4. DISCUSSION
In this study, only local variations of three parameters – initial soil moisture, hydraulic conductivity, and the exponent of the VIC model – are tested independently. However, the interactions among these parameters are not explored herein. Global sensitivity analysis, such as the Morris method used in the previous study (Li et al., 2021b), can measure the variation of each parameter relative to other parameters, so it provides a clearer picture of the parameter interactions. Needless to say, initial soil saturation state is the dominant controller for the differences between the simulations with and without re-infiltration process. When the soils are fully saturated, the with and without re-infiltration scenarios are almost identical if other parameters are the same. Combined with our previous study that underlies the importance of infiltration and initial soil moisture for flood inundation modeling, we highly recommend taking into consideration the initial soil moisture state, as it has not been well-recognized in the hydraulic model community. This can be achieved via three ways: 1) warm up the model for a relatively long period prior to the simulation period (Chen et al., 2020); 2) parameterize the initial soil moisture and calibrate it, similar to the way we treat initial in-channel water depth (Xue et al., 2013); 3) approximate it using observations or other model simulations, like what has been done in the real case study in Section 3.2 (Flamig, Vergara, & Gourley, 2020). The first approach is ideal because it eliminates uncertainties in parameterization (such as equifinality) or error propagation from observations/simulations to models; it is, however, the most computationally expensive approach for hydraulic modeling compared to the other two. Approach two and three are more pragmatic, while both inherit uncertainties or errors. We prefer the third approach if the data source is found to be trustworthy. For instance, in our case study, we used the simulated soil moisture product from the operational CREST/EF5 model which shares the same land surface processes as the CREST-iMAP.
The results relating to different rainfall events are not factored in this study, yet different event characteristics interact with soil infiltration dynamics, thereby causing differences between the two schemes (Zhang et al., 2020). One could envision the differences increase with rainfall rates if they did not exceed the maximum infiltration capacity and the soils are not saturated. However, once reaching the maximum infiltration rates or soil saturation, the differences are unlikely to further enlarge. Other parameters regarding terrain characteristics are also effective in re-infiltration. For instance, an increase in slope will leave less room for surface water to re-infiltrate, which explains why re-infiltration compromises its importance in hillslope hydrology (Corradini et al., 2002; Zhang et al., 2020).