SOURCES OF VARIATION IN RADIOMETRIC SURFACE-TEMPERATURE OVER A TALLGRASS PRAIRIE

Numerous studies have noted a strong negative correlation between radi ometric surface temperature and spectral vegetation indices such as th e NDVI, and have suggested that this relationship might be exploited i n strategies to model land surface energy balance from satellites. The se studies have been largely empirical in nature and the relationships among remotely sensed data, land surface properties, and land surface energy balance that produce this phenomenon remain unclear. We studie d the relationship between radiometric surface temperature and NDVI ov er a tallgrass prairie in northeastern Kansas. The study site included a mix of landcovers, with fractional vegetation cover and exposed soi l backgrounds over much of the site. We observed a persistent negative correlation between radiometric surface temperature and NDVI, but fou nd that the relationship was highly date- and time-specific. In this c ontext, the relationship between surface temperature and NDVI was obse rved to depend on landcover type, and a significant proportion of the total variance in both NDVI and radiometric surface temperature was ex plained by stratifying the data by landcover class. More importantly, our results show the relationship between surface temperature and NDVI to have little association with surface energy balance for data sets acquired from aircraft and helicopters on several dates during the gro wing seasons of 1987 and 1989. Based on results from a simulation mode l of the soil-canopy-sensor system, we hypothesize the observed covari ance between radiometric surface temperature and NDVI to be largely ca used by temperature differences between the soil background and vegeta tion canopy and by variation in fractional vegetation cover. This hypo thesis is supported by evidence showing soil moisture to be an importa nt secondary control on radiometric surface temperature due to its eff ect on soil thermal inertia, rather than as a limiting control on late nt heat flux, as might be expected. These findings indicate that inver tible surface energy balance models must account for the effects of la ndcover, soil background temperatures, and soil moisture before therma l infrared imagery can be effectively used to estimate land surface fl uxes.