WATER QUALITY FOR MICROIRRIGATION

By Farouk A. Hassan, Ph.D

Evaluation of quality of the source water is essential for the design, operation, and maintenance of microirrigation systems. Laboratory analysis provides the necessary information for appropriate evaluation. Neglecting to analyze the water and provide adequate treatments could result in the failure of the microirrigation system to function properly.

Water Quality Analysis

Why is it needed?

A: Irrigation System Considerations

Water to be used for microirrigation should be carefully examined to assess any potential clogging problems. The stream of water carries inorganic particles such as sand, silt and clay as well as organic particles like algae and weed seeds. These particles can block emitter flow passages or settle out in the lateral lines or filters. High loads of suspended particles in the irrigation water may require prefiltration treatment. A settling pond may be used for this purpose where sand and silt size particles are separated by sedimentation. Clear water is then pumped through the filter of the microirrigation system. Larger filtering capacities may also be considered in the design. Fine silt and clay particles which pass through the filter may settle out of the water in the laterals or emitters. Lateral lines should be flushed regularly to remove these sediments.

Bacteria carried with irrigation water can grow within the system producing a mass of slime and may cause iron and sulfur to precipitate out of the water. The slime may bind fine silt and clay particles in aggregates large enough to cause emitter clogging. Algae may also grow within the system and cause clogging problems when washed in the laterals and emitters. Water rich in nitrate encourages microbial growth in the system. Microbial growth can be controlled by chlorine injection on continuous basis to achieve residual free chlorine concentration of 1-2 ppm or on intermittent basis at concentration of 10-20 ppm for 20-30 minutes.

Chemical constituents such as calcium, bicarbonate, iron, manganese, and sulfide commonly found in well water may precipitate and clog emitters. Bicarbonate is common in surface and ground waters. At pH of 7.5 or higher and bicarbonate concentration of 2 milliequivalent per liter (meq/l), the bicarbonate is susceptible to precipitation as calcium carbonate (lime) if comparable levels of calcium are naturally present in the water or if compounds containing calcium are injected in the system. Calcium carbonate precipitate is the most common cause of chemical clogging. The usual treatment of lime precipitation is to acidify the water to lower the pH below 7.0.

Iron and manganese are often present in groundwater in soluble forms and they may precipitate out of the water due to changes in temperature or pressure, in response to a rise in pH, or through the action of bacteria. The result is a massive sludge or slime capable of incapacitating the entire irrigation system. Iron precipitate is of rusty reddish color while manganese precipitate is dark brown or black. These elements may create emitter clogging problems at concentrations as low as 0.1 ppm. At high concentrations of iron and manganese chemical precipitation can be mitigated by water aeration, settling the precipitates in a pond and then clear water is pumped through the system filter. At low concentrations, iron and manganese may be maintained in a soluble state by injecting acid, e.g. hydrochloric acid, in the system to lower the pH of irrigation water. Cost effectiveness may determine which method to be used.

The presence of oil in water will very rapidly block sand media and screen filters, clog emitters, and degrade other system components. Oily water should not be used for microirrigation. An appropriate maintenance program should be implemented right after the installation of the system to secure clear clean water and prohibit the development of clogging material in the system.

B: Soil Considerations

The concentration and composition of dissolved salts in a water supply can affect soil properties, productivity of crops, and determine the suitability of water for irrigation. Under microirrigation high soil moisture content is maintained in the root zone and salts are "pushed" to the outer edges of the wetted volume of the soil. Salts accumulate midway between emitter lines and in the surface layer above buried drip lines. Irrigation with saline water increases the amount of accumulated salts. Because of the absence of deep percolation under microirrigation, there will be practically no vertical leaching of salts unless the system capacity is large enough to supply the crop water requirements plus the leaching requirements necessary for maintaining soil salinity in the root zone within the range of crop tolerance. On the other hand, water of low salt content (electric conductivity, EC < 0.5 mmho/cm) applied to non-saline soils of low to moderate sodium adsorption ratio (SAR) can cause deterioration of soil permeability. Mixing solution grade gypsum into the irrigation water could maintain a favorable permeability.

Bicarbonate is common in natural water. The use of water rich in bicarbonate (usually of pH 8.0 or higher) for irrigation repetitively on the same field could result in developing sodic conditions in the soil. Sodic soils often have restricted infiltration and poor crop vigor. As the moisture content of the soil is reduced by evaporation and transpiration, soluble calcium bicarbonate loses carbon dioxide and precipitates as calcium carbonate (lime). A similar reaction takes place with magnesium bicarbonate. As calcium and magnesium are removed by precipitation sodium becomes the dominant cation in solution. This provides the opportunity for sodium to replace calcium on the surface of the soil particles. In this way high bicarbonate irrigation water can deplete the calcium content of the soil solution and change a calcium dominant soil into a sodium dominant soil (sodic soil). Acid injection is advisable with this water quality to maintain low pH and avoid carbonate precipitation. Irrigation water of high SAR could lead to developing sodic conditions in the soil in a manner similar to that described above specially if water EC is low. Applying solution grade gypsum in the water is recommended to guard against developing soil sodicity.

