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Causes of Stream Flow Increase and Decline
James Koelliker created a graph to help illustrate the idea of how much various activities affect the stream flow. Because it is easy to draw conclusions that are not warranted, I have simplified the graph so that we can highlight key points. His study can be found here. A conservation analysis of the study can be found here. The combined blue and green areas in graph 2 below show the total stream flow. The reason for the large green mound, according to Koelliker, is an increase in agricultural activities that created large amounts of run off. From 1850 to 1925, there was a large increase in stream flow caused by land management practices. After the 1930s, the government mandated erosion control practices (such as terraces, retention ponds, and leaving residue on the fields). These land management practices resulted in a decrease in the amount of run off and a resulting decrease in the stream flow, as seen on the graphs. The stream flow in 1850 was a combination of aquifer seepage and runoff from precipitation. The flow back then was unaffected by man. The dip in the blue line, beginning around 1980, is the reduction in stream flow caused by a reduction in aquifer seepage. This reduction is caused by a lowering of the water table and a reduction in the springs that feed the streams. Note that almost all numbers on these graphs are Koelliker's estimates. The flows prior to 1900 are guesses, as there is no data recorded prior to that period of time. Also note that the baseflow number is also a guess, as there is no good way to estimate what portion of the stream flow comes from the aquifer and what ran over the surface of the ground before washing into the stream.
In Graph 3, we highlight, in red, the amount of water that is not showing up in the stream due to aquifer pumping. Once again, I would like to emphasize that these are Koelliker's estimates. What Koelliker is saying is that the increased stream flows from 1850 to 1980 or so were unnatural and were due to poor land management practices that caused unnatural run off. As conservation practices were put into place, these unnatural stream flows were eliminated and returned to normal. However, the stream flow is less now than in the 1850s because of aquifer depletion. This lessening is represented by the red area.
To measure the responsibility of aquifer pumping on stream flow, one would need to compare today's stream flow amounts with the stream flow amounts in the 1850s. As can be seen from the graph above, this results in the red area. Most people wanting to show the large decrease in stream flows and wanting to blame these reductions on aquifer pumping are comparing today's stream flow to that of the 1930's. Yet, according to the Koelliker study, this is an inaccurate comparison. One should compare current flows to 1850 flows in order to get a more accurate picture. The problem we have is knowing for sure what the flows were before the 1900's. There were no stream gauges in the 1850s and, hence, any comparison is a guess. But, if we assume that Koelliker is close to right in his guess, then we should realize that the increased stream flows from 1850 to 1980 were abnormally high and that natural stream flow amounts are close to the 1980 flows. For the Republican River, this would result in the following graph.
The drop of the red line below the blue line is the result of aquifer pumping. This is a significant amount of water. Yet, it is far less than what is typically blamed on ground water irrigators. Remember that those against irrigation often compare 1930s flows to current flows, rather than referring back to the estimated 1850-era flows. The trend is for the red line to continue down and, thus, action will be required if the stream flow is to be returned to its natural level (blue line). |
