“My tank is empty before the end of the watering round, do I need a bigger pump?” In most cases, no. A more powerful pump fills a tank that is still too small faster, and the problem comes back at the consumption peak. Tank capacity can be calculated, and the calculation takes three operations: the crop’s peak daily need in millimetres, its conversion into cubic metres over the real area, then the choice of an autonomy. The design FAO uses for solar pumping is to send a full day’s demand into a reservoir placed at height.
Peak demand takes two multiplications
A crop’s water need is the product of two terms: the reference evapotranspiration of the zone, written ETo, and the crop coefficient of the plant at its current stage, written Kc.
For ETo, the FAO manual on crop water needs gives indicative values by climatic zone and by mean daily temperature, in millimetres per day:
| Climatic zone | Under 15 degrees C | 15 to 25 degrees C | Over 25 degrees C |
|---|---|---|---|
| Desert or arid | 4 to 6 | 7 to 8 | 9 to 10 |
| Semi arid | 4 to 5 | 6 to 7 | 8 to 9 |
| Sub humid | 3 to 4 | 5 to 6 | 7 to 8 |
| Humid | 1 to 2 | 3 to 4 | 5 to 6 |
For pepper, the FAO crop water information sheet gives a Kc of 0.6 at the initial stage, 1.05 at mid season and 0.9 at the late stage. The same sheet gives close field markers: 0.4 after transplanting, 0.95 to 1.1 at full ground cover, and 0.8 to 0.9 at harvest. The stage lengths used are 25 to 30 days for the initial phase, 35 days of crop development, 40 days of mid season and 20 days of late season, for a total of 125 days, within a cycle range of 120 to 150 days from sowing to the last harvest.
Sizing is always done on the most demanding stage, so on the mid season Kc. That is when the tank has to hold, not on a cycle average.
From millimetres to cubic metres
One millimetre of water over one hectare is 10 cubic metres. That is the only conversion to remember. Peak demand in millimetres per day is therefore multiplied by 10 to get cubic metres per hectare per day, then by the area actually planted, in hectares.
Over the whole cycle, the FAO sheet puts the total need for pepper between 600 and 900 mm, and up to 1 250 mm for long cycles with several pickings. That total serves to check the consistency of the daily calculation and the availability of the resource across the season, but it does not size the tank. The tank is sized on the peak day.
One caution: the figure obtained is the crop’s net need. The volume actually abstracted depends on the efficiency of the network installed, which is not the same under drip, under sprinklers or with a watering can. That coefficient is determined on the plot, from the equipment actually fitted.
Capacity: one full peak day
The FAO presentation on solar powered water lifting for irrigation, prepared by Ahmed Abdelfattah in 2017, describes the most widespread configuration: a stand alone system that sends a full day’s demand into a reservoir placed at height, with the water then released by gravity and no booster pump. The benefit is direct: the time of pumping, which depends on the sun, is decoupled from the time of watering, which depends on the crop.
One full peak day is therefore the baseline target. Two situations justify going beyond it: a run of heavily overcast days in the rainy season, and the need to secure flowering. On that second point the FAO pepper sheet is clear: at the beginning of flowering, soil water depletion in the root zone should not exceed 25 percent. The largest yield loss is observed with a continuous shortage up to the first picking, and the yield response factor to water for the whole season is 1.1. A watering skipped at flowering is not made up later.
Storing water rather than electricity
The same FAO presentation compares the two ways of getting through the night and cloudy spells. The battery option allows full day operation, but according to that document the batteries and their mandatory replacement over time make the system very cost intensive. The FAO chapter on water lifting devices points the same way on service life: in tropical conditions it is often limited to four or five years, and unsealed batteries need regular topping up with distilled water.
A correctly sized tank plays the same role with no wearing part. That is why a solar pumping budget almost always gains from being settled in favour of water storage. For the choice of the pump itself, flow rate and total dynamic head, see our article on sizing the pumping of an onion plot.
Stand height gives the pressure, not tank volume
A 30 cubic metre tank sitting on the ground does not put a network under pressure. What creates pressure at the emitter is the height of the water surface above that emitter. Volume decides autonomy, height decides whether the system works.
The FAO chapter on drip irrigation notes that water is applied there at very low rates, 2 to 20 litres per hour per emitter, through waterways 0.2 to 2.0 mm in diameter, and that irrigation water must be free of sediments. Two practical consequences for a storage tank. First, the stand height is worked out from the rated pressure of the dripper chosen, not at random. Second, a filter at the tank outlet is essential, because an open tank collects dust, debris and algae, and waterways two tenths of a millimetre wide block with very little material. The other causes of blockage on a small plot are set out in our article on the mistakes that make small drip schemes fail.
