Drought fatigue is hitting California. Although the state got some limited relief from El Niño, the drought continues in much of the state, and people are being asked to continue their quest to save water.
Many Californians are looking at their brown lawns knowing that 80 percent of the freshwater consumed in the state goes toward farming, and wondering why all farmers in California (and across the country) don't make the switch to the most sustainable form of crop watering - drip irrigation. That would free up more water for cities, right?
The reasons why farmers use one irrigation system over another are varied and sometimes complex. Each form of irrigation has pros and cons that make it more or less appropriate for any particular farm and any given crop. It's worth a look at different irrigation systems to understand why drip systems, despite their reputation for water efficiency, aren't always appropriate for every situation.
Types of Irrigation Systems
The USGS defines irrigation as "the controlled application of water for agricultural purposes through manmade systems to supply water requirements not satisfied by rainfall." Put simply, if we rely on rainfall without irrigation, we'll never grow enough food to feed the world's ever increasing population. Because it can be expensive to install and operate, farmers have to determine that increased crop yield and better quality will result in enough of an increase in income to offset the cost of installing and operating an irrigation system. In places that are experiencing droughts (like California), many farmers have to choose between either irrigating their fields or letting them go fallow. It's a tough choice to have to make when faced with feeding a nation.
Worldwide, there are many irrigation methods used but they can all be divided into four classes: drip, subsurface, surface (flood and furrow) and sprinkler (center-pivot, traveling gun, etc.). Before deciding what method of irrigation to use, farmers have to know the quantity and quality of water available, the soil type and slope of their crop fields and what crops they will irrigate.
Some major forms of irrigation include:
You might be familiar with this concept if you're a home gardener. This is a water-efficient system that applies water directly to plant roots through applicators that operate under low pressure, either on or below the surface of the ground. The system delivers water either at the plants or along rows and requires clean water so the nozzles don't get clogged. Even still, the nozzles require regular maintenance to prevent clogging from algae growth or mineral buildup. The amount of water can be tailored to the needs of individual crops and the system works well on sloping or oddly shaped fields.
In this system, water is applied below the ground surface either by raising the water table within or near the root zone or by using a buried perforated pipe that brings water directly into the root zone. This type of irrigation can be very efficient at delivering water directly where it is needed.
Flood and Furrow Irrigation
Flood irrigation is what it sounds like - water is pumped to fields that are flooded to a certain depth so the entire soil surface is covered by ponded water. This type of system is inexpensive to install, has low energy costs and is less affected by water quality. Flood irrigation is much less water-efficient than other systems and can waste a good deal of water if the fields aren't level, which allows water to run off into local water ways. Poor drainage is especially problematic in the Western states where the water has a higher saline content; salt build-up can eventually ruin fields.
Furrow irrigation is similar to field flooding where water is applied in furrows or rows that contain irrigation water between rows. The pros and cons are similar to those of flood irrigation.
In this system, water is not pumped but flows through canals and is distributed by gravity from higher to lower areas. This is a good way to get water to smaller fields. Silt in the canal water can act like free fertilizer and the quality of water is generally good. This is a cheaper form of irrigation but is inefficient, and canals require regular maintenance.
If you've ever flown over fields of green circles in farm country then you've seen this type of irrigation. This is an automated sprinkler system with a rotating sprinkler pipe that supplies water from a well in the center of a circular field. The pipe is powered along a circular path on wheels, and water is sprayed onto plants at a uniform rate. Because the water is sprayed into the air it is subject to evaporation from wind and low humidity environments. The amount of water applied depends on how fast the watering arm moves around the circle. A single center-pivot system normally irrigates about 130 acres.
Traveling Gun Irrigation
In this system, a single large nozzle (that looks like a big gun) rotates and is self-propelled, similar to small home automatic sprinkler heads. The big guns typically discharge between 100 and 600 gallons per minute and will irrigate to a radius of 80 to 250 feet. These systems use more energy than center-pivot systems because of the pressure required to "shoot" the water out. They are also more labor intensive. Like other spray systems, they lose water easily to evaporation.
Linear Move Systems
These are similar to center-pivot systems but rather than rotating around a fixed point, the systems move laterally across rectangular fields. Most systems are supplied with water from a ditch that runs the length of the field or from a large hose that is dragged along through the field. These are best suited to level fields. Because they have internal guidance systems to keep them straight, these systems require regular maintenance.
Irrigation Efficiency vs. Water Productivity
In many cases, drip irrigation is more sustainable due to how precisely water can be delivered without much waste. Spray irrigation is generally less efficient and loses about 65 percent of water applied but can be made highly efficient (about 90 percent) by using hanging pipes instead of spraying water at high pressures. Furrow or flood irrigation is widely used in many parts of the world, including the US, but is the least efficient, losing about 50 percent of water applied).
At the same time, efficiency has its limits in terms of sustainable water use for agriculture because it can drive intensification that can, in turn, lead to problems like over-tilling, poor soil health, erosion, overuse of fertilizers and pesticides and greater water use, all of which undo efficiency gains. "Water productivity" is a better measure of effective use of water because it pairs efficiency with other sustainable practices like monitoring soil moisture, conducting farm water audits, keeping up with best irrigation practices, planting cover crops, rotating crops, planting water-climate-smart crops, etc.
Unfortunately, switching all farmers to drip irrigation is not the easy, simple solution we all wish it would be. How, when and why farmers irrigate is a complex set of decisions made even more complex by extreme drought situations that we face all too frequently around the country. As water consumers, we all have a part to play in using water wisely, even if that means occasionally watching our lawns turn brown. The next time you're enjoying some fresh California produce and wondering where all the water is going in California, just think about the complex set of variables and decisions it took to get that produce to your plate.