Traditionally, a graph of sediment concentration versus time,
,
was drawn by hand
[ Porterfield, 1972].
After sample values were plotted on the graph, the line was drawn on the
basis of hydrologic judgment, sediment transport curves, and experience.
The load estimator,
,
was then evaluated numerically
[ Press et al., 1986, Chapter 4]:
The quality of 
as an estimator depends on
the hydrologist's skill and training.