Avocado farmer, Kurt Bantle, has employed the Internet of Things (IoT) to automate water management for his avocado crop in Southern California.
It takes 74 gallons of water to produce one pound of avocados and drought-stricken California produces 95% of avocados grown in the United States. The southern coastal region is locked in a drought and largely cut off from the flow of surface water from the state’s big irrigation projects. Avocado groves have been hit badly with sky-high water costs and reliance on water pumped from underground aquifers.
Water consumption is regulated in California with the state entering its fourth year of drought, resulting in water regulators imposing sweeping and draconian restrictions on the use of water. Some avocado farmers in California have turned to new methods for growing avocados, such as higher density planting, which in some cases enables twice as much fruit to be produced for the same amount of water.
An alternative approach has been demonstrated on a plantation of 900 young avocado trees on a farm in Fallbrook, Southern California. The case study explored the potential for exploiting the Internet of Things (IoT) to optimise soil moisture content and fertiliser application for avocado cultivation. The aim was to see whether avocados could be grown more efficiently using less water through soil moisture monitoring and automated irrigation.
The case study explored the potential for exploiting the Internet of Things (IoT) to optimise soil moisture content and fertiliser application for avocado cultivation.
Automating irrigation water management has the potential to increase yields and improve crop quality. More importantly, it helps to conserve water, which is critical in drought-stricken California. Some additional benefits are reduced runoff, optimised fertiliser application and energy savings in general. For avocados, 80-90% of the rooting zone is in the upper 8 inches of soil and the trees are not very efficient at absorbing water. Keeping the water in this zone is critical to fruit production. Typically, it is recommended that avocado trees receive 4 acre feet of water per acre per year. Given the average cost of water per acre foot ($1400) and the average price per pound of fruit ($0.75), that means 7500 pounds of fruit per acre is required just to cover the water bill.
There are several options available for irrigation scheduling. A key ingredient for improving irrigation water management to help conserve water resources is crop water information, often referred to as evapotranspiration (ET). This information can be used by growers and their advisers to understand daily crop water use for scheduling irrigations and to determine the amount of water to apply to replenish soil water depletion.
For avocados, other options are typically:
• Soil moisture measurements (volumetric, tension)
• Advice from neighbouring farmers (i.e. four acre feet per acre)
This article focuses on scheduling based on soil moisture sensors.
Soil moisture monitoring
Utilising soil moisture readings is a good way to determine when and how much to irrigate. Avocados do not like to work hard for their water but do not like to be moist all the time either. Irrigation scheduling through soil moisture analysis improves efficiency and can lead to improved quality, yield and profits.
Soil moisture monitoring provides insight into how hard the trees are working for water, gives detailed information about the changing soil moisture status and indicates when crops are at risk of stress and require irrigation. Soil moisture data gives information about movement of water down through the root zone. Irrigation using soil moisture readings applies water when it is needed and where it is needed in the area where roots are established or desired i.e. in the upper 8 inches for avocados.
The sensors are very easy to install. They are inserted into individual bore holes at two depths, one for the moisture in the rooted zone and a second to indicate when the water front has moved through.
Managing root zone moisture not only helps use water efficiently, it helps make decisions on when to apply fertilisers. When decisions are made based on information gathered through monitoring, nutrients remain in the rooting zone until absorbed by the plants instead of being pushed out through excessive irrigation. By not over irrigating when applying fertiliser, the fertiliser is prevented from leaching into ground water.
Monitoring with soil moisture sensors also allows growers to get the best performance from low-application rate irrigation systems, such as micro-sprinklers, which continuously supply water to replace soil moisture. When the application rate of the irrigation system is very low, it can be difficult to compensate for scheduling errors because it can take a long time to get enough water on the crop with micro-sprinklers. If a weather event is imminent (heat wave), the soil moisture sensors can determine whether the ground has enough available water.
Distribution of the water into the avocado root zone depends on the soil’s ability to transmit water laterally, which can be problematic in decomposed granite soils. Emitters with a large spread pattern are required to ensure water will move throughout the root system. Soil moisture monitoring helps to take the guess work out of this process.
Monitoring can also help protect crops from salinity. The majority of the irrigation water comes from the Colorado river and is relatively salty. Given that avocados are not a salt tolerant crop, it is critical to keep salinity in check. In areas where salinity is a factor, the outward and downward wetting front pushes salts away from the effective root zone. If this process is not continued, salts can move back into the root zone and damage crops.
