Irrigation and ET₀ Guide¶

Weather Station Core provides two ET₀ (reference evapotranspiration) sensors for irrigation scheduling and water budget management.

ET₀ is the water demand of a reference grass surface under current atmospheric conditions. It is the standard input for computing crop water requirements.

ET₀ sensors¶

Sensor	Formula	When available	Accuracy
`sensor.ws_et0_daily`	Hargreaves-Samani (1985)	Always (once coordinates are set)	±15-20% vs Penman-Monteith
`sensor.ws_et0_pm_daily`	FAO-56 Penman-Monteith	When a solar radiation (W/m²) sensor is mapped	±5-10% vs lysimeter

Both sensors update daily. Penman-Monteith is preferred when available; use Hargreaves-Samani as a fallback if you do not have a solar radiation sensor.

Hargreaves-Samani (1985)¶

Requires only temperature and latitude-derived extraterrestrial radiation:

ET₀ = 0.0023 · Ra · (T_mean + 17.8) · (T_max − T_min)^0.5

Ra: extraterrestrial radiation, computed from latitude and day-of-year
Uses today's temperature high, low, and mean from ws_core's rolling statistics

Tendency to overestimate in humid climates; underestimate in arid conditions.

Reference: Hargreaves, G.H. & Samani, Z.A. (1985). Reference crop evapotranspiration from temperature. Appl. Eng. Agric., 1, 96-99.

FAO-56 Penman-Monteith¶

The internationally standard reference method (Allen et al. 1998 / FAO-56):

ET₀ = [0.408·Δ·(Rn − G) + γ·(900/(T+273))·u₂·(eₛ − eₐ)] / [Δ + γ·(1 + 0.34·u₂)]

Requires: - Temperature (T), mean - Relative humidity (for eₛ and eₐ) - Wind speed at 2m height (u₂), automatically converted from 10m sensor - Net radiation (Rn), derived from the mapped solar radiation sensor - Soil heat flux (G), approximated as 0 for daily timestep

Reference: Allen, R.G., Pereira, L.S., Raes, D., Smith, M. (1998). Crop evapotranspiration — Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56. FAO, Rome.

Using ET₀ for irrigation decisions¶

A simple daily water budget:

Irrigation needed (mm) = ETₒ × Kc − effective_rainfall

Where Kc is a crop coefficient (typically 0.6-1.2 depending on crop type and growth stage). See FAO-56 Annex tables for crop coefficients.

The sensor.ws_irrigation_deficit entity (part of the Comfort Indices group) computes this value automatically using the Penman-Monteith ET₀ and today's rainfall accumulation.

Smart Irrigation integration¶

See the Smart Irrigation Bridge guide for step-by-step configuration of ws_core's ET₀ as an input to the Smart Irrigation HA integration.

Soil sensor integration (v2.1+)¶

When soil moisture or temperature sensors are available, enable the Soil Sensors feature group in Configure → Features.

Enabling soil sensors¶

Go to Settings → Devices & Services → Weather Station Core → Configure
On the Sources step, map your soil moisture sensor and/or soil temperature sensor
On the Features step, enable Soil sensors
Restart is not required — sensors appear on the next coordinator update

Sensors added¶

Sensor	Description
`sensor.ws_soil_moisture`	Volumetric moisture %. Accepts 0–100% or 0–1 (auto-detected)
`sensor.ws_soil_temperature`	Soil temperature in °C
`sensor.ws_soil_moisture_deficit`	Difference between 40% field capacity and current moisture
`sensor.ws_irrigation_need`	Text label: None / Low / Moderate / High / Critical
`sensor.ws_irrigation_need_score`	0–100 demand score

Irrigation need score calculation¶

score = min(100, soil_deficit × 1.5 + max(0, ET₀_today − rain_today) × 5)

Score	Label
0–9	None
10–24	Low
25–49	Moderate
50–74	High
75–100	Critical

Example automation: irrigate only when needed¶

alias: "Garden: irrigate when soil needs water"
trigger:
  - platform: time
    at: "06:00:00"
condition:
  - condition: state
    entity_id: sensor.ws_irrigation_need
    state_not: "None"
  - condition: state
    entity_id: binary_sensor.ws_rain_expected_1h
    state: "off"
action:
  - service: switch.turn_on
    target:
      entity_id: switch.garden_irrigation