H20 Choices for the Farm

February 07, 2012

By:  Ira Myers, Georgia’s Finest Organic Farm

Water is by far the most important issue on the farm. Let’s face it, without it all you can do is hope Mother Nature will provide it. We in Georgia had 148 of the 149 counties declared a drought area in 2011. The year before was not much different and the temperature was abnormally high also. This means you need to know the numbers, amount and water timing of your drip irrigation.

Let us assume you have 27,000 running feet of beds and your drip tape emits .47 gals per hour per emitter and each is set at 12 inches apart. Let’s assume that we can get by with one gallon per day per plant. This is easy math meaning we need 27,000 gallons. Multiply that by $6 per 1000 and you get $162 per day or $4,860 per month or 18 cents per plant per month. Assume from planting to harvest 60 to 90 days and water alone can be drowning. Add your inputs and labor along with the rest of your cost and you begin to see this is not the method of choice. Note has to be made here that many counties or cities will not allow or have the ability to provide this volume of water on a continual basis due to drought. They may under extreme circumstances do so for a short term and may even then cut you off. That is why I didn’t even consider the cost of construction of the line to the property in this equation.

You also must have an appropriate PSI (pounds per square inch) to properly operate your drip irrigation system. Again it’s the math. Depending on your emitter and length of your drip tape will dictate the requirements. Low emitters in general need at least 8 to 10 PSI. What will happen if this is not me?  You will have an inconstancy in watering, meaning only some of your plants will get watered. You must know the volume per minute that your water source is giving. This will then dictate how much area you can water at one time. Using the sample above you need to have 225 gallons per minute if watering all at once. Not many wells can produce this volume. You will then set zones your source is capable of watering at one time. Just because you have a known factor of the PSI at the source does not mean it will be the same at all junctures in the system due to friction and elevations. The water running through a pipe will reduce the PSI along with the consideration of elevations.  It’s important to measure each drip line end to insure proper flow. To do this, take a one inch diameter hose two foot long and close one end. At the other end attached a metal coupling connector which is threaded so to be able to attach a PSI gauge.

All my drip tape ends are closed by using a drip tape connector with a valve turn off.  It’s the easy way to close the end of the drip tape line. This method allows you to monitor the pressure with the gauge. The hose I made has a hole punched in the middle of the line made by the punch tool used for drip line correctors to attach to the line place at the head of your bed.

What you now know is how many beds you can run at one time so that the proper solenoids can be in place to control watering by the controller. What you will do is run all the beds, then start to turn them off one by one until you find that each line has the necessary PSI. It will take you some time to do this, but you'll know for a fact, you have what you need. There is just no other way to know for sure.

Next is pond water. This is a science unto itself and would require more space than available for this paper. The internet has tons of paper on the construction, reduction from seepage, evaporation, method of collection, concerns of contaminates with appropriate controls and requirements for distribution. Let’s assume that you would have ample supply for the sake of our discussion. The major concerns are the cost of construction for distribution and higher risk of contamination by human pathogens.  Wildlife including birds can carry Salmonella spp., E. coli and parasites. When their feces enter pond water and you irrigate with the pond water, your produce can become contaminated.  The highest risk is for leafy greens or other fruit and vegetables that are not cooked.  Experts recommend that you test pond water at least quarterly for fecal coliforms to help minimize the risk.  I would hope this audience is already aware of the cantaloupe issue that killed about thirty people and why monitoring is important. Personally, I do not want this risk or the many other issues associated with using a pond as an irrigation source.

You've got to have the pump to move the pond water. Here is where your need to know what horsepower is required to move the water and maintain the proper PSI. We have already discussed how to calculate your needs so there are charts a plenty on the internet that give required horsepower necessary. The question then is source of power, generator or electric. Your location will dictate. A plot far from a power line will require a cost study to figure which method of energy source will be cost effective, IE cost of bringing a line in vs. cost for the generator and fuel.

Now to the well. Here too is not a short or simple discussion. Your farm plan will dictate you water needs. Your main concerns are drilling cost and pump requirement. First consideration is your acreage size which will begin to push you toward the size of your borehole. We will limit our discussion to a farm which would be cultivating 30 acres or less. You will need to bore a minimum of a six inch diameter hole which should allow for ample water. This can get complicated due to you can't stick your head in the ground to tell if there is water there. I've studied and seen the surest thing is have a seismologist study the area at a cost of a few thousand to pin point the best area and I've seen the old divining rod do the same believe it or not. Cost for drilling varies on several conditions which more or less depends on you location. The Piedmont will be going thru solid rock and the depth will depend on passing thru at least three fractures ample to supply a volume suitable to irrigate. Remember you only have 24 hours in a day, so your needed water could mean you need two or more wells to supply you. Further south and you hit the aquifers and here you need to go down to the third layer to insure two things. First, a constant supply, as we all know wells can run dry due to drought condition.  Second, contamination is in the first layer, believe it or not.

You can figure at least drilling cost of $12 to $15 a foot and a depth not uncommon of 400 to 600 feet or more. Your pump needs to be sized to the water flow. You don't want a under sized or over sized pump. A 2.5 Hp pump will do max 30 gallons a minute and this is going to be dictated by what is called the static level. Static line is the distance from ground level down the bore hole to where your water is when pumping. This will give 30 gallons when your static level is about 30 feet and only about 12 gallons if the static level is at 180 feet. So plan on $12,000 to $15,000 at the cheapest and the price can go anywhere. This does not include your power source. A constant flow of 30 gal per minute will only irrigate approximate area of 3,600 running feet when using a .47 gal emitter every 12 inches of drip tape. This is also assuming that you are maintaining a PSI of a minimum of 10.

The other factors are friction from pipe and land elevation. So you’re in between a rock and a hard place because you don't know how to truly set up your zones until you know how much water you have. What I mean is you want to automate the water system so you don't have to manually turn on or off beds for proper watering. You will lay out the beds and PVC and have to wait to cover them up to see have many solenoids you'll need to automate. One more consideration is a holding tank which will give you some flexibility in adequate watering by not stressing your well. A well is no guarantee of continuous water as they can go dry or recovery may not be as quick as needed.

Additional notes for micro irrigation from Deena Roberts, Area Engineer, Georgia Natural Resource Conservation Service:

1)    When planning your micro irrigation system, you should have the area surveyed or take some elevation shots to ensure your rows and drip tape do not exceed 2% up gradient or 3% down gradient.  If the slope is greater than this, you will get non-uniform distribution of water along your row.  If you cannot change your row layouts to decrease slopes you will need to use pressure compensating drip tape to ensure uniform distribution of water along the row.

2)   Surface water can be used with drip tape but additional filtration is required to prevent sediment or algae from clogging the drip tape.  The type of filtration for surface water would be a sand media filter which would be similar to a filter for a swimming pool.  

3)   Wells are a good source of water for micro irrigation systems but until a well is drilled and you know the flow (gallons per minute) of the well a micro irrigation system cannot be designed or a pump size determined for the well.  Wells in the Piedmont region of Georgia can vary from 5 gallons per minute or less up to 35 to 40 gallons per minute.  Wells that only produce 5 gallons per minute will most likely need to incorporate a storage tank to allow more acreage to be irrigated at a time.  

4)   You should get an engineer or irrigation designer to design your micro irrigation system.  This will ensure that the pump, pipeline, and drip tape are sized appropriately for uniform distribution of water.     

Farmers