no. 0.506 |
Irrigation Water Quality Criteria
by T.A. Bauder, R.M. Waskom and J. G. Davis1Quick Facts...
- Knowledge of irrigation water quality is critical to understanding management for long-term productivity.
- Water with electrical conductivity (ECw) of only 1.15 dS/m contains approximately 2,000 pounds of salt for every acre foot of water.
- In many areas of Colorado, irrigation water quality can influence crop productivity more than soil fertility, hybrid, weed control and other factors.
Corn plant damaged by saline sprinkler water. |
Salt-affected soils develop from a wide range of factors including: soil type, field slope and drainage, irrigation system type and management, fertilizer and manuring practices, and other soil and water management practices. In Colorado, perhaps the most critical factor in predicting, managing, and reducing salt-affected soils is the quality of irrigation water being used. Besides affecting crop yield and soil physical conditions, irrigation water quality can affect fertility needs, irrigation system performance and longevity, and how the water can be applied. Therefore, knowledge of irrigation water quality is critical to understanding what management changes are necessary for long-term productivity.
Irrigation Water Quality Criteria
Soil scientists use the following categories to describe irrigation water effects on crop production and soil quality:
- Salinity hazard - total soluble salt content
- Sodium hazard - relative proportion of sodium (Na+)
to calcium (Ca2+) and magnesium (Mg2+)
ions
- pH
- Alkalinity - carbonate and bicarbonate
- Specific ions: chloride (Cl), sulfate (SO42-), boron (B),
and nitrate-nitrogen (NO3-N).
Other potential irrigation water contaminants that may affect suitability for agricultural use include heavy metals and microbial contaminants.
Salinity Hazard
The most influential water quality guideline on crop productivity is the water salinity hazard as measured by electrical conductivity (ECw). The primary effect of high ECw water on crop productivity is the inability of the plant to compete with ions in the soil solution for water (physiological drought). The higher the EC, the less water is available to plants, even though the soil may appear wet. Because plants can only transpire "pure" water, usable plant water in the soil solution decreases dramatically as EC increases.
Table 1. Suggested criteria for irrigation water use based upon conductivity. | |
Classes of water | Electrical Conductivity |
(dS/m)* | |
Class 1, Excellent | ≤0.25 |
Class 2, Good | 0.25 - 0.75 |
Class 3, Permissible1 | 0.76 - 2.00 |
Class 4, Doubtful2 | 2.01 - 3.00 |
Class 5, Unsuitable2 | ≥3.00 |
*dS/m at 25ºC = mmhos/cm 1Leaching needed if used. 2Good drainage needed and sensitive plants will have difficulty obtaining stands. |
Table 2. Potential yield reduction from saline water for selected irrigated crops.1 | ||||
% yield reduction
|
||||
Crop |
0%
|
10%
|
25%
|
50%
|
ECw2
|
||||
Barley |
5.3
|
6.7
|
8.7
|
12
|
Wheat |
4.0
|
4.9
|
6.4
|
8.7
|
Sugarbeet3 |
4.7
|
5.8
|
7.5
|
10
|
Alfalfa |
1.3
|
2.2
|
3.6
|
5.9
|
Potato |
1.1
|
1.7
|
2.5
|
3.9
|
Corn (grain) |
1.1
|
1.7
|
2.5
|
3.9
|
Corn (silage) |
1.2
|
2.1
|
3.5
|
5.7
|
Onion |
0.8
|
1.2
|
1.8
|
2.9
|
Beans |
0.7
|
1.0
|
1.5
|
2.4
|
1Adapted from Quality
of Water for Irrigation. R.S. Ayers. Jour. of the Irrig. and
Drain. Div., ASCE. Vol 103, No. IR2, June 1977, p. 140. 2ECw = electrical conductivity of the irrigation water in dS/m at 25oC. 3Sensitive during germination. ECw should not exceed 3 dS/m for garden beets and sugarbeets. |
The amount of water transpired through a crop is directly related to yield; therefore, irrigation water with high ECw reduces yield potential (Table 2). Beyond effects on the immediate crop is the long term impact of salt loading through the irrigation water. Water with an ECw of only 1.15 dS/m contains approximately 2,000 pounds of salt for every acre foot of water. You can use conversion factors in Table 3 to make this calculation for other water EC levels.
