Water Quality for Agriculture a Primer

8/13/2019 Water Quality for Agriculture a Primer

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IRRIGATION WATER QUALITY STANDARDS AND SALINITY

MANAGEMENT

Nearly all waters contain dissolved salts and trace elements, many of which result

from the natural weathering of the earth's surface. In addition drainage waters from

irrigated lands and effluent from city sewage and industrial waste water can impactwater quality. This paper discusses water quality considerations for irrigation. In

most situations, the primary concern is salinity levels, since salts can affect boththe soil structure and crop yield. However, a number of trace elements are found in

water which can limit its use for irrigation.

Generally, salt is thought of as ordinary table salt !sodium chloride". However

any types of salts e#ist and are commonly found in waters. $ost salinity problems

in agriculture result directly from the salts carried in the irrigation water %s water

evaporates, the dissolved salts remain, resulting in a solution with a higher

concentration of salt. The same process occurs in soils. &alts as well as other

dissolved substances begin to accumulate as water evaporates from the surface andas crops withdraw water.

Water Analysis: Units, Terms and Samplin

Numerous parameters are used to define irrigation water quality, to assess salinity

haards, and to determine appropriate management strategies. % complete waterquality analysis will include the determination of(

). the total concentration of soluble salts,

*. the relative proportion of sodium to the other cations,

+. the bicarbonate concentration as related to the concentration of calcium and

magnesium, and

. the concentrations of specific elements and compounds.

The amounts and combinations of these substances define the suitability of water

for irrigation and the potential for plant to#icity.


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-hen taing water samples for laboratory analysis, eep in mind that water from

the same source can vary in quality with time. Therefore, samples should be tested

at intervals throughout the year, particularly during the potential irrigation period.

T!e T"# Types #$ Salt %r#&lems

There are two types of salt problems which are very different, those associated

with the total salinity and those associated with sodium. &oils may be effected only

by salinity or by a combination of both salinity and sodium.

Salinity 'a(ard

-ater with high salinity is to#ic to plants and poses a salinity hazard. &oils with

high levels of total salinity are call saline soils. High concentrations of salt in the

soil can result in a physiological drought condition. That is, even though the fieldappears to have plenty of moisture, the plants wilt because the roots are unable to

absorb the water. -ater salinity is usually measured by the T/& !total dissolved

solids" or the 01 !electric conductivity". T/& is sometimes referred to as the totalsalinity and is measured or e#pressed in parts per million !ppm" or in the

equivalent units of milligrams per liter !mg2l".

01iw is the electric conductivity of the irrigation water. 01e is the electric

conductivity of the soil as measured in a soil sample !saturated e#tract" taen from

the root one. 01d is the soil salinity of the saturated e#tract taen from below the

root one. 01d is used to determine the salinity of the drainage water whichleaches below the root one.

S#di)m 'a(ard

Irrigation water containing large amounts of sodium are of special concern due tosodium's effects on the soil and poses a sodium hazard. &odium haard is usually

e#pressed in terms of &%3 or thesodium adsorption ratio. &%3 is calculated from

the ratio of sodium to calcium and magnesium. The latter two ions are important

since they tend to counter the effects of sodium. 4or waters containing significant

amounts of bicarbonate, the ad5usted sodium adsorption ratio !&%3ad5" issometimes used.


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1ontinued use of water having a high &%3 leads to a breadown in the physical

structure of the soil. &odium is adsorbed and becomes attached to soil particles.

The soil then becomes hard and compact when dry and increasingly impervious towater penetration. 4ine te#tured soils, especially those high in clay, are most

sub5ect to this action. 1ertain amendments may be required to maintain soils underhigh &%3's. 1alcium and magnesium, if present in the soil in large enough

quantities, will counter the effects of the sodium and help maintain good soil

properties.

Soluble sodium percent!&&6" is also used to evaluate sodium haard. &&6 is

defined as the ration of sodium in epm!equivalents per million" to the total cation

epm multiplied by )77. % water with a &&6 greater than 87 percent may result insodium accumulations that will cause a breadown in the soil's physical properties.

I#ns, Tra*e Elements and Ot!er %r#&lems

% number of other substances may be found in irrigation water and can cause to#ic

reactions in plants. In addition to sodium, of most concern are chloride and boron.In certain areas of Te#as, boron concentrations are e#cessively high and render

water unsuitable for irrigation. 9oron also can accumulate in the soil.

1rops grown on soils having an imbalance of calcium and magnesium may also

e#hibit to#ic symptoms. &ulfate salts affect sensitive crops by limiting the uptae

of calcium and increasing the absorption of sodium and potassium, resulting in adisturbance in the cationic balance within the plant. The bicarbonate ion in soil

solution harms the mineral nutrition of the plant through its effect on the uptae

and metabolism of nutrients. High concentrations of potassium may introduce a

magnesium deficiency and iron chlorosis. %n imbalance of magnesium and


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potassium may be to#ic, but the effects of both can be reduced by high calcium

levels.

+lassi$i*ati#n #$ Irriati#n Water

&everal different measurements are used to classify the suitability of water for

irrigation, including 01iw, the total dissolved solids, the per cent sodium, and

&%3.

+lassi$i*ati#n #$ Salta$$e*ted S#ils

9oth 01e and &%3 are commonly used to classify salt:affected soils . &aline soils

!resulting from salinity haard" normally have a pH value below ;.


