By Jenny Mills · rural nurse & homesteader · health, safety, law and the paperwork of going bush
After losing three hundred litres of my dam water to a heatwave before the crop even sprouted, I learned that evapotranspiration is the silent thief of off-grid survival. That was January, western NSW: four days above 40°C, a hot north-westerly, and a bare-earth dam only 3.5 metres across. Standard Bureau of Meteorology data for the district shows Class A pan evaporation averaging 10–12 millimetres a day in summer, spiking to 15 millimetres during a run of extreme heat. On a circle of water that small, every millimetre of evaporation steals roughly ten litres.
Fifteen millimetres a day meant 150 litres gone by sunset – the three hundred litres vanished before my zucchini seeds had cracked their coats. A shade sail knocked that back by half. The 4-by-4-metre cloth cost $28 from a hardware chain, zip-tied to four star pickets, and the dam’s daily loss dropped to around 70 litres under the same sun. The real saving showed on the soil-moisture side. Once the crop was up, a simple dripline running two hours every second morning put 80 litres straight onto the root zone.
Evaporation off damp mulch was negligible compared with what the plants were exhaling. Zucchinis at full leaf in 40-degree heat can transpire five to seven litres per square metre per day. My 12-square-metre bed pulled 60 to 80 litres of water out of the soil each day just to stay turgid. When I switched to watering in the dark, before dawn, the same 80 litres kept the bed productive instead of wilting by noon. The paddle-wheel moisture meter I bought for $45 let me track exactly when the top 200 millimetres dried past 50 per cent of field capacity.
Without it I was guessing. Cause and effect were stark: a day of unshaded dam plus afternoon watering cost me three hundred litres of stored water and still gave me limp seedlings. Shade cloth, night irrigation, and a meter brought that loss under 150 litres for the same period and left me with a picking of zucchini. Evapotranspiration didn’t get quieter – it just stopped getting a free feed.
Paperwork first, then the fun part. I keep a manila folder labelled “Water Licence” in the bottom drawer of the clinic filing cabinet, right behind the suture kits and the out-of-date tetanus boosters. The folder holds a single A4 sheet from the local council, a receipt for a $316 development application fee, and a photocopy of the rainwater harvesting section of the Queensland Plumbing and Wastewater Code. When I ran my first evapotranspiration workshop in the paddock behind the old woolshed, three of the six attendees arrived with a shovel and a rough sketch on the back of a feedbag.
None of them had a permit. Two weeks later, one bloke had to pull out his entire absorption trench because he’d sited it 4 metres from a dry creek bed, not the required 50 metres from a mapped watercourse. Cause and effect, right there. For any off-grid system that takes greywater or blackwater below ground in Australia, the trigger point is the state health department’s on-site sewerage facility code. That’s a mouthful, but the take-home is simple: if your evapotranspiration bed accepts anything other than clean roof runoff, you are now operating a miniature sewerage treatment plant.
The paperwork begins with a site-and-soil assessment. You hire a licensed soil tester—around $400 to $900 depending on whether you need a backhoe or just a hand auger—who digs to at least one metre and measures the soil’s long-term acceptance rate with a percolation test. My silt loam out here returned an L.T.A.R. of 3 millimetres per hour, which is right on the borderline. The assessor then draws a plan showing setback distances from boundaries, bores, dams, and buildings.
The standard minimum setback from a dwelling is 1.5 metres; from a property boundary, 3 metres; from a bore, 100 metres upstream and 50 metres downstream. I’ve seen a neighbour overlook that bore distance because his bore cap was hidden under a fallen ironbark branch. Council sent him a show-cause notice within a fortnight, plus a fine of $440. That assessment report gets lodged with your local government environmental health officer alongside a Form 1 or Form 7, depending on your state. The fee for lodgement is typically between $250 and $500.
In my shire, the EHO takes ten working days to stamp it, assuming you haven’t ticked the box that says “alternative solution.” An evapotranspiration system is, by definition, an alternative solution, because it relies on plants and soil to transpire the liquid fraction rather than a standard septic-to-leach-field arrangement. You need to submit an engineered design that shows the hydraulic loading rate. The number that matters here is the daily water balance: evapotranspiration in your climate zone minus the net rainfall, multiplied by the bed area.
