Estimating sub‐canopy shortwave irradiance to melting snow on forested slopes

Estimates of shortwave irradiance energy beneath needle-leaf forests over complex terrain are needed to drive energy balance snowmelt models and to evaluate the potential hydrological impacts of forest-cover change in mountain regions. This paper outlines and evaluates a physically-based model designed to estimate sub-canopy shortwave irradiance to snowcover under needle-leaf forest-cover with respect to surface slope and azimuth. Transmission of above-canopy irradiance was estimated using forest-surveys and hemispherical photographs to determine the fractions of forest-cover occupied by non-transmitting trunks, partially-transmitting crowns and fully-transmitting gaps with respect to above-canopy diffuse and direct beam shortwave irradiance. Simulations were conducted for continuous, uniform lodgepole pine forests on level site and a north facing slope and a discontinuous, non-uniform forest on a southeast facing slope during snowmelt at the Marmot Creek Research Basin, Alberta, Canada. Mean observed daily transmissivity values of irradiance were 0·09 at the north-facing forest, 0·21 at the level forest and 0·36 at the southeast-facing forest. Modelled and observed results indicate that potential snowmelt energy from sub-canopy shortwave irradiance is likely to exhibit the greatest variation with change in cloudiness and forest-cover density under south-facing forests and the least variation under north-facing forests. Comparisons of simulations to observations indicate that the model can explain much of the difference in daily shortwave transmission amongst sites, performing relatively poorest at the north-facing forest where fluxes were small and relatively best at the south-east facing forest where fluxes were large. However, simulation errors in terms of absolute irradiance were greatest at the southeast-facing forest, having a root mean square error (RMSE) 0·64 MJ m -2 d -1 compared to 0·44 MJ m -2 d -1 at the level forest and 0·27 MJ m -2 d -1 at the north-facing forest.

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