⇦ Back to Water Resource Management Home
¶ Introduction
There are a number of different calculations and methods of interpreting water analysis results for predicting the suitability of a specific water for long-term irrigation use. Each set of interpretations depends on a given set of assumptions, including irrigation type, crop to be irrigated, soil type to be irrigated, etc.
Figure 1 is a diagram that illustrates the system used by Servi-Tech Laboratories to interpret irrigation water quality for agronomic field crops. It shows a graphical representation of the computer code used to generate interpretations found on laboratory reports.
The two primary criteria used to estimate water quality are the electrical conductivity and the adjusted sodium adsorption ratio of the irrigation water (ECiw and SARadj). We assume that irrigation management will be or has remained fairly constant, so the ECiw and the SARadj will reach an equilibrium steady-state with the irrigated soil, as measured by the soil extract electrical conductivity, the soil SAR, and the exchangeable sodium percentage (ECe, SARsoil, and % Na).
Increasing electrical conductivity values represent an increasing salinity hazard to the plants or crops due to salt accumulation in the irrigated soil. The final degree of potential yield loss or crop damage will depend on the relative salt tolerance of the plant species, irrigation type, irrigation amount, irrigation timing, weather, soil drainage, and other factors.
Increasing SARadj values represent an increasing permeability hazard to the irrigated soil due to the accumulated sodium. We assume the sodium that accumulates on the clay exchange surfaces will result in deflocculation (i.e., loss of soil “granulation”), dispersion, loss of soil pores, crusting, sealing, etc. The exact degree of potential problems will depend on the soil clay content, organic matter, clay type, and overall management.
The ECiw and SARadj values are used to place the water into general quality categories ranging from “Excellent” to “Very Poor”. These categories are intended to predict the combined impact of the water on both the soil and crop that are to be irrigated.
The chloride and boron concentrations may indicate additional limitations to water quality beyond the limitations that might be present due to salinity and/or sodium accumulations. The final impact of any irrigation water will depend on many unforseen factors so the best use of the interpretations would be to compare water sources or to plan management strategies.
The interpretations in Figure 1 assume the following assumptions:
- agronomic crops with moderate salt tolerance
- medium-textured soils, C mixed clay mineralogy
- medium organic matter levels, and
- 75% to 90% irrigation application efficiency
Interpretations will differ for other irrigation systems, like drip irrigation, nursery crops, salt-tolerant or salt-sensitive crops, etc.
The chloride interpretations are the projected impact for irrigation water that will be applied through overhead sprinklers. Chloride can penetrate through the leaf cuticle and accumulate in the foliage. Leaf damage or toxicity may occur from the chloride that accumulates to excess. The chloride interpretations are not appropriate for surface-irrigation systems.
The boron interpretations are the potential toxicity from boron that may accumulate in the soil being irrigated. Toxicity may result from excess boron accumulating in the plant tissue following uptake by the plant root. Different crops and different varieties may respond differently to soil boron accumulations.
More specific information is found in the preceding Crop files of this series.
¶ Figure 1. Irrigation Water Suitability Classifications - General Agronomic Quality
¶ Table 1. Irrigation Water Chloride Interpretations for Sprinkler Irrigation |
||
| Chloride, Cl mg/L | Rating* | Interpretation |
| 0 - 35 | Very low | No foliar injury expected. |
| 35 - 70 | Low | Not likely to cause foliar injury, unless crop or plant species are very sensitive. |
| 70 - 150 | Medium | Acceptable for most crops and plants. Foliar injury may occur when applied to sensitive plant types (e.g., certain tree species, ornamentals, etc.). |
| 150 - 350 | High | Likely to cause foliar injury when applied to moderately sensitive crops. Foliar damage expected with sensitive plant types. |
| 350 - 550 | Very high | May cause foliar injury when applied to moderately tolerant crops. Foliar injury may be severe in sensitive and moderately sensitive plant types. |
| 550 - 700 | Extremely high | May cause foliar injury when applied to tolerant crops. Foliar injury may be severe with moderately tolerant plant types. Not advised for use with sensitive or moderately sensitive plant types. |
| over 700 | Not advised | Likely to cause foliar injury to many crops and commercially grown plants. |
| *Chloride injury problems may be more common during high temperature and low humidity conditions. | ||
¶ Table 2. Irrigation Water Boron Interpretations |
||
| Boron, B mg/L | Rating | Interpretation |
| less than 0.5 | Very low | Considered suitable for all field crops. |
| 0.5 - 0.75 | Low | Suitable for most field crops. |
| 0.75 - 1.0 | Medium | Slight to moderate injury may occur in many field crops. |
| 1.0 - 2.0 | High | Moderate to severe injury hazard for many field crops is expected. |
| 2.0 - 6.0 | Very high | Presents severe injury hazard for many field crops. |
| over 6.0 | Extremely high | May cause severe injury to most field crops. |