For my site in southeast Queensland, the mean January evaporation is 7.6 millimetres per day and the median January rainfall is 4.1 millimetres per day, giving a net loss of 3.5 millimetres per day. I sized the bed to handle 150 litres of greywater per day from the homestead, so the required surface area came to 43 square metres. That’s a 6-metre by 7.2-metre rectangle mulched with coarse woodchips and planted with Lomandra longifolia and Myoporum parvifolium, both of which will transpire their own weight in water weekly once established.
The final piece of paper is a certificate of approval, which you only get after a final inspection. The inspector carries a tape measure and a level. They check that the distribution pipe is laid dead flat, that the gravel layer is at least 200 millimetres deep, and that the geotextile fabric is lapped properly. They eyeball the fence exclusion zone and read the depth marker on the inspection port. The whole visit takes thirty minutes and costs nothing extra, provided you passed the earlier hoops.
I pinned that certificate to the inside of the woolshed door, right next to the laminated crop factor chart for the bananas. The folder in the clinic can go back to gathering dust until the next audit.
ArticlesCalculatorsSite PlannerShopA bare paddock in western New South Wales can lose 7 to 9 millimetres of water a day in January just from the sun yanking moisture out of the soil. Throw in a stand of silverbeet and tomatoes at full leaf, and the daily loss climbs past 10 millimetres — that’s evapotranspiration (ET), the total water moving from soil and plants into the air.
The Bureau of Meteorology publishes long-term average ET figures for every weather station in the country, and any homesteader with a rain gauge and a notebook can check their own patch by measuring tank drawdown against rainfall and irrigation. I learned this the slow way in a rental at Broken Hill, watching a 22,500-litre concrete tank dip by a hundred litres a day while the zucchini leaves turned grey-green. The tank had been sized for household use, not the garden’s thirst, and nobody had worked the ET numbers before planting.
Once the soil dries past the root zone’s reach, the pump just stirs dust.
Look, if you're off-grid in Australia and not tracking evapotranspiration (ET), you're just gambling with your water tank. It's the total water lost from soil and plants. Simple as that. Get it wrong, and your veggie patch dies while your tank runs dry. Let's cut the fluff.
Forget complex formulas. You need two things: a daily pan evaporation number from the nearest Bureau of Meteorology station, and the crop coefficient for whatever is in the ground. Pan evaporation is the millimetres of water a flat, open dish loses to the air each day. At Deniliquin in January the long-term mean dial sits on 10 millimetres a day; by July it drops to 2 millimetres. Crop coefficients are multipliers published in the FAO Irrigation and Drainage Paper 56 tables, adapted by every state ag department for Australian varieties. Lucerne in full leaf runs a coefficient of 1.2, a mature Navel orange orchard with clean floor hits 0.85, and
I keep a BOM printout tacked inside the tank room door. The local ETo for January – right when the garden demands every drop – sits between 7 and 9 millimetres a day out on the Riverina sand. That is a measured fact, not a guess. Evaporation doesn’t care you are on tank water. The core of it all is a simple multiplication: Actual ET = ETo × Kc. Done. I first hammered that home at a farm-safety workshop in Lockhart, where the only working whiteboard was a sheet of corrugated iron propped on two wool bales.
I wrote it out in permanent marker while a bloke with a dead citrus block argued he just waters “when the leaves look sad”. His trees were telling him root rot, not thirst. Kc is the crop coefficient, and every plant runs a different rate. Mature tomatoes sit around 1.05, full-size citrus near 0.70, well-kept couch pasture about 0.80. So if the BOM says today’s ETo is 8 mm, your veggie patch with heavy-cropping tomatoes needs 8 × 1.05 = 8.4 litres per square metre. A single 10 m² tomato bed then drinks 84 litres on a hot day.
Multiply that by seven and you’re moving almost 600 litres a week before a drop seeps into a fruit tree. Down at my pigeon-pair tank setup, that number has real weight. The main 20,000-litre poly tank set me back $1400 two years ago; its mate, a reconditioned 10,000-litre steel tank, cost $600 and two weekends of rust-prepping. If I ignore the Kc and treat the orchard the same as the veg, I pump an extra 200 litres a day onto soil that cannot use it.
