Estimation of soil heat flux from net radiation during the growth of alfalfa

Abstract

Soil heat flux studies have indicated that the instantaneous daytime flux can be estimated as a fraction of the net radiation, the ratio ranging from 0.1 to 0.5, depending on the amount of vegetation present and on the time of day. Soil heat flux and net radiation were measured for an alfalfa crop over two regrowth cycles during the fall growing season. For both sparse alfalfa stubble and full vegetative canopy, the surface soil water content did not significantly affect the fraction of net radiation consumed as soil heat flux. The ratio of soil heat flux to net radiation around midday was found to be a linearly decreasing function of crop height only for heights up to 450 mm. As crop growth continued beyond this height, the ratio remained nearly constant at 0.1. The ratio data were also found to be well-fitted by a linearly decreasing function of a spectral vegetation index (near-IR to Red ratio) over both regrowth cycles. These results indicate that both crop height and spectral vegetation indices can be used to estimate soil heat flux from net radiation measurements.