Climate
The Boise-Payette-Weiser subbasins are located on the western edge of the central mountain mass of the Salmon River and Boise Mountains. The forested watersheds of these subbasins drain southwesterly from elevations above 10,000 feet to 3,200 feet along the major rivers. Surrounding mountains rise about 4,000 feet above the valley floors. The subbasins encompass a strong climatic gradient from the hot, dry Continental climate of the Snake River Plain to the Pacific maritime climate of upland montane slopes and ridgecrests.
Pacific maritime-influenced climatic regimes, characterized by cool, moist winters and warm, dry summers, are prevalent in all but the Lower Boise River and Payette River watersheds. Extreme Continental climatic conditions are prominent in the low elevation valleys, of Lower Boise River and Payette River watersheds. Intermediate climatic regimes occur on the foothill slopes, and upper valleys of Boise-Mores Creek, Lower Boise River, mainstem Payette River, and Weiser River watersheds. The climate of the subbasins is summarized by watershed using the Koppen climatic classification system (Godfrey 1999) (Table 2).
Table 2. Climatic regimes of watersheds within the Boise-Payette-Weiser subbasins: the proportional representation of varying climatic regimes (using the Koppen climatic classification system, described by Godfrey 1999) within the subbasins is summarized by watershed.
Koppen climate class
|
Description
|
Boise-Mores
|
Lower Boise
|
Middle Fork Payette
|
North Fork Payette
|
North and Middle Fork Boise
|
Payette
|
South Fork Boise
|
South Fork Payette
|
Weiser
|
BSk
|
very dry Continental Climate; most precipitation occurs in summer
|
|
79.7
|
|
|
|
24.1
|
|
|
2.9
|
Dfb
|
warm summers, cold winters; precipitation is relatively evenly distributed between winter and summer
|
|
|
|
2.1
|
|
|
|
|
|
Dfc
|
warm summers, cold winters; precipitation is relatively evenly distributed between winter and summer; summers are relatively short
|
|
|
|
2.7
|
6.8
|
|
2.7
|
2.7
|
|
Dsa
|
warm summers, cold winters; extreme differences occur between summer versus winter precipitation (summers are much drier); summers are relatively long and hot
|
6.6
|
14.3
|
|
|
|
20.9
|
|
|
21.3
|
Dsb
|
warm summers, cold winters; extreme differences occur between summer versus winter precipitation (summers are much drier); summers are relatively warm
|
90.9
|
6.0
|
94.4
|
79.7
|
74.8
|
53.5
|
79.9
|
74.0
|
74.2
|
Dsc
|
warm summers, cold winters; extreme differences occur between summer versus winter precipitation (summers are much drier); summers are relatively short and cool
|
2.5
|
|
5.6
|
15.5
|
18.4
|
1.4
|
17.5
|
23.3
|
1.6
|
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