• Function and appropriateness

• Rate and Duration / requirements

• System design considerations

‐ Climate‐ Soil Characteristics, and ‐ Hydrologic Properties

• Performance monitoring

• Methods – filtration ‐ supply

Irrigation Methods

Climate

• In humid to tropical climates or in densely planted crops, transpiration accounts for 55 to 90 percent of evapotranspiration 

(depending on plant density ‐ and the actual evaporation closely mimics the potential evaporation in wetlands)  Bachand et al., 2013… Humidity is high and PE is low…

What about in a semi arid climate?• In semi‐arid climates with dense native vegetation, plants have been shown to account for a max of 4 to 7 percent of total ET  – i.e. 90 + percent is lost by evaporation. Balugani et al., 2017

• Native, desert plants do well here because they can pull large matric suctions, outside of the PAWHC and they can simultaneously shut down day‐time transpiration – our cultivated crops can not!

•

A well designed and managed irrigation system 

regardless of method should provide for

optimal plant growth

while minimizing losses to

• evaporation • deep percolation• runoff

Key Irrigation Questions:

1) Rate and Duration2) Timing3) Method and Uniformity

Why do I care? Efficiency = more profit, less pollution 

higher yield with fewer inputs

How do I answer these?Sample your soilsMonitor results

hydrology – a few definitions

Infiltration - The vertical entry of water into the immediate surface of soil or other materials. Water moves under gravity and capillary forces (“suction”).

Infiltration Capacity - The maximum rate at which water can infiltrate into a soil under a given set of conditions.

The rate of infiltration is limited by the capacity of the soil and rate at which water is applied to the surface until Ksat is achieved

Percolation - Vertical and Lateral Movement of water through the soil by gravity.

Flow-through occurs when gravity dominates grain boundary forces and surface tension effects.

Irrigation Rate

Conceptualized Active Soil System

Irr = ET ± R ‐ NP ± S

Goal #1: Hold water in soil for crops• Maximize infiltration• Minimize Evaporation, • Minimize Runoff and erosion,• Minimize NP

losses

retention

Maximize Storage  ‐ we have to understand soil properties and calculate an ideal irrigation rate

Understanding Infiltration Rate

Single or Double ring InfiltrometersMeasuring Infiltration Rate

Flooded or Tension InfiltrometersMeasuring Infiltration Rate

Application ≤ Infiltration rate Application ≥ Infiltration rate

• Maintains soil structure

• Eliminates surface water buildup

• Minimizes soil compaction

• Small soil particles remain disbursed

with larger particles

• Maximizes and maintains the soil 

infiltration capacity

• Can alter soil structure ‐ silt and

clay particles flocculate forming a sealing        layer 

• Can result in nutrient loss/mobilization

• Can result in runoff and erosion 

• Can result in non‐uniform irrigation and  greater cost

• Sediment and nutrient rich returns can damage waterways

What about duration?Have to understand the Soil ‐Water relationships

Types of available soil moisture

Adsorbed and membranous water (hygroscopic water)

Capillary water

Gravitational water

Available vs. Actual – Soil Water

Soil water availability

Gravity drainage

We can simply apply water based on a plant response method “tried and true”, it does work…

however,

2. Identifying Irrigation Triggers

With limited water and tight profit margins, irrigation triggered by soil moisture relationships and can directly equate to “cash in hand” through increased yield

Modified from: Oliveira, M.R.G., Calado, A.M., and Portas, C.A.M. 1996

66% 55%51%

30% 44%47%

Approximatepoint of plant response

Two simple methods:

1) Calculate time – est E and Plant uptake, PSD, Literature values

Marginally better than plant response method because of seasonal variations in ET.

2) Identify soil-water relationships for each soil/field/unit, monitor real time volumetric water content or matric suction.

Low cost, simple way to trigger irrigation for cash crops…so how do you do this?

Identifying Irrigation Timing

Soil Water potential

Total soil water potential:t=m+s+g

The amount of work that must be done per unit of a specified quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a specified source to a specified destination.

Water moves from areas of high potential to areas of low potential

Soil Water potential

SoilA

SandSoil

10%

SoilB

Clay Soil15%

SoilSoilRoot

‐7

‐2

‐3‐.4

‐8

Saturated soil: moving force is the gradient of a positive pressure potentialUnsaturated: moving force is the direction of a negative matric potential

TensiometersMatric Suction 

Time Domain ReflectometerVolumetric Water Content

Data Needs 

• Flux calculations based on sensor data

• Collect sensor data at a 4‐hr frequency and real‐time outflow 

• Plot short duration, high resolution hydrographs following irrigation

• Plot pressure and water content data as depth profiles following irrigation

HDS – TC SensorsMatric Suction

Matric Suction from HDS 

• Plot pressure and water content data as depth profiles following irrigation

• Flux calculations based on sensor data

Combine TDR and HDS

Higher quality data, easier automation but more expensive 

Soil 1Soil 2Soil 3

Soil Water Characteristic CurvesThe relationship between matric potential or pressure head and volumetric water content for a particular soil is known as a soil water characteristic curve

The shape of soil water characteristic curves reflect the distribution of pore sizes in the soil - which is in turn controlled by grain-size distribution and sorting.

This is the single most important relationship to understand the soil-water relationships of a soil.

Unsaturated Hydraulic Conductivity

•Unsaturated soils have a lower hydraulic conductivity because some of the pore space is filled with air and thus cannot transmit water

•Soil moisture in the unsaturated zone travels through only the wetted cross section of pore space

•Unsaturated hydraulic conductivity is a function of water content of the soil K=K(θ)

WALKING

TROUT

FARM

• 1800’s Acequia dry and in disrepair• Limited flow in the river• 2‐inch surface pipe from spring• 12,000 gallons in storage, 125,000 secondary

• 4‐inch buried pipe to each field / greenhouse

• Gravity and pressurized options –limited flood

What about the Challenges?

Level Furrow

• Very limited use to small one field – crop specific• Not practical with our water supply• Typical setting ~ 5‐10 gpm/furrow and a length of ~100ft• High application rate, labor intense• Intensity>  than the steady state infiltration rate• Poor coefficient of uniformity

• Used for outdoor cover crops and shoulder season orchards• Typical setting ~ 50 gpm and a radius of ~50 ft (0.6 in/hr)• Medium application rate – nozzle dependent • Application rate <<  than the steady state infiltration rate• Fair to good coefficient of uniformity….  (+/‐wind speed)

Komet 163 Big Gun Sprinklers   (0.4 – 0.9 in/hr) 

• Used in greenhouses and high tunnels• Typical setting ~ 2.5 gpm/head and a radius of ~15ft• Low application rate • 0.5 in/hr <<<  than the steady state infiltration rate• Excellent coefficient of uniformity

Senninger I‐WOB

• Used in high tunnels and outdoor melon production• 2 tapes per 36 inch bed• Variable application rate based on crop and field• Moderate coefficient of uniformity over bed width

Drip under mulch

Drip under mulch

• Gravity or pressurized• Outdoor annuals

• Filtration?