Notes:
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 A^{B} 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
C=Min(99,((12.5+B*0.6345-pow(B,2)*0.002234))+(10.4-A*0.366+pow(A,2)*0.00997))
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
C=Max(0,pow(max(0, (A-0.20*(1000/B-10)*25.4)),2)/(A+0.88*(1000/B-10)*25.4))
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
C=Max(0,pow(max(0, (A-0.25*(1000/B-10)*25.4)),2)/(A+0.75*(1000/B-10)*25.4))
is an expression of the ordinary CN-formula for metric units (mm) with an initial abstraction value of 0.25
Note:
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).
The formula
C=A-0.8*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).
The formula
C=max(0,(A-0.2*B))
is complementary to the adjustment of the AMC.