Smart Irrigation: Sensors, Weather Data, Automation

A traditional irrigation timer waters on a schedule whether or not the plants need it. Smart irrigation replaces that fixed schedule with a measurement: either how wet the soil actually is, or how much water the weather has pulled out of it. The idea works the same in a backyard vegetable bed and on a quarter-section of corn. What changes with scale is the hardware and the cost of getting it wrong.

Measure need instead of guessing

Plants use water at different rates depending on heat, sun, wind, humidity and growth stage. A timer set in May over-waters in a cool, rainy June and under-waters in an August heat wave. Smart systems close that gap in one of two ways. Soil moisture sensors check the supply side, the water still available in the root zone. Weather-based controllers estimate the demand side, how much water the plants and soil have lost. The best setups use both.

Soil moisture sensors

Soil moisture sensors fall into two families, and the difference matters when you read their numbers. The University of Minnesota Extension's guide to soil moisture sensors for irrigation scheduling lays them out:

• Volumetric water content sensors report what share of the soil volume is water. Capacitance and time domain reflectometry probes are common types.
• Soil water tension sensors report how hard roots must pull to extract water, in centibars. Tensiometers and granular matrix sensors are common types. A higher reading means drier soil.

Cost differs sharply between the two families. The UMN guide, last reviewed in 2019, lists granular matrix sensors at $40 to $50 each and tensiometers at about $80, against $250 to $350 for a capacitance or TDR sensor, before data loggers. Treat those as relative prices and get current quotes. On coarse-textured soils, about half the plant-available water is gone by 25 to 45 centibars, so the guide says to irrigate before the sensor reaches that range. It also warns that tension readings are soil specific and can be inaccurate, and that capacitance sensors work best with a site-specific calibration.

Placement decides whether a sensor tells you anything useful. For field crops, UMN recommends installing sensors at one-third and two-thirds of the crop's root zone depth, at two or more locations, in representative soil away from high points, depressions and slopes. Make sure each sensor has direct contact with the soil, with no air gaps around it. At home, the same logic applies on a smaller scale: bury the sensor in the root zone of a typical spot in the zone it controls, not beside a downspout or under a tree canopy.

Weather-based (ET) controllers

Evapotranspiration, or ET, is the total water that moves from the land to the air through evaporation from soil and transpiration from plants. The USGS notes that transpiration rises with temperature and with wind. On a hot, windy day, ET is high and plants need more water. On a cool, cloudy day, it is low.

A weather-based controller estimates daily ET from local weather data, subtracts rainfall, and adjusts run times to replace roughly what was used. Colorado State University Extension's lawn irrigation scheduling guide describes controllers that, for a small annual fee, download actual ET and rainfall from a local weather station system and adjust watering day by day.

For home systems, the EPA's WaterSense program is a useful filter. It labels two kinds of irrigation controllers: weather-based models that use local weather and site conditions to set schedules, and soil moisture-based models that prevent watering when the soil does not need it. The EPA says replacing a standard clock-based controller with a WaterSense labeled model can save an average home up to 15,000 gallons of water a year.

A weather-based controller only works as well as its setup. It needs to know, for each zone, the plant type, soil type, sun exposure, slope and how fast the sprinklers or emitters apply water. Skip those settings and it will adjust the wrong baseline with great precision.

Drip versus spray

How water gets delivered matters as much as when.

Spray and rotor heads throw water through the air over an area. They suit lawns and dense ground covers, but wind blows the spray around, some evaporates before it lands, and water that falls faster than the soil can absorb it runs off onto pavement.

Drip and micro-irrigation puts water slowly at the base of the plant through emitters or tubing. The EPA's WaterSense microirrigation sheet says microirrigation systems use 20 to 50 percent less water than conventional sprinkler systems. Drip suits vegetable rows, shrubs, trees and containers. The same sheet says a microirrigation system should have, at a minimum, pressure regulators, filters and flush end assemblies to clear the lines. Emitters can still clog and leaks hide under mulch, so check it by walking the lines while it runs.

On farms, sprinklers are the most widely used method. In USDA's 2023 Irrigation and Water Management Survey, summarized in its highlights report, US farms irrigated 53.1 million acres. Sprinklers covered 30.8 million acres, gravity systems 18.3 million, and drip, trickle and low-flow micro systems 6.4 million. Those add up to more than the total because some acres use more than one method.

Automation rules that save water

Whatever controller you use, these rules do most of the work:

1. Skip after rain. A rain sensor or weather feed should cancel or shorten the next cycle when meaningful rain has fallen.
2. Gate on soil moisture. Let a sensor veto scheduled runs when the root zone is still wet.
3. Cycle and soak. On slopes and clay soils, break one long run into several short ones with soak time between, which CSU recommends because water is often applied faster than it can soak in.
4. Water at night or early morning. CSU notes that many water providers ask homeowners to water between 9 PM and 9 AM, when winds are lighter and less water evaporates.
5. Size each run to the soil, not the clock. Use the zone's measured application rate (see the catch-can test below) to set how long a run takes to wet the root zone, then let the sensor or ET data decide how often.
6. Skip on freezing mornings. If your controller has a freeze-skip setting, turn it on so the system never sprays water that can ice a sidewalk or driveway.
7. Cap run times and watch flow. A maximum run time per zone and a flow sensor that alerts on unusual flow will catch a broken head or a split line before it runs all night.

The math in inches and gallons

Irrigation depth is measured in inches of water, the same unit as rainfall. Converting depth to gallons is simple arithmetic. A cubic foot holds about 7.48 gallons, so 1 inch of water over 1,000 square feet is about 83.3 cubic feet, or roughly 623 gallons.

At farm scale, the same USDA irrigation report defines an acre-foot as the water needed to cover one acre one foot deep: 325,851 gallons. Divide by 12 and one acre-inch is about 27,154 gallons. A center pivot applying 1 inch across 125 acres is moving nearly 3.4 million gallons.

To know how many inches your system applies, measure it. CSU's catch-can test takes six identical straight-sided, flat-bottomed cans set randomly between the heads of one zone. Run the zone for exactly 10 minutes, pour all six cans into one, and measure the depth with a ruler. Six 10-minute samples add up to an hour of catch, so that depth is the zone's rate in inches per hour. If it reads 1.5 inches, a 0.5-inch watering takes 20 minutes. For planning, CSU estimates about 1.75 inches per hour for pop-up spray heads and about 0.5 inch per hour for rotors. Before you pour, compare the cans: big differences mean uneven coverage, which no controller can fix.

Home garden and farm scale compared

Where to start

Before buying anything, run a catch-can test on each zone and walk the system while it runs. Fix broken heads, misaligned sprays and leaking drip lines first, since a smart controller cannot correct water that lands on the driveway. Then add one change at a time, such as a rain skip or a soil moisture sensor in your thirstiest zone, and compare the water bill or meter reading against the same month last year.