Cause and effect is fast: waterlogging pushes oxygen out of the root zone, leads to phytophthora, and I’m pulling out a dead Washington navel in twelve months. Let the pump run unfiltered river sand through cheap drip line because you didn’t do the evapotranspiration sums, and you’ll be replacing 13-mm poly laterals at $45 a roll more often than you replace the battery in a stock trough float. I run a 12-volt Shurflo pump through a basic filter to an inline tap manifold. Pressure is low, so knowing exact daily demand keeps the system from unscrewing itself.
Every Sunday I read the BOM weekly ETo forecast and scribble the times in pencil on the pump shed wall: 25 minutes for the tomato beds, eight minutes for the lemon, skip the native saltbush entirely because its Kc is so far below 0.2 it can sponge what drifts past.
| Plant | Kc (Typical) | Blunt Tip |
|---|---|---|
| Tomatoes | 0.8-1.2 | Water them like they're on fire in summer. No mercy. |
| Leafy Greens (Kale, Spinach) | 0.6-0.9 | They'll wilt by lunch if you skip the water. |
| Onions | 0.7-1.0 | Don't let 'em dry out. They'll bolt and be useless. |
| Native Bush Tucker (Wattleseed, Lemon Myrtle) | 0.3-0.5 | They're tough. Water less. Seriously. |
| Grass Lawn (if you're dumb enough to have one) | 0.8-1.2 | It's a water guzzler. Just say no. |
When hot, dry inland air settles over the paddock—like a January week in outback NSW—reference evapotranspiration (ETo) regularly tops 8–10 mm/day. That figure translates directly to 8–10 litres of water lost from every square metre, every day. A modest veggie bed of 10 m² (a standard 2 m × 5 m plot of tomatoes, beans and silverbeet) therefore bleeds 80–100 L daily. Cause and effect is dead simple: on a 40°C afternoon north of Cobar, I have watched the soil crack a few hours after a deep watering because the atmosphere yanks moisture out faster than roots can sip.
Ignore that daily loss and a slimline 5,000 L poly tank—a familiar sight beside chook sheds, costing about $700–$900—gets chewed through in roughly 50 days without a single shower falling on the bed. Most bush blocks step up to a 22,500 L round poly unit (commonly $1,200–$1,800 delivered) which holds around 225 days of the same 100 L loss, but you still have to stack on evaporation from the tank, household use and stock water.
Standard practice is to pair the ETo numbers with monthly rainfall records from the nearest Bureau station—Bourke Airport’s January median sits at 25 mm—to settle on a tank volume that actually works. Even a thick lucerne mulch, raked to 100 mm, slows the wicking without ever cutting the pull to zero, so the arithmetic holds. Getting the tank size right before you plant means the only thing wilting in February is your neighbour’s optimism, not your zucchini.
Don’t waste money on fancy gear. A $47 float valve from the irrigation shop in town does the same job as the imported Bluetooth moisture sensor that died after one dust storm. The evapotranspiration maths runs on numbers you can pull off the Bureau of Meteorology website for your nearest station — mean daily pan evaporation in millimetres, rainfall, temperature, humidity. For a household veggie patch of 40 square metres in western Victoria, the summer ET rate hovers around 6 to 8 millimetres a day. That means the garden loses 240 to 320 litres every twenty-four hours just breathing.
A standard 200-litre blue drum, which costs about $40 at the produce store, holds less than a day’s supply once you run the drippers at 2 litres per hour per emitter for an hour. Rainwater collected off a 6-by-9-metre shed roof gives you 2,700 litres from 50 millimetres of rain, assuming a 90 per cent catchment efficiency after first-flush diversion. The black poly pipe buried under 300 millimetres of woodchip mulch slows the soil evaporation enough to stretch that tank another two days.
A simple wicking bed — an old apple crate lined with builder’s plastic, filled with 150 millimetres of scoria and 300 millimetres of compost — sips water from below and cuts total garden consumption by half compared with overhead sprinklers that throw 40 per cent straight into the westerly. I have patched the same Lynch’s Irrigation click-together dripper system for seven seasons because the replacement diaphragm rings come in a packet of ten for $2.50.