A worked example on 2 500 square metres of pepper
| Item | Value | Origin |
|---|---|---|
| Sub humid zone, mean daily temperature above 25 degrees C | ETo of 7 to 8 mm per day | FAO crop water needs table |
| Pepper at mid season | Kc of 1.05 | FAO pepper sheet |
| Net peak need | 7.4 to 8.4 mm per day | ETo multiplied by Kc |
| Need per hectare | 74 to 84 cubic metres per hectare per day | 1 mm equals 10 cubic metres per hectare |
| Planted area | 2 500 square metres, or 0.25 ha | Plot |
| Peak daily volume | 18.5 to 21 cubic metres | Calculation |
| Target usable capacity | One full day, or 21 usable cubic metres | FAO solar pumping design |
The same calculation leads to very different tanks depending on the zone. In a humid zone above 25 degrees C, ETo falls to 5 to 6 mm per day in the FAO table, peak demand for pepper to 5.3 to 6.3 mm, and the daily volume on 2 500 square metres to 13 to 16 cubic metres. In a semi arid zone above 25 degrees C, with an ETo of 8 to 9 mm, it rises to 21 to 24 cubic metres. A capacity copied from a neighbour’s plot in another climatic zone is therefore wrong by construction.
Usable capacity is lower than nominal capacity
A tank advertised at 20 cubic metres does not deliver 20 usable cubic metres. Three losses have to be deducted at the point of choosing:
- The dead volume below the outlet connection, which never reaches the network.
- The air gap at the top, needed so that filling does not overflow.
- The sediment layer at the bottom, which cuts the workable volume between two cleanings and is better left undisturbed rather than sent to the filter.
Count capacity as usable volume, that is between the high filling level and the outlet connection, and not as the advertised volume. An opaque cover also limits algal growth, which lengthens the interval between two filter cleanings.
Upkeep of the pump and tank together
- Daily: read the level morning and evening, and check that filling took place during the day.
- Weekly: clean the outlet filter, check the float or fill cut off, inspect the stand and its fixings.
- Monthly: open the bottom drain for a few seconds to flush deposits, check the cover and the debris screens.
- Each season: empty, brush and rinse the tank, flush the network laterals, check the stand anchors.
The panels and pump set follow their own rhythm, set out in our article on solar pumping upkeep week by week. For the agronomic management of the crop itself, nursery, density and calendar, see the pepper crop technical sheet.
Frequently asked questions
What tank capacity suits a solar irrigated plot?
The FAO design for stand alone solar pumping sends a full day’s demand into a reservoir placed at height. One full peak day is therefore the baseline target, to be increased if the plot sees runs of heavily overcast days.
How is the peak daily need of a pepper plot calculated?
Multiply the ETo of your zone, read from the FAO crop water needs table, by the crop coefficient of pepper at mid season, which is 1.05. Then convert the result into cubic metres, one millimetre being 10 cubic metres per hectare, and multiply by the planted area.
How much water does a pepper cycle use?
The FAO sheet gives 600 to 900 mm over the cycle, and up to 1 250 mm for long cycles with several pickings, for a length of 120 to 150 days from sowing to the last harvest.
Are batteries needed with a solar irrigation pump?
The FAO presentation on solar pumping states that the batteries and their mandatory replacement make the system very cost intensive, and favours water storage in an elevated reservoir. The FAO chapter on water lifting devices adds that in tropical conditions battery life is often limited to four or five years.
Is an elevated tank enough to run a drip system?
The pressure available corresponds to the height of the water surface above the drippers, so the stand height is calculated from the rated pressure of the dripper chosen. A filter at the outlet is essential: FAO notes that emitters deliver 2 to 20 litres per hour through waterways of 0.2 to 2.0 mm and that the water must be free of sediments.
Can a watering be skipped at flowering?
No. The FAO pepper sheet states that at the beginning of flowering soil water depletion in the root zone should not exceed 25 percent, and gives a yield response factor to water of 1.1 for the whole season.
Having your installation sized
JOGOO sizes pump, tank and network sets from the records taken on the plot: available resource, head to overcome, area and crop. See the Irrigation & Pumping page for the range, and send your area, your crop and your water point through the Contact page to receive a costed proposal.
Sources
- FAO, Irrigation water management, Crop water needs, chapter 2, table 2 of average daily needs
- FAO, Crop water information, Pepper sheet
- FAO, Ahmed Abdelfattah, Solar Powered Water Lifting For Irrigation, 2017
- FAO, Water lifting devices, section on photovoltaic pumping
- FAO, Irrigation water management, chapter 6, Drip irrigation
