Designing: How to design a Drip irrigation System

 Introduction:

As drip irrigation is already introduced in "introduction to drip irrigation". This article aims to explain a general design for an efficient drip system. This  will involve some engineering concepts, fundamentals of fluid mechanics and mathematics.

Considerations:

Following steps are fundamental to design drip irrigation system for a given crop, area and water availability:

• Survey of the area

Find the total area that needs to be irrigated using the drip system. Then determine the topography and relative position to the highest point from the pumping unit. Determine the type of crop (vegetables and drip system are a perfect match) and find the power source, also, locate where the pumping system is located. Determine where Main lines, submain lines and lateral will be laid out. Refer to the image below.

 

• Water quality consideration

Emitters in a drip system have very small openings and can be easily clogged by dust particles, salts and other solid media. Analysis of water tells about the presence of chemicals like Calcium carbonate, calcium sulfate, carbonates, silicates, sulfides etc., and suspended solids like sand, silt, clay, algae and bacteria. Calcium and iron precipitates are a potential problem with most of the well water. If bicarbonate level is higher than 2.0 meq per liter coupled with pH > 7.5, it is a major problem for emitter clogging. Although filter media, hydro-cyclone filters and flush valves can solve this problem it is usually a standard practice to analyze water quality.

• Soil type

In light soils distribution of water will be narrow and deeper while in heavy soils distribution will be spherical, wider and shallow.

• Emitter selection

The emitter selection is based on the soil type i.e infiltration of the soil ( how well the soil allows water to penetrate).

The table above describes the required discharge (Lph) for each type of soil with a suitable value of emitter spacing (cm).

• Peak water requirement

The next step is to determine the system flow requirement or required discharge that needs to be provided to meet the peak water requirement of the crop. The equation to calculate this is a function of  Area (m^2), Emitter flow (lph), Emitter spacing (m) and row to row spacing between laterals.

• Design of lateral, sub-main and main lines

As a rule of thumb the head loss ( Pressure loss) in the main and sub-main line should not exceed 1m/100m of pipe length whereas, velocity of water inside the pipe should not exceed 1.5m/s.

The head loss in the lateral should not exceed 20% of operating pressure of the emitter.

Note: The operating pressure of the drip irrigation should be about 10-30 psi. 

Head loss can be calculated using the Dacry- Weisbach formula.

In the above formula the L is the length of the pipe (m), Q is the dischage (m^3/s) and D is the diameter of the pipe (mm). F is the friction factor and can be calculated using different formulas.

It depends on the internal roughness of the pipe, Reynold's number (Re) and internal diameter of the pipe.

Most importantly, The formula to calculate f (friction factor ) will change depending on the value of reynold's number (Learn more about Reynold's number here!).

• Selection of Pump

Discharge and Head are the main terms which are considered when selecting a pump for this purpose.

The discharge which should be needed to be produced by the pump can simply be calculated by multiplying the discharge of one emitter by the number of emitters in the irrigation zone.

Moreover, the pump should be able to overcome all the energy losses (head loss) in the main line, submain line, and laterals.

The selection is usually made using a pump characteristic curves (Q-H curves).

For a required head against a given discharge one can find the suitable pump by finding the RPM required for a pump. This information can be given to a pump manufacturer to pin-point the suitable machine.

• Selection of Prime Mover

Prime mover which is the motor required to power/run the pump can be found using a simple formula given below.

Hp = (Q.H)/(76*efficiency of system)

where, Q = discharge( lps)

            H = Head (m)

Sample Problems

1. 

2. 

               

These examples build up and add on the concepts previously discussed and are a part of the calculation of an actual system based in Pakistan.

Conclusion:

A drip irrigation system design based on the above steps should be sufficient to cover most applications of this technology. However, some considerations/ assumptions may be needed for specialized cases.