THE EFFECTS OF LATERITE AND ASSOCIATED TERRAIN COMPONENTS ON PBMR RESPONSE IN HAPEX-SAHEL

Terrain characteristics such as roughness and vegetation have been sho wn to significantly affect the interpretation of microwave brightness temperatures (TBS) for mapping soil moisture. This study, a part of th e 1992 HAPEX-Sahel experiment (Hydrologic Atmospheric pilot Experiment in the Sahel). aimed to determine the effects of laterite and associa ted terrain components (i.e. vegetation, soil, and exposed water bodie s) on the Ts response of the Pushbroom Microwave Radiometer (PBMR, L-b and, 21 cm wavelength), using the NS001 Thematic Mapper Simulator data as a surrogate for ground data. Coincident PBMR and NS001 data acquir ed from the high altitude (about 1500 m) long transect flights were pr ocessed to obtain TBS and radiances, respectively. The transects cover ed a range of moisture conditions. For this preliminary evaluation, no atmospheric corrections were applied, and the data sets were aligned by matching the acquisition times of the data records. NS001 pixels (a bout 3 m) were averaged to approximate the resolution of the PBMR (abo ut 450 m), before their flight line data were compared. The laterite p lateaux were found to have a surprisingly strong effect on the PBMR T- B response. T-B variations along the flight line could largely be expl ained by a combination of density and dielectric properties of laterit e. The effect of surface moisture was distinguishable from the laterit e effect, with the distinction apparently related to the occurrence of ephemeral pools of water after rainfall. Model simulated TBS agreed r easonably well with the observed TBS.