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=4 >leaching fraction : the fraction of applied irrigation water that must be

leached through the root one

0ciw >electric conductivity of the irrigation water

0ce>the electric conductivity of the soil in the root one

0quation !)" can be used to determine the leaching fraction necessary to maintain

the root one at a targeted salinity level. If the amount of water available forleaching is fi#ed, then the equation can be used to calculate the salinity level that

will be maintained in the root one with that amount of leaching. 6lease note thatequation !)" simplifies a complicated soil water process. 01e should be checed

periodically and the amount of leaching ad5usted accordingly.

S)&s)r$a*e Drainae

?ery saline, shallow water tables occur in many areas .&hallow water tables

complicate salinity management since water may actually move upward into the

root one, carrying with it dissolved salts. -ater is then e#tracted by crops and

evaporation, leaving behind the salts.

&hallow water tables also contribute to the salinity problem by restricting thedownward leaching of salts through the soil profile. Installation of a subsurface

drainage system is about the only solution available for this situation. The originalclay tiles have been replaced by plastic tubing. $odern drainage tubes are covered

by a soc made of fabric to prevent clogging of the small openings in the plastic

tubing.

% schematic of a subsurface drainage system is shown in 4igure *. The design

parameters are the distance between drains !=" and the elevation of the drains !d"

above the underlying impervious or restricting layer. 6roper spacing and depthmaintain the water level at an optimum level, shown here as the distance mabove

the drain tubes.


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Seed %la*ement

@btaining a satisfactory stand is often a problem when furrow irrigating with saline

water. Growers sometimes compensate for poor germination by planting two orthree times as much seed as normally would be required. However, planting

procedures can be ad5usted to lower the salinity in the soil around the germinatingseeds. Good salinity control is often achieved with a combination of suitable

practices, bed shapes and irrigation water management.

In furrow:irrigated soils, planting seeds in the center of a single:row, raised bed

places the seeds e#actly where salts are e#pected to concentrate !4igure +". Thissituation can be avoided using salt ridges. -ith a double:row raised planting bed,

the seeds are placed near the shoulders and away from the area of greatest salt


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accumulation. %lternate:furrow irrigation may help in some cases. If alternate

furrows are irrigated, salts often can be moved beyond the single seed row to the

non:irrigated side of the planting bed. &alts will still accumulate, but accumulation

at the center of the bed will be reduced.

-ith either single: or double:row plantings, increasing the depth of the water inthe furrow can improve germination in saline soils. %nother practice is to use

sloping beds, with the seeds planted on the sloping side 5ust above the water line.&eed and plant placement is also important with the use of drip irrigation. Typical

wetting patterns of drip emitters and micro sprinlers are shown in 4igure . &alts

tend to move out and upward, and will accumulate in the areas shown.

Ot!er Salinity Manaement Te*!ni-)es

Techniques for controlling salinity that require relatively minor changes are morefrequent irrigations, selection of more salt:tolerant crops, additional leaching,

preplant irrigation, bed forming and seed placement. %lternatives that require

significant changes in management are changing the irrigation method, altering thewater supply, land:leveling, modifying the soil profile, and installing subsurface

drainage.

Resid)e Manaement

The common saying salt loves bare soils refers to the fact that e#posed soils have

higher evaporation rates than those covered by residues. 3esidues left on the soil

surface reduce evaporation. Thus, less salts will accumulate and rainfall will bemore effective in providing for leaching.

M#re .re-)ent Irriati#ns

&alts concentrations increase in the soil as water is e#tracted by the crop.Typically, salt concentrations are lowest following an irrigation and higher 5ust

before the ne#t irrigation. Increasing irrigation frequency maintains a moreconstant moisture content of the soil. Thus, more of the salts are then ept in

solution which aids the leaching process. &urge flow irrigation is often effective at

reducing the minimum depth of irrigation that can be applied with furrow irrigationsystems. Thus, a larger number of irrigations are possible using the same amountof water.

-ith proper placement, drip irrigation is very effective at flushing salts, and watercan be applied almost continuously. 1enter pivots equipped with =06% water

applicators offer similar efficiencies and control as drip at least the costs. 9othsprinler and drip provide more control and fle#ibility in scheduling irrigation than

furrow systems.

%replant Irriati#n


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&alts often accumulate near the soil surface during fallow periods, particularly

when water tables are high or when off:season rainfall is below normal. Ander

these conditions, seed germination and seedling growth can be seriously reduced

unless the soil is leached before planting.

+!anin Irriati#n Met!#d

&urface irrigation methods, such as flood, basin, furrow and border are usually not

sufficiently fle#ible to permit changes in frequency of irrigation or depth of waterapplied per irrigation. 4or e#ample, with furrow irrigation it may not be possible toreduce the depth of water applied below three to four inches. %s a result, irrigating

more frequently might improve water availability to the crop but might also waste

water. 1onverting to surge flow irrigation may be the solution for many furrow

systems. @therwise a sprinler or drip irrigation system may be required.

+!emi*al Amendments

In sodic soils !or sodium affected soils", sodium ions have become attached to and

adsorbed onto the soil particles. This causes a breadown in soil structure andresults in soil sealing or cementing, maing it difficult for water to infiltrate.

1hemical amendments are used in order to help facilitate the displacement of these

sodium ions. %mendments are composed of sulphur in its elemental form or related

compounds such as sulfuric acid and gypsum. Gypsum also contains calcium

which is an important element in correcting these conditions. &ome chemicals

amendments render the natural calcium in the soil more soluble. %s a result,calcium replaces the adsorbed sodium which helps restore the infiltration capacity

of the soil. 6olymers are also beginning to be used for treating sodic soils.

It is important to note that use of amendments does not eliminate the need forleaching. 0#cess water must still be applied to leach out the displaced sodium.


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Water Quality for Agriculture a Primer