1. Soil-Water-Plant Relationship
Natural Resources Conservation Service
NRCS
United States Department of Agriculture

2. Soils Check the following Texture Structure Intake Salt
Water holding capacity
***Lateral movement

3. Soil Components V Vg Vp = Vg + Vw Vw Vs Air Water Soil Particles
V = Soil Volume
Vg = Volume of air
Vw = Volume of water
Vs = Volume of Soil Particles
Vp = Volume of Soil Pores
Air

4. Soil Water Saturation Field Capacity MAD Permanent Wilting Point
Gravitational Water, rapid drainage
Field Capacity
MAD
TAW
Capillary Water, very slow drainage
Permanent Wilting Point
Hydroscopic Water,
essentially no drainage
Oven Dry

5. Available Water Capacity
Available water capacity - the amount of water that a soil can store that is available for use by plants.
Field capacity – amount of water left after free water has drained because of gravity. ~1/3 bar potential
Permanent wilting point – soil water content at which most plants cannot obtain sufficient water to prevent tissue damage. ~15 bars potential

6. Available Soil water

7. Available Water Holding Capacity
Limits the amount of water that can be applied at any one irrigation .
For a given crop determines frequency of irrigations

8. Review
Given: Root zone = 3.5 ft, WHC = 1.6”/ft, MAD = 40%, Pw =50% Find: Available water

9. Solution
AW =RZ * WHC * MAD * Pw = 3.5*1.6*.4 *.5= 1.2 inches

10. Salinity All irrigation waters have salts Three potential problems
Salt can accumulate in the soil
Certain types of salt cause surface sealing and poor infiltration
Specific types of salts are highly toxic to plants even in low concentrations
Soil and water chemistry should always be examined before doing an irrigation design

11. Salt distribution -Surface drip irrigation

12. Crop
ECe (mmhos/cm)
Min
Max
Field crops
Alfalfa
Barley
Cotton
Sugarbeet
Wheat
Sorghum
2.0
8.0
7.7
7.0
6.0
6.8 28 27 24 20 18
Corn
Flax
Broadbean
Cowpea
Bean
1.7
1.6
4.9
1.0 10 12
8.5
6.5
Fruit and nut crops
Date palm
Fig, olive
Pomegranate
Grapefruit
Orange
Lemon
Apple, pear
Walnut
Peach
4.0
2.7
1.8 32 14 8
Apricot
Grape
Almond
Plum
Blackberry
Boysenberry
Avocado
Raspberry
Strawberry
1.5
1.3 6 7
5.5 4
Vegetable crops
Beets
Broccoli
Tomato
Cucumber
Cantaloupe
Spinach
Cabbage
Potato
2.8
0.9
1.1
2.2
3.2 15
13.5
12.5 16
Sweet corn
Sweet potato
Pepper
Lettuce
Radish
Onion
Carrot
2.5
1.2
10.5 9
7.5

13. Lateral movement What affect lateral movement:
Varying soil textures throughout a field (e.g., sand vs. loam vs. clay).
Varying installation depths of the tape.
Varying soil compaction or stratification.
Different soil chemistry conditions
Beginning soil moisture

14. Silt Loam soil

15. Clay Loam Soil

20. Surface(DI) vs. buried(SDI)

21. Percent Wetted Area For a lone tree Pw=Aw/Arz Root zone Emitter Aw
Pw is the “average horizontal area wetted in the top 6 to 12 inches of the root zone as a percentage of the total crop area.” (NEH 623, Ch. 7)
For a lone tree Pw=Aw/Arz
Root zone
Emitter Aw
Tree trunk
Surface area wetted
Arz
Actual area wetted

22. Percent Wetted Area and emitter spacingSw
e = # of emitters/tree
Se = emitter spacing
Sp = plant spacing
Sr = row spacing
Sw = diameter of the circular area wetted by a single emitter Se Sp Sr

23. Wetted Area

25. Desired wetted area
For widely spaced crops such as vines, bushes, and trees, a reasonable design objective is to wet at least one-third and up to one-half of the horizontal cross-sectional area of the root system.
smaller Pw is favored for economic reasons.
rows spaced less than 6 ft. (1.83 m) apart, the Pw may approach 100 %.

26. Soil wetted area Kind of soil layers Soil Depth And Texture
Homogeneous
Varying layers,
Generally
Low density
Medium density
S’e x Sw = Aw (ft2)
2.5 ft
Coarse
1.2 x 1.5 = 1.8
2.0 x 2.5 = 5.0
2.8 x 3.5 = 9.8
Medium
2.4 x 3.0 = 7.2
3.2 x 4.0 = 12.8
4.0 x 5.0 = 20.0
Fine
4.0 x 5.0 = 20
4.8 x 6.0 = 28.8
5 ft
3.6 x 4.5 = 16.2
3.2 x 4.0=12.8
5.6 x 7.2 = 39.2
7.2 x 9.0 = 64.8
4.0 x 5.0=20.0
5.2 x 6.5 = 33.8
6.4 x 8.0 = 51.2
1 Based on an emitter flow rate of 1 gph (3.785 L), the estimated Aw is given as a rectangle with the wetted width (Sw) equal to the maximum expected diameter of the wetted circle and the optimum emitter spacing (S’e) equal to 80 percent of that diameter.

