Florida Crystals Drip Irrigation Project Design and Pricing Assumptions Including Experimental Design Considerations

Transcription

1 Florida Crystals Drip Irrigation Project Design and Pricing Assumptions Including Experimental Design Considerations The project intention is to test drip irrigation on approximately 160 acres in Martin County, Florida. The following are the assumptions used when designing and pricing the system. All costs are estimates at this point, pending final design and material pricing at the time of purchase. Irrigation Supply Consumptive Use of.275 acre inches/day. The lowest flow reliable emitter flow appears to be about.25 gallons/hour. The 15 mil tape thickness is resistant to damage, and is tolerant of higher pressures, which is useful when designing the flushing system. A two zone set operating schedule allows pulsing the irrigation through the daylight hours one zone is operating while the other is off to allow aeration of the wetted area under the emitters. Plants to not transpire after sunset, so applications of drip irrigation to root balls under the emitters during the night can be damaging. Therefore, the application period per day would be 7 hours per zone 14 hours total for the longest days, which is also the period of peak demand. Given the soil texture in the fields, the emitter spacing in the lines would be 18 (Sugar cane roots will grow to the wetted areas with emitter spacing up to 36 in the lines). This results in a 50% overdesign of the system (see section below on emitter spacing, and accompanying worksheet). There are 11 fields, which cannot be divided evenly into two operating zones. Assuming that an operating zone set would involve 6 fields in one zone set and 5 in the other, the maximum flow required would be about 2,000 gallons per minute. If the plumbing was designed to serve only two zones, the pipe sizes would have been extremely large. Therefore, the field was divided into 4 zones, with three fields in three of the zones, and two in the other. The maximum pipe size for any run would be 10, which can be laid and glued with normal procedures. Larger pipe is more difficult to install, and may require welded HDPE, which is much more expensive than PVC. Breaking the fields up into more zones avoids this situation, but would require more main lines, flush lines, etc. The limitation is still the two-zone operating set, so two zones would operate at once. The 4-zone setup will allow additional flexibility when determining operating schedules, etc. This will also aid in fertigation, since the smaller pipe sizes will allow for better distribution of the fertilizer. Mains, submains, and flush lines would be located in roadways on the north, center, and south of the fields, or along north-south ditchbanks. All field

2 components would be below ground to avoid damage. Pipes crossing the ditches would be only in the centers of the fields, and protected from damage. Submain and flush line connections to drip tape nearest the soil surface would be to polyethylene oval hose manifolds. The poly oval hose is flexible, and will withstand heavy equipment travel with shallow depth. PVC supply lines would be installed with a minimum of 18 soil cover, with thrust blocks at 90 s and tees. Installation prices assume no rock encountered when trenching in the pipe. If rock is encountered, the trenching cost could be higher for those areas. Pump Station The system would use a diesel motor powering a line shaft turbine pump, installed in wet well connected to the north canal. The filtration would be provided by Netafim Apollo self-flushing disc filters. Sand media filters would be about the same cost, but would be higher maintenance. Pump start and stop, zone and flush line control, fertilizer injection, and filter flushing would be completely automated. Fertilizer will be supplied from 4,600-gallon A and B tanks containing open hydroponics liquid fertilizer solutions. Normal setup would be 6 individual tanks, allowing custom blending of the major elements, but the A and B setup can work with frequent deliveries. If nutrient program experiments are desired, the 6 tank setup would allow different schedules. Pump station would have two sections, one for the filter and control, and the other, with a higher roof, for the two fertilizer tanks. The fertilizer tanks would be in containment. General Considerations Installation labor costs are based on Arapaho Agricultural Services prices. Owner or contractor supplied labor would probably cost about the same. If electric power were available, the cost of the diesel motor would be deducted, and replaced by an 85 hp electric motor. Some additional electrical gear would be required. The remaining components would be about the same cost. The installation cost of the drip tape assumes the amortization of any necessary new equipment over the cost of this job. Experimental Design The desire is to find the optimal design and operation systems to minimize cost and maximize sucrose production, thereby enhancing profits when growing sugar cane on sand land. Accordingly, a number of possible variables could be researched in the initial installation. Experiments with different layouts or operating regimes should be

