Solar Pump for Brackish Water: Size for the Salt, Not Just the Head

The short answer: a solar pump for brackish water is the same machine it would be in a fresh well — and a completely different project. The pump does not know or care that the water is salty; what the salt changes is who drinks the water and what it costs to keep them safe. Three numbers move before the pump is even discussed. First, the FAO leaching requirement: every cubic metre of salty irrigation water carries salt onto your field, and washing it past the root zone takes 5 to 20 percent extra water depending on your crop — water your pump must lift every single day. Second, the 40:1 rule: where fresh water floats over salt, lowering the pumping water level by a quarter of a metre can raise the salt interface underneath by ten metres, so pump depth becomes a precision decision. Third, materials: salt water conducts electricity well enough to run real corrosion currents, which is why the stainless-steel ladder — 3DSC and 4DSC three- and four-inch pumps, up to the 6DSC and 8DSC six- and eight-inch machines — exists in our catalogue alongside the plastic 2DPC and 3DPC series. Size for the salt first, then buy the pump that serves that number.

Everything below is published arithmetic. The leaching fractions, the blending numbers and the interface figures can all be checked with a pocket calculator, and the source ladders (USGS salinity classes, WHO taste ratings, FAO crop thresholds, National Academy of Sciences livestock limits) are cited where they appear.

What counts as brackish — the ladder that decides everything

“Brackish” is not one thing. The USGS classification, used across groundwater reports worldwide, splits dissolved solids into classes, and each class behaves differently:

Two reference points anchor the ladder. Sea water sits at about 35,000 mg/L, the top of the scale. And drinking-water palatability collapses much earlier than irrigation: the WHO taste panels rate water “excellent” below 300 mg/L, “good” to 600, “fair” to 900, “poor” to 1,200 and unacceptable beyond. So a single aquifer can serve a household and defeat a maize field at the same time.

For irrigation the crop, not the palate, sets the bar, and the conversion between water salinity and soil salinity is where the real arithmetic lives.

The leaching requirement: salt adds a second, invisible water demand

Every litre of irrigation water carries dissolved salt onto the field. The plant drinks the water and keeps the salt. Unless part of every irrigation passes clear through the root zone and carries that salt away with it, soil salinity climbs season after season until yield falls. The portion of applied water that must drain through is the leaching requirement, and the FAO irrigation standard (Ayers and Westcot, Irrigation and Drainage Paper 29, after Rhoades 1974) gives it in one line:

LR = ECw / (5 x ECe - ECw)

ECw is the salinity of the irrigation water, ECe the soil-salinity threshold your crop tolerates (from the Maas-Hoffman tables: barley 8.0 dS/m, wheat 6.0, sugar beet 7.0, alfalfa 2.0, maize 1.8). Run the formula at three realistic water salinities and the crop ranking does the talking:

Read that table as a pump-sizing document, because that is what it is. Each percentage is extra water beyond the crop’s own demand, forever. A wheat field on 2.5 dS/m water needs about 9 percent more water than the evapotranspiration tables claim — and if maize was the plan on the same water, 38 percent extra is usually where the project stops being an irrigation project.

The leaching percentage converts to litres mechanically. Take the worked example we will finish in the sizing section: 1 hectare of wheat, peak-month demand 6.5 mm/day, so the salt tax at ECw 2.5 adds 9 percent:

• Crop demand: 6.5 mm/day x 10 = 65 m3/day per hectare
• Salt tax: 65 x 0.09 = 5.9 m3/day
• Water the pump must actually lift: 70.9 m3/day, not 65

That 5.9 m3/day difference is invisible in every evapotranspiration table and decides the model on the ladder. It is also free to compute, which is why the leaching check belongs before the panel count, not after it.

Who is drinking: crops and animals have very different limits

Salt tolerances are species-specific, and the gaps between them are where brackish water becomes an asset instead of a liability. Crop thresholds (Maas-Hoffman, the tables behind FAO practice):

Livestock run on a different scale entirely, set by the National Academy of Sciences (1972, 1974) tables that the FAO reproduces: total dissolved solids under 3,000 mg/L are satisfactory for all classes of livestock and poultry; 3,000-5,000 stays acceptable for cattle, sheep, swine and horses but is poor for poultry; 5,000-7,000 remains usable for ruminants on dry feed while excluding poultry; beyond 10,000 mg/L the risk is judged too great to recommend for any animal. Adult sheep top the tolerance table, poultry sit at the bottom:

The practical consequence: a brackish well that no vegetable crop will accept can still run a sheep enterprise or a beef herd without any treatment at all — which is why the livestock watering and cattle ranch guides matter on salty country. Match the enterprise to the water before matching the pump to the enterprise.

