1. IRRIGATION PRINCIPLES
ERT 349 SOIL AND WATER ENGINEERING
IRRIGATION PRINCIPLES

2. Introduction

3. Importance of Irrigation
Definition
“the supply of water to crops and landscaping plants by artificial means”
Estimates of magnitude
world-wide: 544 million acres
(17% of land  1/3 of food production)

4. Purpose
Raise a crop where nothing would grow otherwise (e.g., desert areas)
Supply water to root zone
Grow a more profitable crop (e.g., alfalfa vs. wheat)
Increase the yield and/or quality of a given crop (e.g., fruit)
Increase the aesthetic value of a landscape (e.g., turf, ornamentals)

5. Reasons for yield/quality increase
Reduced water stress
Better germination and stands
Higher plant populations
More efficient use of fertilizer
Improved varieties

6. Other Benefits of Irrigation
Leach toxic elements from soils.
All water contains salts so irrigation adds salts to soil.
Evaporation of water from soil surface carries salts to through soil to surface.
Allow extra irrigation water just for leaching purposes to dissolve soil salts and flush them away in drainage water.
saline soils = contain Ca2+ and Mg2+ salts. These reduce available water to plants causing plants to wilt and burn. May show up as white crust on soil surface.
sodic soils = contain Na+ salts. Sodium damages soil tilth and structure, can lead to formation of hardpans that resist penetration of water and plants roots so get poor plant growth, stunted.
Frost protection
When water freezes it releases latent heat to the air.

7. Other Benefits of Irrigation
Plant/soil cooling
Misting increases relative humidity around upper portion of plant thereby reducing plant stress, evapotranspiration, temperature.
Chemical application
Supply pesticides (chemigation); fertilizers and liquid animal manure (fertigation) more efficiently since control timing of release, amount, and to some degree, placement.
Wind erosion control

8. An Historical Perspective
Nile River Basin (Egypt) B.C.
Tigris-Euphrates River Basin (Iraq, Iran, Syria) B.C.
Yellow River Basin (China) B.C.
Indus River Basin (India) B.C.
Maya and Inca civilizations (Mexico, South America) B.C.
Salt River Basin (Arizona) B.C.
Western U. S ’s
Involvement of federal government (only about 3 million acres then)

9. Types of Systems Sprinkler
pressurized irrigation through devices called sprinklers (water is discharged into the air and hopefully infiltrates near where it lands)
used on agricultural and horticultural crops, turf, landscape plants
Many types including: - single sprinkler systems
- boom sprinkler systems: single boom (arm) has many nozzles)
- multiple sprinkler systems: side roll, center pivot etc.
- permanent systems, ex. orchard
- movement may be via hand, tractor or self-propelled

10. Types of Systems Surface Micro (drip, trickle)
Irrigation water flows across the field to the point of infiltration
most common method of irrigation world-wide, esp. in developing nations
primarily used on agricultural crops and orchards
Types: - flood = total immersion for long period of time, ex. rice field-
- border irrigation. Water - -introduced at one end of field and allowed to disperse and travel down to other end.
- furrow irrigation. Water introduced through tubes from canal directly into individual furrows.
Micro (drip, trickle)
frequent, slow application of irrigation water using pressurized systems
used in landscape and nursery applications, and on high-value agricultural and horticultural crops

11. Types of Systems Subirrigation
water is applied below ground surface via drain tile/tubes or through deep surface ditches
goal is to increase the height of the watertable
Requirements:
- very permeable soil so that water can move upwards
- impermeable layer or natural water table near root zone
- low hazard due to salt accumulation since no leach provided by this irrigation method

12. Assignment From your reading on textbook and other references:
List and describe the factor affecting you to choose the irrigation method for your farm.
With the chosen crops, consider the best economical method with high crops production.

13. Irrigation Water Requirement

14. Evapotranspiration Terminology Evaporation Transpiration
Process of water movement, in the vapor form, into the atmosphere from soil, water, or plant surfaces
Transpiration
Evaporation of water from plant stomata into the atmosphere
Evapotranspiration
Sum of evaporation and transpiration (abbreviated “ET”)
Consumptive use
Sum of ET and the water taken up the plant and retained in the plant tissue (magnitude approximately equal to ET, and often used interchangeably)

15. Magnitude of ET
Generally tenths of an inch per day, or tens of inches per growing season
Varies with type of plant, growth stage, weather, soil water content, etc.
Transpiration ratio
Ratio of the mass of water transpired to the mass of plant dry matter produced (g H2O/g dry matter)
Typical values: for wheat for alfalfa

16. Plant Water Use Patterns
Daily Water Use: peaks late in afternoon; very little water use at night
Alfalfa: Ft. Cobb, OK
June 26, 1986

17. Plant Water Use Patterns
Seasonal Use Pattern: Peak period affects design
Corn Water Use Pattern
Irrigation system must be able to meet peak water use rate or the crop may be lost.

