What is Evapotranspiration?

Evapotranspiration (often abbreviated ET) quantifies the amount of water that is lost from the soil and plants themselves due to weather conditions.  ET is the sum of evaporation and transpiration.

  • Evaporation: the loss of water from the soil, to the air.
  • Transpiration:  the loss of water from a plant’s leaves and body, the the air.

The amount of evapotranspiration will depend on the temperature, humidity, wind, amount of sun exposure, and the type of plant.  ET is expressed in inches of water.

ET describes the amount of water that is lost in the environment; water is replenished via precipitation or rainfall, and irrigation.

spruce ET evapotranspiration

How does Spruce use ET?

Spruce generates recommendations for watering using an evapotranspiration model. The coefficients for this model are generated using common values for the nozzle, soil, and landscape types, which can be set for each zone in the Devices section of the app.

Based on these coefficients, and the calculated evapotranspiration, Spruce will keep track of the water balance for each zone in your yard.  When the water balance reaches the calculated threshold value, Spruce will refill the water balance by starting the sprinklers at the scheduled time.

In the chart below, the shaded blue is the water balance.  As moisture is lost each day as represented by the orange line, the water balance depletes.  The moisture loss is due to evaporation and transpiration through the plant foliage.  The water balance is filled by both rain and the sprinklers.  The Spruce system tracks the depletion until a moisture “refill” is needed.  This is when the water balance reaches the threshold value.  At this point, the Spruce system will completely fill the zone to the calculated moisture level for the zone type.

  • The moisture loss changes each day based on weather conditions so the time to depletion will vary
  • Rainfall will effect how long it takes for the depletion to reach the threshold
  • The sprinkler frequency changes depending on moisture loss and rainfall
  • Sprinkler duration is the same, but the frequency changes.  This behavior can be modified by selecting a different schedule mode.

Custom ET Coefficients

To tune recommendations for your zones, you may also enter custom evapotranspiration coefficients:

Spruce Custom Coefficients

Crop Coefficient

Crop Coefficient is a property of a plant. It is a ratio which compares the ET loss for a given plant vs a reference ET (typically a grass is used for the reference).  It combines various factors of a given plant, such as ground cover, canopy, height of the plant, wind aerodynamics, and properties of the leaves and stomata.  Crop coefficients can vary dramatically from plant to plant, as well as within the plant’s lifecycle.

  • Lower coefficient means the plant “loses” less water, therefore requiring less frequent watering
  • Higher coefficient means the plant “loses” more water, therefore requiring more frequent watering

Allowed Depletion

Allowed Depletion is the allowed percentage of water loss that you are comfortable sustaining before watering your plant. In any given system, there is a maximum finite amount of water available to the plant. As days progress, and water is lost through ET, Spruce will trigger watering when the amount of water lost is greater or equal to the allowed depletion percent.

  • Generally, we don’t recommend to change this setting. It’s important that plants are allowed to deplete so that oxygen can reach the roots. However, you don’t want to allow too much water loss, as this brings the water level closer to the plant’s permanent wilt point, potentially causing stress to the plant.
  • Lower allowed depletion means the available water limit is higher, therefore causing watering to occur more frequently.  Setting this value too low could result in over-watering, and prevent oxygen from reaching the plant’s roots.
  • Higher allowed depletion means the available water limit is lower, therefore causing watering to occur less frequently. Setting this value too high could result in excessively dry soil between watering events, and potentially cause your plants to die.

Root Depth

Root Depth is measured in inches, and is the length of a plant’s roots.  A deeply rooted plant has access to more soil moisture than a shallow rooted plant, which allows the deeper rooted plant to go longer between watering events. Like crop coefficient, root depth can vary greatly from plant to plant, and can depend on a plant’s maturity. Root depth can be influenced by watering style (ie, long and less frequent vs short and more frequent), and can therefore depend on previous watering exposure.

  • Lower root depth means that the plant has less water available to access, thereby necessitating more frequent watering.
  • Higher root depth means that the plant has more water available to access, thereby necessitating less frequent watering.

Available Water Percent

The amount of water available to a plant is a function of soil texture. The available water percent can be defined as the difference between field capacity and wilt point.  It can be thought of as the equivalent percentage of water accessible to the plant’s roots, in any given profile of soil.  To obtain total available water, multiply the available water percent by the root depth.

  • A higher available water percent will result in less frequent watering, but more watering per event. Cumulatively, it does not change the overall watering amount.
  • A lower available water percent will result in more frequent watering, but less watering per event. Cumulatively, it does not change the overall watering amount.

Absorption Rate

Absorption rate is a property of a soil, and describes how quickly a soil can absorb any water that is applied. It is expressed in inches per hour. Sandy soils (which, by definition, have higher spacing between soil particles) tend to absorb water the quickest, and clay-type soil (which have less spacing between soil particles) will absorb water the slowest.

If the precipitation rate of irrigation is greater than the absorption rate of the soil, some portion of the watering will run off the surface and be wasted.

If this is the case, and the schedule settings are set to “auto” cycles, Spruce will break the day’s watering into multiple cycles, which gives the soil time to absorb the water.

  • A higher absorption rate will result in less cycles
  • A lower absorption rate will result in more cycles

Precipitation Rate

The precipitation rate is the flow rate of a watering nozzle, and is expressed in inches per hour.

  • Determining the value:
    • Physically measure the amount of water being delivered by collecting the water in a cup. Measure the inches dispensed and divide by the amount of time watered.
    • Alternatively, refer to the nozzle manufacturer’s spec sheet.
  • Lower precipitation rate means the nozzle dispenses less water, resulting in longer water times
  • Higher precipitation rate mean the nozzle dispenses more water, resulting in shorter water times
  • Be careful if changing this value. It is tempting to change this setting in order to ‘force’ Spruce to water more or less. In practice, yes, changing this setting will alter overall watering times. But be aware that Spruce only uses this value to convert from inches of water to minutes of watering.  It’s important that this setting is only changed to reflect the correct precipitation rate for your nozzle so that Spruce can accurately schedule to balance natural rainfall, and so that the gallons calculation is correct.


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