Table 3. Conversion factors for irrigation water quality laboratory reports. | |||
Component |
To Convert
|
Multiply By
|
To Obtain
|
Water nutrient or TDS |
mg/L
|
1.0
|
ppm
|
Water salinity hazard |
1 dS/m
|
1.0
|
1 mmho/cm
|
Water salinity hazard |
1 mmho/cm
|
1,000
|
1 µmho/cm
|
Water salinity hazard |
ECw (dS/m) for ECw <5
dS/m
|
640
|
TDS (mg/L)
|
Water salinity hazard |
EC (dS/m) for EC >5 dS/m
|
800
|
TDS (mg/L)
|
Water NO3N, SO4-S, B |
ppm
|
0.23
|
lb per acre inch of water applied
|
Irrigation water |
acre inch
|
27,150
|
gallons of water
|
Definitions |
|
Abbrev. | Meaning |
mg/L | milligrams per liter |
meq/L | milliequivalents per liter |
ppm | parts per million |
dS/m | deciSiemens per meter |
µS/cm | microSiemens per centimeter |
mmho/cm | millimhos per centimeter |
TDS | total dissolved solids |
Other terms that laboratories and literature sources use to report salinity
hazard are: salts, salinity, electrical conductivity (ECw), or total dissolved
solids (TDS). These terms are all comparable and all quantify the amount
of dissolved salts (or ions, charged particles) in a water
sample. However, TDS is a direct measurement of dissolved ions and EC
is an indirect measurement of ions by an electrode.
Although people frequently confuse the term salinity with common table salt or sodium chloride (NaCl), EC measures salinity from all the ions dissolved in a sample. This includes negatively charged ions (e.g., Cl¯, NO¯3) and positively charged ions (e.g., Ca++, Na+). Another common source of confusion is the variety of unit systems used with ECw. The preferred unit is deciSiemens per meter (dS/m), however millimhos per centimeter (mmho/cm) and micromhos per centimeter (µmho/cm) are still frequently used. Conversions to help you change between unit systems are provided in Table 3.
Sodium Hazard
While ECw is an assessment of all soluble salts in a sample, sodium hazard is defined separately because of sodium's specific detrimental effects on soil physical properties. The sodium hazard is typically expressed as the sodium adsorption ratio (SAR). This index quantifies the proportion of sodium (Na+) to calcium (Ca++) and magnesium (Mg++) ions in a sample. Calcium will flocculate (hold together), while sodium disperses (pushes apart) soil particles. This dispersed soil will readily crust and have water infiltration and permeability problems. General classifications of irrigation water based upon SAR values are presented in Table 4.
meq/L = mg/L divided by atomic weight of ion divided by ionic charge (Na+=23.0 mg/meq, Ca++=20.0 mg/meq, Mg++=12.15 mg/meq) |
Table 4. General classification of water sodium hazard based on SAR values. | ||
SAR values | Sodium hazard of water | Comments |
1-9 | Low | Use on sodium sensitive crops must be cautioned. |
10-17 | Medium | Amendments (such as gypsum) and leaching needed. |
18-25 | High | Generally unsuitable for continuous use. |
≥26 | Very High | Generally unsuitable for use. |
However, many factors including soil texture, organic matter, crop type, climate, irrigation system and management impact how sodium in irrigation water affects soils. Additionally, at the same SAR, water with low ECw (salinity) has a greater dispersion potential than water with high ECw. Sodium in irrigation water can also cause toxicity problems for some crops, especially when sprinkler applied. Crops vary in their susceptibility to this type of damage as shown in Table 5.
Table 5. Susceptibility ranges for crops to foliar injury from saline sprinkler water. | ||||
Na or Cl concentration (mg/L) causing foliar
injury
|
||||
Na concentration | <46 | 46-230 | 231-460 | >460 |
Cl concentration | <175 | 175-350 | 351-700 | >700 |
Apricot | Pepper | Alfalfa | Sugarbeet | |
Plum | Potato | Barley | Sunflower | |
Tomato | Corn | Sorghum | ||
Foliar injury is influenced by cultural and environmental conditions. These data are presented only as general guidelines for daytime irrigation. Source: Mass (1990) Crop salt tolerance. In: Agricultural Assessment and Management Manual. K.K. Tanji (ed.). ASCE, New York. pp. 262-304. |
pH and Alkalinity
The acidity or basicity of irrigation water is expressed as pH (< 7.0 acidic; > 7.0 basic). The normal pH range for irrigation water is from 6.5 to 8.4. Abnormally low pHs are not common in Colorado, but may cause accelerated irrigation system corrosion where they occur. High pHs above 8.5 are often caused by high bicarbonate (HCO3-) and carbonate (CO32- ) concentrations, known as alkalinity. High carbonates cause calcium and magnesium ions to form insoluble minerals leaving sodium as the dominant ion in solution. This alkaline water could intensify sodic soil conditions. In these cases, a lab will calculate an adjusted SAR (SARADJ)to reflect the increased sodium hazard.
