Chapter22 - CM Box User Guide

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Revision as of 12:36, 15 September 2006

Calibrating Yield

The yield function is a statistically derived function relating the water balance parameters (which constitute the outputs of the water balance model) and the other factors (farm inputs, trend) or NDVI with station yield. Once this function has been established, it can be used for early crop yield forecasting.
Although many different equations are possible, the most widely used one is the outcome of a multiple linear regression procedure:

Y = a + b₁X₁ + b₂X₂ + b₃X₃

where b1 to b3 are the corresponding X coefficients.
Using an example for Malawi, first it will be established which water balance parameters are good predictors for yield. Then the multiple linear regression will be performed.

The input data file for Malawi contains multiple lines for stations and years. In every line, besides yield, all possibly relevant water balance output parameters are stored. The file can be downloaded here

Finding the relevant parameters with a principle component analysis

Activate the Tools - Principle Component Analysis function. A settings window appears.

All parameters are selected.

The output log file gives the most valuable results. The first, second and third component explains over 50% of te variance. The equation will be based on those 3 components.

Taking all parameters with correlation above 0.70 or below -0.70 the following list of regression parameters is obtained:
WRSIfin
DEFtot
DEFflow
DEFrip
ETAveg
ETArip
WEXTot
Xveg

Retrieved from "http://hoefsloot.com/wiki/index.php?title=Chapter22"

 * DEFflow
 * DEFrip
-                                   ETAtot
+* ETAveg
-                                   ETAini
+* ETArip
-                                   ETAveg
+* WEXTot
-                                   ETAflow
+* Xveg
-                                   ETArip
-                                   YEAR
-                                   WEXtot
-                                   WEXini
-                                   WEXveg
-                                   WEXflow
-                                   WEXrip
-                                   DEFini
-                                   DefVeg

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Activate the Tools - Principle Component Analysis function. A settings window appears.
All parameters are selected.
The output log file gives the most valuable results. The first, second and third component explains over 50% of te variance. The equation will be based on those 3 components.
Taking all parameters with correlation above 0.70 or below -0.70 the following list of regression parameters is obtained: WRSIfin DEFtot DEFflow DEFrip ETAveg ETArip WEXTot Xveg