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| __NOTOC__ | __NOTOC__ | ||
| - | ==7.2. Calibrate crop yields against water balance outputs and other variables.== | + | ==7.1. Calibrate crop yields against water balance outputs and other variables.== |
| ------------------------------------ | ------------------------------------ | ||
| + | {| style="background-color:#F5F5F5; border-collapse:collapse" cellspacing="7" border="1" bordercolorlight="#0000FF" bordercolordark="#0000FF"> | ||
| + | |style="border-style: solid; border-width: 1px"|''Peter Hoefsloot'' | ||
| + | |- | ||
| + | |} | ||
| - | 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. | + | In the context of agrometeorological crop yield forecasting, the yield function is a statistically derived function relating the water balance parameters (which constitute the outputs of the water balance model) and possibly other factors (like farm inputs, trend or NDVI) with station yield. Once this function has been established, it can be used for early crop yield forecasting for a number of seasons. Recalculation of this formula seems appropriate at least every 5 years to cater for changing climatological circumstances. |
| Although many different equations are possible, the most widely used one is the outcome of a multiple linear regression procedure: | Although many different equations are possible, the most widely used one is the outcome of a multiple linear regression procedure: | ||
| + | |||
| <blockquote> | <blockquote> | ||
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| where b1 to b3 are the corresponding X coefficients. | 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. <br> | + | In this chapter an example for Malawi is used. In two steps the fomula will be calculated. * Step 1. Determination of the water balance parameters which are good predictors for yield. |
| - | <br> | + | * Step 2. Performing the multiple linear regression. |
| + | |||
| {|"class=prettytable" cellpadding="15" border="1" style="border-collapse:collapse" | {|"class=prettytable" cellpadding="15" border="1" style="border-collapse:collapse" | ||
| - | |width="225"|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 [http://www.hoefsloot.com/downloads/sample.csv here]||[[Image:graph92.jpg|400px|]] | + | |width="225"|The input data file for Malawi contains multiple lines for stations and years. The file is in FAO format and can be read by Excel as CSV file. In every line, besides yield, all possibly relevant water balance output parameters are stored. The file can be downloaded [http://www.hoefsloot.com/downloads/sample.csv here]||[[Image:graph92.jpg|400px|]] |
| |} | |} | ||
| <br> | <br> | ||
| - | ===Step1. Finding the relevant parameters with a principle component analysis=== | + | ===Step1. Finding the relevant parameters with a Principle Component Analysis (PCA)=== |
| <br> | <br> | ||
| {|"class=prettytable" cellpadding="15" border="1" style="border-collapse:collapse" | {|"class=prettytable" cellpadding="15" border="1" style="border-collapse:collapse" | ||
| - | |width="300"|Activate the ''Tools - Principle Component Analysis'' function. A settings window appears.||[[Image:graph93.jpg|400px|]] | + | |width="300"|Activate the ''Tools - Principle Component Analysis'' function. The settings window appears.||[[Image:graph93.jpg|400px|]] |
| |---- | |---- | ||
| - | |width="300"|All parameters are selected.||[[Image:graph94.jpg|graph94.jpg]] | + | |width="300"|All parameters are selected to be included in the PCA analysis.||[[Image:graph94.jpg|graph94.jpg]] |
| |---- | |---- | ||
| - | |width="300"|The output log file gives the most valuable results. The first, second and third component explains over 50% of the variance. The equation will be based on those 3 components.||[[Image:graph96.jpg|400px|]] | + | |width="300"|In this case, the output log file gives the most valuable results. Note that the first, second and third component explain over 50% of the variance. Therefore we will base the equation on these 3 components.||[[Image:graph96.jpg|400px|]] |
| |---- | |---- | ||
| - | |width="300"|Taking all parameters with correlation above 0.70 or below -0.70 the following list of regression parameters is obtained: | + | |width="300"|Within these components, the parameters with correlation above 0.70 or below -0.70 will be selected for use in the yield formula. This constitutes the following list: |
| * WRSIfin (final index) | * WRSIfin (final index) | ||
| * DEFtot (total deficit) | * DEFtot (total deficit) | ||
| Line 40: | Line 46: | ||
| * WEXTot (Total Water Excess) | * WEXTot (Total Water Excess) | ||
| * WEXveg (Water excess during vegetative stage) | * WEXveg (Water excess during vegetative stage) | ||
| - | |||
| - | |||
| ||[[Image:graph97.jpg|400px|]] | ||[[Image:graph97.jpg|400px|]] | ||
| |} | |} | ||
| + | |||
| + | |||
| + | At the end of this first step, 8 parameters have been identified that will be part of the regression equation established in step 2. | ||
| ===Step 2. Performing the Linear Regression Analysis=== | ===Step 2. Performing the Linear Regression Analysis=== | ||
| - | <br> | + | |
| {|"class=prettytable" cellpadding="15" border="1" style="border-collapse:collapse" | {|"class=prettytable" cellpadding="15" border="1" style="border-collapse:collapse" | ||
