Changes in Flow in the Upper North Canadian River Basin of Western
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Transactions on Ecology and the Environment vol 50, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 Changes in flow in the upper North Canadian River basin of western Oklahoma, pre-development to 2000 K.L. Wahl US. Geological Survey, Lakewood, Colorado, USA Abstract Water levels have declined in the southern part of the High Plains aquifer of the central USA since the mid-1960s in response to extensive irrigation development. The North Canadian River originates in western Oklahoma, and most of the basin is underlain by the High Plains aquifer. Average river flow in the headwaters near Guymon, Oklahoma, has decreased fiom about 0.9 m3/s before 1970 to near zero at present. Canton Lake, on the North Canadian River near Seiling, about 250 km downstream from Guymon, is a source of water supply for Oklahoma City. Precipitation data and streamflow data for gages upstream from Canton Lake were divided into an "early" period ending in 1971 and a "recent" period that begins in 1978. The early period represents conditions before ground-water levels had declined appreciably in the High Plains aquifer, and the recent period reflects the current condition, including the effects of storage reservoirs. Tests for trend and comparisons of flows between the early and recent periods show that the total annual volume of flow and the magnitudes of instantaneous annual peak discharges measured at most locations in the North Canadian River basin have decreased. Precipitation records for the area, however, show no corresponding changes. The decreases in average annual flow, expressed as a percentage of the average flows for the early period, ranged from 91 percent near Guymon to 37 percent near Canton Lake. A major contributing factor in the decreased flows appears to be the large declines in water levels in the High Plains aquifer. 1 Introduction Optima Lake on the Beaver River near Guymon, Fort Supply Lake on Wolf Creek near Fort Supply, and Canton Lake on the North Canadian River near Canton (fig. 1) provide storage of public-water supplies for western Oklahoma Transactions on Ecology and the Environment vol 50, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 River Ba sin Management Transactions on Ecology and the Environment vol 50, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 River Basin Management 75 and for the Oklahoma City metropolitan area. The Beaver fiver and Wolf Creek join to form the North Canahan Rwer, which flows into Canton Lake. The drainage area at Canton Lake is 32,318 km2 and includes most of northwest Oklahoma (54 percent), along with a small part of northeastern New Mexico (6 percent), and the northern part of the Texas panhandle (40 percent). The principal industry in the basin is agriculture, with the land area about evenly divided between cropland and rangeland. Average annual precipitation in the study area ranges from about 400 mm in the west to 610 mm near Canton Lake in the east. The majority of precipitation occurs from spring through summer. Annual runoff to Canton Lake mm. averages about 10 Most runoff occurs between April and August, and the smallest streamflows usually occur from December through February. More than 90 percent of the Beaver-North Canadmn basin above Canton Lake is underlain by the High Plains aquifer, which underlies an area of about 450,500 km2 in eight States [l]. The High Plains aquifer is primarily a water-table aquifer recharged by precipitation. The estimated recharge rate for the High Plains aquifer in the Beaver River basin upstream from Guymon is 1.4 mm per year [2];this represents less than 1 percent of the mean annual precipitation. Prior to the start of large-scale pumping for irrigation in the 19601s,the aquifer was in equilibrium [3]. The introduction of the center-pivot sprinkler system in the early 1960's resulted in a rapid increase in the use of ground water for irrigation and consequent declines in ground-water levels (fig. 2). Long-term water-level monitoring for the High Plains aquifer indicates declines averaging 7.6 mm per year from 1940 to 1980 [4]; however, most of the change occurred after 1970. Water levels continued to decline from 1980 to 1993, but at a lower rate, averaging 4.9 mm per year. Local deches were greater; maximum declines in northern Texas and western Oklahoma were as much as 15 to 30 a The decline with in in ground-water levels, in combination changes land-use practices, has been recognized as a primary cause of decreases in the discharge of the Beaver- North Canalan River in Oklahoma 14, 5,6]. 