Irrigation Scheduling for Agricultural Crops: It’s Not Just a Flip of a Switch!

For this second post in the “Sustainable Agriculture on Cleantech Blog” series, I decided to invite Dave Doll, UC Cooperative Extension Farm Advisor, and fellow blogger at Almond Doctor Blog, to share his expert knowledge about irrigation management for agricultural crops.
Agricultural use of water within California use has been a media magnet these days. With the reductions of pumping into the California Aqueduct from the Delta, California in its third straight year of a drought, and an increasing population that is putting strain on an aging infrastructure, it is not much of a surprise to find that water is on many people’s mind. In a normal year, 48% of the water is used for environmental reasons, 41% for agricultural purposes, and 11% for urban uses. In drought years, these percentages change, usually with reductions facing both the environmental and agricultural uses. Most water “rights” discrepancies come in terms of river restoration and/or protection of native species, which usually reduce water to local growers who then rely more heavily on groundwater to maintain agricultural production. One can see that battles between growers and environmentalist are common and fierce. An example of these can be found with court rulings of the Delta Smelt and the restoration of the San Joaquin River.
Being with water in high demand, are there ways that the water used for agriculture be used more efficiently? The answer is “Yes.” Agricultural water use efficiency can be improved by delivering water to the right place, at the right amount, and at the right time. The “Three Rs” is not a new concept: the most primitive irrigation systems established over 5000 years ago were reliant upon these same principles. Growers would water when the plants showed some sign of water stress (i.e. wilting), and water would be delivered to the root zone at an amount that appeared to wet the soil to the appropriate level. Thankfully, through the use of certain technologies, we can increase the efficiency of our irrigations through tools to that help refine the three Rs.
If the “Three Rs” have worked for 5000 years, why change now?
The current face of agriculture is changing. Water costs are increasing. In drought years, water prices may be over $500 an acre foot in some production areas of the West side of the San Joaquin valley. Increased rates are not just due to droughts; rates throughout California are increasing as urban and environmental water demand increases while supply has not increased. Secondly, the costs involved to apply the water are also high. Fuel and electricity for pumps, cost of irrigation filters and lines, and irrigation maintenance are not cheap and require hours of labor to install and repair. Furthermore, especially within the San Joaquin Valley, water must be properly applied to prevent run-off, prevent plant diseases, ensure adequate soil penetration, encourage leaching and prevent accumulation of salts, reduce evaporation, and produce maximum profits/yields. Knowing all of this, it is easy to understand why wasting of water is unacceptable as well as the reduction of yields caused by under irrigating.
So, how do we do maximize yield but reduce water waste?
Proper irrigation is achievable through monitoring the plant-soil-environment complex. The amount of water within the soil and its ability to be accessed by the plants roots can be measured/estimated through a variety of technologies. These include the low cost feel method, to the more accurate neutron probe. For most irrigation water management systems, one of the several electrical resistance or tensiometers systems are used. These are connected to data-loggers and can be transmitted wirelessly to computer software programs to help growers monitor soil moisture. Soil moisture readings are often used by themselves to schedule irrigations, but they are most valuable when used with data that takes in consideration the water demands influenced by the environment and plant.
Plant water use varies by the stage of growth of the plant. Typically, water use is the highest when the plant is fully leafed out, with maximized leaf surface. This is because the more leaf surface transpiring, the more water is lost through the opening of the stomates. As stomates open and close, water vapor, which is at a high concentration within the plant, is released into the low moisture environment through diffusion. This is also why plant water use is the highest on days with high temperatures and low humidity. To simplify the plant-environment water interaction, the term evapotranspiration is often used. This term encompasses the loss of water by both the evaporation off of the surface of the plant and soil, and the water lost through transpiration. This value is determined by weather stations and multiplied by the respective crop and crop growth stage to determine the water use. Throughout California, these values are recorded and calculated from over 100 weather stations and made available through the California Irrigation Management Information Systems.
Outlined above were brief explanations of the tools available to calculate how much water is in the soil, and how much water is used by the plant and environment. Knowing this information, how can we use the “Three Rs” to reduce water use by increasing irrigation efficiency? By viewing the soil profile as a reservoir for the plant’s water, and calculating the daily water needs of the plant, we can determine how long the plant can survive off the water available within the soil profile. When the soil profile is close to depletion, a timed irrigation of the proper amount can refill the profile, restarting the cycle. This is the premise of basic irrigation scheduling. As one can see, proper crop irrigation encompasses more than “just a flip of the switch.”
Complexities within soil texture and soil water holding capacity, variance in efficiencies of different irrigation systems, plant water potential, and regulated deficit irrigation are all topics

that increase irrigation efficiency and will be discussed in later articles.

Marguerite Manteau-Rao is VP Marketing for Terraqualo, a new venture in precision irrigation for growers of specialty crops. Marguerite is the creator of La Marguerite, a popular environmental blog, and has written extensively for a number of other blogs, including Huffington Post Green. She has a multidisciplinary background as an engineer, marketer, and social worker.