| - | |width="300"|The multiple regression is done in Excel. First the columns that are no longer needed are deleted. The file is saved under another name to prevent data loss (sample1.xls).||[[Image:graph98.jpg|400px|]] | + | |width="300"|The multiple regression is done in Excel. First the columns that are no longer needed are deleted. To prevent data loss the file is saved under the name sample1.xls.||[[Image:graph98.jpg|400px|]] |
| |---- | |---- | ||
| |width="300"|A second worksheet is created to hold the regression parameters. An array formula is entered : | |width="300"|A second worksheet is created to hold the regression parameters. An array formula is entered : | ||
| <blockquote> | <blockquote> | ||
| =LINEST(sample!B3:B1367,sample!C3:J1367, TRUE, TRUE)</blockquote> | =LINEST(sample!B3:B1367,sample!C3:J1367, TRUE, TRUE)</blockquote> | ||
| - | The regression parameters are calculated. | + | Now Excel calculates the regression parameters. |
| ||[[Image:graph99.jpg|400px]] | ||[[Image:graph99.jpg|400px]] | ||
| |---- | |---- | ||
| - | |width="300"|The regression formula is now: | + | |width="300"|The parameters now lead to the following regression formula: |
| yield = WRSIfin*10.95832936 + | yield = WRSIfin*10.95832936 + | ||
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| ||[[Image:graph100.jpg|400px|]] | ||[[Image:graph100.jpg|400px|]] | ||
| |---- | |---- | ||
| - | |width="300"|A graph is created in which measured and predicted yield are combined. This gives an idea of the accurracy of the regression formula calculated. The sample workbook can de downloaded [http://www.hoefsloot.com/downloads/sample1.xls here]||[[Image:graph101.jpg|400px|]] | + | |width="300"|A graph is created in which measured and predicted yield are combined. This gives an idea of the accurracy of the regression formula calculated. The sample workbook (sample1.xls) can de downloaded [http://www.hoefsloot.com/downloads/sample1.xls here]||[[Image:graph101.jpg|400px|]] |
| |} | |} | ||
| + | </Blockquote> | ||
| - | + | -------------------------------------- | |
| - | + | ||
| - | + | ||
| - | + | ||
| - | + | ||
| - | + | ||
| - | + | ||
| - | + | ||
| - | </Blockquote> | + | |
Revision as of 13:09, 6 October 2006
7.1. Calibrate crop yields against water balance outputs and other variables.
Peter Hoefsloot In the context of agrometeorological crop yield forecasting, the yield function is a statistically derived function relating the water balance parameters (which constitute the outputs of the water balance model) and possibly other factors (like farm inputs, trend or NDVI) with station yield. Once this function has been established, it can be used for early crop yield forecasting for a number of seasons. Recalculation of this formula seems appropriate at least every 5 years to cater for changing climatological circumstances.
Although many different equations are possible, the most widely used one is the outcome of a multiple linear regression procedure:
Y = a + b1X1 + b2X2 + b3X3where b1 to b3 are the corresponding X coefficients.
In this chapter an example for Malawi is used. In two steps the fomula will be calculated. * Step 1. Determination of the water balance parameters which are good predictors for yield.
- Step 2. Performing the multiple linear regression.
The input data file for Malawi contains multiple lines for stations and years. The file is in FAO format and can be read by Excel as CSV file. In every line, besides yield, all possibly relevant water balance output parameters are stored. The file can be downloaded here 
Step1. Finding the relevant parameters with a Principle Component Analysis (PCA)
Activate the Tools - Principle Component Analysis function. The settings window appears. 
All parameters are selected to be included in the PCA analysis. 
In this case, the output log file gives the most valuable results. Note that the first, second and third component explain over 50% of the variance. Therefore we will base the equation on these 3 components. 
Within these components, the parameters with correlation above 0.70 or below -0.70 will be selected for use in the yield formula. This constitutes the following list:
- WRSIfin (final index)
- DEFtot (total deficit)
- DEFflow (total deficit during flowering)
- DEFrip (total deficit during ripening)
- ETAveg (ETA during vegetative stage)
- ETArip (ETA during ripening)
- WEXTot (Total Water Excess)
- WEXveg (Water excess during vegetative stage)
At the end of this first step, 8 parameters have been identified that will be part of the regression equation established in step 2.
Step 2. Performing the Linear Regression Analysis
The multiple regression is done in Excel. First the columns that are no longer needed are deleted. To prevent data loss the file is saved under the name sample1.xls. 
A second worksheet is created to hold the regression parameters. An array formula is entered : =LINEST(sample!B3:B1367,sample!C3:J1367, TRUE, TRUE)Now Excel calculates the regression parameters.
The parameters now lead to the following regression formula: yield = WRSIfin*10.95832936 + DEFtot*-19.25393401 + DEFflow*20.00715655 + DEFrip*15.96960614 + ETAveg*-1.232280314 + ETArip*1.210208146 + WEXtot*0.14383726 + WEXveg*-0.629481819 + 73.68884436The predected yield for the first data row (third line) is:
=Regression!A$1*J3+Regression!B$1*I3+Regression!C$1*H3+Regression!D$1*G3+Regression!E$1*F3+Regression!F$1*E3+Regression!G$1*D3+Regression!H$1*C3+Regression!I$1
A graph is created in which measured and predicted yield are combined. This gives an idea of the accurracy of the regression formula calculated. The sample workbook (sample1.xls) can de downloaded here 
