OpenSprinkler can leverage online weather data to make daily adjustments to the station water time automatically. The data is retrieved from Darksky and is available for most locations in the world. Below we describe how to configure weather adjustment methods and settings.


From the homepage, go to Edit Options and then open the Weather and Sensors tab, shown below. The Weather Adjustment Method drop-down list shows several adjustment methods explained below. Each method has associated parameters which you need to set appropriately.

  • The default method is Manual, which means you can manually set a watering percentage (the acceptable range is 0% to 250%). If a sprinkler program is set to 'Use Weather Adjustment', then when it starts, the programmed water time will be scaled by the watering percentage to determine the actual water time for each zone.

  • The Zimmerman method is one of the methods that will determine watering percentage automatically from online weather data, specifically the temperature, humidity, and precipitation of your location. When this method is selected, the % Watering input will be disabled, indicating this percentage is now being automatically calculated. Details about the Zimmerman method and related options are explained below.

  • The Auto Rain Delay method uses online weather forecast data to automatically determine a rain delay time period. The more precipitation the longer the rain delay time. This method does NOT affect the watering percentage.

  • The Evapotranspiration (ET) method is a sophisticated method that automatically calculates the watering percentage using online weather data. It's similar to the Zimmerman method but it leverages more weather parameters, including your geolocation, altitude, solar radiation, and yearly baseline ET value. When selecting this method, you need to set the Baseline ETo value(you can click Detect Baseline ETo, which will detect the default baseline using your location) and elevation. This method is more aligned with industry standard. Details about ET and related options are explained below.

  • The Monthly method is similar to Manual method, but allows you to set a fixed watering percentage per month. This method is only available on firmware version 2.2.0 and above.

Set a Sprinkler Program to Use Weather Adjustment

Each sprinkler program can be individually set to use or not use weather adjustment. The option is available in the program editing page. Please note that by default, when you create a sprinkler program, Use Weather Adjustment is unchecked, therefore if need to set this checkbox if you want the program to be affected by weather adjustment. 

Technical Details on Weather Adjustment Methods

1. Zimmerman Method

This method uses temperature, humidity, and precipitation to calculate watering percentage. By default, it uses a baseline of 70 °F (21 °C) temperature, 30% Humidity and 0 inch precipitation. Under the baseline condition, the formula returns 100% water time (i.e. no change to the water time). The percentage increases or decreases as the weather conditions deviate from the baseline. Specifically:

  • Temperature: using average temperature of the previous day: +4% for each degree Fahrenheit above 70, and -4% for each degree Fahrenheit below 70.
  • Humidity: using average humidity for the previous day: above 30% is subtracted from the weather adjustment, below 30% is added to it. 
  • Precipitation: using total precipitation of the previous day: -2% for each hundredth of an inch precipitation. 
  • The final result is then clamped to between 0% to 200% so that it never exceeds this bound.

Summary of formula: Watering Percentage = (100 + (T-70)*4 + (30-H) - 200*(P-0)) %

where T, H, and P are average temperature (F), average humidity, and total precipitation (inches) of the previous day. 

Example: let's say yesterday's T, H, P are 86 F, 68%, and 0.12" respectively. According to the formula, the watering percentage will be 100 + (86-70)*4 + (30-58) - 200*0.12 = 112%

Tweaking Baseline Parameters and Weights: due to the quality of weather data, the default baseline parameters may not be appropriate for your local weather. For example, the weather service may consistently underestimate your local temperature, resulting in low watering percentage. To address these variations, we allow the baseline parameters to be adjusted using the Adjustment Method Options. For example, you may want to decrease or increase the temperature baseline (default value is 70F) if the weather data consistently under- or over- estimates the temperature. You can make similar adjustments to the Humidity baseline and Precipitation baseline.

In addition, you can adjust how much weight each of the three weather parameters is factored into the computation. For example, if you feel humidity is taking too much weight into the formula, you can adjust the Weight of Humidity, between 0% to 100%. If the humidity weight is 0, that means the formula does not consider humidity at all. 

Summary of Final Formula: accounting for the configurable baseline and weight parameters, the final formula is:

Watering Percentage = (100 + (T-BT)*4*WT + (30-BH)*WH - 200*(P-BP)*WP) %

where BT, BH, BP are baseline T, H, P values, and WT, WH, WP are the weights for T, H, and P respectively.

2. Evapotranspiration (ET) Method

Evapotranspiration, or ET, measures how much water is lost from the ground due to evaporation and transpiration. The ET adjustment method calculates the watering percentage by comparing a reference ET index (ETo) and precipitation from the previous day to the average ETo (i.e. baseline ETo value) for your specific location using the formula
(yesterdayETo - yesterdayPrecip) / baselineETo and clamping the result between 0% and 200%. The baseline ETo value for your specific location can be obtained by clicking the Detect baseline ETo button in Adjustment Method Options. You can additionally configure your altitude or leave it to the default value if you don't know.

How is ET different from Zimmerman? ET is a more sophisticated method. In addition to temperature, humidity, precipitation, it also leverages other parameters, including your geolocation, altitude, solar radiation. The formula is described in detail by this article: FAO-56 Penman-Monteith method.

Precautions when transitioning from Zimmerman to ET? If you are used to Zimmerman adjustment method in the past, and are switching to ET for the first time, you may notice that the watering percentage values calculated by the two methods can differ significantly. This is not surprising as the formulas are quite different. This means you may need to adjust your sprinkler programs to decrease or increase the programmed water times accordingly. Ideally in the programs you want to set the water times based on the yearly average (as opposed to the water times during the summer only). This way, during the summer seasons when it's hot, the watering percentage is likely more than 100%, increasing the programmed water times; while during spring and fall it's likely to be less than 100%. 

Adjusting the baseline ETo value: if you feel the ET method consistently over- or under- estimates the watering percentage, you can adjust the baseline ETo value to counter-act the inaccuracy. For example, if you feel it's over-estimating watering percentage by 20%, you can increase the current baseline ETo value by 20%, which according to the formula, will cancel out the over-estimated amount. For example, if the current baseline ETo is 0.112, you can adjust it to 0.134 (20% increase from 0.112). Because the calculated ETo is divided by the baseline ETo, this will counter-act the inaccuracy and bring the watering percentage down to your expected value.

If you decide to manually calculate an ETo value, there are a few important things to note:

1. You should use the potential evapotranspiration, not the actual evapotranspiration.

2. The ETo should be calculated using the FAO-56 Penman-Monteith method.

3. The ETo should use data from the same time of year that the watering schedule was designed for. If you use the same watering schedule year-round, you should use the annual ETo; if you change the watering schedule with the seasons, you should use the ETo for the current season.