Geoscience Reference
In-Depth Information
water-management. consequences.. The. analysis. of. Bales. et. al.. (2006). shows. that.
within.certain.elevation.bands.in.the.Cascade.and.Sierra.Nevada.Ranges,.over.half.
of.the.annual.precipitation.historically.falls.in.this.temperature.range..The.vulner-
ability.to.a.change.from.snow.to.rain.is.next.greatest.in.the.mountains.of.Idaho.and.
parts.of.the.Great.Basin,.and.least.in.the.highest.and.thus.coldest.parts.of.Colorado.
and.the.southern.High.Sierra.Nevada.
Consistent. with. the. physical. sensitivity. analysis. of. Bales. et. al.. (2006),. several.
studies.have.demonstrated.a.statistical.connection.between.luctuations.and.trends.in.
temperature.and.luctuations.and.trends.in.various.hydrologic.variables..Knowles.et.
al..(2006).used.weather.station.data.and.reported.that.rain/snow.ratios.have.increased.
in.most.of.the.West.since.about.1950,.with.spatial.patterns.resembling.those.of.tem-
perature.change.and.the.temperature.sensitivity.noted.by.Bales.et.al..(2006)..Mote.et.
al..(2005).and.Hamlet.et.al..(2005),.using.observations.and.modeling,.demonstrated.
that. springtime. mountain. snowpack. declined. at. roughly. 75%. of. locations. in. the.
West.since.the.mid-twentieth.century..These.changes.were.dependent.on.elevation.
(and.thus.temperature,.because.temperature.usually.decreases.with.elevation),.with.
warmest.locations.losing.the.largest.fraction.of.snow..Stewart.et.al..(2005).showed.
that.streamlow.in.much.of.the.West.has.changed.in.a.manner.consistent.with.the.
observations.of.declining.mountain.snowpack..In.basins.with.a.signiicant.snowmelt.
contribution,. winter. and. early. spring. lows. generally. increased,. summer. and. late.
spring.lows.generally.decreased,.and.the.date.of.peak.spring.snowmelt.shifted.ear-
lier.by,.on.average,.2.weeks..Stewart.et.al..(2005),.Hamlet.et.al..(2005),.and.Mote.
(2006). all. evaluated. the. possible. contributions. of. changes. in. precipitation. and. of.
changes. in. atmospheric. circulation. over. the. Paciic. Ocean. and. concluded. that. the.
dominant.factor.in.western.trends.in.hydrology.was.the.widespread.increase.in.tem-
perature.unrelated.to.atmospheric.circulation.
Increases.in.temperature.with.no.change.in.precipitation.can.cause.evapotrans-
piration. (ET). to. increase.. Hidalgo. et. al.. (2005). estimate. an. average. temperature.
increase.of.+3°C.could.increase.potential.evapotranspiration.(i.e.,.what.evapotrans-
piration. would. be. if. not. limited. by. water. availability). by. about. 6%. in. California..
However,.the.physiological.response.of.plants.to.increased.CO
2
.concentration.would.
likely.act.to.reduce.water.loss.
g
lobal
T
eleConneCTions
Spatial. patterns. of. climate. variability. in. the. western. United. States. are. correlated.
with.patterns.of.climate.variability.in.other.parts.of.the.world..For.example,.winter.
precipitation. in. the. West. frequently. exhibits. a. “dipole”. pattern. (wet. in. the. Paciic.
Northwest.and.dry.in.the.Southwest,.or.vice.versa),.and.that.this.pattern.is.strongly.
related.to.tropical.Paciic.Ocean.temperatures.and.to.atmospheric.pressure.patterns.
in.the.Southern.Hemisphere.(Redmond.and.Koch.1991)..The.sense.of.the.relationship.
is.such.that.the.phenomenon.known.as.El.Nĩo.is.associated.with.wet.winters.in.the.
Southwest.and.dry.winters.in.the.Northwest.and.northern.Rockies,.and.that.La.Nĩa.
is.associated.with.dry.winters.in.the.Southwest.and.wet.winters.in.the.Northwest.and.
northern.Rockies.
