Using Field Observations to Guide Model

ParametersMichael J. Rush

Graduate Research Assistant – Boulder Creek Critical Zone ObservatoryHydrology, Water Resources, & Environmental Fluid Mechanics

University of Colorado - Boulder

Overall Research Questions: • How does permafrost control groundwater flow?

• How does aspect (north-facing vs. south-facing) control thermodynamics of the subsurface?

• How will alpine ecosystems respond to climate change?

Field Site – Gordon Gulch

Guiding Hypothesis: North-Facing Slope • Frozen soil layer in the subsurface directs snowmelt to

the stream:

Guiding Hypothesis: South-Facing Slope • Absence of frozen soil in the subsurface allows

snowmelt to percolate deep into the subsurface (arriving later at the stream):

The Surface Energy Balance • In order to understand the formation of permafrost in the

shallow subsurface, we need to calculate the so-called “surface energy balance”

• Calculate incoming radiation, reflected shortwave radiation, incoming longwave radiation, incoming heat from precipitation, ground heat flux, outgoing longwave radiation, heat lost by convection, heat absorbed by evaporation… Complicated!!!

Tarboton and Luce, 1996

Utah Energy Balance Model:

Tarboton and Luce, 1996

Easy to calculate incoming radiation…

Duguay, 1993

But radiation calculations are complicated by interception… • High clouds reflect incoming radiation back to the

atmosphere, while low clouds reflect outgoing radiation back to Earth’s surface • Forest canopies reflect incoming radiation, insulate the

soil surface, and remove heat from the soil via evapotranspiration• Snow reflects incoming radiation and insulates the soil

surface

Albedo Function a• Can adjust incoming radiation by a reflection factor to calculate

actual incoming radiation:

• A number between 0 and 1 that quantifies the reflection of radiation by surfaces• Based on type of surface, can look up textbook values: (Lee, 1980)

• Snow ~.9 (very reflective) • Clouds ~.4-.7 • Grass ~.2-.3• Trees ~.1-.2 (not very reflective)

• In seasonally snow-covered catchments, albedo can change throughout the year, i.e. a=f(t)

Canopy Interception Function f• Can adjust incoming radiation by a interception factor to

calculate actual incoming radiation:

• For lodgepole pines, the canopy interception function f depends on the fraction of the sky obscured by the canopy F: (Dunne and Leopold, 1978)

Field Exercise! • Head to Gordon Gulch!• Hike to north- and south-facing meteorological stations

(where we are measuring incoming radiation)• Observe:

1. Soil surface (Pine needles? Grass? Bare soil? Snow?)

2. Canopy cover (just observe and take notes)

Modeling Exercise! (when we return) • Use surface observations to estimate albedo throughout

the year • Use canopy observations to estimate interception • Match solar radiation model with data!