Soil moisture measurement makes it easier to manage many irrigation issues.
Soil moisture measurement makes it easier to manage many irrigation issues. To be effective, sensors must be placed in the active root zones in locations that will accurately represent how the field or area is wetted by the system. In tree crops like avocados, sensors should be located at or near the drip line where the active roots are found.
Proper placement of soil moisture sensors, ideally in each irrigation block, will indicate both when and how long to run the irrigation system so that the maximum benefits can be achieved. Some blocks dry out at a much quicker or slower pace than others. The avocados are grown in hilly terrain and logic would suggest that the lower blocks will dry slower since water from the upper blocks will percolate down. This is not always the case. The aim is to conserve resources while yielding the greatest profit from the crop.
Valves need to be operated remotely. Typically, a valve uses a 24VAC solenoid to operate. Unfortunately, this is not a suitable choice at the avocado farm as it requires extensive cable to be laid to power and control the valves. Fortunately, the farm has DC latching solenoids and they are perfect for deploying in a wirelessly controlled farm automation system.
Data is moved around the farm without the use of miles of copper cable. Today a plethora of wireless options are available including:
• Proprietary 900Mhz.
LoRa, ZigBee and RPMA are extremely power efficient in sending small amounts of data around the farm. Since the remote nodes are battery/solar powered, power consumption is key. Cellular and WiFi are used for backhaul technologies since power consumption is a not a concern and higher bandwidth is required.
Given the wide variety of choices available, it is necessary to focus on what can be accomplished via the radio, taking the terrain into account. In some areas mesh may help to get down further into a canyon. Microcontrollers can be used for processing sensor readings out at the node or a raw sensor reading can be taken and dealt with by gateway. With a lot of sensor readings, it may be more efficient to do some of the processing at the nodes. Load balancing the system and distributing the load across time are also important. A low power radio node will not be efficient at transmitting video. The cellular phone most people carry has undergone extensive testing to ensure performance. In today’s IoT, while the radio technologies themselves undergo this type of testing, often the end systems do not.
Reading the sensors
A microcontroller is used to read the soil moisture sensor and apply calibration factors to the reading. The system measures soil moisture every 15 minutes and reports it back to the system cloud through a gateway.
Triggering an irrigation event
When the upper sensors dry to a set point (measured in Centibars kPa), an irrigation event is triggered. The valve is opened and the pump is turned on. The irrigation event is run until the lower sensor detects a change in tension. This is done to push the salts down below the primary rooting zone of the avocados. As stated above, avocado trees are not tolerant to salts and the irrigation cycles need to account for this.
Controlling the valves
The valves are open and closed via DC latching solenoids. In this configuration, a microcontroller is not required and utilising the DIO pins on the radio is sufficient. In designing the system, every effort was made to use the bare minimum amount of hardware in an effort to save power and cost. Most of the modern radio solutions have plenty of DIO pins to accomplish a task like turning valves on and off.
In addition to running the irrigation system, it was possible to inject fertiliser through the irrigation lines. This allows for precisely controlled application, based on moisture content of the rooting zone, to ensure that the fertiliser stays in the rooting zone and is not flushed out.
These results pave the way for small, medium and large-sized farms around the globe to cut the cost of growing fruit and vegetables.
Expenditure for LoRa stations with soil moisture sensors, valve controllers, LoRa gateway and cellular backhaul to soil moisture monitor and automation for justin-time irrigation amounted to $8,200.
The results of the case study proved staggering. The annual cost of watering 900 avocado trees was initially $47,336. By connecting the trees with IoT technology, the annual water bill dropped to just $11,834 - a 75% cost reduction. The hardware investment was recovered within the first six months. The usage will climb as the trees get bigger; the goal is to reach a 50% reduction of water usage when they are fully grown.
Scheduling irrigation based on soil moisture has saved the avocado farm, allowing the crop to be produced close to its intended market. Without it, the operation cost (water) is just too expensive to remain viable.
These results pave the way for small, medium and largesized farms around the globe to cut the cost of growing fruit and vegetables. While there is a cost associated with installing a system of this type, the payback is very rapid and the benefits (no runoff, no fertiliser leaching) far outweigh the cost. These systems also have infinite expansion possibilities. Other types of in situ measurements, e.g. pH, are already under investigation.
Kurt Bantle is a Senior Solution Manager at Spirent Communications
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