Irrigation Water Management

Irrigation Efficiency & Distribution Uniformity

On-farm efficiency is generally lower than basin-wide efficiencies, because within a basin, the water lost or wasted on one farm can be reused by another farm. For example, surface runoff from a farm may not be wasted. It may be captured directly for use by another farm, or run back to a river or surface water source, where it can be reused by another farm, or it may move downward through soil strata to an aquifer, from which it can be pumped for use on another farm. Thus, even if the on-farm efficiency for the first farm is low because of the uncollected runoff, it may be higher for a group of farms or a watershed basin.

Still, there are plenty of reasons to worry about on-farm Irrigation Efficiency, and to try to improve it if possible:

• Water quality
  • Usually degraded with reuse
• Energy
  • Pumping water more than once wastes energy
• Fertilizer
  • If applied with irrigation water, fertilizer is wasted when the water is
  • If applied "dry" it may be leached by excess water applications
• Chemicals
  • If applied with irrigation water, agricultural chemicals are wasted when the water is
  • If applied "dry,"
    • the chemicals may be washed away by excess water applications
    • the chemicals may suffer reduced effectiveness when water is over applied
• Capacity
  • Reservoirs, pumps, and valves, all must be over-sized to handle the excess water for an inefficient system
• Drainage
  • In areas where natural subsurface drainage won't suffice, the artificial drainage systems required to handle excess deep percolation must be oversized to handle the extra water

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Beneficial Use

Irrigation water that serves an agronomic purpose, that benefits the crop being grown, is called a beneficial use. Note that for the special purpose of defining irrigation efficiency (or application efficiency), 'beneficial' is limit in scope to the crop being grown. Other uses might serve some useful societal purpose, but unless it serves the crop being grown, it is not called beneficial.

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Evapotranspiration (ET)

Probably the most significant beneficial use of irrigation water is for crop evapotranspiration (ET). ET is the sum of plant Transpiration (T) and Evaporation (E) from wet plant and soil surfaces.

Usually, irrigation water infiltrated into the soil is absorbed by the plant roots, taken into the plant, and transpires from the leaves as a result of the process of photosynthesis. Some water 'substitutes' for transpiration as it evaporates from soil or plant surfaces because it cools the surrounding air mass, and thereby reduces the transpiration that would otherwise take place.

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Leaching Requirement (LR)

The second most significant beneficial use of irrigation water is water that percolates through and below the root zone and washes away (leaches) salts that have accumulated in the root zone that may be harmful to the plant.

All irrigation water contains at least some amount of salt. As more and more irrigation water is added to the field, more and more salt is added as well. Plants take up essentially pure water, leaving the salts behind. If that salt is not washed away, it will eventually accumulate to the point that crop production suffers.

To remove this salt from the crop root zone, more water is added to the soil than the plant itself needs. The extra water flushes the salts below the root zone. The amount of water necessary to maintain a proper salt balance in the crop root zone is called the Leaching Requirement (LR). LR is usually expressed as a fraction: that fraction of the applied irrigation water that is required for leaching purposes.

If more irrigation water than LR goes below the root zone, it is called 'deep percolation' and is considered non-beneficial.

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Other Beneficial Uses

Other beneficial uses include water used to achieve some agronomic purpose or to support non-crop plants that achieve some agronomic purpose. Examples include

• climate control
  • crop cooling (protection from heat)
  • crop cooling (prolong dormancy)
  • frost protection
• softening the soil to aid germination
• water to germinate weeds seeds as part of a program to make cultivation more effective
• for ET of windbreaks
• for ET of cover crops when the purpose is
  • to provide habitat for beneficial insects
  • enhance crop quality (prevent fruit burn)

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Non-Beneficial Uses

Not all uses of irrigation water are beneficial

• Runoff that is not collected for reuse
• Excess deep percolation (more than LR)
• Weed/phreatophyte ET
• Evapotranspiration from water in canals or reservoirs
• Canal seepage or spill over

Note that excess deep percolation is a major problem, due in part to

• Non-uniform application of the water
• Poor "timing" [irrigating too soon and/or too long]

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Irrigation Efficiency (IE)

IW here means irrigation water. Note that we only count the Irrigation Water (IW) applied and the uses of Irrigation Water (IW). If there is some rainfall, the rainfall amount is not considered part of the total, nor is any rainfall that happens to go to a beneficial use (crop ET for example) counted in the numerator. The change in Irrigation Water stored in the root zone can be significant over short time periods, but is usually assumed to be zero on an annual basis.

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Application Efficiency (AE)

Technically, Irrigation Efficiency (IE) refers to an annual evaluation or assessment of Irrigation Efficiency, or a time period long enough that the ultimate fate of water stored in the root zone is known (was it used by the crop, or did it become deep percolation as the result of an poorly-timed irrigation later in the season?).

For individual irrigation events, we use the term Application Efficiency (AE).

The 'target' here refers to the management goals for the irrigation, such as 'replenish the soil moisture deficit plus additional water for leaching requirement.'

Example 1 How many inches of the applied water are "beneficially used" if Application Efficiency is 75%? The water supply delivers 10 cfs to a 40 acre field for 24 hours. 1 cfs = 1 ac-in/hr approximately 10 cfs = 10 ac-in/hr volume = rate x time = 10 ac-in/hr x 24 hr = 240 ac-in depth = volume/area = 240 ac-in/40 ac = 6 inches So far, we are talking about water that was delivered. So this is a Gross amount. To find the net amount or the amount beneficially used, use the expression relating gross, net and efficiency Eff. = (Net / Gross ) x 100 Net = Gross x AE/100      = 6 in (gross) x 75/100 = 4.5 inches (net)

Example 2 Water is delivered to a 40 acre field at 10 cfs. AE = 75%. How long must the water be brought in for to deliver a net benefit of 8 inches to the crop? time = volume/rate The rate is 10 cfs = 10 ac-in/hr. So all we must do is compute the amount, which in this case is the gross amount delivered by the pump. Gross = Net/[AE/100] = 8 in/[0.75] = 10.7 inches gross 10.7 in x 40 acres = 428 ac-in gross time = volume/rate = 428 ac-in/10 ac-in/hr = 42.8 hours

Example 3 Over the course of the year, an irrigation system applies a gross amount of 48 inches of water. An evaluation determines that: 34 inches goes to satisfy crop evapotranspiration ET crop; 8 inches runs off and is not collected for reuse; 5 inches passes through the root zone as deep percolation (total deep percolation), of which 3 inches is beneficial leaching for salt removal; and 1 inch is ET from weeds. What is the irrigation efficiency (%)? Irrigation Efficiency(%) = 100 x [beneficial IW]/[Total IW]      = 100 x [ET crop + Leaching]/[Total IW]      = 100 x [34 + 3]/[48] = 100 x 37/48 = 77%

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