Post on 13-Jul-2020
Irrigation Fundamentalsfor ASTE 1010 Class
October 28, 2014
Soil-Water-Plant Relationships
Soil Water Budget
Soil water storage in the crop root zone is affected by infiltrated irrigation water and rainfall, drainage and evapotranspiration
EvapotranspirationRain
Irrigation
Deep Percolation Upward Flow
SurfaceRunoff
Soil-Water-Plant RelationshipsClasses and Availability of Soil Water
AvailableSoil Water
(ASW)
Capillary Water – Slow drainage
Gravity Water – Rapid
drainage
Saturation
Field Capacity (FC)
Permanent Wilting Point (WP)Hygroscopic Water – No
drainage
Oven dry
WP is a function of soil texture, crop, ET rate, soil
salinity.
50% irrigate guideline
Soil-Water-Plant RelationshipsAvailable Soil Water-Holding Capacity (Typical)
Soil Texture Available Soil Water
inch/inch inch/foot
Sands and fine sands 0.04 - 0.06 0.5 - 0.7
Very fine sands, loamy sand 0.07 - 0.08 0.8 - 1.0
Sandy Loam 0.1 - 0.13 1.2 - 1.6
Loam,silt loam 0.16 - 0.17 1.8 - 2.1
Silty clay loam 0.16 - 0.17 1.8 - 2.1
Clay loam, sandy clay loam 0.14 - 0.17 1.7 - 2.1
Soil-Water-Plant RelationshipsTypical Crop Rooting Depths
Crop Typical active root zone depth, feet
Alfalfa 5 - 6
Corn 3.5 - 4.5
Small Grains (Wheat, Barley, Oats) 3 - 3.5
Pasture 1.5 - 2.5
Potatoes 2 - 3
Vegetables 1.5 - 3
Soil-Water-Plant Relationships
Estimating Available Root Zone Soil Water
ASW = Root Depth x Soil Water Holding Capacity
Example: Pasture Crop on Sandy Loam Soil
Root Depth = 2.5 ft
Soil Water Holding Capacity = 1.5 in/ft (sandy loam)
ASW = 2.5 ft x 1.5 in/ft = 3.75 inches
Filling the Soil – Water ReservoirVolume = q x t
q is flow rate, cfst is time, hrs
Volume = a x d
a is area, acres
d is depth, inches
area, a
d
q
time, t
q t = a d (q, cfs; t, hours; a, acres; and d, inches)
q x t = a x d
Irrigation Application Rates
• Surface Irrigation (flow usually in cubic feet per second)
In./hr. = cubic feet per second (cfs) / acres
Example: 4 cfs / 5 acres = 0.8 in/hr
• Sprinkler Irrigation (flow is usually in gallons per minute)
In./hr.=96.24 *gallons per minute(gpm)/area (ft^2)
Example: 96.24*7 gpm / (40 ft*60 ft) = 0.28 in/hr
• Drip Irrigation (flow per emitter is usually in gallons per hour)
In./hr.=1.6 *gallons per hour(gph)/emitter spacing (ft^2)
Example: 1.6*.5 gph / (1 ft * 2.5 ft) = 0.32 in/hr
• Conversions
1 cfs = 448.8 gpm
1 gpm = 60 gph
1 acre = 43,560 feet^2
Measuring Soil-Water
Kick the dirt
Estimate soil water content by feel
Use sensors (such as tensiometers or moisture blocks)
Sandy loam and
Fine sandy loam soils
Soil Water by Feel
Soil Water by Feel
Sandy clay loam,
loam, and
Silt loam soils
Example Utah Et Estimates
Monthly Alfalfa and Pasture Evapotranspiration at Santaquin. Thirty year average for period 1961-1990.
SeasonMar Apr May Jun Jul Aug Sep Oct Total
Alfalfa Water Use, Inches2.37 5.91 7.00 6.62 5.49 3.85 1.01 32.25
Pasture Water Use, Inches0.26 2.02 4.22 5.36 5.84 4.81 3.26 1.34 27.10
Adapted from: Consumptive Use of Irrigated Crops in Utah, Utah Agricultural ExperimentStation Research Report No. 145. Oct. 1994.
Irrigation Systems – Application
Method
Wheel move on alfalfa
Center pivot on alfalfa
Irrigation Application Efficiency -
Definition
Irrigation Application Efficiency (Ea)
Equals
Water Stored in the Root Zone During the Irrigation
(Wrz)
Divided by
Water Delivered to the Field (Wd)
Irrigation Application Efficiency -
Equation
Ea = 100 Wrz/Wd
Ea as a percent
Irrigation Requirements
Irrigation Requirements = Crop ET
divided by Application Efficiency
(%/100)
Irrigation Requirements - Example
Irrigation Requirements = 3 divided by
70/100
Irrigation Requirements = 4.3 inches
Crop Et = 3 inches
Application Efficiency = 70%
Application Efficiency - Factors
• Surface Irrigation– Soil type (clay, silt, sand, gravel)
Soil serves as intake medium and water transport
– Slope
– Length of run
– Stream size
– Duration (set run time, uniformity across field)
– Type (wild flood, furrow, border, basin, surge)
– Other?
