Transcript Irrigation and Water Quality

Irrigation and Water Quality
Section F
SWES 316
S alts are often in trod u ced
in to soils in irrigation w ater
S alts ten d to m ove tow ard soil
su rface b ecau se of m ovem en t of
w ater d u e to evap oration an d
tran sp iration
A d eq u ate d rain age
If soils are p oorly -d rain ed , salts
an d leach in g can h elp
m ay m ove u p w ard w ith w ater
p reven t b u ild u p of
from a sh allow grou n d w ater
excess salts in th e
tab le
crop root zon e
Salt Moves to the Terminus of the
Wetting Front!
Border/Flood Irrigation
Sprinkler
Furrow Irrigation
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:


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
Salinity
(measured by EC)
Sodium
(measured by SAR)
Potential toxic ions
(Na, Cl, B)
Alkalinity or HCO3-
( Na  )
SAR=
( Ca
2
)  ( Mg
2
)
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:




ClNa+
B (H3BO3)
Bicarbonate - precipitates Ca2+ from
soils, makes Na problems worse.
General Guidelines
Degree of Restriction on Use
Potential Problem
Units
None
pH
Slight to Moderate
Severe
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
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
SAR
<3
3-9
>9
meq/L
<3
>3
Surface irrigation
meq/L
<4
4 - 10
Sprinkler irrigation
meq/L
<3
>3
mg/L
<0.7
0.7 – 3.0
Infiltration
Specific ion effects
Sodium
Surface irrigation
Sprinkler irrigation
Chloride
Boron
>10
>3.0
Specific Guidelines
Crop
ECe
ECw
LR
ECe
0%
ECw
LR
ECe
10%
ECw
LR
ECe
25%
ECw
LR
50%
ECw
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
leach adequate quantities of water
soils is to __________________________
through soil
_________________.
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
not all water added can leach
because ___________________________.
Leaching Requirement
Definition: The percentage of water
(rain + irrigation) applied that must
move below the root zone to control
salt buildup.
 Equation:

LR 
( EC w )
5( EC e )  EC w
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 to
avoid 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
Ca2+ must be added to irrigation
______
water.

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.
What does gypsum do?
Sodium level (SAR)
Based on irrigation water analysis
25
20
15
Poor soil
structure
10
Good soil
structure
5
0
0.5
1.0
1.5
2.0
2.5
Electrical conductivity (dS/m)
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 CO32- and HCO3- to
CO2.
Water Quality and Nutrient
Management (1)

Ammonia
Volatilization
Water pH
NH4+  NH3 + H+


Addition of NH4+
fertilizers in alkaline
water will encourage
this equilibrium
toward the right.
Solution: acidify
water first
% 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 + H2O
Ca++ + CO3-2 > CaCO3(s)
Constant H2SO4 injection
keeps water pH low and
prevents 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