Understanding Irrigation Water Quality

Irrigation water quality is a critical factor in managing salt-affected soils to maintain long term productivity.

The development of salt-affected soils depends upon 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 mitigating 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.

Water quality is relatively inexpensive to assess when costs are calculated on a per acre basis. A complete analysis from a laboratory will range from $30 to $70. Because irrigation water (especially ground water) is less variable over time than soil, sampling every year is unnecessary once the quality of an irrigation water source is determined. Water quality results can also be used to direct soil analysis needs. Potential problems in the soil can be predicted and monitored when first detected in the irrigation water.

In spite of these reasons for assessing irrigation water quality, most Colorado producers do not use this practice. Although 66% of producers reported in a 1997 statewide irrigation survey that they soil sampled, only 7% of respondents reported sampling their irrigation water.

Criteria
Soil scientists use the following categories to describe irrigation water effects on crop production and soil quality:

• Salinity hazard - electrical conductivity (EC) or total dissolved solids (TDS)
• Sodium hazard - expressed as SAR or ESP
• pH and alkalinity - (carbonate and bicarbonate)
• Specific ions: chloride (Cl^-), sulfate (SO₄^2-), 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 parameter on crop productivity is the salinity hazard (as measured by EC). The primary effect of high EC 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 a field may appear wet. Because plants can only transpire “pure” water, usable plant water in the soil solution decreases dramatically as EC increases. The amount of water transpired through a crop is directly related to yield and therefore irrigation water with high EC reduces yield potential (Table 1). Beyond effects on the immediate crop being irrigated, is the long-term impact of salt loading through the irrigation water. Water with an EC of only 1.15 dS/m contains 2,000 pounds of salt for every acre foot of water. You can use conversions e. and f. in Table 2 to make this calculation for other water EC levels.

Other terms used to report salinity hazard are: salts, salinity, electrical conductivity (EC), 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 (See article by Jim Self ). For simplicity, we will use EC for the remainder of this article. Although people frequently confuse 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 (eg. Cl^-, NO₃^-,) and positively charged ions (eg. Ca²⁺, Na⁺). Another common source of confusion is the variety of unit systems used with EC. The preferred unit is deciSiemans per meter (dS/m), however millimhos per centimeter (mmhos/cm) and micromhos per centimeter (µmhos/cm) are still frequently used. Conversions to help you change between unit systems are provided in Table 2.

Sodium hazard
While EC is an assessment of all soluble salts in a sample, sodium is defined separately because of its detrimental effects on soil permeability and tilth. The sodium hazard is defined by an index called the sodium adsorption ratio (SAR). This is the proportion of sodium (Na⁺) to calcium (Ca²⁺) and magnesium (Mg²⁺) ions in a sample. Calcium will flocculate (hold together soil particles), while sodium disperses soil and causes crusting and permeability problems. (The differences between saline and sodium affected soils are explained in the June 1998, Vol. 12, Issue 6. Agronomy News) 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 3.

pH and alkalinity
The acidity or basicity of an 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 pH’s are an uncommon problem, but may cause accelerated equipment corrosion. High pH’s above 8.5 are often caused by high bicarbonate (HCO₃^-) and carbonate (CO₃^2-) concentrations (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 to reflect the increased sodium hazard.

Chloride
Chloride is a common ion in Colorado irrigation waters. Although chloride is essential to plants in low amounts, it can cause toxicity to sensitive crops at high concentrations (Table 3). Like sodium, high chloride concentrations cause more problems when applied with sprinkler irrigation (Table 4). 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.

Boron
Boron is another element that is essential in low amounts, but toxic at higher concentrations (Table 5). In fact, toxicity can occur on sensitive crops at concentrations less than 1.0 ppm. Although some yield response to boron fertilization on alfalfa has been claimed in Colorado, many irrigation waters contain enough B that additional B fertilizer is not required. Because B toxicity can occur at such low concentrations, an irrigation water analysis is advised for ground water before applying additional B to crops.

Sulfate
The sulfate ion is a major contributor to EC in many of Colorado irrigation waters. However, toxicity usually is not an issue, except at very high concentrations where high sulfate can 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 on most crops. Exceptions are sandy fields with < 1% organic matter and < 10 ppm SO₄2-S in irrigation water.

Nitrogen
The nitrate ion often occurs at higher concentrations than ammonium in irrigation water. Nitrogen in irrigation water (N) is largely a fertility issue and nitrate-nitrogen (NO₃-N) can be a significant N source in the S. Platte, San Luis Valley, and parts of the Arkansas River basins. 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 whenever the concentration exceeds 10 ppm NO₃-N. Table 2 provides conversions from ppm to pounds per acre inch.

In many areas of Colorado, irrigation water quality can influence crop productivity more than soil fertility, hybrid, weed control and other factors that receive much more attention. Farm managers should be encouraged to get a chemical analysis of their irrigation sources. This basic knowledge is essential in making the right decisions for their irrigated production.

Table 1. Potential Yield reduction from saline water for selected irrigated crops.¹

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.
² EC = electrical conductivity of the irrigation water in dS/m at 25^oC.
³ Sensitive during germination. EC_e should not exceed 3 dS/m for garden beets and sugar beets.

by Troy Bauder
Extension Water Quality Specialist