Introduction
Designing an irrigation system for drip irrigation requires understanding several key principles and calculations to ensure water efficiency, economic feasibility, and the correct delivery of water to crops. Below is an integrated guide on how to design and dimension a drip irrigation system.
1. Defining Objectives
When setting up a drip irrigation system, the first step is to define your goals. This includes identifying:
- Types of crops: The water needs vary significantly between crop types.
- Area of land: Knowing the exact area helps in calculating the required volume of water.
- Water availability: Understanding the peak water demand during the most water-intensive periods helps design an efficient system.
2. Units Used in Irrigation Design
- Pressure: Typically measured in pounds per square inch (PSI) for U.S. applications.
- Conversion: 1 bar = 14.5 PSI.
- Flow Rate: U.S. systems often measure flow rates in gallons per minute (GPM) for ease.
- 1 gallon per minute (GPM) = 0.06308 liters per second (L/s) = 3.785 liters per minute.
- Pipe Sizing: Pipe diameters in U.S. systems are often given in inches.
- 1 inch = 2.54 cm = 25.4 mm.
3. Calculating Pressure Loss
Pressure loss occurs due to friction as water flows through pipes, and it can be broken down into two types:
- Linear Losses: Caused by consistent friction along the pipe.
- Singular Losses: Caused by sudden changes in water flow direction or speed (e.g., bends or valve transitions).
The Blasius formula estimates the linear pressure loss in a pipe:
J=0.478×Φ−4.75×Q1.75J=0.478×Φ−4.75×Q1.75
Where:
- JJ = pressure loss per unit length (in feet or meters)
- ΦΦ = pipe diameter (in inches)
- QQ = flow rate (GPM or L/s)
4. Cavitation in Pumps
Cavitation happens when the available Net Positive Suction Head (NPSH) is insufficient. When pressure drops below the liquid’s vapor pressure, vapor bubbles form, leading to noise, vibration, and potential mechanical damage. Ensuring the system’s NPSH exceeds the pump’s requirement prevents cavitation.
5. Flow Rate and Pressure Relationship
The emitter’s flow rate and ramp pressure are related by:Q=K×HXQ=K×HX
Where:
- QQ = nominal emitter flow rate (GPM or L/h)
- KK = constant
- HH = pressure (feet or PSI)
- XX = exponent characterizing emitter tolerance to pressure variations
6. Water Needs of Crops
Water needs depend on soil and climate factors:Ib=InEa=Kc×Kr×ET0EaIb=EaIn=EaKc×Kr×ET0
Where:
- KcKc = crop coefficient
- KrKr = regional coefficient
- ET0ET0 = reference evapotranspiration
7. Irrigation Dose
The net irrigation dose is calculated using soil properties:DNM=f×(HCC−HPF)×Z×PSHDNM=f×(HCC−HPF)×Z×PSH
Where:
- ff = crop-dependent factor
- HCCHCC = soil field capacity
- HPFHPF = wilting point
- ZZ = root depth
- PSHPSH = percentage of soil effectively wetted
8. Irrigation Frequency
The interval between irrigation applications can be calculated by:I=Net Dose (in inches)ET0 (in inches/day)I=ET0 (in inches/day)Net Dose (in inches)
9. Irrigation Duration
The total irrigation time is:T=Peak Daily Water Need (in inches/day)Emitter Precipitation Rate (inches/hour)T=Emitter Precipitation Rate (inches/hour)Peak Daily Water Need (in inches/day)
10. Minimum Number of Irrigation Zones
Calculate the minimum zones needed:Ns=DfTNs=TDf
Where:
- DfDf = maximum system operation time per day
- TT = maximum irrigation duration per day
11. Equipment
Filtration System: Essential for keeping emitters and pipes free from clogs. Systems include hydrocyclones, sand filters, and screen filters.
Emitters: Choose between non-regulating (sensitive to pressure variations) and self-regulating (providing consistent flow despite pressure changes).
Injection Systems: Used to add fertilizers or other chemicals to the irrigation water.
Accessories: Components like valves, elbows, reducers, and end caps are essential for system construction.
12. Uniform Water Distribution
Assess uniformity by measuring the flow at multiple emitters and calculating a coefficient of uniformity (CU):
- If CU > 90%: No intervention needed.
- If 80 < CU < 90%: Clean the system.
- If CU < 80%: Investigate and address the underlying issue.
13. Pump Selection
Total Dynamic Head (TDH) is calculated by adding pressure requirements and head losses due to friction and fittings. To calculate pump power:Pp=TDH (feet)×Flow Rate (GPM)102.2×Pump EfficiencyPp=102.2×Pump EfficiencyTDH (feet)×Flow Rate (GPM)
This guide provides a structured approach to design a drip irrigation system. Adjustments can be made based on the specific needs of the crops, climate, and available resources.