18. Evaporation Rate and Time Since Irrigation Energy or Water Availability as the Limiting Factor in ET Rate

19. Evapotranspiration Modeling
Estimation based on:
climate
crop
soil factors
ETc = Kc ETo
ETc = actual crop evapotranspiration rate
ETo = the evapotranspiration rate for a reference crop
Kc = the crop coefficient

20. Evapotranspiration Modeling
Reference Crop ET (ETo)
ET rate of actively growing, well-watered, “reference” crop
Grass or alfalfa used as the reference crop (alfalfa is higher)
A measure of the amount of energy available for ET
Many weather-based methods available for estimating ETo
(FAO Blaney-Criddle; Jensen-Haise; Modified Penman; Penman-Montieth)
Crop Coefficient (Kc)
Empirical coefficient which incorporates type of crop & stage of growth (Kcb); and soil water status-- a dry soil (Ka) can limit ET; a wet soil surface (Ks) can increase soil evaporation
Kc = (Kcb x Ka) + Ks
Kc values generally less than 1.0, but not always

21. One way to track real-time water use of your lawn and garden is to use the evapotranspiration model on the Mesonet Agweather page. Agweather is a free site with lots of management information for farmers and homeowners. Agweather is organized according to interest areas, such as Weather, Livestock, Crops, and Horticulture. Clicking on Horticulture . . .

22. . . . takes you to a page where you can select among Fruit & Nut, Ornamental, Turf and Vegetable options. Selecting Turf takes you to several more options, including an OSU Pest Diagnostics link, and one titled Evapotranspiration.

23. Selecting the Mesonet station nearest your location can be done either from the alphabetized, drop-down menu of stations or from the map. The dots representing the station locations on the map are active and will select the station if you click on them. Select your type of turf, either warm-season (Bermuda) or cool-season (fescue, ryegrass, etc.). Estimate the date that began actively growing (probably around mid-March for Bermuda grass). Then click “Get Turf Grass Data”.

24. Using the ET Table to Schedule Lawn Irrigation
This brings us a table which shows the daily estimate of ET for the type of turf selected, showing the most recent days at the top of the table. The “ET” column is the water use for the day shown. The “ET_ACC” column show the cumulative ET for all days from the day at the top of the table (yesterday) going back successive days. For the example shown, yesterday’s ET was 0.19 inch of water, and the cumulative ET is also 0.19 inch. Going back to September 21 (day before yesterday) the ET is 0.20 inch and the cumulative ET is 0.19 (for yesterday) plus 0.20 (for day before yesterday) which equals 0.39 inch. The value of this information is– suppose it is the morning of September 23 and you last irrigated your lawn on September 17. Looking down the table you see the estimated cumulative water use from September 17 through yesterday is 1.23 inch. If you have turf rooted 12 inches deep in a Vanoss silt loam with a total of 1.20 inches of available water stored in the root zone after it is well irrigated– you need to water today. The water balance is meant to show accumulated water use, less any rainfall received, which should be the amount of irrigation required. However, in Oklahoma, if you are more than a few hundred yards away from a rain gauge the measured rainfall is of little value in judging your water balance. I would urge you to have one or more rain gauges on your property and use your local rainfall measurements rather that the Mesonet rainfall in any water balance irrigation scheduling work.

25. Effective Rainfall
Effective Rainfall = portion of rainfall that contributes to ET (some does not wet soil deep enough / goes to runoff / lost to deep percolation)
Pe = estimated effective rainfall for a soil depth of 75 mm (mm)
Pm = mean monthly rainfall (mm)
ET = average monthly evapotranspiration (mm)
D = soil water deficit = net irrigation depth (mm)
f(D) adjustment factor
Note: Pe <= lowest of ET and Pm

26. Moisture Accounting Soil Water Reservoir
AW = (FCv-PWPv)Dr
Where
AW = Available water
FC = volumetric field capacity
PWP = volumetric wilting point
Dr = depth of root zone or depth of layer of soil within the root zone
Refer to Table 15-2 pg. 337.

27. Soil Water Reservoir RAW = MAD x AW RAW = readily available water
From Table 15-4
MAD = management allowed depletion

28. System Planning
Because irrigation is a major water user, it is essential that irrigation system be planned, designed, and operate efficiently.
Determine need - estimate crop use vs. rainfall
Examine site
topography (slope, changes in elevations)
soil characteristics (root zone depth, water holding capacity, infiltration rate)

29. System Planning
Availability of water - quantity & quality. Generally groundwater is better quality than surface water. Need well with sufficient pumping capacity if bringing in water through an irrigation canal.
Economic analysis - compare cost of installing and operating irrigation system vs. expected increase in yields. In Indiana, generally need an increase of the magnitude of 50 bushels of corn per acre per year to justify expense.

