How to set up a cumulative sum of degrees day?

Growing degree days (GDD) are a key agronomic tool for tracking crop development and optimizing the timing of field interventions. Weenat lets you calculate and monitor your GDD accumulations directly from your field data — in real time and over historical periods.

In this article, we use the first nitrogen application on grasslands as a concrete example:

• Base temperature (lower threshold): 0°C
• Ceiling temperature (upper threshold): 18°C
• Trigger target: 200 GDD

How does Weenat calculate growing degree days?

Weenat uses the hourly average method, considered the most accurate as it draws on all temperature readings throughout the day.

For each hour of the day, the measured temperature is clipped between the defined lower and upper thresholds. Values outside the range are brought back to the limits. The average of these 24 hourly values gives the GDD for that day, which are then added to the running total.

Formula: Daily GDD = Average(hourly temperatures clipped between Tbase and Tceiling)

Example — For a day with temperatures ranging between -2°C and 22°C, values below 0°C will be raised to 0°C and values above 18°C will be capped at 18°C. Only the active range contributes to the accumulation.

Good to know: If you export your data or use the API, you can apply your own calculation method (for example the (Tmax + Tmin) / 2 method) using the raw temperature data.

Via alerts

Alerts let you be notified automatically as soon as your GDD threshold is reached. This is the most convenient method to never miss the right time to intervene.

1. Go to the "Alerts" section
2. Click on "Create an alert"
3. Select "Simple alert"
4. Choose "Growing degree days accumulation"
5. Fill in (for example) the following parameters:
   • Lower threshold (base temperature): 0°C
   • Upper threshold (ceiling temperature): 18°C
   • Accumulation start date: the date from which the calculation begins (e.g. January 1st)
   • Trigger value: 200 GDD
6. Select the relevant plots
7. Confirm to activate the alert

Via the widget

The growing degree days accumulation widget lets you visualize the progress of the accumulation on your plot in real time. Handy for a quick daily visual check.

1. From a plot page, click on "Add a widget"
2. Select "Growing degree days accumulation" from the list of available widgets.
3. Fill in the accumulation parameters:
   • Lower threshold: 0°C
   • Upper threshold: 18°C
   • Accumulation start date
4. Click "Confirm" to finalise the configuration
5. You can move your widget wherever you like

Via the history

The history lets you retrospectively view a growing degree days accumulation over a past period. 

1. From a plot page, open the "History" tab
2. Select the "Growing degree days accumulation" metric
3. Set the period and parameters:
   • Lower threshold: 0°C
   • Upper threshold: 18°C
4. The chart displays the day-by-day accumulation over the selected period

Via data export

The export lets you retrieve your raw temperature data to calculate your growing degree days using the method of your choice in a spreadsheet.

1. From a plot page, click on the "History" tab
2. Click on "Export my organisation's data"
3. Choose the desired period
4. Prefer the hourly time step (recommended for calculating GDD)
5. Select the plot(s) you want to analyse
6. Select the Temperature metric
7. Choose CSV or XLSX format
8. You will receive an email with a link to download a .zip file containing one file per plot

Via the API

The Weenat API lets you integrate your growing degree days data directly into your own tools, platforms or calculation scripts — for advanced or automated use cases.

Retrieving data via the API

You can query the API to retrieve:

• Raw hourly temperatures (recommended for calculating your own GDD)
• The GDD accumulation calculated by Weenat (hourly average method), available only on /data/plots (plot level) — not available on /data/devices (sensor level).

To use the API, read: 

• [LINK]
• https://dev.weenat.com/ (swagger)
• https://dev.weenat.com/v3/ (detailed documentation)

2 methods for calculating growing degree days in your spreadsheet

Two methods are commonly used. 

You can choose the one that matches your practices or the recommendations of your local agricultural chamber. If you are comparing against historical values, also check which method was used to ensure you are comparing like with like.

Method 1: Hourly average (Weenat method)
For each hour, clip the temperature between 0°C and 18°C, then calculate the average of the 24 hourly values. Sum the daily values to obtain the accumulation over the period.

Method 2: (Tmax + Tmin) / 2
Calculate the average of the daily maximum and minimum temperatures (clipped between 0°C and 18°C). Sum the daily values to obtain the accumulation.

The two methods may give slightly different results. Make sure to use the same method consistently from one season to the next when comparing your data. The hourly average method is generally more accurate, especially in situations with highly variable temperatures throughout the day (morning frosts, afternoon heat peaks).