Spectral estimates of net radiation and soil heat flux

Conventional methods of measuring surface energy balance are point measurements and represent only a small area. Remote sensing offers a potential means of measuring outgoing fluxes over large areas at the spatial resolution of the sensor. The objective of this study was to estimate net radiation (R_n) and soil heat flux (G) using remotely sensed multispectral data acquired from an aircraft over large agricultural fields. Ground-based instruments measured R_n and G at nine locations along the flight lines. Incoming fluxes were also measured by ground-based instruments. Outgoing fluxes were estimated using remotely sensed data. Remote R_n, estimated as the algebraic sum of incoming and outgoing fluxes, slightly underestimated R_n measured by the ground-based net radiometers. The mean absolute errors for remote R_n minus measured R_n were less than 7%. Remote G, estimated as a function of a spectral vegetation index and remote R_n, slightly overestimated measured G; however, the mean absolute error for remote G was 13%. Some of the differences between measured and remote values of R_n and G are associated with differences in instrument designs and measurement techniques. The root mean square error for available energy (R_n - G) was 12%. Thus, methods using both ground-based and remotely sensed data can provide reliable estimates of the available energy which can be partitioned into sensible and latent heat under nonadvective conditions.