farmers need to know their sprinkler application and evapotranspiration rates, ET, before they can do any irrigation scheduling.
farmers can use assistance to;
- a. soil test to determine type of soil and water holding capacity,
- b. check flow rates of intake and pipes
- c. check nozzle flow rates and pressures, all within 80%
- d. determine application rate and irrigation set times
- e. obtain irrigation manuals

Infiltration rate - movement of water through the soil surface into the soil

- decreases as the length of the irrigation increases

Percolation rate – movement of water through the soil, dependent on soil texture and structure, water drains down 1-3 days after an irrigation

Maximum sprinkler application rate – set by the infiltration and percolation rates

Sod and cultivated soils – water can be applied more rapidly to sod and trash covered soil than to bare or cultivated soil.

1. Water holding capacity of the soil – soils will hold varying amounts of water. Silt loam soils will hold about 2.5 times the water as sands and are therefore safer and easier soils to manage.

AW – available water – amount of water in the soil available for plant use in inches

- determined from irrigating to field capacity and drying or using the water to the permanent wilting point; potatoes

- loamy sand will hold 1.2 inches of water and silt loam 2.5 inches per foot of which 50% is available for alfalfa/grass, 35% for potatoes and 40% for tree fruits.

Available forage soil moisture – safe minimum of AW is 50%

Rooting depth of grass 1-2 feet, alfalfa 1-4 ft (1-2 ft are were nutrients are)

E.g. Forage in sand, roots 2 ft deep provides 1” of AW; 3-7 days of water

Alfalfa in silt loam, roots 3 ft deep provides 3.5” of AW water; 12-20 days of water

Water extraction from the soil profile 1ft-40%, 2ft–30%, 3ft-20%, 4ft-10%

Evapotranspiration rate – rate of removal of water from the soil by plant growth, plant transpiration and evaporation from the soil surface.

Application efficiencies – peak ET rates per day and annual irrigation

Maximum soil water deficit

1”/.30/42”, 2”/.28/32” 3”/.26/27” 4”/.24/24” 5”/.24/21”

8 gpm 7 gpm 6.5 gpm system design flow rates

- sprinkler systems are not 100% efficient in application

Guns 68%, wheel moves 72%, pivots 80%

- affected by evaporation, wind drift, runoff and length of set

- affected by operating nozzle and line water pressures

- larger pipes reduce friction and improve pressure distribution through system,
within 80%

- wind affects low pressure pivots less than wheel and hand moves and less than guns

Irrigation scheduling - interval between irrigations; how to determine when to irrigate

- hand soil ball method, form a ball and see if it will bounce

- crop water use, water available in the soil less the daily ET
(atmometer, evaporation pan - #2 washtub, automated climate station)

- soil moisture equipment – tensiometers, soil moisture blocks

Irrigation uniformity - affected by

- wind speed, wind direction,

- sprinkler pressure and sprinkler spacing – square, rectangle, or triangle

- spacing as a % of wetted area and wind speed

< 6 km/hr 60%

6-12 km/hr 50%

>12 km/hr 40%

- in rectangular spacings run the long side the direction of the wind

- nozzle pressure 35-50 psi, pipe pressure 45-65 psi

Sprinkler spacing – governed by crop irrigated, application rate, sprinkler diameter and wind

8. Sprinkler pressure and nozzle size

- operate within manufacturers specifications for best distribution.

- 20A / 1/8” / 30-40psi,

- 30E / 5/32” / 40-50 psi, (25 psi – 3.5 gpm & 88’ diameter, 45 psi – 4.7 gpm & 96’ diameter)

- low pressure causes large droplets, soil compaction, seedling damage, runoff

- high pressure causes misting, poor wind penetration

- generally use same nozzle size through out the system

Application rate and time – major decision

- based on how often you want to move pipes, change sets, usually 12 or 24 hrs

- really based on 11.5 or 23.5 hrs as it takes some time to drain or move the system

- the application rate must fit to the irrigation time

- a system designed for 12 hrs should not be run for 24 hrs and overfill the soil, waste water and cause leaching and nutrient loss

- a system designed for 24 hrs and run for 12 hrs can work if irrigated twice as often but will be less efficient in water use.

- Guns operate different than conventional sprinkler systems due to high application rates, and shorter set times, 3-4 hrs. Never operate 2 guns, side by side together.

2. Good Irrigation Management

The key to irrigation management is to keep sufficient water in the root zone to avoid stressing the crop while not overfilling the soil and wasting water and leaching nutrients.

Irrigation system maintenance

- check nozzles for size, repair and replace if worn

- use drills to check nozzle wear

- use matched nozzles and flow regulators

- repair & replace worn out sprinklers

- check water pressure and correct if require

- use cans to check sprinkler uniformity, should be within 80%

- for wheel moves repair or replace leaking connector hoses, leaking gaskets and ensure that sprinkler drains seal under pressure

- for pivots repair leaks at pivot and make sure end gun is working correctly

Fertilize and irrigate – if you irrigate then fertilize

Fertilize specific to the crop

- alfalfa – optimize levels of sulphur, boron, phosphorus and potassium

- grass – spend 90% of your fertilizer money on nitrogen

- alfalfa-grass – fertilize as a grass when grass makes up 50% of the stand

- spring forage growth or first hay cut has the highest fertilizer requirement

Critical growth/irrigation periods –seeding, early spring and after each cutting

- seeding – only requires the soil surface to be wetted to germinate the seed followed by frequent shallow irrigations. Requires the irrigation system to be moved every couple of hours. Seedlings and future stand density are dependant on adequate irrigation during this period. Good producing fields are made at this time.

