Irrigation Management and Hydraulic Calculations

Posted on Jun 20, 2026 in Other university degrees and diplomas

Flow and Measurement

• Velocity = Distance / Time
• Q = V × A
  • Q = Flow Rate
  • V = Velocity
  • A = Area

Evapotranspiration and Scheduling

• ETc = ETo × Kc

Irrigation Efficiency

• IE (%) = (Beneficial Water Used / Total Applied) × 100

Application Efficiency

• AE (%) = (Water Reaching Target / Water Applied) × 100

Available Water Holding Capacity

• AWHC = FC − PWP

Maximum Soil Moisture Depletion

• SMDmax = AWHC × Root Zone × MAD

Gross Irrigation Depth

• Gross Depth = Net Depth / IE

Volume Calculations

• Volume = Area × Depth

Key Soil Water Terms

Saturation

• All pore space filled with water
• Tension = 0 bar

Field Capacity (FC)

• Water remaining after free drainage
• 0.1 bar = 10 cb

Permanent Wilting Point (PWP)

• Plants permanently wilt
• 15 bar = 1500 cb

Available Water Holding Capacity (AWHC)

• Water available to plants
• FC − PWP

Management Allowed Depletion (MAD)

• Percentage of AWHC allowed before irrigation

Irrigation Scheduling

Two Primary Questions

1. When to irrigate?
2. How much to apply?

Scheduling Requirements

• ETc
• MAD
• Efficiency

Crop Water Use

• ETc = Kc × ETo

Irrigation Timing

• Irrigate before SMD exceeds SMDmax

Efficiency and Uniformity

Distribution Uniformity (DU)

• Measures how evenly water is distributed

Efficiency

• Measures beneficial use of applied water
• Always less than 100%

Beneficial Uses

• Crop ET
• Salt leaching
• Climate control
• Soil preparation

Non-Beneficial Uses

• Excess deep percolation
• Excess tailwater
• Phreatophyte ET

Important Relationship

• Perfect scheduling: Efficiency ≈ DU × 100

Hydraulics

Pressure

• Force exerted on an area
• 1 psi = 2.31 ft of water head

Static Conditions

• Water is not moving
• No friction losses

Dynamic Conditions

• Water is moving
• Friction occurs
• Higher velocity leads to higher friction loss

Bernoulli Principle Concept

Pressure Head + Elevation Head + Velocity Head = Constant − Losses

Flow Measurement

Basic Equations

• Velocity = Distance / Time
• Q = VA

Device Types

1. Velocity devices
2. Head-loss devices
3. Critical-flow devices

Common Units

• cfs (cubic feet per second)
• gpm (gallons per minute)
• acre-ft/yr
• MGD (million gallons per day)

Water Rights

California Dual System

Riparian Rights

• Land touches the water source
• Priority claim
• No permit required

Appropriative Rights

• Permit-based system
• First in time = First in right
• Use it or lose it

Water Right Definition

Legal permission to use water for beneficial purposes.

Irrigation Systems

Surface Irrigation

• Furrow
• Border Strip
• Basin

Furrow Typical Values

• Length: 300–2600 ft
• Slope: 0.05–0.3%
• Flow: 5–50 gpm

Factors Affecting Distribution Uniformity

• Soil type
• Slope
• Land grading
• Flow rate
• Infiltration rate
• Opportunity time

Sprinkler Systems

• Hand Move
• Side Roll
• Solid Set
• Center Pivot
• Linear Move
• Traveling Gun

Distribution Uniformity Problems

• Wind
• Pressure variation
• Poor spacing
• Nozzle wear or clogging

Drip and Micro Systems

Components

• Pump
• Filter
• Mainline
• Manifold
• Laterals

Core Concepts

• Frequent small applications
• Deliver water directly to the plant
• Do not wet the entire soil surface

Major Distribution Uniformity Problems

• Pressure variation
• Plugging
• Poor filtration

Unit Conversions

Length

• 12 in = 1 ft
• 3 ft = 1 yd
• 5280 ft = 1 mile

Area

• 43,560 ft² = 1 acre

Volume

• 1 acre-ft = 43,560 ft³
• 1 acre-in = 1/12 acre-ft

Pressure

• 1 psi = 2.31 ft head

Exam Quick Solving Order

Scheduling Problems

1. ETc = ETo × Kc
2. AWHC = FC − PWP
3. SMDmax = AWHC × RZ × MAD
4. Days until irrigation = SMDmax ÷ ETc
5. Net Depth = ET replacement
6. Gross Depth = Net Depth ÷ IE

Flow Problems

1. Velocity = Distance / Time
2. Calculate Area
3. Q = VA

Soil Problems

1. Find FC
2. Find AWHC
3. Find SMD
4. Compare with MAD threshold