## 3 Results and Discussions
### Rainfall Simulations by WRF-Only and the Fully Coupled WRF/WRF-Hydro
#### 3.1.4 Spatial Variation of the Cumulative Rainfall  
Figures 7 and 8 illustrate the spatial variation of the 24 h cumulative rainfall in the Fuping and Zijingguan catchments. From left to right are the observed spatial patterns of the cumulative rainfall from the rain gauges, the simulated rainfall distribution of WRF-only and the fully coupled WRF/WRF-Hydro, and the spatial variations of the cumulative rainfall simulated by the two modes (i.e., the fully coupled WRF/WRF-Hydro minus WRF-only). Among all the subfigures (c), the spatial variations of Event 3, Event 4, and Event 6 were relatively more significant than the other events.  
In particular, the coupled system simulations showed greater variations than WRF-only, which can be seen in the spatial variations in the subfigures (c) of Event 3, Event 4 and Event 6 (Type 3) and the fully coupled WRF/WRF-Hydro model improved WRF-only at the critical storm center due to its spatial redistribution of the rainfall. Spatially, the fully coupled WRF/WRF-Hydro and WRF-only showed essentially the same patterns for the cumulative rainfall distributions for both Type 1 and Type 2 events, and the simulations for the two events were better fitted with rain gauge observations than those of the Type 3 events. The storm centers of Event 1 and Event 2 were captured relatively well by the fully coupled WRF/WRF-Hydro as well as WRF-only. However, for Type 3 storm events, when matched with the observations, some parts of the catchments with high rainfall accumulations were missed by the fully coupled WRF/WRF-Hydro and WRF-only.  
Figure 7: Spatial distributions of the 24 h accumulative rainfall for the three storm events in the Fuping catchment, from left to right: gauge observation; (**a**) WRF-only; (**b**) the fully coupled WRF/WRF-Hydro; (**c**) the fully coupled WRF/WRF-Hydro minus WRF-only.  
In comparison with WRF-only, Event 4 in the fully coupled WRF/WRF-Hydro was apart, with variations ranging from \(-60\) mm to \(60\) mm as shown in Figure 8c, which raised the overall rainfall in the eastern and southern parts of Fuping and decreased the overall rainfall elsewhere. As a heavy convective historical storm, the rainfall intensity of Event 5 increased sharply in a very short period of time, with the maximum gauge cumulative values reaching up to \(355\) mm. The fully coupled WRF/WRF-Hydro underestimated this storm, and the WRF-only performed even worse, indicating a failure to capture these kinds of storm events in the Zijingguan catchment. The WRF-only had likewise the worst performance in reproducing the spatial rainfall distribution of Event 6, the storm center of which was dislocated. In stark contrast, the coupled system raised the rainfall in the north more than WRF-only for Event 6, with differences ranging from \(-60\) mm to \(40\) mm, which showed a wider range than the differences of all other events, except those of Event 4. Event 4 and Event 6 occurred in relatively dry soil conditions in the previous period and there was no obvious rainfall in the catchment for nearly 15 days before the occurrence of the two storm events.