3 established prior to finalizing the design and pricing, since flows may change, requiring different pipe sizes, etc. When considering possible experiments, the variables that could be researched can be divided into two categories; A) The relationship of water and nutrient requirements to the sugar cane plant, and B) The physical characteristics of the irrigation system itself. Possible examples would include: A) Water and nutrient requirements of the sugar cane plant with drip irrigation 1. Peak water use demand for sugar cane during the various growth events (phenology) of the crop cycle. The figure used for design is.275 inches per day peak demand. During the development of the cane plant, the demand will only rarely reach this level most of the time it will be less. What is the accurate estimate of water demand as related to the phenology of cane for Florida? 2. Actual phenology-based nutrient requirements for sugar cane. This is not the amount of fertilizer usually applied it is the amount of nutrients the plants actually need. The amount applied compensates for soil effects, whereas with open hydroponics drip systems, the soil is only peripherally involved, allowing direct application and subsequent uptake of nutrients that approximate the actual plant requirements. We may know the amount of nutrients necessary per ton of cane, but do we know what percentage of that total is necessary each month? With drip, the application can be fine-tuned accordingly. 3. Nutrient balances for various stages of the crop cycle growth events (phenology) of sugar cane. For example, the plants may need different amounts of nitrogen relative to phosphorous, potassium, and other nutrients during each growth event. Drip irrigation on sandy soils allows the adjustment of the nutrient balances on essentially a daily basis, so growth and sucrose levels can be optimized, and cost reduced. The fertilizer mixes in the A and B tanks would always contain all the necessary nutrients for plant growth but the ratios would be different through the year, usually on a monthly basis, based on the phenology of the growth cycle. 4. Operating schedules. The pulse schedule will depend on the depth of wetting from a single application. In sand, most of the root systems will be in the top 12 of soil, with most of the remainder being in the next 6. Few roots will occur below 2 feet deep. Therefore, the optimal pulse length will be that necessary to achieve about an 18 depth of wetting. The next question would be the time between the end of one pulse and the beginning of the next. This is based on the percolation rate of the soil, and evapotranspiration (mostly the water extraction by the roots since the system is buried). Sufficient time between pulses must be allowed to achieve good aeration in the wetted zone. It is the air management, not the water management, that allows roots to grow immediately under the emitters. The pulse:rest relationship changes as the plants grow. At first, as the plants are small, the pulse length will be short, and the delay between pulses will be long. As the plants grow to their maximum size, the pulses will become longer

4 since the plants are taking up water that previously percolated down, and the delays become shorter because both the water use of the plants and the percolation allow aeration of the wetted areas. Therefore, the pulsing schedule will be an ongoing challenge. It must be monitored and changed frequently, sometimes every few days. There is no right amount, so that is not the experimental challenge. Rather, there are a number of ways to decide whether the pulse schedule in use at any time accurately reflects the demands of the plant and characteristics of the soil. As such, a series of experiments that include different ways of monitoring and calculating water use would be the most important operational consideration. B) Physical characteristics of the irrigation system itself 1. Depth of drip tape installation. This is perhaps the single most critical consideration. The largest risk to this installation, in my opinion, is the possibility of damage to the drip tape. In the systems we ve installed, there has been damage from animals to the tape installed at a depth of about 4-6. However, this is the depth recommended by the drip irrigation engineers in virtually every country where drip irrigation is used in sugar cane. It is presumed that if the tape is installed at a depth of about 8 inches, the performance will be retained and the damage will be avoided, but this is unknown. Therefore, some thought should be given to comparing depth of installation, with shallower depths perhaps using a heavier wall (and more expensive) tape. 2. Distance from the cane rows. The danger here is that the root systems of sugar cane are fibrous and vigorous, so if the lines are too close to the plant, the roots will pinch the drip tape shut, thereby cutting off the flow to the remainder of that line. The line spacing from the plant will therefore be a trade-off between irrigating the seed pieces vs. avoiding root pinching. Conventional thought would be to space the lines 12 to 18 away from the cane row. The closer the lines, the better they will wet the seed pieces. The further away, the less likely the roots will pinch the lines. Therefore, it would appear to be a good idea to try different distances from the cane rows, as well as different depths. 3. Emitter spacing. This is actually a very complex issue. The emitter spacing along with the emitter flow and row spacing determines the flow rate of the system, so changing the emitter spacing has major impacts on all other considerations. Ideally, the flow rate of the system would be the amount of water required to supply the peak needs of the plants in 14 hours of operation, which is the day length of our longest days that typically occur during peak demand periods. Any amount of water applied in excess of the plant needs in 14 hours is an overdesign. Once the plant needs are supplied, applying more water would be damaging, so the application period would have to be adjusted accordingly. The emitter spacing is one of the variables that determine the flow rate per acre. The optimal spacing can be derived by multiplying the number of emitters per acre by their flow rate per hour, and then multiplying that by 7 hours (2 zones

