The module "Functions" can be used to modify the various
functions used by Surfer Grid Math in the modelling process.
Functions which can be modified:
Only two grids (A and B) can be processed at one time, the results are written to grid C.
This results in the form C = f(A,B).
The syntax must be compatible to the built-in functions of Surfer.
The most important difference to ordinary programming languages,
is that AB is not written as A**B or A^B but as pow(A,B)
- Actual Curve No.
Calculates the actual curve number out of standard (surface) curve number (grid A)
and the total rainfall of the preceding 5 days (grid B).
By not using an AMC index (antecedent moisture condition), a much better fit is obtained,
and sudden jumps from one condition to another are avoided. The function
was obtained under Excel by regression analysis.
- Potential Runoff. Calculates the surface runoff from daily rainfall (grid A) and the actual (surface) curve number (grid B). The function
is an expression of the ordinary CN-formula for metric units (mm) with an initial abstraction value of 0.20.
- Deep infiltration. Calculates the deep infiltration out of shallow infiltration (grid A)
and the (deep infiltration) curve number (grid B). The function
is an expression of the ordinary CN-formula for metric units (mm) with an initial abstraction value of 0.25
Shallow infiltration is calculated by subtracting direct runoff from daily rainfall.
The CN-values for deep infiltration are not adjusted, but kept constant.
- Evap. Adjustment AMC.
Reduces the rainfall total of the 5 preceding days (grid A) by a portion of potential evapotranspiration (grid B).
exposes a rather strong effect,
particularly if mean monthly data are used for the assessment of daily evapotranspiration.
- Evap. Reduce SI. Reduces shallow infiltration (grid A) by a portion of potential evapotranspiration (grid B).
is complementary to the adjustment of the AMC.
None. Functions are stored in the file
runoff.cfg in the current directory.