Abstract:
Purpose Visible and near-infrared (Vis-NIR) reflectance measurements may be an alternative technique to identify
suspended sediment sources in streams of headwater catchments. In this study, we examined if Vis-NIR reflectance
measurements are capable of estimating sediment source contributions to sediment yield and compared this technique with
a more conventional (i.e. geochemical) technique.
Materials and methods Two headwater catchments in
Ethiopia, Unta (2,052 ha) and Desera (1,657 ha), were
analysed with the same techniques in order to find similarities
and differences in the results obtained. The first technique
used Vis-NIR spectral analysis as a fingerprint, using a partial
least squares regression model. The second technique was a
quantitative composite fingerprinting technique using geochemical analysis of source materials and suspended sediment
samples. As a comparison, the partial least squares model was
also used on the geochemical data. In August and September
2009, 30 soil samples of three different land uses (landslides,
croplands, and grazing lands) and 21 suspended sediment
samples at the catchment outlet were collected. Source samples were sieved to <63 μm. Geochemical analyses consisted
of total element concentrations, percentage carbon, percentage
nitrogen, and atom percentage 15N and δ13C. Reflectance
measurements were taken on dried source samples with a
spectrometer.
Results and discussion Neither technique was able to predict
the contributions of the three land use types; they could only
distinguish between landslide and topsoil material. The agreement between the results of both techniques was significant
for the Unta catchment (R2
=0.80) but not for the Desera
catchment (R2
=0.39). The uncertainty of the technique using
Vis-NIR reflectance measurements was slightly higher than
with the geochemical approach. Both techniques revealed that
topsoil erosion played an important role during storm runoff
discharges. Using the partial least squares model for the geochemical data revealed that uncertainty can differ greatly
when using other statistical techniques.
Conclusions The quantitative composite fingerprinting technique using spectral signatures from both source and
suspended sediment samples was able to quantify the contribution of two source materials (landslides and topsoil). It
provided a faster and more cost effective alternative to the
conventional geochemical procedure.