27. Factors affecting wetted area
Lower application rates gives best horizontal water distribution
Wet soil gives the narrowest water distribution
Management can provide only minor improvement in water distribution
Soil texture is the primary determinate factor

28. Crop ET/System flow rate
Water requirement
MAD/stress – Root zone
Salt Tolerance – leaching requirement
Frequency

29. Water use ETc = ETo x Kc Where:
ETc = Crop Evapotranspiration rate, in/day, (mm/day)
ETo = Reference evapotranspiration, ETo or ETr, in/day, (mm/day)
Kc = Crop coefficient for specific crop
References (NEH Chapter 2 Irrigation water requirements, irrigation Guide)

30. Instructor Note: Make the point that a crop water needs change during the season and to avoid deep percolation the water application may need to be changed also.

31. ET for Trees Big tress need more water than small trees
Mature tress on close spacing need same amount of water per acre as large trees on wider spacing
So if there is several blocks of the same type of tree using the same flow rate per tree, but on different spacing. Each block needs to be design for a different number of hours per week.

32. Pruning – if the same percent canopy cover exists on two blocks of the same type of plant – use the same ET
“Percent canopy cover” is defined as % of ground surface shaded at noon.
Cover crop come in all sizes shapes and types and may have an additional ET component – upwards %
For cover crop grown all season long, the micro/drip system needs to be a microspray that wets a large percentage of the surface area

33. To crop or not to cover crop?

34. Drip systems –
the ground surface is moist almost all the time which increase evaporation
The small frequent irrigations contribute to little or no plant stress
These two factors may increase the ET by as much 15% above published rates.

35. Designing for less than Peak ET
Regulated Deficit Irrigation (RDI)
Wine grapes (increase sugars)
Almonds (start of hull split)
Tomatoes (increase solids)
Regulate early growth of trees and vines ( trying to avoid spindly mature trees)

36. Frequency Maximum Frequency Example: AWC(MAD)=1.2, ETc=0.30
Is MAD still applicable with a drip system?

37. Net Application
NIR = ETc * f - Pe e. g. If f = 2 days, ETc =.3”/day, and Pe=0.0 Then NIR =.6”

38. Considerations on Published ET
Loam or heavier textured soil with at least 60% wetted volume
Design for the peak month of a normal year
Situations of low soil water capacity
Design flow rates may need to be % higher
Low water holding capacities are caused by:
Small percentage wetted area
Sand or rocky soils
Shallow soils
Shallow root systems (e.g. avocadoes or some produce crops)

39. Transpiration ratios - Unavoidable losses
Table Seasonal transpiration ratios for arid and humid regions with various soil textures and rooting depths.
Climate zone and root depth
TR1 for indicated soil texture
Very course
Coarse
Medium
Fine
Arid
<2.5 ft (.75 m)
2.5 to 5.0 ft ( m)
>5.0 ft (1.5 m)
1.15
1.10
1.05
1.00
Humid
1.35
1.25
1.20
1Seasonal transpiration ratios (TR) are for drip emitters. For spray emitters add 0.05 to TR in humid climates and 0.10 in arid climates

40. Emission Uniformity – Distribution Uniformity
is a mathematical expression used to quantify differences in the amount of water received by the plants throughout the field

41. Emission Uniformity Poor EU Good EU
Instructors note: Explain DU using visual examples
Good EU

42. Emission/Distribution Uniformity
What’s the highest EU that you can have?

43. Actual vs. Potential EU
Actual EU’s are lower than the Potential and are dependent on the following –
System Design
Suitability
Management

44. EU Deteriorate with time
Allowable pressure difference determined by brand new design EU
Required system capacity by estimated EU after several years

45. What can cause Poor EU? Pressure Differences Uneven spacing
Unequal drainage
Other
Plugging etc.
Wear

46. Emitter type
Spacing
ft,
Topography
Slope %
EU range
Point source on
perennial crops
>13
uniform ,
<2
90 to 95
steep or undulating
>2
85 to 90
perennial or
Semi permanent
crops
<13
Uniform
Steep or undulating
80 to 90
Line source on
annual or
All
uniform
70 to 85
Spray

47. Leaching ratio
LR is the faction of applied water required to maintain a desired salinity level in the soil at the spot that receives just enough water to match ET
The complete LR may be satisfied by rainfall in some areas

50. Gross application
Gross amount of water to be applied at each irrigation, (Fg), (in.), includes sufficient water to compensate for the system non- uniformity and unavoidable losses, and to provide for salt leaching

51. Gross Application or

52. Review Given: NIR = .6”, Du = 0.90, LR = 0.15,
from table 7-15 Tr = 1.0
(arid, RZ=3.5’, medium soil)
Find: Fg

53. Solution

54. Practice problem

55. Gallons per plant

56. Review
Given: Fg= 0.78, f = 2 days, Tree spacing 15’, row spacing 20’, 7 emitters/tree Find: q per tree and emitter

57. Solution

58. Practice problem

59. Class design problem