Crops to be Irrigated

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Crops to be Irrigated

• Factors for consideration
• Rooting depth of crop ( Irr. Guide pp.. 3-8,3-9)
  
• Are there any soil barriers rock, hard pan, etc.
• establishment and shallow root depths require
  more frequent irrigations
• Crop Height
• wheel or riser height
• above canopy or within canopy application
• Water tolerance
• water sensitive crops develop diseases (beans)

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What is Management Allowable Depletion (MAD)?

• Mad is defined as the percentage of the available
  soil water that can be depleted between
  irrigations without serious plant moisture
  stress. MAD is expressed as
• a percentage of the total Available Water Content
  (AWC) the soil will hold in the root zone
• a soil-water deficit (SWD) in inches, or
• an allowable soil-water tension level
• More information Irrigation Guide pg.. 3-7

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Example

• Given Silt Loam soil (AWC 2.1/ft), growing
  potatoes with rooting depth of 2.5and a MAD of
  35.
• Find The maximum amount of water depletion
  before irrigation is necessary?
• 2.12.5.35 1.84

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Evapotranspiration Overview

• Recently with more weather station and a greater
  demand for the available water there has been a
  great push to update ET values. The new ET
  values are needed to provide a more accurate
  picture of what is actually happening in the
  field.

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What is EVAPOTRANSPIRATION?

• Definition
• Evaporation of water from the soil and plant
  surfaces and transpiration from the stomatal
  cavities of plants

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What are some methods for determining ET?

• Estimated crop evapotranspiration ETc
• Blaney-Criddle, etc.
• Direct measurement
• aerodynamic method
• detailed soil moisture monitoring
• lysimetry
• plant porometers
• regional inflow-outflow measurements

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What influences the method you would select?

• Type, accuracy, and duration of available
  climatic data
• Natural pattern of evapotranspiration
• Intended use of the evapotranspiration estimates

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Climatic Data

• Type
• Temperature, radiation, wind, humidity
• Quality
• Length of Record

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Natural Pattern of Crop Water Use

• Crop ET varies from day to day
• Fluctuating climatic
• Plant growth
• Daily average Vs. average for a period
• 1 day Vs 5 day

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Frequency Distributions
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ET for an Averaging Period
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Intended Use

• Irrigation Scheduling
• System Design
• Reservoir operation

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Various Methods

• NRCS endorses four methods
• Penman-Monteith
• Radiation method
• Temperature Method
• Class A evaporation pan

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Penman-Montieth Method

• More reliable for any length period
• daily, monthly, or seasonal
• If adequate data available

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Radiation Method
Not for daily ET, but for average daily ET over a
period of days 5day period Good for monthly and
Seasonal
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Temperature Method
ET0 evapotranspiration for grass reference
crop Ce elevation adjustment factor at
climate adjustment factor bt climate
adjustment factor p mean daily percent of
annual daytime hours T mean air temperature
Not for daily ET, but for average daily ET over a
period of days 5day period Good for monthly and
Seasonal
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Evaporation Pan Method
ET0kpEpan
ET0 evapotranspiration for grass reference
crop kp pan coefficient Epan evaporation
from pan
Good for monthly and Seasonal
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Things to Consider before changing ET Values

• Legal ramifications
• Quantity of water supply
• System efficiency

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SCS TR21

• Where does TR21 fit in?
• Water Rights of many States Based on TR21
• Less Accurate

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What is the difference between ET0 and
Consumptive Use?

• CU Crop coefficientET0
• Use reference ET for specific Crop

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Crop Curves

• NRCS has switched from an Alfalfa based crop
  reference to a Grass crop reference
• To convert use a multiplier factor , usually 1.15
• New ones and procedures found in NEH part 623
  chapter 2

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Consumptive Use Calculations

• Field by field
• CU crop coefficient ET
• Farm CU - multiple fields, multiple crops
• weighted CU based on percentage of crops
• Project CU
• gpm/acre weighted by percent

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Weighted Consumptive Use
CUw 80/230.31100/230.2150/230.25
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Net Irrigation Requirements

• Fn ETc Aw - Pe - GW - DSW
• Fn net irrigation requirement for season
• ETc crop evapotranspiration
• Aw auxiliary water - leaching, temperature
• modification, crop quality
• Pe effective precipitation
• GW ground water contribution
• DSW soil water depleted during season

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Effective precipitation

• The part of rainfall that can be used to meet the
  evapotranspiration of growing crops.
• Does not include surface runoff or percolation
  below the crop root zone

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System Sizing

• Simply put
• Q FgA/t
• Q to system flowrate
• Fg Gross irrigation requirement
• A irrigated area
• t time to irrigate the field

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What is the difference between Net and Gross?
Cg Cn Ea(1-Dt )
100
Cg gross system capacity Cn net system
capacity Ea application efficiency Dt system
downtime
Many efficiencies come in to play Field
efficiency Farm efficiency Conveyance
efficiency Project efficiency etc.
Things influencing Field efficiency Deep
percolation Surface runoff Spray, drift losses
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Where to get more information

• NRCS NEH 623 Chapter 2 Irrigation Water
  Requirements