Reducing uncertainty in sediment yield through improved representation of land cover: Application to two sub-catchments of the Mae Chaem, Thailand

A sediment source, transport, and deposition model known as SedNet has been applied to two sub-catchments of the Mae Chaem River in Thailand, the Mae Suk (95 km²) and the Mae Kong Kha (91 km²). The applied models were analysed to determine the dominant sources and sinks of suspended sediment in these catchments, and to examine the uncertainty in model inputs and results. Two landuse grids were available for these catchments; one in 1995 was derived from satellite imagery and contained a relatively undifferentiated landuse classification (lumping all forest types into one category); the other in 2003 was derived from a mix of satellite imagery, ground truthing, and mapping, and better differentiated between landuse types (dividing forest types into evergreen, deciduous, and pine plantations). Results indicate significant differences in sediment export in 2003 compared to 1995, with exports from the Mae Suk reduced by 10 kt/y, and exports for the Mae Kong Kha increased by 6 kt/y. It is difficult to say whether these changes were due to actual changes in landuse between 1995 and 2003, or due to the improved landuse classification in 2003. Given the range of uncertainties associated with these predictions (Figure 1), these relatively small increases / decreases are not significant. The source areas of suspended sediment also changed significantly between 1995 and 2003, and these changes in source area can clearly be linked to the improved mapping of landuse in 2003. The improved landuse classification in 2003 also led to significant reductions in the uncertainty associated with the export of suspended sediment from both catchments. The uncertainty in suspended sediment export associated with the landuse classification for the Mae Suk reduced by 23%, while that for the Mae Kong Kha reduced by 17% (Figure 1). These reductions in uncertainty were primarily due to improvements in the classification of forest type, with the range of uncertainty in sediment yield from forests declining from 28.99 kt/y to 12.92 kt/y in the Mae Suk and 26.91 kt/y to 11.44 kt/y in the Mae Kong Kha. (Figure Presented) Despite these improvements in landuse classification, significant uncertainty in suspended sediment yield still exist for both sub-catchments. Much of this uncertainty is due to the uncertainty in suspended sediment yields from field crops and fallow fields. There are two reasons for this, firstly, these landuses produce the vast majority of the suspended sediment from the Mae Suk and Mae Kong Kha sub-catchments (75% and 54% respectively), and secondly, the landuse grids do not distinguish the type of crop being grown. This leads to a large range in the possible values of the C-factor used in the USLE grid, from 0.25 to 0.79. Further improvements in identifying the total volume, source areas, and uncertainty in suspended sediment yields will best be achieved through an improved landuse coverage which identifies the type of crop being grown.