The patch that comes back in the same shape
A whole lawn going off evenly in a July drought is a scheduling conversation, and the client has usually had it with themselves before they call anybody. What brings a technician out is the other thing: grass that fails in the same shapes every year — the metre-wide strip inside the south fence, the kidney two paces off the corner of the patio, the pale ring around one rotor — while the turf on the other side of that line stays green enough to be worth mowing. The edges are the interesting part. Weather does not draw edges. Something on this lawn is repeatable, and anything repeatable can be measured.
The difficulty is that half a dozen unrelated faults all produce brown grass and the same phone call. A nozzle a third blocked with grit off a repair, a water-repellent band sixty millimetres down in a sandy profile, a wheel rut compacted into the subsoil during a build six summers ago, a beech hedge quietly drinking the top of the bank, chafer grubs shearing roots off at the crown — from the path they are one symptom with one obvious remedy, which is more water. That remedy has usually already been tried. The controller has been walked up in five-minute increments for two seasons and the patch is still there, which is the most useful thing anybody has told you so far: a fault that more water has failed to fix is not fundamentally a fault of run time.
So the order of work is set by what each test costs. A soil probe and a long screwdriver answer the ground questions in twenty minutes for nothing. A catch-can audit answers the water question in a morning for the price of the cans and one wasted cycle. Only when both are done does anyone know whether the job is a nozzle change, a trench, a hollow-tining and a bag of surfactant, or a rewritten programme — and those four invoices are not remotely alike. Quoting before the plugs come out is quoting on a guess.
Pull a plug before you touch the controller
Take two cores, not one. A hundred-millimetre plug out of the middle of the patch tells you very little by itself; the same plug laid beside one cut from healthy grass three metres away tells you nearly everything, because every difference between them is a candidate and every similarity is eliminated. Cut both after a full irrigation cycle rather than in the dry — you are looking at where the water went, and there has to have been some. Split each plug down its length with a knife and read the profile: colour, moisture, root depth, where the moisture stops, and what the probe hit on the way down.
The signature worth learning first is dry underneath wet. If the top twenty millimetres is damp and everything below it is loose, pale and dusty, water is being applied and is refusing to enter. On sandy and sand-dominated rootzones this is soil water repellency, an organic coating on the grains that makes them shed water sideways to the nearest wettable route instead of admitting it. It is easy to confirm without a laboratory: place a droplet on the broken face and time it. On a wettable soil it disappears in a few seconds. If it sits as a bead for a minute or more, the soil is strongly repellent by the classification the soil-science literature works to, and no amount of run time is going to change its mind. The condition also builds year on year in the same places, which is exactly why the patch has an outline the client can point at.
Thatch and compaction are the next two, and they fail in opposite directions. A spongy fibrous mat sitting above the soil intercepts a light application, holds it, and dries out between cycles, so the rootzone below stays thirsty however often the valve runs; the trade generally starts worrying at around twelve millimetres of it, which is a working rule rather than a published limit, and scarification is the answer. Compaction shows as a plug that the probe fought for, with roots turning sideways at a definite depth and moisture stopping flat at the same line. Push a screwdriver in at ten or twelve points across the patch and the same number across the good grass — where it stops, and how hard, maps the compacted ground faster than any instrument on the van.
Then the causes the irrigation cannot reach at all. A probe that halts on brick, stone or a slab of old bedding at eighty to a hundred and twenty millimetres has found builder's spoil, a shallow fill over a footing, or the backfill above the mainline itself; that ground holds a fraction of the water the surrounding profile holds, empties first in every dry spell, and will do so forever. Fine roots crossing the core horizontally, or a plug that is simply full of them, is competition rather than supply — a hedge or a mature tree is taking the water before the grass gets a turn, and the cans over that patch will read the same as everywhere else. Both of these fail the same visual test as an irrigation fault and neither one has an irrigation fix.