Factors influencing application efficiency:
Application Efficiency - Factors
• Sprinkle Irrigation– Sprinkler spacing and nozzle type (impact,
spray)
– Pressure (pressure differences)
– Wind
– Air temperature and humidity
– Duration (set run time, uniformity across field)
– Soil type
– Type (hand line, wheel move, solid set, pivot)
– Other?
Factors influencing application efficiency:
Water Distribution Uniformityis a measure of how evenly
the water is distributed
across the field
Water Distribution Uniformity
Wheelmove evaluation with catch cans
Sprinkler Uniformity - Example
Wheelmove
water
distribution, no
offset, CU of
62%
Dmax
Dmin
40 foot by 60 foot
sprinkler spacingCU = 62%
Sprinkler Uniformity – Example 2
Wheelmove
water
distribution
with 20 foot
offset in 2nd
irrigation
(CU of
87%)DminDmax
CU = 87%
Typical irrigation application efficiencies
Surface Sprinkle Low flow/dripwild flooding 15-25% handline 30-65% micro sprinkler 75-85%
furrow 35-55% wheelmove 45-70% drip 85-90%
border 45-65% center pivot 75-85%
level basin 85-90%
Irrigation Application Efficiency
Sprinkler Application Efficiency
Application Efficiency = 60 - 70%
Application Efficiency = ??
Center Pivot Application Efficiency
Application Efficiency = 70-75%
Application Efficiency = 80-85%
Center Pivot Application Efficiency
Irrigation Scheduling
When to irrigate and how much water to apply
OBJECTIVE:
Apply only the water needed to refill the root
zone, while accounting for seepage, runoff
losses, and leaching requirements
Irrigation Scheduling –
General Rule:
Field crops (pasture, alfalfa, …) should be irrigated whenever the soil water depletion approaches 50% of the available water in the root zone
Irrigation Scheduling
Irrigation timing and amount depend on:
• Available soil water capacity
– Soil texture (sand, loam, clay)
– Crop root depth
• Irrigation system characteristics
(design, maintenance, management and operation)
Common irrigation scheduling
approaches include:
• Irrigation on fixed intervals or following a simple calendar, i.e., when a water turn occurs or according to a predetermined schedule
• Irrigating when the neighbor irrigates
• Observation of visual plant stress indicators
Common approaches, continued
• Measuring (or estimating) soil water by use of
instruments or sampling techniques
– Estimate soil water content by feel, or
– use soil moisture sensors (ie. Watermark)
• Following a soil-water budget based on weather
data and/or pan evaporation
• Some combination of the above
Irrigation Scheduling – Soil Water
Budget (ET estimates)
Irrigation timing and
amount depend on:
• Evapotranspiration
– weather conditions
– crop growth progress
Irrigation Interval – Pasture
Root Depth = 2.5 ft, MAD = 50%,
Peak ET = 0.21 in/day
Soil TypeAWHC
in/ft
Root Zone
Available Soil
water, inches
MAD
(50%)
refill
(inches)
Irrigation
Interval, days
Sand 0.6 1.5 0.75 4
Fine sandy
loam1.0 2.5 1.25 6
Loam 2.0 5.0 2.5 12
Irrigation Interval – Pasture
Root Depth = 2.5 ft, MAD = 50%,
What if the Peak ET were 0.25 in/day,
instead of 0.21 in/day, how would the
irrigation interval change from the previous
table?
Irrigation Interval – Pasture
Root Depth = 2.5 ft, MAD = 50%,
Peak ET = 0.25 in/day
Soil TypeAWHC
in/ft
Root Zone
Available Soil
water, inches
MAD
(50%)
refill
(inches)
Irrigation
Interval, days
Sand 0.6 1.5 0.75 3
Fine sandy
loam1.0 2.5 1.25 5
Loam 2.0 5.0 2.5 10
Maximum Days Between Irrigation for Typical
Soils and Various Daily Et rates.
Average daily crop Et, inches per day
0.15 0.20 0.25 0.30 0.35
Soil Type Days to deplete 50 percent of SW in top two
feet
Sands and fine sands 4 3 3 2 2
Very fine sands, loamy sands 6 5 4 3 3
Sandy loam 9 7 5 5 4
Sandy clay loam 12 9 7 6 5
Loam, silt loam, silty clay 13 10 8 7 6
loam, clay