2 Background and methods of analysis The U.S. Geological Survey has collected streamflow data in the study area since about 1937. Wahl and Wahl [6] used those data for all gages in the basin upstream from Beaver, Oklahoma, to defme changes in the flow regime. The average flow of the river near Guymon was about 0.9 m3 before 1970, but began to deche in the early 1970's. When I .e gage at Guymon was discontinued in 1993, the channel was dry more than 90 percent of the time (fig. 3), and the average annual flow for the past 10 years was near zero. Wahl and Tortorelli [7] examined flow changes upstream from Canton Lake using data for the 25 gages in the basin with at least 10 years of data. Base flows were estimated from daily mean discharge using a FORTRAN program, BFI, developed by Wahl and Wahl [6, 81. The program defines the Base Flow Index as the ratio of base flow to total flow. Transactions on Ecology and the Environment vol 50, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 76 Elver Basin Management 2,500 .,,,,m,.,,,,,l,,l~..,l,,,,l,,,. 5BW U y 2,000 1 W 3 1,500 1 - W ELLS 8 -,-m-., 8 1,000 m € 2 500 0 ""'"" II,I,I, 1930 1940 1950 1980 1970 1980 1990 2000 YEAR Figure 2: Growth in numbers of large-capacity wells in western Oklahoma, and decline in water levels in observation well 363033 101440701. Figure 3: Annual percent of no-flow days for the Beaver River near Guymon, Oklahoma. A non-parametric test was used to test for the presence of trends. Trend tests were limited to data collected after 1942 for sites downstream from Fort Supply Lake, which began storing water in 1942. Optima Lake was completed and began storing water in 1978, but storage in Optima Lake has been minimal. Moving averages of the data were also used to damp the fluctuations so trends were more readily apparent to the eye. Finally, data were divided into an "early" period ending in 1971, representing conditions before ground-water levels had declined appreciably and the "recent" period beginning in 1978, reflecting the condition of declining ground-water levels and the effects of storage reservoirs. Medians and averages of precipitation and streamflow for the "early" and "recent" periods were compared. Precipitation data collected at the Goodwell Research Station near Guymon between 1936-1986 were tested by Wahl and Wahl [6]. In addition, Transactions on Ecology and the Environment vol 50, © 2001 WIT Press, www.witpress.com, ISSN 1743-3541 River Basin Management 77 monthly and annual averages of precipitation for 1895-1994 for the climate division that covers western Oklahoma were tested by Wahl and Tortorelli [7]. The Guymon gage (07232500) is not affected by reservoirs, but the other three mainstem gaging stations are. The amount of annual change in storage in the upstream reservoirs is small in relation to the annual volume of flow. However, annual volumes were adjusted for change in reservoir contents. The medians of the adjusted and unadjusted annual volumes of flow differed by about 2 percent at the Beaver (07234000) and Seiling (07238000) gages, and by about 9 percent at the Woodward gage (07237500). The last year of record used in the adjusted record was 1993 because change-of-contents data for the lakes are not available for the 1994 water year. 3 Results Comparisons of flows for the early period (ending in 1971) with those for the recent period (1978-1994) show that the magnitudes of peak discharges and the total annual volume of flow measured at most gaging stations in the Beaver-North Canadian Rwer basin have decreased in recent years. These changes are most pronounced in the headwaters upstream fiom Woodward, but also are evident at Woodward and near Seiling, whlch represents the inflow to Canton Lake. The melans of the annual peak discharges decreased fiom the early period to the recent period by the following percentages: near Guymon (98), at Beaver (86), at Woodward (80), and near Sehg(53). The year-to-year variation of precipitation over the area is large, but no discernible long-term trends were found, and differences between the "early" and "recent" periods were negligible [7]. Analyses of annual streamflow, however, show that the flows measured at most gaging stations in the Beaver-North Canadian River basin are smaller in the recent period. These results are summarized in Table 1 for the four main-stem Beaver-North Canadian River gages. Changes in annual peak discharges, annual volume of flows, and base flow index are shown in figures 4 and 5 for the gages near Guymon (07232500) and Selling (07238000). These gages represent the respective inflows to Optima Lake and Canton Lake. Annual rates and volumes of flow have deched at most gaging stations in the basin [6, 71. The average daily rate of flow of the river near Guymon reported in 1960 for 23 years of record (water years 1938-1960) was 0.91 m3/s; the 10-year moving average was only 0.02 m3/s by 1993.