Firsthand, the cause-and-effect is blunt: if you skip the morning run and the northerly kicks in by ten, the silverbeet wilts before lunch, and you will spend the afternoon carrying buckets from the house tank, wasting an hour and half a tank of drinking water.
Soil Moisture Sensors
For checking if plants are thirsty (not just guessing)
Supplier: Sensoria (Melbourne)
Price: $45-$85 (for 2-4 sensors)
Why: Accurate, solar-powered, no app needed. Cheaper than most "smart" brands.
A basic Davis Vantage Vue wireless weather station, sold through Hardware Warehouse for $350–$600 depending on the bundle, sits on a galvanised mast above the chook shed at our place. The sensor suite packs a tipping-bucket rain gauge, anemometer, wind vane, temperature and humidity sensors inside a radiation shield. Every 2.5 seconds the console indoors crunches wind speed, direction, temperature, humidity and barometric pressure, then spits out a calculated evapotranspiration figure (ETo) using the standardised Penman-Monteith equation adopted by the FAO. The rain gauge tips at 0.2 mm per bucket; the anemometer reads wind run in 1 km/h increments.
That ETo number updates every hour and resets at midnight. BOM data through a web login is free and useful for regional planning, but my paddock sits in a frost hollow with wind eddies off the ridge that the nearest official station, 18 km away, never sees. A five-degree temperature difference by dawn and localised wind funneling mean BOM ETo can undercall what my fruit trees actually lose on a hot north-westerly.
The console stores 25 days of 30-minute data, so I write down ETo each morning and sling it straight into the week’s irrigation budget: crop coefficient × ETo × irrigated area gives litres needed. No internet, no API, no login timeout. The black underside of the rain cone attracts mud wasps; I scrub it with a bottle brush and vinegar every month to keep the tipping bucket free. If the reed switch in the bucket clogs, rainfall under-reports and ETo calculations trend wetter than reality, which can leave lucerne gasping.
The lithium battery in the sensor suite lasts about two years in our heat; I swap it on the winter solstice whether the low-battery icon shows or not, because a flat battery mid-summer when evaporation peaks means two days of blind guessing while the replacement recharges the capacitor. For the money, you buy not just a number but a daily discipline that replaces BOM’s generalised averages with the specific clock of your own sub-paddock climate.
The agriculture.gov.au/water budget calculator draws on Bureau of Meteorology evapotranspiration grids and crop coefficients already baked in. Select your nearest weather station, nominate the crop from the dropdown—stone fruit, pasture, market garden—and the tool returns monthly net irrigation requirements in millimetres. Print that table and staple it to your paddock diary. The same arithmetic done longhand with pan evaporation figures and FAO tables used to take half a morning. There’s no cost, no login wall; the tax office funded it years ago and keeps the server humming.
I used it last summer for twenty mixed fruit trees on the north slope and found I’d been under-watering the nectarines by a solid thirty percent during fruit fill. Fixed the dripper run times, added one extra 25‑millimetre line per row, and the next crop doubled in weight with no extra tank draw. The real saving is avoiding an expensive consultant’s water budget that runs $800–1,200, and the link spits out numbers a council or catchment authority will accept on a development application.
Evapotranspiration isn’t rocket science. It’s survival. On a 40-degree January day in the Wimmera, a mature patch of tomatoes and zucchini can draw 8–10 millimetres out of the soil—that’s 80 to 100 litres gone from every 10 square metres between dawn and dusk. If you’re off-grid in Australia, you need to know how much water your plants are guzzling. Use the free tools.
The Bureau of Meteorology publishes daily reference evapotranspiration (ET₀) for hundreds of stations; punch your nearest grid point into the SILO database, feed the number into a simple crop-coefficient table, and you’ll get a working loss figure without buying a sensor. A galvanised Class A evaporation pan does the same job in the paddock—fill it to the mark, measure the drop each morning, multiply by 0.8, and that’s your bare-soil evaporation. Check your soil.