30. System Planning Available Water: AW = (FC - PWP)Dr / 100
AW = available water (mm, in)
FC = volumetric field capacity (decimal)
PWP = volumetric permanent wilting point (decimal)
Dr = depth of root zone or depth of soil layer of interest (mm, in)

31. System Planning
Leaching Requirement (LR) = extra water applied to dissolve and carry away salts in the soil.
value will be given if needed
expressed as a portion of the total irrigation water applied
ex. LR = 0.2 and total applied = (1+LR) x soil water deficit
*From Figure 15-3 (pg 344) textbook

32. System Planning Irrigation Requirement: IR = [(ET - Pe)(1 + LR)] / Ea
Pe = effective rainfall
Ea = application efficiency
ET = u from Blaney-Criddle evapotranspiration equation

33. Efficiencies and Uniformities
Efficiency:
Output divided by an input an usually expressed as a percentage.
There are 3 basic efficiency concept:
1. Water Conveyance Efficiency:
Can be applied along any reach of a distribution system
Example: A water conveyance efficiency could be calculated from a pump discharge to a given field or from a major diversion work to a farm turnout

34. Efficiencies and Uniformities
Water Conveyance Efficiency, Ec
Where
Wd= water delivered by a distribution system
Wi = water introduced into the distribution system

35. Efficiencies and Uniformities
2. Water Application Efficiency, Ea
The efficiency may be calculated for an individual furrow or border strip, for an entire field or entire farm/project.
When applied to areas larger than a field, it overlaps the definition of conveyance efficiency.

36. Efficiencies and Uniformities
Application efficiency (Ea)
Ws = water stored in the root zone by irrigation
Wd = water delivered to the area being irrigated
fraction or percentage

37. Efficiencies and Uniformities
3. Water Use Efficiency, Eu
Where
Wu = water benefecially used
Wd = water delivered to the area being irrigated

38. Example
Refer textbook:
Pg. 345
Example 15-5

39. Application Uniformity
Coefficient of Uniformity (UC)
n = number of observations (each representing the same size area)
d = average depth for all observations
yi = depth for observation i
Popular parameter for sprinkler and microirrigation systems in particular
For relatively high uniformities (CU > 70%), Eq. 5.4 and 5.5 relate CU to DU

40. Turf Sprinkler Uniformity Test
(catch cans placed on a 5 ft x 5 ft grid)

41. Adequacy
Because of nonuniformity, there is a tradeoff between excessive deep percolation and plant water stress
Adequacy: the percent of the irrigated area that receives the desired depth of water or more

42. Water Losses
Water losses
Evaporation
Drift
Runoff
Deep percolation

43. Water Losses

44. Irrigation Scheduling

45. Irrigation Scheduling
If water is available, schedule so as to achieve maximum yields
If water is limited / expensive then schedule so as to maximize economic return
Typically start irrigation when available water = 55% maximum

46. General Approaches Maintain soil moisture within desired limits
direct measurement
moisture accounting
Use plant status indicators to trigger irrigation
wilting, leaf rolling, leaf color
canopy-air temperature difference
Irrigate according to calendar or fixed schedule
Irrigation district delivery schedule
Watching the neighbors

47. Irrigation Timing/Period
Actual irrigation interval, (days)
de = effective depth of irrigation, (in. or mm)

48. Irrigation Period
# days over which irrigation cycle must be complete. Equals the time it take for field at FC to reach 55% AW without rainfall occurring
Example:
Root zone depth = 1 m, allowable depletion = 40% AW, AW = 150 mm/ m depth, ave. ET = 8 mm. day
IP = [150 mm/m (1m) (0.4)] / 8 mm/day = 7.5 days
So can divide up entire area to be irrigated such that repeat irrigation at same site every 7.5 days.

49. Possible Irrigation Scheduling Management Objectives
Maximum yield/biomass production
Maximum economic return
Functional value of plants (e.g., athletic fields)
Aesthetic value of plants (e.g., landscapes)
Keeping plants alive

50. Plant Root Zones
Depth used for scheduling vs. maximum depth where roots are found
Influenced by soil characteristics
Soil texture
Hardpan
Bedrock
Perennial vs. annual plants

51. Components of Crop Root Zone Water Balance

52. Other Irrigation Scheduling Methods
Soil Water Measurement
Need measurements at several locations
Need measurements throughout root zone depth
Doesn’t indicate how much water to apply

53. Other Irrigation Scheduling Methods
Plant Status Indicators
Leaf water potential (energy status of leaf water)
Use pressure chamber or thermocouple psychrometer
Measured at mid-day; many samples needed
Foliage/Air temperature difference
Well-watered plants cooler than air
Use infrared thermometer
Leaf appearance
Color, wilting, etc.
Indicators show up too late
Irrigate at critical growth stages (e.g.: flowering)