SPDSS_Task1_Research-ID-StudyPeriod

• Title: SPDSS Task 1 - Research and Identify Appropriate Study Period
• Description: The objective of this task was to verify or refine the recommended study period as follows: 1. Review the availability and reliability of digitized data required for the development of DSS components; 2. Determine that the period includes wet, dry, and average periods; represents the long-term average hydrology of the basin; and allows for cost-effective modeling efforts; 3. Document study periods used in other modeling efforts in the South Platte Basin.
• Decision Support - Doc Type: Task Memorandum
• Date: 10/26/2003
• DSS Category: Surface Water
• DSS: South Platte
• Basin: South Platte
• Contract/PO #: C153954
• Grant Type: Non-Reimbursable
• Bill Number: SB01-157, HB02-1152, SB03-110, HB04-1221, SB05-084, HB06-1313, SB07-122
• Prepared By: Leonard Rice Engineering

As shown on Figure 6, extreme extended droughts (PDSI -4.0 or less), lasting more than
one year, have occurred four times during the available period of record, including the
current period from 2001 through 2002. The most recent extreme drought of significant
duration ended in 1957. The longest drought took place in the 1930s. The most extended
continuous wet period took place between 1905 and 1925. As shown on Figure 6, the
selected study period of 1950 to 2002 does not include the extremes of record, but it
captures extended periods of extreme drought and extreme wet.
For the entire period of record the average PDSI is 1.37. For the current period of 1950
to 2002, the average PDSI is 0.57. While the selected period of record is overall drier
than the total period of record, the period of 1950 to 2002 is closer to a normal
classification as defined by the PDSI Index.
Correlation Between PDSIIndex and streamflow
A comparison of the PDSI Index for the South Platte Basin was made to each of the
representative gages shown in Table 2. The annual streamflow data were plotted such
that the average streamflow corresponds to near normal conditions as defined by the
PDSI Index classification. Figure 7 shows the measurements at Middle Boulder Creek at
Nederland as an example. As presented, climatic conditions represented by the PDSI are
generally mimicked in the annual streamflow records. The correlation for each of the
gages is not perfect, which indicates that fluctuations in the streamflow are not accurately
represented in the basin wide PDSI Index. This is to be expected because streamflow
data are only representative of a single year, while the PDSI Index is dependent on the
current weather patterns plus the cumulative patterns from previous months regardless of
the year. Therefore, one would expect to see extended dry or wet periods lasting for a
shorter duration in the streamflow records than in the PDSI Index.
As noted above, both the analysis of PDSI Index and representative streamflow gages
indicate that the selected study period of 1950 through the most current year with
available data is representative of long-term climatic conditions and corresponding
hydrology in the South Platte basin.
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