Salt loading from irrigation results from water percolating through soils of marine origin carrying dissolved salt to the river system. The amount of salt picked up in this manner is assumed to be dependant only on the amount of water that returns to the river and the soil through which it has percolated. It has long been assumed that the salt concentration of return flow is relatively constant, depending only on the solubility of the salt in the water. An essentially infinite source of salt is available in the soil.
Irrigation contributes to salt loading when relatively fresh water is diverted from a stream, applied to the soil, and a fraction of the applied water percolates through the soil and returns to the river with its load of freshly dissolved salt.
Of the total amount of water applied to the soil, part is used by the crop for its biological needs (CU, consumptive use), part may evaporate or run-off back to the river and the balance percolates below the root zone (deep percolation) carrying dissolved salt. Part of the deep percolation may be consumed by deep-rooted plants on its way to the river; the balance (return flow) carries dissolved salt into the river.
Application efficiency is CU ÷ the total amount of water applied. As irrigation systems improve, application efficiency also improves. See figure 1. Deep percolation/salt loading can be reduced dramatically by installing and using higher efficiency irrigation systems.
Distribution Uniformity (DU)
Distribution Uniformity is a numerical indication of the uniformity of water application on a field. A gentle, steady rain would be expected to be very uniform, while dumping a truckload of water in the center of a field would have very poor distribution and uniformity.
Uniformity for a sprinkler system is simple to measure, but less so for flood or drip irrigation systems. Distribution Uniformity for sprinkler systems is measured by:
- Placing an array of rain gauges (catch cans) in a field, typically on a 10 foot by 10 foot grid
- The system is turned on for a measured amount of time
- The depth of water collected in each rain guage is recorded and analyzed
- DU = the average of the lowest quarter of the readings ÷ the average of all readings, expressed as a percentage
The implication is that in order to apply adequate water to the driest areas, the wettest areas must be over-irrigated somewhat, resulting in some deep percolation. See figure 2. In addition, a small amount of deep percolation is required on an annual basis to leach excess salt from the root zone (leaching fraction).
The Christiansen Coefficient of Uniformity (CCU) uses a different mathematical approach for analyzing catch can data.
CCU = the sum of deviations from average, of each reading. Sprinkler system uniformity is a function of system design and proper maintenance. Uniformity is adversely affected by:
- Water pressures outside the design range for the sprinklers
- Nozzle plugging or wear
- Other equipment malfunctions
Any given system can have very good DU, and still have poor application efficiency. Conversely, a poorly operated system can have poor distribution and high efficiency, while severely under-irrigating the crop. See figures 3 and 4.
|Colorado River Basin Salinity Control Program - Uniformity/Efficiency Standard|
|Minimum CCU||Minimum DU||Minimum Efficiency|
Irrigation Water Management (IWM)
To minimize salt in the river system and maximize crop production requires careful management of irrigation water application. Effective IWM is putting the right amount of water at the right place at the right time. To effectively irrigate, one must be able to:
1. Determine how much water is needed. This can be done using CU tables or near real-time weather calculations
2. Determine how much water the soil can absorb and store using soil evaluations and soil moisture monitoring with manual or electronic methods
3. Measure the amount of water applied by meter, weir, calculation, etc.
4. Apply water using an efficient irrigation system with adequate DU designed in
5. Maintain irrigation equipment in good working order
6. Keep good irrigation records
7. Evaluate performance
Water Tables and Salt Contamination
Another serious result of inefficient irrigation is high water tables and associated salt contamination of the soil. Visible salt on the soil surface is generally associated with a water table very near ground level. As water evaporates, salt is concentrated at the point of evaporation.
While not the only cause, the elevation of the water table can be greatly increased by inefficient irrigation, adding water to the soil faster that it can drain away. Frequently, improvement in application efficiency results in lowering the water table and reducing salt damage to crops.
Improving irrigation systems and operational procedures results in increased crop production and operating efficiency. In addition, large reductions in salt loading to the river system are realized. Both the landowner and the environment benefit.
The simplest form of delivery is to divert water from the stream, using a bank cut-out or canvass or earthen dike. There is little or no control or measurement of flow. DU and efficiency are poor. Deep percolation is uncontrolled. Potential enhancements include:
1. Permanent water control structures with head gates
2. Water measurement systems
3. Careful water management including record keeping
4. Corrugations and furrows
5. Land leveling
6. Gated pipe
7. Surge systems allowing higher flow rates over shorter periods of time
8. Border systems which assure no runoff
Sprinkler systems allow improved distribution and efficiency with less surface disturbance, since mild slopes and topography do not adversely affect their performance. To maximize performance, careful management and maintenance are required. Sprinklers are a good compromise between installation cost and effectiveness and are the practice of choice for most salinity control projects. Where high value crops are produced, drip and micro-spray systems can improve efficiency beyond that of typical sprinkler systems. High tech modifications can be made to center pivots that also enhance DU and efficiency.
Examples of existing irrigation systems can be viewed in the Irrigation Systems Gallery of this website. As improved irrigation systems are installed, additional photos will be added to the gallery.