Another conceptual rainfall–runoff model used in this study is HYMOD, which has five parameters. The model consists of a simple rainfall excess model based on the probability distributed principle (Moore 1985) and has been applied by several recent studies (Boyle 2001; Wagener et al. 2001; Vrugt et al. 2003; De Vos et al. 2010). The model parameters are described in Table 2 and the model structure is illustrated in Figure 3. The cumulative distribution function of the water storage capacity C is in the following form:
8where 
is the maximum soil moisture storage capacity in the catchment and controls the degree of spatial variability of the soil moisture capacity. The excess rainfall is treated as the runoff which is divided into quick flow and slow flow based on the partitioning factor a. The runoffs are routed through three identical quick flow tanks and a parallel slow flow tank. The flow rates are determined by the recession coefficient for quick flow tank and slow flow tank
Table 2
HYMOD model parameters
| Parameter | Unit | Range | Description |
|---|---|---|---|
| Cmax | mm | 1–500 | Maximum soil moisture storage capacity |
| bexp | – | 0.01–1.99 | Spatial variability of soil moisture capacity |
| α | – | 0.01–0.99 | Quick/slow flow distribution factor |
| Rs | day | 0.01–0.99 | Recession coefficient for slow flow tank |
| Rq | day | 0.01–0.99 | Recession coefficient for quick flow tank |
| Parameter | Unit | Range | Description |
|---|---|---|---|
| Cmax | mm | 1–500 | Maximum soil moisture storage capacity |
| bexp | – | 0.01–1.99 | Spatial variability of soil moisture capacity |
| α | – | 0.01–0.99 | Quick/slow flow distribution factor |
| Rs | day | 0.01–0.99 | Recession coefficient for slow flow tank |
| Rq | day | 0.01–0.99 | Recession coefficient for quick flow tank |
Figure 3
Structure of the HYMOD model (adopted from Vrugt et al. (2003)).