Answer: I am pleased to provide you with information regarding designing a gravity fed drip irrigation system.
For every 1' (12 cm) of elevation above the system you will gain .433 PSI (.030 bar). This means that if your water source is 10 above the system you will have 4.33 PSI (.30 bar) at the start of the system. (.433 x 10 = 4.33). Mind you, this is significantly lower than most drip irrigation guides will recommend. You can compensate this by creating a slope from the beginning to the end of your field, as 10’ is quite high for you to place your barrels. Another consideration is the platform in which the barrels will reside. This will be withstanding a lot of weight, so it must be very sturdy. The 100’ distance is a significant amount of distance to travel with low pressure and little to no slope. Reducing the bed length would help alleviate the worry of your irrigation water not reaching, and thus reducing yields, towards the end of the bed.
Water flow rate is typically specified in gallons per minute per 100 feet of tape (gpm/100 ft) or by the emission rate of a single emitter in gallons per hour (gph). Tape flow rates typically range from 0.2 to 1.0 gpm/100 ft. For vegetable production, tapes with flow rates around 0.5 gpm are often used. Maturing vegetables grown in the northeastern United States require about 2 to 3 hours of irrigation per day during hot summer days when a 0.5 gpm/100 ft tape is used. Most drip tapes emit water at about 25 gallons per 100 feet per hour when operated at l0 psi pressure, so you can cut that in half for your gravity fed system to 12 gallons/ 100 ft. per hour given the pressure that I mentioned above.
For how long should you water?:
The NCAT Guide to Efficient Irrigation has a handy calculation to determine this correct set time=net water application (inches) x irrigated area (sq. ft.) / flow rate (gpm) x 96.3 (this is a conversion factor) x system efficiency. Using this calculation, I have come up with the following set time for one 100’ bed on your farm:
Time(hours) = 1” x 300 sq. ft / .5 gpm x 96.3 x 85%= 7 hours. Given this calculation, each 100’ bed will require approximately 84 gallons/ week if you do not have any rain to supplement your irrigation. For more information on accounting for rain in your system, please see the ATTRA publication Soil Moisture Monitoring.
Drip Tape size:
Diameter of the drip tape is important to consider in system design and is chosen based on row length. Row length directly affects both the flow rate through the tape and pressure loss in the tape. The decrease in tape diameter causes an increase in pressure in the drip system. For 100’ length bed 5/8” diameter is often used.
Since you will have very little pressure, you will want to limit the number of beds that you irrigate at one time. This can be done by installing a ball valve at each bed. The number of beds that you can feasibly water will take your experimentation. It may only be 1-2 beds. This will be apparent if the water is not appearing at the end of the drip line.
The Pennsylvania Ag Alternatives Center has developed a good overview of establishing a drip system This will at least give you an idea of the components that you will need to purchase and the relative cost of them. I have listed the PDF link to this publication at the following link:
The Dripworks company has consultants that can help you design a system. They have a good introduction and tutorial to many of the concepts I outline in this letter at the following link.
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