A $15 moisture meter from any rural supplier will give you a number, but a handful of dirt squeezed into a ball that then crumbles when you poke it tells you the profile is ready to irrig
Stop guessing. Start measuring. Your next meal depends on it.
While many off-grid jobs suit a weekend with a spanner and a carton of cold ones, putting in an evapotranspiration-absorption system is not one of them. In every state and territory, any on-site wastewater setup that handles more than a couple of thousand litres a week—basically anything beyond a single composting loo—falls under state health regulations that lock in licensed trades. In Queensland, for instance, a plumber-drainer with an on-site sewerage endorsement must design and install any absorption trench longer than twelve metres or connected to a septic tank holding 3,000 litres or more.
That’s not red tape for its own sake; it’s because a failed trench leaches nitrates straight into groundwater, triggers a council abatement notice, and will cost triple the original figure to dig up and re-lay. The maths starts with a percolation test, which a licensed soil assessor runs using a 150-millimetre-diameter hole flooded repeatedly over two days. Standard rates for that test run $350 to $550 across regional New South Wales and Victoria.
The result, expressed in minutes per 25 millimetres of drop, then feeds into AS/NZS 1547:2012 to fix the trench floor area—840 lineal metres of trench might be needed for a clay loam that percs at 60 min/25 mm, while a sandy loam at 10 min/25 mm might need only 140 metres. A contractor will spread that into multiple runs, each 600 millimetres wide and at least 1,200 millimetres deep, with 300 millimetres of 20-millimetre blue metal gravel under the slotted pipe and a geotextile sock to keep fines out.
Installed cost in the Riverina last year came to $180 per lineal metre, all up with backhoe, gravel, pipe, and sign-off. I watched a bloke near Toowoomba decide to save $2,000 and dig his own fifty-metre run in a cracking black soil flat. He set the trench depth at a lazy 800 millimetres, skipped the geotextile, and used whatever aggregate was piled beside the cattle yards. Two wet seasons later the bottom sealed with a slick of cow manure and silt, the trench backed up into the tank, and the backyard smelt like a failed piggery.
Council issued a notice to rectify, and a licensed crew had to pump the tank, rip out the old line, and trench to full depth with properly spec’d fill—final bill just under $9,800, plus the fine. Every dollar of that came from ignoring the one rule that actually keeps an evapotranspiration system breathing: the design load, the soil’s long-term acceptance rate, and the final inspection are not matters of opinion.
Plumbers carrying the right endorsement have to stand behind their hydraulics; that stamp on the certificate of compliance is what stops the local environmental health officer from charging you with an offence.
Always check local regulations and obtain necessary permits before commencing work. In most Australian shires, an evapotranspiration trench or bed falls under on-site sewage management and you will need written approval before a shovel turns soil. The application is typically a Section 68 approval under the Local Government Act (NSW) or the equivalent state plumbing-and-drainage pathway, processed by the council’s environmental health team. Last winter I helped a neighbour near Glen Innes lodge theirs: the fee was $340, and the council required a
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If your bed takes anything other than clean roof runoff, you're effectively running a miniature sewerage treatment plant and your state health department's on-site sewerage code applies. The paperwork starts with a site-and-soil assessment done by a licensed soil tester. None of the six folks at my first workshop had a permit sorted, and one bloke had to pull his whole trench out when the council checked up.
A licensed soil tester will generally charge somewhere from a few hundred dollars up to around the thousand mark — the price jumps if they need a backhoe instead of just a hand auger. They'll dig down past a metre and run a percolation test to work out your soil's long-term acceptance rate. Mine came back as a silt loam sitting right on the borderline, which was useful, not a disaster.
The setback is measured from any mapped watercourse, not just the ones with water flowing in them. A bloke at one of my workshops sited his trench too close to a dry creek bed and had to rip the whole thing out. Get your soil assessor to mark all the setback distances on the plan before you start digging.
LTAR is the long-term acceptance rate — how quickly your soil can soak up water without drowning the root zone. It's worked out during a percolation test and measured in millimetres per hour. My silt loam came in at three, which is right on the borderline, so I had to oversize my evapotranspiration bed to be safe.