Frank is one of the most progressive farmers we’ve met, a pioneer in water conservation and sustainable management practices. He upgraded all his acreage to drip irrigation in the 1990s, utilizes deficit irrigation strategies, and is constantly seeking to improve the Mullers’ operation. When we first met
to discuss the concept for the project, although his team had already implemented RDI, he told us how he thought they could probably water even less without sacrificing tomato yields, and increase their soluble solids, to gain a premium on quality. Despite frequent scouting to assess canopy development, he felt that the team was still struggling to pinpoint the right moment to implement RDI strategy, due to varying tomato genetics, planting dates, and field conditions. It’s not an easy task, since dialing back water too early can dramatically reduce yield, making it financially very risky to start RDI too early. To mitigate the risk, the team decided to wait until they could actually see water in the field before dialing back the irrigation, with the tradeoff being a loss of premium on quality. Arable and Muller Ranch collaborated with Dan Johnson, an irrigation engineer at USDA NRCS, to build out an RDI guide to address this issue. To inform our guide, we decided to collect the following information at the level of individual irrigation management zones:
- Daily monitoring of phenological progress by variety and field to identify optimal timing to transition period from regular irrigation to an RDI strategy.
- Measuring ETc by irrigation management zone to assess irrigation needs relative to evaporative demand.
- Accurately measuring and forecasting field-level weather events to better evaluate potential future risks and scenarios.
- Accurately measuring the amount of irrigation applied as a final check that the strategy has been successfully implemented.
At the start of the project, we sat down in Frank’s office, and he explained what a successful RDI system would look like:
“Ultimately, the technology that will rise to the top will be able to show, in real time on a site-specific basis, what has happened recently in the field, what is the current status, how many hours of water is applied on a daily basis, what is the projected ET-based need for the following day and week, and how many hours of irrigation should be put on today. It will be a decision-maker, and will communicate the conclusions it has reached based on the site-specific data it has gathered. Predictive capability is the key.”
Challenge accepted. Here’s how we did it:
- We installed Arable Marks every 20 acres in a representative portion of the field to measure ETc by combining energy budget information from a four-way net radiometer (up, down, shortwave, and longwave radiation) with static properties of air (temperature, humidity, pressure), and a direct measurement of crop temperature.
- Arable calculates heat units, or cumulative growing degree days (CGDD), by capturing air temperature every five minutes and recording daily maximum and minimum temperatures. The accumulation of CGDD is what primarily drives crop metabolic responses. We also measure other attributes of weather (relative humidity, pressure and precipitation), and build field-level weather forecasts to help us understand impending risks on a field-by-field basis.
- Through a seven-band spectrometer (which captures wavelengths from the visible spectrum to the near infrared spectrum), the Mark then calculates a daily Normalized Difference Vegetation Index (NDVI), which determines the density of green in a plant canopy. This index acts as a visual proxy for what is happening in the tomato fields; for example, as tomatoes enlarge and ripen, the NDVI values will lessen as the colors reflected will start looking less green and more pink, orange, yellow, red, or purple.
- Using Arable’s ability to integrate other third-party sensors, we connected Badger Flow Meters to monitor water application through the drip tape, allowing us to verify the quantity of water applied in each field through a single platform.