Some Information about Irrigation

Dave Bjorneberg

Irrigation is important for crop production in Idaho and the United States.  According to the 1997 Agriculture Census, 13% of the total cropland in the United States is irrigated, but irrigated land produces almost 40% of the crop value.  The impact of irrigation is even greater in Idaho.  Approximately 55% of the state's cropland is irrigated and this land produces almost 80% of the total crop value.  Few crops are grown in southern Idaho without irrigation (90% of the cropland in Twin Falls County is irrigated).

About half of the irrigated land in the United States is surface irrigated while 45% is irrigated by sprinklers and 5% irrigated by drip or other irrigation methods.  In Idaho, 67% of the irrigated land is sprinkler irrigated and 31% is surface irrigated.  Table 1 shows irrigated land area for the top seven irrigated states.  All other states have less than 200,000 acres of irrigated land.

Table 1.  Irrigated Area by State and Method
Irrigated Area (acres)
State Total Sprinkler Surface Drip
California 8,140,000 1,528,000 5,820,000 1,022,000
Nebraska 5,692,000 3,686,000 2,020,000 -
Texas 5,236,000 3,196,000 2,015,000 55,400
Arkansas 4,043,000 579,000 3,495,000 480
Idaho 3,188,000 2,187,000 992,000 1,080
Colorado 2,942,000 1,290,000 1,664,000 -
Kansas 2,650,000 2,054,000 614,000 830


There are tradeoffs between various irrigation systems (Table 2).  Surface irrigation has lower initial costs but requires more labor.  Sprinkler irrigation, especially a center pivot, generally requires less labor but costs more to install and operate.  Drip irrigation can be automated to greatly reduce labor and carefully manage water, but this increases initial costs and requires operators to have more technical skills.  Some typical disadvantages of each type of irrigation are erosion with surface irrigation, wind loss with sprinkler irrigation, and plugged emitters with drip irrigation.

Table 2.  Relative Irrigation System Costs
System Type Initial
Cost		 Operating
Cost		 Labor
Requirement
surface moderate low high
sprinkler - hand or wheel move moderate moderate moderate
sprinkler - center pivot, solid set high moderate low
drip high moderate low


Soil texture is also a factor in choosing irrigation systems.  Soil texture refers to the amount of sand, silt and clay particles in the soil.  Sand is the largest particle (<0.05 mm) and clay is the smallest (>0.002 mm).  As a reference, consider sand to be the size of a beachball, silt the size of a golf ball, and clay the size of a BB.

Soil texture affects how we irrigate soil.  A sandy soil allows water to infiltrate faster than a clay soil so water can be applied faster.  However, sandy soil holds less than 1 cup of water per gallon of soil so it must be irrigated more frequently than a silt loam that can hold 3 cups of water per gallon of soil.  (Loamy soils contain uniform amounts of sand, silt and clay.)

Application efficiency is another important characteristic of irrigation systems.  Drip irrigation tends to have greater application efficiency than surface or sprinkler irrigation.  Application efficiency can be greater than 95% with buried drip irrigation because all of the water is applied in the root zone.  Center pivots, on the other hand, tend to have application efficiencies of 75 to 85% compared to 30 to 50% for surface irrigation.

Application efficiency refers to the percent of applied water that is stored in the crop root zone.  For example, a 75% application efficiency means for every gallon of water applied only 0.75 gal. is stored in the soil for plants to use.  The remaining 0.25 gal. of water is not used by the crop, but is not necessarily wasted.  Runoff from a surface irrigated field is often applied to another field downstream.  Water that leaches through the soil can recharge the aquifers.  Much of the groundwater in southern Idaho is the result of "inefficient" irrigation in the past.  Without this recharge, the water table will decline and spring flows with decrease.  Our concern, however, is sediment, nutrients or contaminant that move with this recharge water.

References

USDA - ARS - Pacific West Area
Northwest Irrigation and Soils Research Laboratory
3793 N. 3600 E.,  Kimberly, ID  83341-5076
Voice: 208-423-6521  Fax: 208-423-6555  E-mail: bdavid@kimberly.ars.pn.usbr.gov

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