Four surface runoff model options are available:
· None
· Linear, available for all crops (i.e. for all types of Soil model)
· Modified SCS, available for all crops except rice (i.e. for all types of Soil models except the ZIMsched for Rice field)
· Modified SCS for Rice field available only in combination with the ZIMshed for Rice field Soil model
No runoff model option applied.
The linear runoff model assumes a linear relationship between the rainfall [mm] and the amount of surface runoff. The runoff will be generated according to two specified linear formulas, valid when the accumulated rainfall is above or below a given threshold. The parameters for the linear model are:
· When Rainfall is lower than. It is the threshold of the accumulated rainfall [mm] on the field below which runoff occurs. In this case the runoff follows the formula specified in the line below:
Runoff = linear coefficient [ ] (fraction to be inserted in the formula) x Rainfall + constant (to be inserted in the formula) in [mm]
· When Rainfall is greater than. It displays the threshold of the accumulated rainfall [mm] on the field above which runoff occurs. If the rainfall is above the threshold, the runoff follows the formula specified in the line below:
Runoff = linear coefficient [ ] (fraction to be inserted in the formula) x Rainfall + constant (to be inserted in the formula) in [mm]
Regardless of the specified parameters, the runoff will never be negative.
Note: When including the Linear runoff model, for any crop including rice, setting both coefficients in the linear equation to zero will have the same effect as not including a surface runoff model. In that case, any excess surface water will to return to the river.
The parameters to be specified for the Modified SCS runoff model are:
· Soil moisture integration depth. The soil moisture integration depth [m] which has an influence on the infiltration losses in the modified SCS model.
· Coefficient of initial abstraction. This coefficient determines also the infiltration losses.
When this runoff model is selected, the surface runoff is estimated using the Soil Conservation Service (SCS) storm flow equation (USDA, 1985) /4/ as modified by Schulze (1995) and used in the ACRU agrohydrological simulation model in South Africa.
The modified equation is given by:
(A.3.6) 
Where:
Q: Surface runoff [mm]
P: Rainfall [mm]
c: Coefficient of initial abstraction (default value 0.25) [-]
S: Potential maximum water retention of the soil, taken as the soil water deficit below porosity over the depth Zinte [mm]
Zinte: Integration depth (default Zinte = 0.25 m) [m]
S is calculated from the following formulas:
For 
:
For 
:
Where:
: Water content at saturation [-]
: Water content in the Top Soil [-]
: Water content in the Root Zone [-]
Ze: Depth of the Top Soil layer [m]
Example:
Input P = 100 mm, Soil moisture integration depth = 0.2 m.
Porosity = 0.45, and actual soil moisture content is equal to 0.20.
The runoff is calculated from:
For C= 0 and S = 50 mm Q = 67 mm
For C=0.5 and S = 50 mm, Q = 45 mm
The difference between the Modified SCS for Rice field model and the Modified SCS model is that the space (storage) between the maximum ponded water depth (see the ZIMsched for Rice field model section) and the actual water depth is also taken into account in the calculation of the water volume of the potential maximum water retention of the soil, S.
In addition, excess water above the maximum ponded water depth at the end of a time step will be added to the already calculated surface runoff (can only occur when a specified irrigation application is used).
The input parameters for the Modified SCS for Rice field model are the same as those for the Modified SCS.
