DWU files

This application note is concerned with daily water use (DWU) files - how they are used by PR41 and how they can be created and edited.


There is some confusion between the use of the terms GlobalDWU and Global Formulas, and Global Functions File and DWU file.

DWU file

A DWU file contains long-term daily water use (DWU) behaviour for a particular crop during the growing season.

Global DWU

The Global DWU is calculated from a nominated DWU file, specified on Utilities/Updateoptions at Daily Water Use File:

The Global DWU is the single value of DWU that is calculated for today (using the clock in the computer), and is used to predict the Global Predicted Irrigation Date (GPI) for all sites.

Global Functions File

The global functions file contains up to 16 functions or formulae - global formulae - the results of which will be displayed on every readings screen. Use Utilities/Globals to enter or change the formulae, and when saving give a filename such as AGRITEC.GLB. The globals file name can also be entered or changed on Utilities/Updateoptions.

DWU Files

DWU file names can be used in four places in the software:

1 Utilities/Updateoptions (a single GlobalDWU value for today for all sites)

2 Keydata Formula (DWU values for the whole season for an individual site)

3 Global Formula (DWU values for the whole season for all sites)

4 Site/EditVariety (a single DWU value for today for an individual site)


Use Utilities/Updateoptions to enter the DWU filename that will be used to calculate the GlobalDWU.

Daily Water Use file: COTTON.DWU

Global DWU: 5.6 mm/day

A DWU file name should be entered here (just COTTON.DWU, no + sign). When <F10> is pressed to update the PROBE.INI settings, the software will calculate a single value of GlobalDWU for todays date. This single value is then used to predict the Global Predicted Irrigation date (GPI) for all sites (but see 4 below for exception). The nominated file should contain data for the principal crop being monitored.

The calculated value of todays DWU from this file - the GlobalDWU - will be displayed next time you choose Utilities/Updateoptions, and can be changed.

In addition, the GlobalDWU is used as the 'default' for the EstimatedDWU each time probe readings, irrigations and rainfalls are posted. Therefore, if the value is never changed, the values of DWU in all rows will be from this file.

2 Keydata formula

A DWU filename can be used as a keydata formula on SiteEdit.


Bolls and Squares will allow data collected in the field to be manually recorded, but +COTTON.DWU will calculate values of DWU from this file for each column in the readings screen. The :11 tells the software to display the data in the units chosen on Utilities/Preferences.

Use this method to draw a time graph of data from the DWU file when each site has a different crop.

Note that if a new keydata formula is entered using Site/Edit, then the whole of the readings screen needs updating with <F8>Update.

Global formula

Entered using Utilities/Globals, and stored in the globals file.

Select Parameter Type Graph Graph Graph
(Formula) (0-21) Factor Offset Type
1 * +COTTON.DWU 11 1 0 0
2 * +ProbeDWU/COTTON.DWU 1 1 0 0

This is similar to entering as a keydata formula, but in this case it is entered only once but the results will appear on every readings screen.

In the example above:


will calculate the corresponding value of DWU using the data in COTTON.DWU for every column of each readings screen.


will calculate the ratio of actual DWU this year to the long term value, and display to 2 decimal places.

A time graph of the values of DWU from COTTON.DWU for the whole season can now be plotted by selecting with <F6> on any readings screen, and choosing Globals:selected on the time graph dialogue screen.

A DWU filename should be entered as a global formula only for a crop that is grown on most sites.

The readings display is slowed down as the global formulas are calculated. To disable the global values display use Utilities/UpdateOptions and delete the name of the globals file. Global values will no longer be displayed, although the formulas in the global file are still there and can be re-activated by re-entering the globals file name.


Each site can also have an individual 'Global' DWU calculated, allowing for different crops. Entering the name of a DWU file in Site/Edit at Variety:


If a DWU filename is entered here (just COTTON.DWU, no + sign), then the software will use the value of DWU from this file for today to predict the Global Predicted Irrigation date (GPI) for this site, instead of using the GlobalDWU value calculated at 1 above.

Entering a DWU filename at Variety: should be done if different crops or varieties are being monitored - each site can have its own, individual 'GlobalDWU' based on the long-term behaviour of each crop.

On the sites screen, in the GlobalDWU column, (scroll to the right with arrow keys) sites using individual DWU filenames will show different values of GDWU for each site

Creating and editing DWU files

DWU files are standard DOS text files, and can be created and edited using EDIT, XTREE, or any word processor that will save unformatted, or ascii, files.

A DWU file called COTTON.DWU is supplied, but can be edited and renamed to reflect the standard behaviour of other crops when the probe has been used for several growing seasons.

A DWU file must have at least 3 lines, with format...

273 ...number of days since Jan 1 to planting
40, 2.0 ...days-since-planting, DWU in mm/day
60, 2.5
80, 3.8
90, 5.0 ...28 December, 5.00 mm/day
100, 6.0
120, 8.0
130, 8.0
160, 5.5
180, 4.2

The units must be in days, and mm/day. The software will display in in/ft or other user selected units set with Utilities/Preferences.

DWU files will work in any year because the date information is relative to Jan 1st, not to any particular year. To use the same data for crops with different planting dates, or to compare with data from another hemisphere, simply change the 'planting date number' in the first line and save with a new file name.

Daily Water Use of cotton

The data in DWU files should be of the long-term behaviour of a crop, built up from use of the probe. To generate values from your own data, plot an overlaid time graph of ProbeDWU for all farms and all sites. Using your judgement, sketch an average curve (or perhaps a 'high' and 'low' curve), and enter the data points into a DWU file.

Crop water use data is available from many Departments of Agriculture and Universities, some of which is available in DWU format from Research services New England. If users develop their own DWU data, RSNE would be interested in the results for dissemination to other users.

Probe for Windows has an in-built editor for creating and editing DWU files

Other uses for DWU files

DWU files can be used to display other data. For eaxmple a 'DWU' file called EVAP.DWU could contain pan evaporation data, entered into the file using EDIT as the data becomes available from a weather station. Using UtilitiesGlobals, entering the formulas:

Select Parameter Type Graph Graph Graph
(Formula) (0-21) Factor Offset Type
1 * +EVAP.DWU 11 1 0 0
2* +EVAP.DWU/ProbeDWU 2 1 0 0

will display evaporation values on every readings screen, and also the ratio of evaporation to actual crop water use.

Growing Degree Days

If growing degree days are required, a file called CUMGDD.DWU could contain growing degree days (GDD) calculated from weather station data and added each each day to the end of the file. A global formula of:


will calculate and display growing degree days for every site. If a value of, say, 67 was displayed at the actual planting date for a particular site, then a Keydata formula should be entered for that site:


and a value of 0 will now be displayed at the site planting date, and all following columns will have the correct value of growing degree days.

Growing degree days could also be plotted as the horizontal axis of time graphs, to allow comparisons of crop development.

Crop Growth Models

Many agronomic models use the equation:

Crop Daily Water Use = Et0 * Kcrop

Where Kcrop varies with the growing season, and Et0 is the reference or pan evaporation. The Kcropinformation could be entered into a file called, say, COTTON.KC. If Et0 or pan evaporation data was in a file called ET0.DWU, then the formula:


will calculate and display the model predicted crop daily water use.

DWU files for evaporation and growing degree days can be produced automatically from downloaded weather data files using the programme CONVERT, available from RSNE.