The 40:1 rule: where the pump hangs decides what it drinks

Many brackish aquifers are not uniformly salty — they are stratified. Fresh recharge floats on older, denser saline water the way oil floats on vinegar, and the equilibrium is governed by the density difference. The classical Ghyben-Herzberg relation for coastal aquifers gives the ratio:

depth of interface below sea level = 40 x freshwater head above sea level

The factor 40 falls out of the densities (1.025 / 0.025). It is a coastal formula, but the stratification logic travels inland: in many basins the freshest water sits nearest the water table, and salinity climbs with depth. Two consequences for pump placement:

1. A shallow-set pump drinks the freshest layer. Setting the pump just below the dynamic water level — rather than as deep as the borehole allows — skims the fresh lens. Our borehole yield-test guide covers the pumping-level arithmetic; the salinity profile is one more reason the screen and the pump belong at the shallow end of what the well can support.
2. Over-pumping pulls salt up to meet the pump. Draw the fresh layer down and the interface beneath rises — in the equilibrium ratio, about forty metres up for every metre of head you remove near the coast, and measurably inland as well. A quarter-metre of extra drawdown “invites” the salt ten metres closer. Wells that turned salty in their first heavy season were usually pumped past their interface, not cursed.

The honest corollary: this is also why salinity sometimes worsens over years, not seasons. If your neighbour’s well two years ago was sweet and today is not, compare pumping levels, not luck.

What salt does to a pump: electrochemistry, not fiction

Salt water is simply better at being an electrolyte. Electrical conductivity climbs with dissolved solids, and every conductivity gain strengthens whatever galvanic and electrochemical cells exist between dissimilar metals in the pump, its fixings and its earthing. The corrosion-protection industry works in exactly these terms: to stop corrosion on steel in contact with an electrolyte, cathodic-protection practice (the criterion used across pipeline standards) shifts the metal potential at least 300 millivolts negative — an entire discipline built on the fact that corrosion in conductive water is a measurable electric current.

Two other salt mechanisms are quieter but just as real:

• Pitting of stainless steel. Not all stainless is equal. The pitting-resistance equivalent PREN = %Cr + 3.3 x %Mo puts ordinary 304 stainless around 18-20 and molybdenum-bearing 316 around 24-26 — which is why “marine grade” means 316, and even 316 pits in warm, stagnant chloride water and loses the stress-corrosion race above about 60 °C. In the moderately saline band, stainless grade and water temperature both matter.
• Crystallization. Wherever brackish water evaporates — splash zones, seals, the waterline inside a tank nipple — it leaves salt crystals that grind clearances mechanically. The wear is abrasive, and the plastic versus stainless impeller trade-off reads differently once you add crystals to sand.

None of this means brackish water destroys pumps. It means the wetted-materials choice is now a rated decision, which is exactly why the catalogue splits the way it does.

The materials ladder: which Trista series for which water

The catalogue separates plastic-impeller pumps (the 2DPC and 3DPC two- and three-inch series) from stainless-steel-impeller pumps (the 3DSC, 4DSC, 6DSC and 8DSC three-, four-, six- and eight-inch series). Plastic does not corrode at all and shrugs off chlorides; stainless resists pitting where mild steel would not survive a season. For brackish duty the honest default is the stainless ladder, with the plastic series reserved for lightly mineralized water where budget rules:

Two readings of the ladder. First, the same head can be served by plastic or stainless at the same power — 3DPC7-46-110-750 and 3DSC6-45-48-750 are near-twins, and the entire difference is what the water touches. That difference is the cheapest corrosion insurance in the catalogue. Second, the six- and eight-inch machines are not just bigger, they are a different electrical platform (three-phase AC380 / DC520, controller-driven), so very large brackish schemes change the panel and controller conversation, not only the pump.

If your water analysis sits in the moderately saline band and the well is deep, shortlist from 4DSC7.5-100-110-1500 upward; if it is a shallow, slightly saline aquifer, the 3DSC and 4DSC models from 400 to 750 W cover most farms. Ask for the wetted-parts material certificate with your enquiry — it is a normal request on salty water, and we answer it with the pump’s own paperwork.

