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<br />I <br /> <br />10 <br /> <br />~ <br /> <br />DISCUSSION AND CONCLUSIONS FROM COMPUTER ANALYSIS <br /> <br />Many factors acted singly or In combination to cause the changes which <br />were observed in the relationships summarized above -- particularly in as <br />diverse and complex a system as the interrelated surface water-groundwater <br />system within the South Platte basin. The natural fluctuations of flow are <br />large without the influence of man's activities. With these influences super- <br />imposed on the natural conditions, and with many of man1s Influences being <br />indirect, unmeasured and gradually changing, an accurate separation and con- <br />clusive quantitative evaluation of each factor is difficult. <br /> <br />The following factors probably had some influence on the changes and <br />trends summarized above: <br /> <br />1. Natural factors, such as long periods with abnormally high or low <br />precipitation, or a shift in precipitation pattern which affected only a part <br />of the area. <br /> <br />2. Changes in crops grown and/or irrigation water management practices. <br /> <br />3. Changes in the character of water use, such as transfers from agri- <br />cultural to municipal. <br /> <br />4. Changes in non-beneficial consumption of water, such as that due to <br />changes in water table levels and the growth of eradication of phreatorhytes. <br /> <br />~ <br /> <br />5. Changes in the amount of water imported into the basin. <br /> <br />6. Changes in the amount of groundwater pumped from the alluvium under- <br />lying the South Platte valley. <br /> <br />7. Changes in river operation practices, particularly in regard to the <br />sequence in which offstream reservoirs are filled. <br /> <br />It would be possible to util ize more sophisticated analyses of the <br />available data to further evaluate the Influence of these factors on the <br />observed flows within calculated confidence I imits. However, it is unl ikely <br />that the accuracy of the data pertaining to these influencing factors is suf- <br />ficient to justify such analyses at this time. A study of the moving average, <br />mass diagram and correlation analyses which were made, along with general <br />knowledge of the development of the influencing factors, has produced the <br />following conclusions: <br /> <br />1. The reduction in annual discharge at the Julesburg station during the <br />period following 1925 compared to the 20-year period prior to 1925 was princi- <br />pally caused by natural factors rather than man-made influences. This Is not <br />to say that man's activities did not have an effect, only that the major <br />effect was due to natural climatic and hydrologic conditions. <br /> <br />,~ <br />\ <br /> <br />2. Manis activities, notably groundwater pumping, have caused a change <br />In pattern of return flow with time which is noticeable principally in the <br />fall months. However, the influence is small in comparison to the total volume <br />of discharge, and because of increases experienced in other months the net <br />annual effect seems to be small. <br /> <br />~ <br />