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" o , .~) ,..'-. ..-' ~ .... N STUDIES OR ANALYSES CONDUCTED BY COLORADO STATE UNIVERSITY ON 'tHE ARKANSAS VALLEY OF COLORADO Physical Relationship Between Surface and Ground Water - In 1961 a cooperative study with USGS and Colorado State University personnel was initiated and funded by the Colorado Natural Resources Department under the direction of Dr. Clark. The purpose of the study was to determine physical relationship between surface and ground water for a hydraulically connected stream-aquifer system such as the South Platte or Arkansas River valleys, The La Junta to Las Animas reach of the Arkansas was selected as a pilot sturly area because the physical system appeared to contain most of the variables and had fairly good records. The multiple I'egression,crop consump- ti ve use, phreatophyte use, and water exchange analyses as described below were all developed as part of the overall study. Funding in 1963 was transferred to the Colorado Water Consel"vation Board. Multiple Regression Analyses - Over 40 regression analyses were made using river gain between La Junta and Las ,\nimas as the depen- dent variable and prf'cipitation, applied irrigation water, pumped water, eonsurnptive US" by phreatophytes, and combinations of these parameters as independent variables. It wa;; hoped that the regression coefficients would indicat.' the physical significance of each variable but the high correlation betwL'cn independent variables often caused the regression coefficient to havc a value that could not be explained physically. This technique did indicate some valuable guidelines in the overall hydrologic anlyses, but was disappointing as a tool to determine Just how much each independent variable affected river gain. River gaIn was defined as outflow from a reach plus diversions in the reach minus inflow and had both positive and negative values. Consumptive Use Analyses - Estimates of the consumptive use by crops and the nonbeneficial use by phreatophytes were made using the Colorado Agricultural Statistics data for historic county acreages for individual crops, the windshield survey data for 19/,1, and phreatophyte data from the respective field study. The Blaney-Criddle technique was used to compute the consumptive use. Monthly values of the consumptive use were computed for the period 10 39-1962. Time Series Analyses of Hiver' Flows - An extensive effort was made to determine if there was a time series relationshil' for river flows at Canyon City, Pueblo, Nepesta or La Junta WIth the river gain. Of course, the annual runoff cycle was quiet evident and there was some indication of a fi ve year cycle. Cycles of greater length or ha ving a period less than twelve months were not core lusive although there appeared to be a time lag relationship between river flow and river gain.