IRRIGATION SCHEDULING


to make accurate irrigation management decisions based on field-specific data. While the process of collecting and analyzing the necessary data to make irrigation decisions can seem complex, the benefits have been consistently proven in the field through decades of field trials and validation by numerous researchers and commercial growers around the world.


With irrigation scheduling, growers have the ability to improve their operation through the following:


• Provide the exact amount of water a crop needs at the time it needs it and help avoid crop stress, which together can ultimately optimize yield.


• Prevent overirrigation, reducing the amount of energy used for pumping and conserving water.


• Eliminate leaching of fertilizers beyond the root zone, reducing loss of costly inputs.


• Help protect ground and surface water from unnecessary pollution.


• Help prevent yield-reducing disease which can be worsened by overwatering.


Typically, science-based irrigation scheduling in North America is primarily achieved in three ways: using soil moisture sensors, the checkbook method and the automated checkbook method. Let’s compare these three common methods.


Soil moisture sensors


Soil moisture sensors, when coupled with site-specific soil information, can provide insights into water availability in the crop root zone, indicating when crops are at risk for stress, when to irrigate and when to stop. These sensors can be stationary, placed at predetermined locations and depths in the field, or handheld to allow measurement of soil moisture at several locations. Depending on the device, data can be collected manually or transmitted wirelessly.


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While soil moisture sensors are typically cost-effective and easy to use, their footprint is limited to the volume immediately surrounding the device. Installing and monitoring multiple sensors can be time- and labor-intensive. Moreover, relying on and making sense of electromagnetic signals for agricultural water management can be a daunting task. Commercial sensors can be subject to soil-specific errors and demonstrate highly variable attributes that mitigate their ease of adoption. In most cases, effective reliance on a soil moisture sensor requires site- specific calibration and frequent, close inspection.


Growers should consider the following when including soil moisture sensors in irrigation decision-making:


• Sampling across the soil profile, monitoring soil moisture across the entire root zone to provide a more comprehensive picture of plant water availability.


• Tracking sensor accuracy, especially if the sensor was installed incorrectly or the field has high levels of clay content or salinity.


• Considering location, placing sensors at a representative spot in the field, out of the way of farm machinery and irrigation systems.


• Comparing wireless versus manual data collection and the cost and convenience associated with each option.


• Looking at the ability to easily and quickly convert collected data to irrigation decisions.


Checkbook method


The checkbook method is a form of soil water accounting that is widely used today in irrigation research. It’s called the checkbook method because it operates just like a bank checking account. Rain and irrigation are deposits while water used by the crops and evaporation from the soil surface are withdrawals. Like a checking account, it’s important to keep a running account of deposits and withdrawals to ensure moisture levels don’t fall below the minimum balance.


To start the checkbook, growers must determine the soil texture, crop type and rooting depth, available water-holding capacity of the soil, minimum allowable balance and an estimate of current soil water balance. During the growing season growers need to monitor and log evaporative demand and measure and log rainfall and irrigation applied to the field. Using a soil water balance spreadsheet, the grower manually enters the data, calculates the new soil water deficit and determines when the next irrigation is needed.


Summer 2022 | Irrigation TODAY 21


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