Estimation of surface net radiation in the boreal forest and northern prairie from shortwave flux measurements

Abstract

The relationships between net radiation and surface shortwave fluxes were explored by using one year of 15 min surface flux data for nine sites in central Canada. The sites were located throughout a 50,000 km2 area, in conjunction with the BOREAS project, across a gradient in surface cover from prairie grassland to boreal forest. When insolation alone was used to estimate instantaneous net radiation, the r2 values at the nine sites ranged from 0.93 to 0.98. The use of the net shortwave flux as a predictor improved the range in r2 values to between 0.96 and 0.99. The root‐mean‐square error across sites varied from 26 to 41 W m−2 without albedo to a range of 18–36 W m−2 when albedo was included. The data for all sites were combined to produce a single linear relationship with an r2 value of 0.97 and a root‐mean‐square error (RMSE) of 29 W m−2 when the surface albedo is included. Cross validations were performed where the data for eight sites were used to predict net radiation at a ninth (excluded) site, revealing little site‐specific bias. Classifications of the data by season and by transmittance provided little improvement in estimated net radiation at any site. Our study suggests that a single equation using net shortwave flux alone can be used to estimate net radiation for large portions of the boreal forest and northern prairie, regardless of spatial and temporal variation in surface cover and atmospheric conditions.