watervulnerability

Sawtooth National Forest Watershed Vulnerability Assessment, Intermountain Region (R4)
CLIMATE CHANGE EXPOSURE MODELS
Water Temperature
Potential effects of increased water temperatures due to climate change to bull trout were evaluated using
a non-spatial multiple regression stream temperature model (Isaak et al. 2010). This model was created
using an extensive, but non-random database of stream temperature measurements within the upper
Salmon River, Upper S.F. Payette and Upper S.F. Boise subbasins on the SNF. More than 450
temperature measurements (Hobo and Tidbit models) were used from numerous resource agencies from
1994–2008. The majority of thermographs were placed in streams before mid-July, geo-referenced, and
retrieved after mid-September. This sample period encompassed the warmest portion of the year when
variation in temperatures among areas is most pronounced and influence on fish growth, behavior, and
distribution is potentially greatest (Scarnecchia and Bergersen 1987, Royer and Minshall 1997).
Predictor variables (i.e., geomorphic, climatic, and categorical) were used to describe spatial and temporal
attributes associated with the stream network. Geomorphic predictors included watershed contributing
area, elevation, and channel slope. Predictors in this category represented relatively static features of the
river network, valley bottoms, and upstream watersheds that were hypothesized to affect stream
temperatures.
Interannual variation in climatically-influenced factors such as air temperature and stream flow have
important consequences for stream temperatures. Air temperature affects stream temperature through
sensible heat exchange near the surface of the stream and by influencing temperatures of near surface
groundwater, which is an important component of summer flows. Stream flow determines the volume of
water available for heating; larger flows have greater thermal capacities and are less responsive to heating
(Hockey et al. 1982, Caissie 2006).
Climate predictors included air temperature measurements derived from extrapolations of the observed 30
year trends at cooperative weather stations (Ketchum and Stanley) on the Sawtooth National Forest, and
the 50 year trends at the USGS gauges (S.F. Boise River near Featherville, S.F. Payette River at Lowman,
and Salmon River below Yankee Fork near Clayton) with the longest records on or near the SNF. The air
temperature data between weather stations was strongly correlated (r 2 = 0.74–0.91), so the individual time
series were averaged and the same summary metrics that were applied to model stream temperatures were
applied (i.e., MWMT). Flow data were obtained from two USGS stream gauges in the basin (Twin
Springs and Featherville gauges). These two sets of data were also strongly correlated (r 2 = 0.97) and were
averaged to calculate annual mean flow (m 3 /s) from 15 July to 15 September.
Air temperature projections, used in the water temperature model, assume climate change will continue at
the same rate that has occurred in the last 50 years on the forest. This likely underestimates the amount of
change (as predicted by or some IPCC climate change scenarios). These scenarios generally predict the
rate of air temperature change to accelerate due to increased carbon dioxide (Isaak/Wegner, pers. comm.).
The advantage of using empirical estimates is that they're based on data from the Forest, are easy to
understand. They provide estimates comparable to those from the IPCC scenarios for future values at
mid-century.
Categorical predictors included effects due to increased water temperature in lake outflows, water
diversions, wildfires, and professional judgment. All upstream wildfires that occurred within the past 20
years were considered. Water diversion effects on water temperatures were coded from zero (when they
diverted less than 5% of flow) to three (when they diverted more than 30% of flow). Diversion effects on
stream temperature were assumed to extend as far as 7 km downstream of the diversion or to a confluence
with a larger river or stream. Finally, lakes larger than 0.1 km 2, or groups of lakes, were considered to
166 Assessing the Vulnerability of Watersheds to Climate Change