“…Many studies on data assimilation into hydraulic models or forecasting systems integrate synthetic, in situ or remote sensing-derived observations of water levels. For example, see Table 1 in Revilla-Romero et al (2016), and also Neal et al (2007), Matgen et al (2010), Hostache et al (2010), Giustarini et al (2011), Yoon et al (2012), Andreadis and Schumann (2014), García-Pintado et al (2015), Hostache et al (2015), and Xu et al (2017). Indeed, water level is a diagnostic variable of any hydraulic model and hence is more straightforward to assimilate than flood extent (Lai et al, 2014), which is a prognostic variable (diagnostic variables are defined as variables that are required to solve the model, that is, state variables, whereas prognostic variables are derived quantities).…”