5 alternating for 14 hours) to determine the peak application rate per day. See the accompanying table for examples of these equations. In this case, the 18 emitter spacing was an arbitrary choice based on the soil type and need for wetting the cane row during germination of the seeds pieces and tillering. The.275 /day/net acre is the highest water use demand recorded in the two must humid climates where cane is grown, Brazil and India, so the assumption is that Florida s is similar. Another variable is the emitter flow rate. A flow rate of.25 gph is generally considered to be the lowest flow emitter that is reliable smaller flow rate emitters can plug up more easily. Therefore, the emitter flow rate becomes a constant in the equation. The other constants are row spacing, which is fixed, and the hours of operation per day, with 7 hours being an ideal time for a system with two operating zone sets so that one set can be applying water while the other is resting. Using these constants and solving for emitter spacing, the ideal emitter spacing for sugar cane in Florida with 5 rows would be 27. This may very well be sufficient for producing cane on sand lands, but there is some concern about the sandy texture of the soil, so the decision was made to use 18 for the emitter spacing in the proposed design. The 18 spacing results in about a 50% overdesign, meaning that the system would not need to operate over the full 14 hours to supply enough water to meet peak demand. As a result, any overdesign translates into higher pumping and material cost, since the flows are higher than necessary. In this case, some overdesign appears to be advisable as a first attempt to make sure that all eventualities are covered. The primary factor that changes in this case is the pulse schedule. The determination of pulse length is based on the time required to move water down to about 18 in depth, followed by a rest period so that root extraction and percolation can allow aeration of the wetted area. This is required to achieve high root densities under the emitters. With closer emitters at the same flow rate/emitter, the amount of water applied per foot of plant row is greater, which means that percolation would be higher since the same amount of water extracted by roots would be less per emitter since there are more emitters per foot of row. Therefore, the pulse lengths would have to be shortened, and the aeration under the emitters would take longer between pulses. If the emitters are moved to 12 spacing, the result would be a 125% overdesign, meaning that the system would have to operate only a few hours a day to supply the peak demand. Pump capacity, main lines, submains, filters, valves, etc., would all have to convey over twice as much water as the plants demand. Along with significantly higher installation and operating costs, the pulse durations would become very short using the.275 /day peak use, the pulse duration would only be about 13 minutes each with 12 emitter spacing at.25 gph. When a zone set is turned on, it takes a few minutes for all the lines to pressurize. If the pulse durations become too short, then there would be

6 variability in each application because some lines pressurize before others. Therefore, 15 minutes is about the shortest pulse advisable, with 20 minutes being a more comfortable duration. The latter happens to be the nominal pulse duration with 18 emitters. The only way to lengthen the pulse duration would be to use lower flow emitters. To optimize the pumping capacity at a 12 spacing, the emitter flow rate would have to be.11 gph. There are no emitters available with this low of a flow rate, so this is not a possibility. There are some emitters that flow at about.16 gph, so if the 12 spacing is to be tested, it should probably be with these very low flow emitters to avoid the need for extremely short pulse durations. Since the flows would indicate that an emitter spacing of about 24 would actually be more appropriate, experiments involving emitter spacing should include that spacing as well. This would be the approximation of an optimized system. There is some evidence that the peak demand may not be as high as the.275 acre inches/day used in these calculations. If the lower rate of.185 inches/day is used, which is more typical of various crops in Florida, the optimum emitter spacing would be 36, as shown on the bottom equation in the accompanying sheet. Therefore, this emitter spacing should be tested as well. Higher flow emitters would reduce the possibility of plugging even.25 gph is pretty small. However, as the equations show, this would result in the optimum emitter spacing extending out to 43 inches with the next highest flow emitter,.4 gph clearly too far apart. So, this option is not considered viable, and would not require testing. 4. Other variables in flows and durations. The optimal application rate essentially has one fixed variable the row spacing - and the rest are assumptions. These assumptions can all be challenged, and experiments installed to test the validity of them. The 7 hour per day figure is used to minimize the necessary flow rates. This has the effect of most closely matching the cost of the system to the required performance. If the system is idle for part of the day during peak use periods, it is over-designed, and therefore the cost is unnecessarily high. However, since sugar cane is a C-4 plant, it may be that there is some water consumption in the evening, too. So, the daily available uptake period may be different with cane, and should be monitored. The Peak Use figure is viewed as a constant, but it may very well be too high. It is not likely that it is too low. If the water demand is less than anticipated, the system flow rate in future installations could be less. The way this would be accomplished would be to move the emitters further apart because the row spacing would still be the same and the emitter flow rate is already the minimum. Or, the answer would be to operate the system for less time per day. That will be the case, actually, in all but the few days each year when the demand is at the peak. If, however, there is substantiation for a lower demand rate for Florida, the flow of the system could be reduced. There are lower flow drip emitters than those specified. The assumption is that they will plug up easier. However, lower flow emitters could be tested to

7 see if they indeed do perform reliably. If so, the emitter distance could be reduced without such a large impact on the flows. The biggest assumption is that the emitter spacing in the row doesn t matter, because the roots will grow to the emitters regardless of spacing, which is the case in every other country where drip irrigation is used in sugar cane. To measure this effect, root growth rates could be monitored to determine how long after planting it takes for the roots to reach the wetted areas of the emitters.