Sprinkler Application Rates & Soil Considerations Bruce Sandoval, P.E. Irrigation Engineer USDA-NRCS

Sprinkler Application Rates & Soil Considerations Sprinkler Application Rates Soil Infiltration Rates Application vs. Infiltration = Runoff ? Management to Eliminate/Reduce Runoff

Application Rates & Application –Instantaneous –Average –Total Application RATES

Impact Sprinkler Pattern Instantaneous Instantaneous Application Rate where water is hitting the ground Area being watered: Average Application Rate

Impact Sprinkler: Pattern Profile of Average Application Rate

Total Application for a Single Impact Sprinkler Total Application = Average Rate x Run Time IF –Nozzle q = 5 gpm –Nozzle throw = 45 ft THEN –The average application rate over the area to which water is applied (the green circle) is inches/hour (if all the water makes it to the ground) AND SO –If the sprinkler is run for 10 hours TOTAL APPLICATION is 0.76 inches

Pivot & Linear-move Application Devices

Instantaneous Application Rate Serrated Deflection Plate Depends on Where The Evaluation Point is

Instantaneous Application Rate Flat Deflection Plate Nozzle Water hitting ground in most of outside part of area

Application Devices with Distinct Streams

Exaggerated Rotator-style Pattern w/ 4 individual streams Instantaneous Instantaneous Application Rate Where water is Hitting the ground Area of Average Application Rate

Pattern Profile Illustrations

Total Application for a Single Applicator Total Application = Average Rate x Run Time IF –Nozzle q = 5 gpm –Nozzle throw = 25 ft THEN –The average application rate is inches/hour AND SO –If the sprinkler is run for 10 hours TOTAL APPLICATION is 2.45 inches (if all the water makes it to the ground)

Reality Check: Consider Overlap & Sprinkler Movement Individual Patterns are Overlapped for Uniformity –Typical hand/wheel line spacing of 40’ (& 50’ or 60’ in solid sets) –Adjacent applicators on a pivot/linear-move Consider the movement of the pattern as the pivot/linear- move advances –the instantaneous application rate the soil “sees” will change because the overlapped pattern is not perfectly uniform –Initially the soil “sees” a small rate, gradually getting more intense until it peaks, then gradually decreases again as the machine moves away

Wetted Diameter Soil One point in the field Instantaneous Application Rate Varies as Machine Moves over the field Application Rate

Instantaneous Application Rates under a pivot or linear-move Low Pressure Spray Moving Applicator High Pressure Impact Application Water Application Time (min) Application Rate (in/hr) Low Pressure Impact Shapes also represent overlapped pattern

Application Rate Summary Instantaneous Application Rate is the rate “seen” by the soil at one point in the field at one point in time – NOT influenced by speed of machine Average Application Rate is the average rate “seen” by the soil at one point in the field over the time it receives water– NOT influenced by speed of machine Total Application is total “depth” of water received by the soil in the field. Equal average rate x time watered

Questions - Comments ?

Soil Infiltration Rates Vary with Soil Texture Vary with Soil Structure Vary with time –Short term –Long term Vary with other factors

Soil texture defined by relative percentages of sand, silt, & clay

Infiltration Rate Varies With Time INFILTRATION TIME, (minutes) INFILTRATION RATE, (in/hr) Initial Rate is high Rate Drops Rapidly Silty soil Sandy soil

Soil Structure: arrangement & grouping of soil particles Good structure in finer grained soils provides small cracks, or macropores, that provide infiltration capacity Other things related to structure: –Compaction degrades structure –Organic Matter aids structure; residue at surface aids infiltration capacity –Soil and Water Chemistry can affect structure

Conditions that May Produce Surface Sealing Salts –Sodium tends to break down structure and produce “dispersed” conditions which lead to reduced infiltration rates –Calcium tends to bind particles together, improving soil structure and, thus, infiltration rates –These impacts typically seen in soils with large percentages of clay particles

Conditions that May Produce Surface Sealing Compaction/Sealing By Equipment By impact of water particles with soil –Intense rainstorm –Application Rate & Energy of Sprinkler Droplet; energy is basically a function of size of droplet –Silty soils more susceptible; aggregates broken down and the silt particles “float” on top, producing a seal –Effect is progressive, getting worse throughout the irrigation season Organic or other material added to soil surface

Sealing Effect on Infiltration Rate INFILTRATION TIME, (minutes) INFILTRATION RATE, (in/hr) Soil before sealing Soil after sealing

Questions - Comments ?

Application vs. Infiltration = Runoff ? There is potential for runoff when Application Rate exceeds Infiltration Rate. Runoff will occur if “surface storage” cannot temporarily hold the water not being infiltrated.

Instantaneous App rate “seen” by soil Surface Storage Potential Runoff Runoff Potential INFILTRATION TIME, (minutes) INFILTRATION RATE, (in/hr) 0.3 Intake Family Curve

Infiltration vs. Application Rates Low Pressure Spray Rotating Spray High Pressure Impact Application Water Application Time (min) Water Application Rate (in/hr) Low Pressure Impact Sand Silt

Questions - Comments ?

What can we do to eliminate/minimize runoff? Reduce Application Rates Choose application device with lower average rate – but be careful to consider droplet size and its effect, as well as pressure & energy costs Utilize booms to one degree or another –Zig-zag offsets –splitters –Mega-booms (structural issues to consider)

Reduce Application Rates: Booms

Booms: Reduction of Peak Application Rate Boom Offset from Pipe Zig-Zag Configuration Reduction* (%) 10 ft5 – 15% 15 ft15 – 30% 20 ft30 – 40% *Varies with application device Based on data from King & Kincaid

Instantaneous App rate “seen” by soil Surface Storage Potential Runoff Runoff Potential INFILTRATION TIME, (minutes) INFILTRATION RATE, (in/hr) 0.3 Intake Family Curve

Water time is increased Total Application is same Application rate “seen” by soil is decreased Potential Runoff` Reducing Application Rate INFILTRATION TIME, (minutes) INFILTRATION RATE, (in/hr) 0.3 Intake Family Curve

Reduce Application Rates: Booms

What can we do to minimize/eliminate runoff? Reduce Total Application per Irrigation Less Total Application takes advantage of initially high, then decreasing, infiltration rates Be careful! –Applying less per irrigation will actually require more seasonal water because of increased evaporation from the frequently wet soil surface. –Irrigating the entire root zone of the crop – getting water to the bottom of the root zone – is good practice. Lesser Total Applications may wet only the upper part of the root zone.

Water Application Depth Increases Runoff

What can we do to minimize/eliminate runoff? Increase & Maintain Infiltration Rates Choose application package that minimizes surface sealing: small droplets & low application rates PAM…& other soil/water amendments? Leave crop residue on & near soil surface Deep tillage If required, irrigate bare soil w/ small droplet and low application rate package

What can we do to minimize/eliminate runoff? Increase Surface Storage Reservoir Tillage –more effective on flat fields than sloping (0-2%, ¾; 2-5%, ½”; >5%,1/4”) “Mini ponds” tend to break down as the season progresses Choose application package that minimizes surface sealing; small depressions hold ponded water at soil surface; usually < 0.25” Maximize surface residue - intercepts and temporarily holds water

Runoff Potential Eliminated Surface Storage INFILTRATION TIME, (minutes) INFILTRATION RATE, (in/hr) No Runoff

Avoid a Wreck! Choose & Manage Wisely