C: Agronomic Considerations

Continuous use of saline water for irrigation without effective leaching could cause soils salinization and deterioration of crop yield. High SAR in the water, even at low salinity, could be harmful to some crops. Boron is an essential element for plant growth in small amounts and it occurs in water in different anion forms. However, a concentration slightly higher than the optimum requirements is toxic to plants. Crops vary in their sensitivity to excess boron. Chloride is found in most natural waters and at high concentrations it is toxic to some plants. Excess nitrogen in irrigation water (e.g. downstream water) may affect production quality of crops. Leaching, water mixing, crop rotation, and use of salt tolerant crops may need to be considered if the quality of available irrigation water is less than favorable.

D: Common Considerations

The pH of the source water may determine whether or not various dissolved solids present in the water, such as iron or calcium carbonate, will precipitate out to cause emitter clogging. Lower pH of irrigation water enhances the biocide action of chlorine. The pH of water also affects the availability of various plant nutrients in the soil. Acid injection in the system may be necessary in some cases to control the pH of irrigation water.

How often the analysis should be done ?

Analysis of irrigation water, from each source, should be requested every other year. Future testing will alert to changes in water quality that may require matching changes in system operation and maintenance. If future tests indicate a significant increase in iron and manganese concentration in the water, this may require using a settling pond for precipitating and separating these potential clogging elements. Also, a measurable rise in the salinity of water may necessitate increasing the leaching requirements and irrigation set time.

What Are The Sampling Requirements ?

* Samples must be representative: Collect water samples from wells after the pump has been running for at least 30 minutes, and collect samples from streams from running water. Samples collected from reservoirs should be taken near the center and below the water surface. When water sources are subject to seasonal variations in quality, samples should be taken when water quality is at its worst.

* Samples must be of sufficient size: The minimum quantity of water required for chemical analysis is about half a gallon (approximately 2 liters); however in some cases larger quantities may be necessary.

* Use proper containers: Collect samples in clean plastic bottles; bottles should be washed or rinsed at least 3 times prior to use.

* Handle samples appropriately: Water should be analyzed within 3 hours after sampling. If analysis cannot be completed within this period of time, samples should be frozen or held below 40o F until analyzed particularly if nitrate or special "pollution" analysis is required.

What should be included in the analysis?

Water quality analysis for microirrigation should include:

- Electrical conductivity (E.C.), in units of dS/m or mmho/cm, as a measure of total salinity or total dissolved salts (TDS),

- pH (acidity range of 1 to 14), 1 is very acidic, 14 is very alkali, and 7 is neutral,

- Cations: calcium (Ca++), magnesium (Mg++), sodium (Na+), in unite of milliequivalent per liter (meq/l),

- Anions: chloride (Cl-), sulfate (SO4--), carbonate (CO3--), bicarbonate (HCO3-), in unite of meq/l,

- Adjusted sodium adsorption ratio (Adj. SAR) to evaluate the potential of sodium in the water for developing soil sodicity, possible deterioration of soil permeability, and toxicity to plants,

- Iron (Fe), manganese (Mn), hydrogen sulfide (H2S), in parts per million (ppm),

- Suspended solid particles (ppm),

- Bacterial population in count per milliliter (ct/ml),

- Nitrate-nitrogen, NO3-N (ppm),

- Presence of oil (in areas close to oil fields).

How can you tell if the analysis is done right ?

Less than accurate analysis can misleading and may result in serious consequences. The quality of the analysis can be evaluated as follows:

1) The sum of the concentrations of cations (Ca + Mg + Na) should be about equal to the sum of the concentrations of the anions (Cl + CO3 + HCO3 + SO4), all expressed in meq/l . If the sums are exact equals, then one of the constituents was obtained by difference.

2) The sum of the cations and the sum of the anions, expressed in meq/l, should each be near equal to 10 times the electrical conductivity (EC) expressed in mmho/cm or dS/m.

3) A pH of 8.0 or higher is usually associated with a measurable concentration of bicarbonate (HCO3-).

4) If the results of the analysis prove to be unsatisfactory, it should be repeated preferably by a different laboratory.

How to use water analysis

A: Scale of Evaluation

Quality of water for microirrigation systems should be evaluated according to the scale presented in table 1.

Table 1: Water evaluation scale for microirrigation

* TDS (approximately) in ppm = 640 x E.C. (dS/m or mmho/cm)

?Adj. SAR is adjusted sodium adsorption ratio. It is calculated based on the concentrations of sodium (Na), calcium (Ca), magnesium (Mg), carbonate (CO3), and bicarbonate (HCO3) to account for dissolution of CaCO3 from the soil or precipitation of CaCO3 from the water.

Sources: D.A. Bucks, F.S. Nakayama, and R.G. Gilbert. 1979. "Trickle irrigation water quality and preventive maintenance". Agricultural Water Management, Vol. 2:149-62;

F.S. Nakayama and D.A. Bucks. 1991. "Water quality in drip/trickle irrigation: A Review". Irrigation Science, Vol. 12:187-92;

Soil and Plant-Tissue Testing in California, H.M. Reisenauer (editor). 1978. Bulletin 1879, Division of Agricultural Sciences, University of California.

