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IRRIGATION PRINCIPLES. ERT 349 SOIL AND WATER ENGINEERING. Introduction. 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). Purpose.

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irrigation principles



importance of irrigation
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)
  • 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)
reasons for yield quality increase
Reasons for yield/quality increase
  • Reduced water stress
  • Better germination and stands
  • Higher plant populations
  • More efficient use of fertilizer
  • Improved varieties
other benefits of irrigation
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.
other benefits of irrigation1
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
an historical perspective
An Historical Perspective
  • Nile River Basin (Egypt) - 6000 B.C.
  • Tigris-Euphrates River Basin (Iraq, Iran, Syria) - 4000 B.C.
  • Yellow River Basin (China) - 3000 B.C.
  • Indus River Basin (India) - 2500 B.C.
  • Maya and Inca civilizations (Mexico, South America) - 500 B.C.
  • Salt River Basin (Arizona) - 100 B.C.
  • Western U. S. - 1800’s
  • Involvement of federal government - 1900 (only about 3 million acres then)
types of systems
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

types of systems1
Types of Systems
  • Surface
    • 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
types of systems2
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

  • 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.
  • Terminology
    • Evaporation
      • 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)
magnitude of et
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: 500 for wheat 900 for alfalfa
plant water use patterns
Plant Water Use Patterns

Daily Water Use:peaks late in afternoon; very little water use at night

Alfalfa: Ft. Cobb, OK

June 26, 1986

plant water use patterns1
Plant Water Use Patterns

Corn Water Use Pattern

  • Seasonal Use Pattern:Peak period affects design

Irrigation system must be able to meet peak water use rate or the crop may be lost.

Evaporation Rate and Time Since IrrigationEnergy or Water Availability as the Limiting Factor in ET Rate
evapotranspiration modeling
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
evapotranspiration modeling1
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
effective rainfall
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
moisture accounting soil water reservoir
AW = (FCv-PWPv)Dr


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.

Moisture AccountingSoil Water Reservoir
soil water reservoir
Soil Water Reservoir
  • RAW = MAD x AW
  • RAW = readily available water

From Table 15-4

  • MAD = management allowed depletion
system planning
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)
system planning1
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.
system planning2
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)
system planning3
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

system planning4
System Planning
  • Irrigation Requirement:

IR = [(ET - Pe)(1 + LR)] / Ea

    • Pe = effective rainfall
    • Ea = application efficiency
    • ET = u from Blaney-Criddle evapotranspiration equation
efficiencies and uniformities
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
efficiencies and uniformities1
Efficiencies and Uniformities
  • Water Conveyance Efficiency, Ec


  • Wd= water delivered by a distribution system
  • Wi = water introduced into the distribution system
efficiencies and uniformities2
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.
efficiencies and uniformities3
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
efficiencies and uniformities4
Efficiencies and Uniformities

3. Water Use Efficiency, Eu


  • Wu = water benefecially used
  • Wd = water delivered to the area being irrigated
  • Refer textbook:
    • Pg. 345
    • Example 15-5
application uniformity
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
Turf Sprinkler Uniformity Test

(catch cans placed on a 5 ft x 5 ft grid)

  • 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
water losses
Water Losses
  • Water losses
    • Evaporation
    • Drift
    • Runoff
    • Deep percolation
irrigation scheduling1
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
general approaches
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
irrigation timing period
Irrigation Timing/Period
  • Actual irrigation interval, (days)

de = effective depth of irrigation, (in. or mm)

irrigation period
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.
possible irrigation scheduling management objectives
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
plant root zones
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
other irrigation scheduling methods
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
other irrigation scheduling methods1
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)