SPDSS_Task39_Phase4_FinalWaterLevelMeasurement

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In addition to not capturing the high and low water level, bi-annual measurements have the potential of being biased by nearby pumping well influences. For example, in Figure 9 during the period from May to August, pumping cycles can cause water levels to fluctuate several feet over a couple of days. If a bi-annual point measurement during a pumping cycle from one year is compared to a bi-annual point measurement in the following year from anon-pumping event, t11e difference between t11e two measurements would not accurately depict the awlual water level trend. Furthermore, a comparison of water level trends from the bi-awlual measurements could be distorted if the tinting of nearby irrigation and pumping were earlier or later in a given year compared to the tinting of a particular water level measurement. Bi-annual measurements conducted in an irrigation well or in a monitoring well located near an irrigation well, should consider the pumping cycles during the growing season. The continuous 1lydrographs in Appendix E were reviewed to help deternune the period of time during which pumping cycles are observed. Figure 10 demonstrates t11e approximate starting and ending months of irrigation pumping in the alluvial aquifer of the SPDSS study area observed from the hydrograplls of the 37 SPDSS alluvial monitoring wells based on the record from 2004 to 2007. Eac11 well is counted once in the figure as t11e starting and ending months were mostly consistent over the period of record for each well. The figure illustrates pumping begins as early as March in a given year but in other areas may not beg>lz wltil July. The most frequent beginning of pumping is >11 April based on these hydrographs. The pump>11g can end as early as September but may extend until November. To minimize the effects of nearby irrigation pumping on bi-annual measurements, the measurements should be conducted when irrigation pumpuzg is at a minimum. Based on the SPDSS monitoring well data this period would extend from the latter part of November through February. This date range is a generalization for the study area that is based on limited observation points and may vary from region to region. Wells in the DWR monitoring program are typically collected in late March and early November, and therefore, t11e November measurement is less likely to be influenced by pumping and irrigation activities. 2.2.4 Comparison of Spring and Fall Annual Alluvial Water Level Changes Previous analyses conducted wider Task 44.3 of the SPDSS have shown that groundwater levels in most portions of the South Platte River alluvial aquifer system have not changed appreciably over time when comparing spring measurements. Concerns were brought up at t11e SPDSS Peer Review Conlnuttee meetings regarding how water use during the growing season affects alluvial aquifer water levels. The claim was that a comparison of spring-to-spring measurements did not show water level trends as well as a comparison of fall-to-fall measurements would. The reasoning was that winter recharge to the aquifer would mask t11e effects of irrigation season pumping by the time the spring measurement was taken. To evaluate this effect, water levels in several wells >11 the alluvial aquifer were analyzed. The average annual change in fall-to-fall water levels in the South Platte River alluvial aquifer system was compared to the average annual change ii1 spring-to-spring water level changes. Water levels from 1996 through 2006 in February to April (spring season) and October to December (fall season) for alluvial wells in Water Division 1 were queried from HydroBase. There are 83 alluvial wells in HydroBase t11at have five or more spring and five or more fall measurements. These wells are located throughout t11e SPDSS alluvial aquifer study area. The SPDSS Phase 4 Task 39 Technical Memorandum-Final 17 O~i20%2008