Chloride
Chloride is a common ion in Colorado irrigation waters. Although chloride is essential to plants in very low amounts, it can cause toxicity to sensitive crops at high concentrations (Table 6). Like sodium, high chloride concen-trations cause more problems when applied with sprinkler irrigation (Table 6). Leaf burn under sprinkler from both sodium and chloride can be reduced by night time irrigation or application on cool, cloudy days. Drop nozzles and drag hoses are also recommended when applying any saline irrigation water through a sprinkler system to avoid direct contact with leaf surfaces.
Table 6. Chloride classification of irrigation water. | |
Chloride (ppm) | Effect on Crops |
Below 70 | Generally safe for all plants. |
70-140 | Sensitive plants show injury. |
141-350 | Moderately tolerant plants show injury. |
Above 350 | Can cause severe problems. |
Chloride tolerance of selected crops. Listing in order of increasing tolerance: (low tolerance) dry bean, onion, carrot, lettuce, pepper, corn, potato, alfalfa, sudangrass, zucchini squash, wheat, sorghum, sugar beet, barley (high tolerance). Source: Mass (1990) Crop Salt Tolerance. Agricultural Salinity Assessment and Management Manual. K.K. Tanji (ed.). ASCE, New York. pp 262-304. |
Boron
Boron is another element that is essential in low amounts, but toxic at higher concentrations (Table 7). In fact, toxicity can occur on sensitive crops at concentrations less than 1.0 ppm. Colorado soils and irrigation waters contain enough B that additional B fertilizer is not required in most situations. Because B toxicity can occur at such low concentrations, an irrigation water analysis is advised for ground water before applying additional B to crops.
Table 7. Boron sensitivity of selected Colorado plants (B concentration, mg/ L*) | ||||
Sensitive
|
Moderately Sensitive
1.1-2.0
|
Moderately Tolerant
2.1-4.0
|
Tolerant
4.1-6.0
|
|
0.5-0.75
|
0.76-1.0 | |||
Peach
|
Wheat
|
Carrot
|
Lettuce
|
Alfalfa
|
Onion
|
Barley
|
Potato
|
Cabbage
|
Sugar beet
|
Sunflower
|
Cucumber
|
Corn
|
Tomato
|
|
Dry Bean
|
Oats
|
|||
Source: Mass (1987) Salt
tolerance of plants. CRC Handbook of Plant Science in Agriculture.
B.R. Cristie (ed.). CRC Press Inc. *Maximum concentrations tolerated in soil water or saturation extract without yield or vegetative growth reductions. Maximum concentrations in the irrigation water are approximately equal to these values or slightly less. |
Sulfate
The sulfate ion is a major contributor to salinity in many of Colorado irrigation waters. However, toxicity is rarely a problem, except at very high concentrations where high sulfate may interfere with uptake of other nutrients. As with boron, sulfate in irrigation water has fertility benefits, and irrigation water in Colorado often has enough sulfate for maximum production for most crops. Exceptions are sandy fields with <1 percent organic matter and <10 ppm SO4-S in irrigation water.
Nitrogen
Nitrogen in irrigation water (N) is largely a fertility issue, and nitrate-nitrogen
(NO3-N) can be a significant N source
in the South Platte, San Luis Valley, and parts of the Arkansas River
basins. The nitrate ion often occurs at higher concentrations than ammonium
in irrigation water. Waters high in N can cause quality problems in crops
such as barley and sugar beets and excessive vegetative growth in some
vegetables. However, these problems can usually be overcome by good fertilizer
and irrigation management. Regardless of the crop, nitrate should be credited
toward the fertilizer rate especially when the concentration exceeds 10
ppm NO3-N (45 ppm NO3¯).
Table 3 provides conversions from ppm to pounds per acre inch.
1 T.A. Bauder, Colorado State University Extension water quality specialist; R.M. Waskom, Extension water resource specialist; and J.G. Davis, Extension soils specialist and professor, soil and crop sciences. 7/03. Revised 3/07.
Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating. CSU Extension programs are available to all without discrimination. No endorsement of products mentioned is intended nor is criticism implied of products not mentioned.
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