The sizing arithmetic, end to end: wheat on 2.5 dS/m water

Here is the complete calculation for one hectare of wheat irrigated from a moderately mineralized borehole, because every number below is checkable:

• Water analysis: ECw = 2.5 dS/m (about 1,600 mg/L TDS — “slightly saline” on the USGS ladder)
• Crop: wheat, threshold ECe = 6.0 dS/m
• Leaching requirement: LR = 2.5 / (5 x 6.0 - 2.5) = 2.5 / 27.5 = 9.1%
• Peak-month crop demand: 6.5 mm/day = 65 m3/day
• Gross demand with the salt tax: 65 x 1.091 = 70.9 m3/day
• Effective sun: 5.5 peak-sun hours/day → design flow = 70.9 / 5.5 = 12.9 m3/h
• Add head: dynamic water level 30 m, tank on a rise at +8 m, pipe friction 4 m → total head 42 m

The duty point is 12.9 m3/h at 42 m. On the ladder, 4DSC16-70-110-1500 (1500 W, 16 m3/h max flow, 70 m max head, 2-inch outlet) carries it with head in hand: at a 42 m duty the pump is running well inside its curve, so the flow the salt tax asked for is there on the year’s worst weeks, not just the average ones. A 750 W machine with the same 42 m of head would flow 6-9 m3/h at best on that curve and leave the field short.

Note what decided the model: not the head — 42 m is easy on the ladder — but the salt tax of 5.9 m3/day. Without it the same field would have shortlisted the 750 W row. This is why the leaching check runs before the panel count: salt moves you across the ladder, and the array follows the pump, at the standard 1.2-1.3 times panel-to-pump wattage, once.

Blending, shallow setting and cycling: making hard water cheaper to pump

Three levers reduce the salt burden before a bigger pump enters the conversation:

Blending. Mixing salty and fresh water changes the arithmetic linearly, and halving beats heroics. Wheat on straight 5.0 dS/m water pays LR = 5.0 / (30 - 5.0) = 20%. Blend that water half-and-half with 0.5 dS/m rain-tank water and the mix is 2.75 dS/m: LR = 2.75 / (30 - 2.75) = 10.1%. Half the salt tax, from stored rain and a tee fitting. For sensitive crops, blending is often the difference between an impossible LR and a routine one.

Shallow setting and skimming. In a stratified aquifer, every metre of pump depth buys salt. Setting the intake just below the dynamic level, with the screen in the fresher band, is a design decision that costs nothing at installation time and pays every pumping day. The watch item is drawdown: the 40:1 logic says aggressive pumping pulls the interface up toward the intake, so a modest, steady duty beats a max-flow sprint in these wells.

Cyclic leaching. The FAO practice notes that leaching does not have to ride on every irrigation: salts accumulate over seasons, and a heavy leaching application when water is abundant — before the season, or after rains — can reset the soil. For a solar system this matters because leaching and crop demand peak together in the hot months; pushing some of the leaching volume into the shoulder season flattens the daily peak and often drops the pump one row on the ladder.

None of these three changes the pump’s job description. All three change which row of the ladder is the right row — and on salty water, the row you buy is the water bill you pay forever.

What never changes: the solar side stays standard

Brackish water taxes the water budget and the materials, but it does not touch the solar design rules. They are the same ones as everywhere else on this site:

• Panel wattage at 1.2-1.3 times pump power, matched to the controller’s VOC window — the arithmetic in the sizing guide and the pump-head guide applies unchanged.
• Store water, not electricity: a tank is still cheaper per stored kilowatt-hour-equivalent than batteries, and leaching water is storable by definition.
• Dry-run protection matters more here than elsewhere: a salty well that over-pumps does two kinds of damage at once, to the aquifer interface and to the pump that overheats out of water.

If the water is destined for drinking rather than fields, the picture flips: the pump becomes the feed stage of a small solar reverse-osmosis train, and rejection flows of 30-70 percent mean the pump must move one and a half to three times the drinking demand. The drinking-water guide covers the treatment side; size the pump for the raw-water number, not the tap-water number.

The decision, in order

1. Measure EC and TDS — an EC meter and, for irrigation water, a lab panel including SAR. Test at the end of the dry season, when the number is worst.
2. Match the enterprise to the water: crop thresholds and livestock limits first, machinery second.
3. Compute the leaching requirement: LR = ECw / (5 x ECe - ECw), and convert it to extra cubic metres per day.
4. Check stratification: ask whether salinity rises with depth in your aquifer, and set screen and pump depth accordingly.
5. Pick the materials row before the power row: stainless ladder for anything beyond lightly mineralized water.
6. Size the pump on ET plus the salt tax at your total head, then follow with the panel count and the controller.

Done in that order, salty water is a design constraint, not a death sentence: the well gives what it gives, the crop or herd matches the water, and the pump — probably a 4DSC with stainless internals — lifts exactly the number the arithmetic asked for. Send me your water analysis (EC or TDS), the crop or animals, the depth to water and the tank height, and I will return the leaching number, a model, and a panel count sized for your salt, not just your head.

Pumping salty water? Send me your EC or TDS reading, the crop or livestock, depth to water and tank height on WhatsApp and I will return the leaching requirement in extra m3/day, a stainless-ladder model and a panel count to match. For the treatment side, start with the drinking-water guide, and for pump placement in stratified wells read the borehole yield-test guide.