- early spring – most root growth is in the spring. The first harvest has the highest water and fertilizer requirement, temperatures are conducive to maximum growth, providing the best yields/production of the year. First harvest is the largest and should be 3 tons. The first cut may require the most water but some of it comes from the winter moisture in the soil and the actual amount of water required per ton for the first cut is less than the 2^nd or 3^rd cuts. Good spring root development can help plants survive later.

- after each cutting – if available soil moisture is restricted plants will not have sufficient nutrients or water to kick in full growth and yield will be decreased.

- fall – forage crops have significant root growth in fall
- plants in moist soils, going into the winter, experience better winter survival
Field productivity – aim for 5-6 T in the first couple of years, 4T overall average and replace before the field is producing 2.5T per year.

harvest time - pre-bloom, high quality feed, lower yield, dairy quality feed

- 25-50% bloom, good to medium quality feed, higher yield, beef cattle feed

- with alfalfa, one harvest during the year should be done when the field is at least in 25% bloom
Weed control – weeds waste moisture. Often weed control is by replanting the field to get rid of grasses and dandelions or aided by rotating in an annual crop or using herbicides.
Soil moisture checking – irrigation timing is accomplished by balancing soil moisture with crop water use and available irrigation. Requires routine checking of the root zone soil moisture. See section on irrigation scheduling. Similar soils at lower elevations will generally have shorter intervals than higher elevations; require more irrigation water.
Maximum application rates – inches / hour

                    sod                cultivated             usual design
Loamy sand     .65               .35                      .40
Silt loam         .35              .20                      .30
Clay loam        .30                .15                      .20

- water drains down, through the soil, for 1-3 days after the irrigation
Irrigation system evaluation – continuously evaluated through out the season. Systems do not apply water with 100% efficiency or uniformity; usually 70% efficient and 80% uniform.

3. Irrigation Strategies

Irrigation can be withdrawn or reduced following the 1^st cut without significantly reducing stand density or production the following year. The stand usually recovers fully with adequate water the next year.

Plant nutrition and water are linked because plant nutrients are usually near the soil surface, the top 1-2 feet. Plants have most of their feeder roots in this top 1-2 feet and this is the area to dry out first. Plants have roots deeper in the soil, alfalfa 4-6 feet, with the ability to pick up water but because this area lacks nutrients plant growth and yield may suffer. These deeper roots may provide water to keep plants alive during periods of no irrigation, i.e. hay cutting. Irrigation scheduling must take into account these variations.

Alfalfa, grass, hay or pasture; all have different management and harvest regimes

Grass/pasture
- do not start grazing too early, greater than 8 inches of growth, enhances root growth

- graze and rest, do not bite twice, take half leave half

- summer fields with good root growth may stand a period of heavy grazing, but reqrazing regrowth will be weakening.

- Grasses generally have shallower root systems than alfalfa and are more susceptible to dry spells, go dormant faster.
Alfalfa
- twice the rooting depth of grass

- more efficient user of water as deeper rooting depth allows longer
irrigation schedules
New planting and seedling irrigation – when only doing short sets (2 hour), turn system off at night to save 8 hours of water and fertilizer leaching.
Good spring root development - schedule first irrigation to bring soil moisture up to field capacity. Time first irrigation to water in fertilizer and before soil moisture at 1 foot is at 50% available water.May require a half set to wet this top foot. Excessive early irrigation can be detrimental; causes cold soils, inhibits root growth and ties up and leaches nutrients. Wet soils prevent oxygen from entering the soil.
Irrigation system flexibility- whether a system is over or under designed and whether it is irrigating sand or loam soils, affects its flexibility. An under capacity system takes longer to irrigate a field after a hay cut or emergency shut down. An irrigation schedule for a loam can be twice as long as a sand or in the case of an emergency shut down a loam soil can go more days without water than a sand.
Irrigation systems are designed to meetpeak ET requirements. Generally the system applies the same amount of water at each irrigation and only the frequency of irrigation is varied according to the water used. The time may be shortened if a field is being replanted or preceding or following a harvest or emergency shut down.
Cutting and irrigation – schedule last day of irrigation to be 2-3 days before haying on sands and 4-5 days on loams. For making silage dry soils are not as important.

- water depletion in soil after hay cut is near zero, takes about 10 days for alfalfa growth to cover ground and full ET water use resumes. Schedule 1-5 days after cutting as zero water use and after 5 days as full water use

- because cutting delays irrigation, fields usually need water as soon after cutting as possible. Alfalfa is most sensitive to water stress when regrowth begins after cutting. Postponing irrigation reduces yields.

- to facilitate a faster irrigation make irrigation sets shorter for the first trip over the field after a cut. (three 8 hour sets)

Avoiding carbohydrate depletion –Irrigation, harvesting or grazing is built around avoiding depletion of carbohydrate root reserves. Spring or after harvest growth is supported by using stored carbohydrates until the leaf growth and photosynthesis can supply the new growth; approximately when the canopy reaches 8 inches. At this time root carbohydrates have dropped to 15%. They accumulate with plant leaf growth to maximize around full bloom at 35%.
Drought induced boron deficiency – boron is a water soluble plant nutrient, used in trace amounts, which aids in cell division and plant growth. Following a summer or winter drought alfalfa, more than grasses, becomes deficient in boron and new growth is restricted. Early spring irrigation, following a fall/winter drought is important to activating the soil boron.
Drought induced dormancy – where drought has induced dormancy but water is available for a fall irrigation; it should be applied late enough so that it does not cause regrowth for regrowth will deplete root carbohydrates.
Irrigation scheduling – With most of the plant nutrients and 70% of the plant moisture coming from the top two feet of soil, forage producers need a monitoring system to balance soil moisture, to crop use, to their irrigation system, matching their 2 foot soil profile.