Irrigation scheduling

Observation method

The most basic form of scheduling relies on visual cues or physical inspection.

• Visual cues: Monitoring plants for signs of water stress (wilting, leaf curling, or color changes) before watering.
• Feel and appearance method: Taking a soil sample by hand to estimate moisture content based on how the soil ribbons or clumps.

Soil moisture measurement

This method uses sensors placed directly in the root zone to measure the volumetric water content or soil water tension.

• Tensiometers: Measure the force (tension) roots must exert to extract water.
• Capacitance Sensors: Measure the dielectric permittivity of the soil to determine water volume.
• Neutron Probes: Used in agriculture for highly accurate deep-soil readings.

These sensors can act as a "thermostat" for irrigation, triggering the system only when moisture drops below a specific Management Allowed Depletion (MAD) threshold.

Meteorological (water balance) method

Also known as the "Checkbook Method," this approach calculates the water balance of the soil. It treats the soil reservoir like a bank account:

• Deposits: Rainfall and Irrigation.
• Withdrawals: Evapotranspiration (ET)—the combined loss of water via soil evaporation and plant transpiration.

The daily soil water balance equation is often expressed as:^[1]

${\displaystyle D_{c}=D_{p}+ET_{c}-P-I}$

Where:

• ${\displaystyle D_{c}}$ = Current soil moisture depletion
• ${\displaystyle D_{p}}$ = Previous depletion
• ${\displaystyle ET_{c}}$ = Crop evapotranspiration (water used by crop)
• ${\displaystyle P}$ = Precipitation (rain)
• ${\displaystyle I}$ = Irrigation applied

This method relies heavily on accurate weather data to calculate daily water loss.

Weather-based (ET) controllers

Evapotranspiration (ET) controllers adjust irrigation schedules daily based on local weather conditions. They calculate the Reference Evapotranspiration (${\displaystyle ET_{0}}$)—the amount of water a standard grass surface would lose given the current temperature, humidity, wind speed, and solar radiation.

• Signal-Based: The controller receives a daily ET value via Wi-Fi or cellular signal from a local weather station.
• On-Site Calculation: The controller uses attached sensors (temperature and solar radiation) to calculate ET directly at the site.
• Historical: The controller uses a pre-programmed curve of historical average weather for the region (least accurate).

Soil-based controllers

Instead of predicting water loss via weather data, these controllers measure actual soil moisture.

• Bypass System: The timer is set to run, but the cycle is interrupted (bypassed) if the soil moisture sensor detects wet soil.
• On-Demand System: The controller initiates irrigation only when the soil moisture sensor reaches a low threshold, regardless of time of day.

Reactive sensors

• Rain sensor: Simple hygroscopic disks that swell when wet, breaking the electrical circuit to the valves during a storm.
• Freeze Sensors: Prevent irrigation when air temperatures drop below 32 °F (0 °C) to prevent ice hazards and plant damage.

Crop coefficient (Kc)

Not all plants use water at the same rate. The "weather awareness" (${\displaystyle ET_{0}}$) must be modified by a Crop Coefficient (${\displaystyle K_{c}}$) to get the actual water requirement (${\displaystyle ET_{c}}$). This relationship is defined by the Food and Agriculture Organization (FAO) Paper 56 standard:^[2]

${\displaystyle ET_{c}=ET_{0}\times K_{c}}$

• ${\displaystyle K_{c}>1}$: High-water users (e.g., Corn, Fescue grass).
• ${\displaystyle K_{c}<1}$: Drought-tolerant plants (e.g., Cactus, native shrubs).

Management allowed depletion (MAD)

MAD is the percentage of available soil water that can be depleted before plant stress occurs.

• Standard Crops: Often set at 50% depletion.
• Sensitive Crops: (e.g., Vegetables) may require watering at 30% depletion.
• Stress Tolerant: (e.g., Established trees) may tolerate 60-70% depletion.^[3]