B: Additional Points On Clogging

* Bicarbonate concentrations exceeding 2 meq/liter and pH exceeding 7.5 may cause calcium carbonate precipitation. Continual acid injection in the system may be necessary.

* Irrigation water containing more than 0.1 ppm sulfides may encourage the growth of sulfur bacteria within the irrigation system forming masses of slime which may clog filters and emitters. Chlorination may need to be done continuously.

* High concentrations of sulfide ions can cause iron and manganese precipitation; iron and manganese sulfides are very insoluble even in acid solutions. Frequent acidification and chlorination is advisable.

* Caution should be exercised when chlorination is practiced with water containing manganese due to the fact that there is a time delay between chlorination and the development of a precipitate, i.e. manganese precipitate may form downstream from the filter and cause emitter clogging. Acidification and chlorination may need to be done concurrently and necessary "precautions" should be taken.

C: Water Quality and Fertilizer injection

* Excess nitrogen (N) may affect production quality of certain crops, e.g. sugar beats, citrus, avocados, apricots, grapes and cotton. A concentration of one ppm of nitrate-nitrogen (NO3-N) is equivalent to 2.72 lb. N/acre-foot of water. The amount of nitrogen in irrigation should be taken into account when determining the fertilizer require- ments of crops.

* It is not advisable to inject fertilizer containing calcium, such as calcium nitrate if irrigation water has more than 2 meq/liter of bicarbonate and the pH of the water is more than 7.5. Under these circumstances calcium carbonate may precipitate and cause emitter clogging. Higher temperature promotes the formation of this precipitate.

* Fertilizers containing sulfate, such as ammonium sulfate, if injected in irrigation water that has more than 20-30 meq/liter of calcium, can cause calcium precipitation as calcium sulfate (gypsum).

* Some phosphate fertilizers may precipitate and lose their effectiveness if the calcium concentration in the irrigation water exceeds 2-3 meq/l.

Example

The analysis of three irrigation waters A, B, and C is presented in table 2.

Table 2: Analysis of irrigation waters A, B, and C

Water Quality Evaluation

Reference to the water analysis presented in table 2, the quality of water samples A, B, and C can be evaluated as follows:

Water A:

Low concentration of bicarbonate (HCO3-) and a pH below 7.0 present no risk of clogging due to calcium carbonate precipitation. Iron (Fe) and manganese (Mn) concentrations also do not pose a potential for clogging from the precipitation of these elements. The low level of salinity (low EC) and low Adj. SAR indicate no future deterioration of soil permeability or injury to most crops from long term use of water of this quality. Boron (B), chlorine (Cl-) and nitrate-nitrogen (NO3-N) concentrations would cause no harm to plant growth or the quality of crop.

Water B:

The concentrations of calcium (Ca++ ) and bicarbonate (HCO3-) together with a relatively high pH suggest a potential for calcium carbonate precipitation and emitter clogging particularly if pH rises due to chemical injection. Acid injection on continuous basis may be needed to maintain the pH of the water below 7.0 to guard against precipitate formation. Iron and manganese concentrations also indicate a serious clogging potential. This may require the use of a settling pond for separating iron and manganese oxides prior to pumping the water into the system. Regular pH monitoring of the water flushed out of the laterals is necessary. The salinity of the water is low (low EC) and would not cause any harm to plant growth but the marginal Adj. SAR may trouble sensitive crops. Crop quality may suffer because of the high level of nitrate-nitrogen (NO3-N) if the concentration of this nutrient in the water is not included in the fertility management plan.

Water C:

High pH and high bicarbonate concentration present a severe clogging potential due to calcium carbonate precipitation. Continuous acidification is recommended. Iron and manganese concentrations are fairly low and would not cause any serious clogging; however, it is necessary to monitor the water flushed out of the laterals for any sign of precipitates. The high EC of the water may lead to soil salinity build up and may have a negative effect on sensitive crops; leaching requirements should be added to the irrigation requirements to control salinity. Excess water may need to be applied periodically (e.g. annually with preirrigation) through surface or sprinkler systems to reduce the accumulation of salts in fields under microirrigation if the amount of rainfall is insufficient to leach excess salts. The frequency of periodic leaching depends on the salt tolerance of grown crops under the prevailing conditions. High Adj. SAR presents a serious possibility for developing sodic conditions in the soil and injury to crops. Growing sodium sensitive crops should be avoided. Boron and chlorine concentrations are marginal and may only affect sensitive crops.

The quality of irrigation water affects the performance of micro-irrigation systems. It also affects soil properties and crop yield. Evidently, the evaluation of water quality is necessary for determining the adequate treatments needed for preserving the efficiency of microirrigation and for taking the appropriate measures to maintain the profitability of irrigated agriculture.

Forouk A. Hassan is an irrigation and soils consultant with Agro Industrial Management, Fresno, California.

Phone: (209) 224-1618, Fax: (209) 348-0721, E-mail: weaim@aol.com

This article is part of the micoirrigation maintainance program provided with irrigation systems designed by AIM.