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