Water use in agriculture is a complex topic to measure. You may water your 8 houseplants once a week, year-round, distributing 1 quart of water approximately equally, between all plants and your biggest concern will be whether the saucers will overflow if you give too much to one or the other. Farms and ranches use water for plenty of reasons, but we’ll focus solely on the topic of irrigation water. Irrigation water is defined by the US Geological Survey as water supplied through artificial means with the goal of maximizing crop yields. This includes water used not only to sustain plant growth, but to pre-irrigate soil before planting, to coat crops for frost protection, to aid in dispensing chemicals such as fertilizer and weed controls, dust suppression and crop cooling, and even in leaching accumulated salts from root zones.
Irrigation water use is calculated based on cumulative volumes withdrawn from groundwater sources (including nearly 500,000 active wells) or surface water (rivers, lakes, constructed reservoirs). While this is a useful marker, it doesn’t consider the percentage of water consumed by the crops themselves, as opposed to water lost to the atmosphere via evaporation from fields and reservoirs or via the soil to the water table.
In 2015, total freshwater irrigation withdrawals were 118 billion gallons/day, including 60.9 Bgal/d from surface waters and 57.1 Bgal/d from groundwater sources. This total volume was used on about 63.5 million acres in total, making an average application rate of 2.09 acre-feet per acre. While these are imperfect measures, over time these figures can help compare past usage and predict future supplies.
Flood and Furrow Irrigation
Flood and furrow irrigation may be the oldest methods of surface irrigation, requiring little more than a bucket or some shovels and a few rocks to set up at the most basic level. With such low barriers to entry, it’s no wonder that it continues to be popular even today, not only in less developed parts of the world where mechanized spray irrigation systems are rare, but in the US as well. In fact, in 2015, about 23.3 million acres in the US were managed using flood/furrow irrigation. This method works well for many crops commonly grown in rows, like corn, cotton, and tomatoes. In its traditional form, this is one of the least efficient irrigation methods. Still, many farmers can make improvements that reduce water waste:
- Leveling the fields: water should be evenly distributed to all corners of a field, so there should be no grade or depression that will concentrate water in one place.
- Surge flooding: Instead of simply directing water onto a field through the growing season, surge flooding involves periodic scheduled releases. This allows water to be provided at crucial intervals during the season while significantly reducing runoff.
- Even with surge flooding, a large amount of water can be lost when water runs off the edges and back of the fields. Farmers may use trenches or pipes to capture runoff and divert that water to storage ponds, where it can be pumped back to the front of the field and reused in the next scheduled flood. This reduces overall use of possibly limited water resources.
An optimized surge irrigation system can be expected to reflect application efficiencies in the 60-90% range.
Sprinkler irrigation
Sprinkler irrigation is widely used in the US and throughout the world. It requires only moderately complex machinery and is somewhat analogous to the way you might water the lawn at home. Agricultural sprinklers come in several models and sizes, some are automated and travel over large fields, while others are stationary and must be turned on/off by hand. In the United States in year 2015, about 34.7 million acres were irrigated with sprinkler systems.
Center-Pivot systems usually draw their water from a well located at the pivot point in the center of a large field. Older models feature high-pressure water guns that would shoot water high in the air, rapidly covering a large area. This is impressive to watch, but inefficient, especially on windy days. As much as 35% of water applied using a high-pressure system can be lost to wind or evaporation. Newer sprinklers operate on a low-pressure system, where water is sprayed gently from a height of about 4 ft above the soil. This reduces water loss to wind and since the water reaches and soaks into the ground more quickly, less can evaporate. Overall, improved low-energy center pivot systems reflect an application efficiency in the 75-95% range.
Drip or micro irrigation
Micro or drip irrigation is a very efficient way to deliver water since it is precisely directed and not subject to evaporation. It’s a modern technique which can be further aided by the addition of computerized controls and specialized sensors that can give foot-by-foot data on soil moisture, permitting highly customized water delivery to the exact location and in the precise amount needed.
If properly managed, micro irrigation has the potential to increase productivity of farmland while reducing water, chemical, and even labor requirements. Both water and fertilizer can be applied directly above the root zones, minimizing the amount of water utilized and also reducing problems with over-fertilization and chemically laden runoff or tailwater. Micro irrigation is well- suited to land that is difficult to manage because of slopes or oddly shaped parcels.
In the United States in 2015, about 5.5 million acres were irrigated with drip or micro irrigation systems. Typical values of application efficiency for point source emitters are in the 75-95% range.
Conclusion
Efficiency in farm irrigation has improved substantially in the past hundred years as new methods have been introduced and old ones improved. Efficiency is a big point of concern because our weather, particularly precipitation and temperature, are increasingly unpredictable. While some places are receiving more rain than expected, a lot of traditional farming regions have recognized that their activities are drawing down available water sources much more quickly than they can be replenished. Other strategies must be devised, but efficient use of the water that is available has become increasingly critical.
