Understanding Application Rates

Understanding Application Rates

Acre Inches

Water is measured in different units depending on the situation: we use gallons, cubic feet, and acre feet in the United States. A gallon is typically the easiest to picture when thinking about a gallon of milk. The emitters we use are made in gallons per hour (GPH) and we measure pipeline flow rates in gallons per minute (GPM). The other units typically take more work to imagine. Cubic feet of water is the unit used for canals and reservoirs, and flow rates of district canals are given in cubic feet per second (CFS). We use the acre inch (Ac In) when talking about irrigation, and the numbers are usually in decimals. One acre inch (Ac In) is equal to 27,154 gallons.

Crop Requirements

Each type of plant needs a certain amount of water to produce a crop, and more water is need in the summer months. The California Irrigation Management Information System (CIMIS) has published a map showing different geographic zones and a table of monthly averages. The amount of water that evaporates into the atmosphere and transpires (given off as water vapor) through a plant are combined to form evapotranspiration (ET). Because crops have different requirements, individual crop evapotranspiration (ETc) numbers have been determined by CIMIS. Tree fruit and nuts, for example, might need 7 inches of water in the hottest months while vines need 6 inches. For comparison, the reference ET number (ETo) is based on grass or alfalfa. A good irrigation design takes into account crop type and location in order to meet seasonal demand.

CIMIS ETo Zones Map
CIMIS ETo Zones Map, developed by California Department of Water Resources and UC Davis. https://cimis.water.ca.gov/SpatialData.aspx

What Affects Application Rate

The main components of application rate are irrigation run time and flow rates of emitters. Factors such as the row spacing and distance between emitters also affect the number. We have a calculator in our resources tab to measure the application rate of a field.

Th double line drip in this almond orchard shows the wetted pattern under each drip emitter.

Here are some examples for how the rate is calculated. These figures are based on the average layout for crops we tend to deal with. The distances between sprinklers and rows change how quickly water can be applied.

For Sprinklers: 10 ft emitter spacing x 18 ft row spacing = 180 sq ft per emitter

43,560 sq ft per acre / 180 sq ft per emitter = 242 emitters per acre

The same goes for drip tubing with set dripper spacing. Divide one acre by the distance between rows to find the length of tubing per acre. Remember to double the number if you are calculating for double line drip.

For Drip: 43,560 sq ft per acre / 22 ft row spacing x 2 hoses per row = 3,960 ft of tubing per acre

3,960 ft of tubing / 2 ft dripper spacing = 1,980 emitters per acre

After finding the total number of emitters in an acre, you can factor in the emitter flow rate. If the drippers or sprinklers are not pressure-compensating, the flow rates will increase at higher pressure. Most manufacturers put technical information for emitters on their websites that show the relationship between pressure and flow rate.

Sprinklers:  242 emitters per Ac x 8.4 GPH = 2,032.8 GPH per Ac

2,032.8 / 27,154 gallons per Ac In = 0.0749 Ac In per Hr

Drip:  1,980 emitters per Ac x 0.53 GPH = 1,049.4 GPH per Ac

1,049.4 / 27,154 gallons per Ac In = 0.0386 Ac In per Hr

For row crops and flood irrigation, you can use the pump flow rate and number of acres to find your gallons per hour per acre. From there, divide by the same 27,154 to get acre inches per hour.

How It Applies to Your Field

Irrigation scheduling should be evaluated often to be sure that changing seasonal needs are met. Based on those requirements and the flow rate of available water, an irrigator can plan their run times for each set. Another thing to note is that a higher application rate is not always better with certain soil types. In sandy soil, heavy watering can accelerate leaching into the soil that will draw water away from the root zone. With more clay in the soil, irrigating faster than the soil can absorb water will cause it to pool on the surface. As with many things in farming, it’s good to keep an eye on the field to make sure your irrigation is dialed in.

If you’re interested in learning more about application rates, the sites below have more information.



Sand Media Filters

Sand Media Filters

How Sand Media Filters Work

Sand media filters are a proven method of purifying water and are preferred by many farmers due to their simple operation and large capacity. Designed to remove organic debris and particles from water, sand media filters extend the life of irrigation systems and prevent emitters from plugging. The filters are set up as a series of tanks filled with a certain type of sand. Contaminants are filtered out of the water as it flows through the media in the tank. The water is pressurized and forced through the top of the tanks, pushed through the sand media, and out of the tank into the irrigation lines.

The media is small and sharp-edged; the jagged edges fit closely together to trap particles more effectively than regular sand. The sharp edges also snag organic contaminants as water flows through the tight spaces. Specific media type and grain size and are not universal, but Streamline has found the best results with #16 silica sand and a base layer of 3/8” gravel. This size of sand provides filtration at around 180 mesh while the large gravel prevents the finer media from leaving the tanks. Sand media choice depends on water quality, filtration objectives, and length of filter runs.

Filtering and Flushing

Sand media filters are good at removing fine solids. Larger particles reduce the effectiveness of the filter because contaminants build on the top layer of the media, resulting in pressure loss. To get rid of filtered particles, sand media tanks regularly back-flush with filtered water. During this process, clean water from one tank is pumped backwards through another tank to remove the accumulated particles from the media. This is why it is necessary to have more than one tank in a filter station. The removed particles are drained through a flushing manifold away from the tanks. This preventative maintenance allows the filter media to be used continuously and quickly resume operation under normal pressure. If the water consistently carries more particles than sand media is able to remove, a separate filter such as a gravity screen may be needed before the tanks.

Benefits of Higher Filtration

These filter systems are considered vital for drip and sprinkler irrigation. Drip systems need good filtration because the drippers can quickly become plugged with sandy water. The ideal scenario is to use the highest level of filtration that is practical. Even if particles do not cause clogging in the system, removing them minimizes the wear caused by grains that would otherwise be pushed through.

Sand, rock, and other grit are not the only particles these filters remove. Organic materials like algae and slime also pose a threat to irrigation systems. In some cases, small fish or freshwater clams have plugged irrigation systems.

Timers, Gauges, and Switches

These filter stations come with a controller to schedule how often the system will backwash, how long, and the time in between each tank. The tubing that runs along the tanks allow the controller to read the pressure difference between the top inlet and bottom discharge manifold. As particles build up and reduce pressure, the differential increases until it triggers a switch that starts a flush cycle. This allows the filters to keep operating with a minimal drop in pressure. A pressure gauge on the outside of the tanks will indicate top and bottom pressures, it is usually located at eye level on the inlet manifold.

The technical parts of tanks might seem daunting at first, but once their operation is learned they require very little input. The main benefits of this system are its dependability, the simple way it removes contaminants, and that it can continue irrigating throughout its cleaning cycle. Most farmers agree that sand media tanks greatly improve their water cleanliness and are worth installing for the maintenance time they save.