Irrigation and Water Quality

Section FSWES 316

Salts are often introducedinto soils in irrigation water

Salts tend to move toward soilsurface because of movement ofwater due to evaporation andtranspiration

If soils are poorly-drained, saltsmay move upward with waterfrom a shallow groundwater table

Adequate drainageand leaching can helpprevent buildup of excess salts in the crop root zone

Salt Moves to the Terminus of the Wetting Front!

Border/Flood Irrigation Furrow IrrigationSprinkler

Subsurface drip

Why Water Quality is Important

Effects on Soil Salinity Effects on Soil Sodicity Bicarbonate content Toxic ion content Effects on nutrients applied by

fertigation

Irrigation and Water Quality

When soils are irrigated, the quality of the water used will eventually determine the salinity and sodicity of the soil: Soils will be at least 1.5 times more saline

than the water used to irrigate them, unless very high (>20%) leaching fractions are used.

The ESP (and SAR) of the soil will eventually equal the sodium adsorption ratio (SAR) of the water.

Irrigation Water Quality

Important Measurements: Salinity

(measured by EC)

Sodium (measured

by SAR) Potential toxic ions

(Na, Cl, B) Alkalinity or HCO3

-

( )

( ) ( )

Na

Ca M g

2 2SAR=

Irrigation Water Hazards

Interpreting Salinity Remember that even with excellent

management, soil EC will be at least 1.5X that of the water used (except in very coarse-textured soils or very high LF).

Crop tolerance varies, however Interpreting sodicity - depends on

clay content and salinity

Irrigation Water Hazards

Concentrations of toxic ions: Cl- Na+

B (H3BO3)

Bicarbonate - precipitates Ca2+ from soils, makes Na problems worse.

General GuidelinesDegree of Restriction on Use

Potential Problem Units None Slight to Moderate

Severe

pH Normal range 6.5 – 8.4

Salinity

EC dS/m <0.7 0.7 – 3.0 >3.0

TDS mg/L <450 450 – 2,000 >2,000

Infiltration

SAR=0-3 and ECw= >0.7 0.7 – 0.2 <0.2

SAR=3-6 and ECw= >1.2 1.2 – 0.3 <0.3

SAR=6-12 and ECw= >1.9 1.9 – 0.5 <0.5

SAR=12-20 and ECw=

>2.9 2.9 – 1.3 <1.3

SAR=20-40 and ECw=

>5.0 5.0 – 2.9 <2.9

Specific ion effects

Sodium

Surface irrigation SAR <3 3-9 >9

Sprinkler irrigation meq/L <3 >3

Chloride

Surface irrigation meq/L <4 4 - 10 >10

Sprinkler irrigation meq/L <3 >3

Boron mg/L <0.7 0.7 – 3.0 >3.0

Specific GuidelinesCrop ECe ECw LR ECe ECw LR ECe ECw LR ECe ECw LR ECw

0% 10% 25% 50% max

Date palm 4.0 2.7 4% 6.8 4.6 7% 10.9 7.3 11% 17.9 12.0 19% 32.0

Fig 2.7 1.8 6% 3.8 2.6 9% 5.5 3.7 13% 8.4 5.6 20% 14.0

Olive 2.7 1.8 6% 3.8 2.6 9% 5.5 3.7 13% 8.4 5.6 20% 14.0

Pomegranite

2.7 1.8 6% 3.8 2.6 9% 5.5 3.7 13% 8.4 5.6 20% 14.0

Grapefruit 1.8 1.2 8% 2.4 1.6 10% 3.4 2.2 14% 4.9 3.3 21% 8.0

Orange 1.7 1.1 7% 2.3 1.6 10% 3.2 2.2 14% 4.8 3.2 20% 8.0

Lemon 1.7 1.1 7% 2.3 1.6 10% 3.3 2.2 14% 4.8 3.2 20% 8.0

Apple 1.7 1.0 6% 2.3 1.6 10% 3.3 2.2 14% 4.8 3.2 20% 8.0

Pear 1.7 1.0 6% 2.3 1.6 10% 3.3 2.2 14% 4.8 3.2 20% 8.0

Walnut 1.7 1.0 7% 2.3 1.6 10% 3.3 2.2 14% 4.9 3.2 20% 8.0

Peach 1.7 1.0 8% 2.2 1.4 11% 2.9 1.9 15% 4.1 2.7 21% 6.5

Apricot 1.6 1.0 9% 2.0 1.3 11% 2.6 1.8 15% 3.7 2.5 21% 6.0

Grape 1.5 1.0 4% 2.5 1.7 7% 4.1 2.7 11% 6.7 4.5 19% 12.0

Almond 1.5 1.0 7% 2.9 1.4 10% 2.8 1.9 14% 4.1 2.7 19% 7.0

Plum 1.5 1.0 7% 2.1 1.4 10% 2.9 2.9 14% 4.3 2.8 20% 7.0

Blackberry 1.5 1.0 8% 2.0 1.3 11% 2.6 1.8 18% 3.8 2.5 21% 6.0

Avocado 1.3 0.9 8% 1.8 1.2 10% 2.5 1.7 14% 3.7 2.4 20% 6.0

Raspberry 1.0 0.7 6% 1.4 1.0 9% 2.1 1.4 13% 4.2 2.1 19% 5.5

Strawberry 1.0 0.7 9% 1.3 0.9 11% 1.8 1.2 15% 2.5 1.2 21% 4.0

Controlling Salinity in Irrigated Soils

The only way to control salinity in irrigated soils is to __________________________ _________________.

