous sandstones, fractured limestones, and impervious shales overlain in most areas of the valley by shallow (generally less than 50 ft thick), unconsoli- dated alluvial deposits that range from a few hundred feet to more than 1 mi in width. A general absence of large-capacity irrigation wells in the study area indicates that these alluvial deposits might not be suitable for ground-water development. The river channel generally varies in width from about 10 to 400 ft and contains a mixture of riverine deposits mostly consisting of gravels and sands. The alluvial deposits near the river might be in hydraulic connec- tion with the river. However, sufficient geohydrologic information to define the relation of ground water to surface water is not available. A more complete description of the geology along the lower Purgatoire River is presented by Qazi and Krason (1976). The study area has a semiarid continental climate typical of the Great Plains province. Mean monthly precipitation and mean air temperature at Las Animas County Airport near Hoehne (fig. 1) for 1947-92 are shown in figure 2. Mean annual precipi- tation on the eastern plains of southeastern Colorado averages about 13 in. (fig. 2), most of which falls during spring and summer thunderstorms that have varying amounts of precipitation from storm to storm and from site to site. Mean monthly air temper- ature increases steadily to a maximum in July then decreases steadily to a minimum in January (fig. 2). Throughout the study area, potential evapotranspira- tion greatly exceeds precipitation. Natural vegetation present on the semiarid prairie throughout the area primarily is low grasses and shrubs, with scattered ~tands of pinon and juniper trees on some of the higher elevation hills. Along the flood plain of the lower Purgatoire River, there are mature starids of salt cedars and other types of woody phreatophytes (Lindauer and Ward, 1968). The average growing season for pasture grasses, alfalfa, and other types of hay crops (Colorado Cooperative Crop and Livestock Reporting Service, 1995) irrigated in the study area is about 8 months (U.S. Department of Agriculture, 1988). Most irrigated land along the lower Purgatoire ~iver extends 35 river miles in the valley downstream from Trinidad and along gently sloping plateaus northeast of Trinidad. About 25 mi farther downstream from this irrigated area, the river flows through the steep-walled (as much as 500 ft) Purgatoire River Canyon, a narrow 40-mi-long reach flanked on either side of the river by semiarid prairie. After leaving the canyon, the river flows about 60 mi through a relatively flat and wide valley with two tracts of irrigated land before its confluence with the Arkansas River near Las Animas. A detailed descrip- tion of surface-water distribution in southeastern Colorado is provided by Abbott (1985); the report describes water operations after 1977 and provides site information about surface-water-withdrawallocations along the lower Purgatoire River between Trinidad and Las Animas. Thirteen irrigation canals in the study area divert streamflow from the Purgatoire River usually from April through the middle of November; peak diversions generally are largest from June through August. At times, there is little surface-water flow in some river reaches because the irrigation canals divert the entire flow of the river. Canals withdrawing streamflow are Antonio Lopez, Baca Joint, Chilili, Enlarged Southside, El Moro, Model, Johns Flood, Hoehne, Bums-Duncan, Lewelling-McCormick, Salas, Ninemile, and Highland. Parshall flumes that vary from small structures (6-ft3/s capacity) to moderately large structures (700-ft3/s capacity) are used to measure streamflow into the irrigation canals. Each canal has a maintained recording device located near the river that records daily streamflow diversions (Henry Marques, Colorado Department of Natural Resources, oral commun., 1995). About 19,700 acres of cropland in the vicinity of Trinidad are irrigated through 11 canals, which are part of the PRWCD (Bureau of Reclamation, 1964a). 4 Evaluation of Streamflow Traveltime and Streamflow Gains and Losses along the Lower Purgatoire River, Southeastern Colorado, 1984-92