“…The method was implemented by the authors in the context of a recent study conducted in four tropical mountain rivers, in Costa Rica where a clear need to estimate bed load transport rates and evaluate input‐variable sensitivity and uncertainty of bed load transport relations with no data available for validation existed. Selection of the MVFOSM method was based on the following aspects: (i) it is quick and easy to implement for any transport relation, especially with the aid of symbolic math packages such as the Symbolic Math toolbox and MuPad [ The Mathworks, Inc ., , b], or open source libraries as SymPy [ Meurer et al , ] which are widely available; (ii) allows assessing sensitivity for a single input only or multiple input variables at the same time; (iii) requires only two values for each input variable, namely a mean and a variance which can be estimated from basic knowledge of the river in question and a one day visit to the site; (iv) as a local sensitivity analysis, it allows evaluating the effect of variations (perturbations) about a base state, i.e., the river's current conditions (slope and grain size distribution), on the variability of the transport estimates; and (v) in places where there is sufficient information for one or more variables, but there is still need to better constrain others, the method is able to point out which of the remaining variables will introduce the largest variability in the transport estimates thus helping with the design of field campaigns in such a way as to better invest the available resources. The methodology, developed to assess input‐variable sensitivity and to better inform field measurement campaigns to complement the transport estimates and associated uncertainty is presented next.…”