If irrigation water EC is <0.75 dS/m, no risk of salt buildup. Normally, the soil is at least 1.5X the salinity of the irrigation water used because ___________________________.

leach adequate quantities of waterthrough soil

not all water added can leach

Leaching Requirement

Definition: The percentage of water (rain + irrigation) applied that must move below the root zone to control salt buildup.

Equation:LR

EC

EC ECw

e w

( )

( )5

Notes on LR

The LR is the amount of total water that should be applied above crop water use. Can be applied at every irrigation, or only

periodically The LR you calculate depends on

assumptions. This is a fairly crude method - also

consider irrigation system characteristics.

LRs above 30% are not very practical.

Other Ways to Live with Salts

Keep soils moist - this keeps salt concentrations more dilute. May require frequent irrigation. Drip irrigation !!!

Plant seeds on the sides of sloping beds. Salts move with water

Use plants that are salt-tolerant

Squash planted on sides of beds toavoid zone of highest salt.

Fun things you can do with EC

Estimate total dissolved solids (TDS) EC (dS/m) x 640 TDS (ppm)

Estimate osmotic potential of soil solution o (bars) EC (dS/m) x (-0.36)

Controlling Sodium

The SAR describes the equilibrium relationship between Na, Ca, and Mg. It should be as low as possible. Over time, the ESP of the soil will equal the SAR value of the irrigation water.

In order to control Na, a source of ______ must be added to irrigation water.

Ca2+

Irrigation Water Treatment (1)

Regular treatment of irrigation water can help prevent the formation of sodium problems.

Irrigation water can be regularly treated with gypsum to lower SAR of water. Typical rates: 100-300 lbs/acre-foot water (326,000 gallons)

There is no effective (economical) water treatment to counteract salinity.

0

5

10

15

20

25

0.5 1.0 1.5 2.0 2.5

Electrical conductivity (dS/m)

What does gypsum do?

Poor soilstructure

Good soilstructure

Sod

ium

leve

l (S

AR

)Based on irrigation water analysis

Soil Amendments and Water Treatments

Soil application of amendments are used for initial reclamation and long-term maintenance of soil quality. Rates are often large and based on economics.

Water treatments are generally intended to alter the chemistry of irrigation water so that no further degradation in soil quality will occur. Rates used for water treatment are usually small and based on solubility and stoichiometry.

Bicarbonate Hazard

Excess HCO3- causes precipitation of CaCO3 thus increasing the Na hazard (SAR) of irrigation water

Irrigation Water Treatment (2)

Carbonate (CO32-) and bicarbonate

(HCO3-) in irrigation water are

detrimental because they: precipate Ca2+ from soil solution and hasten replacement of Ca2+ with Na+

Treatment of irrigation water with H2SO4 to a pH <6.0 will convert all CO3

2- and HCO3

- to CO2.

Water Quality and Nutrient Management (1)

Ammonia VolatilizationNH4

+ NH3 + H+

Addition of NH4+

fertilizers in alkaline water will encourage this equilibrium toward the right.

Solution: acidify water first

Water pH% NH3 volatilized

7.2 1

8.2 10

9.2 50

Water Quality and Nutrient Management (2)

Adding NH3 to irrigation water (as a fertilizer) will raise water pH to 9-10. This will: Cause NH3 volatilization Remove Ca and Mg from water by

precipitating them as carbonates Can irreversibly plug irrigation

systems Solution: Acidify water first

Calcium Carbonate Precipitation

1. High bicarbonate Ca++ + 2HCO3

- > CaCO3 + H2CO3 >H2O + CO2

2.Ammoniated water NH3 + H2O > NH4OH NH4OH > NH4 + OH OH- + HCO3 > CO3

-2 + H2OCa++ + CO3

-2 > CaCO3(s)

Constant H2SO4 injectionkeeps water pH low andprevents formation of CaCO3 in the drip lines, and also dissolves some CaCO3 in the soil, helping to maintain high exchangeable Ca2+ and low exchangeable Na+.

Water Quality and Nutrient Management (3)

Many P fertilizers are not very soluble in water.

Adding them to irrigation water high in soluble Ca will lead to precipitation of Ca-phosphates Loss of P Plugging of irrigation system