WMOD00274 ‎(2)‎

• Title: Hydro-Climate Index Study - Final Report
• Prepared By: HDR Engineering
• Date: 2/14/2008
• County: Statewide
• State: CO
• Weather Modification - Doc Type: Report

A - October to March Precipitation (single season)
This segment of precipitation was left in the October to March format vs. the
December to March format as existing datasets were readily available for this
information. Nevertheless, this analysis should still sufficiently cover the important
periods in the area and the precipitation/climate relationships. This basin was broken up
into three components, given the unique orientation of the basin. The first is the section
of the river upstream of Granite which could be best described as Lake County, the
second is from Granite to Salida, in southern Chafee County and the third is from Salida
to Pueblo Reservoir. For the purposes of this report these sections will be referred to as
the Upper, Middle, and Lower Arkansas sections respectively.
Upper Arkansas:
In this period, the PDO exhibited some correlation between HCI values and basin
average precipitation. For the PDO, in general, negative values of PDO corresponded
better with higher values of precipitation in the basin. However, this basin did exhibit a
modest signal with ENSO variables as well, more notably theMEI. From an ENSO
standpoint La Nina regimes tended to produce greater averages of precipitation while El
Nino’s tended to be drier.
Middle Arkansas:
This portion of the basin also exhibited a similar and more steady relationship
with ENSO in that La Nina years tended to yield higher precipitation values while El
Nino years have drier tendencies. When examining the MEI, this relationship is
somewhat steady in the period leading up to and continuing into the winter. However, as
the seasons progresses into the winter months the relationship between the precipitation
and the NAO begins to rise in importance. In summary positive values of NAO
correlated to higher values of precipitation. Also on the “Atlantic” side of the equation,
early summer/fall values of the AMO exhibited a negative correlation between its value
and precipitation tendencies and unlike the NAO, this relationship actually weakened as
the late fall/early winter approached.
Lower Arkansas:
During this period, this sub-basin resulted in a number of relationships that was
stronger than most basins. However, the most robust relationships were with the NAO
and PDO, in which positive values correlated with higher precipitation values. For both
of these indexes, the relationship was stronger as the winter months approached. Also,
negative values of the NAO had some relationship with higher precipitation values.
Although ENSO was the relatively weakest component, there was some tendency for El
Niño’s to experience above average precipitation, while La Nina exhibited some drier
tendencies.
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