Finally, check that the plant can still drink. Turf that lifts off the soil like loose carpet, with roots sheared off short, is grub damage, and the giveaway is usually secondary: birds, foxes or badgers tearing at the same patch overnight. Disease scars have their own shapes and their own seasons. Both matter here because August is when a weakened plant stops coping, so a patch that has been marginal since spring only becomes visible in the hottest month and gets blamed on the sprinklers. Where treatment is warranted, the product label as approved by the authority having jurisdiction governs what may be applied, at what rate and when — in the United States that label carries the force of law under FIFRA.
| What the plug from the patch shows | What that points to | What it takes off the list |
|---|---|---|
| Damp in the top 20 mm, loose and pale beneath; a droplet beads on the broken face for a minute or more | Soil water repellency shedding water sideways instead of admitting it | Under-application, once the cans show the zone is delivering its depth |
| Wet to 30 mm and dry below, with the wet band ending on a flat line | Rate outrunning intake, or a compacted layer holding water up above the roots | A nozzle or spacing fault, which would starve the surface as well |
| A spongy fibrous mat above the soil that the probe passes through with a squeak | Thatch thick enough to intercept a light application and dry between cycles | Nothing yet — re-read the patch after scarification before blaming anything deeper |
| The probe stops hard at 80 to 120 mm on brick, stone or old bedding | Buried spoil or a shallow profile over something solid, holding a fraction of the water | Anything a controller setting can reach |
| Fine roots crossing the core horizontally, or a core packed with them | Competition from a hedge or mature tree: a demand problem, not a supply one | A distribution fault, if cans over the patch match the rest of the zone |
| Roots sheared short, turf lifting off the soil like loose carpet | Grub damage — the plant cannot take up water that is already there | Irrigation entirely; this is a treatment decision on a product label |
Cans on a grid, and one wasted cycle
Once the ground has been cleared, the question becomes what this zone actually puts down, which is not what the catalogue says it puts down. Published nozzle figures come from still air at a stated pressure and a stated spacing. Your zone has the pressure the supply holds on an August evening, the spacing whoever installed it managed against the boundaries, whatever the last repair left in the ground, and the wind at the hour the programme runs. A grid of identical containers measures all of that at once, in one cycle, and it is the only measurement on this job that argues with nobody.
Set the cans out on a regular spacing across the whole zone and ignore where the heads happen to be — the point is to sample the overlap between patterns, not to prove the heads are running. Push the grid through the dry patch and out the other side so the patch is represented by several cans rather than one, and keep every can the same: same throat, same height, same level seating. Two of the numbers that come out are worth more than the rest of the survey put together, and both are read off the same set of volumes.
- Pick the hour the programme normally runs. A dawn test on a zone that waters at midday measures a different wind and a different supply pressure.
- Lay the cans on a regular grid across the entire zone, seated level, throats clear of grass blades — not clustered around the heads and not concentrated in the dry patch.
- Measure the internal diameter of the opening the cans collect through, once for the whole set. A volume only becomes a depth after it is divided by that area, and on a tapered container the wide part is not the part that collects.
- Record where every can sits, on a sketch, before the water goes on. The positions are half the result and nobody remembers them afterwards.
- Run the valve for a fixed period long enough to give readable volumes and short enough that nothing overflows or evaporates — fifteen minutes suits most spray and rotary zones.
- Time the valve, not the station. A zone that takes twenty seconds to pressurise did not apply water for the whole of its nominal run.
- Measure every can individually into one graduated cylinder and write down each reading. An average alone cannot produce a uniformity figure; the individual numbers are what the low quarter is drawn from.
- Rank the readings, count off the driest quarter, and average that subset separately. Round the size of that subset up if the count will not divide by four.
The grid produces a column of millilitre readings and a run length; put those in alongside the diameter the cans collect through and they convert into the delivered rate and the low-quarter uniformity that the rest of this job is decided on.
Add every can's measured catch together and divide by the number of cans.
Sort the readings, take the driest quarter of them, and average just those.
The internal diameter of the opening each can collects through.
How long the zone ran while the cans were collecting.
How many catch devices were set out across the zone.
Measured precipitation rate
1.321 in/h
The driest quarter of the ground is receiving a high proportion of the average, so run times set from the average will not leave dry patches behind.
- Low-quarter distribution uniformity
- 76.47 %
- Average catch depth over the test run
- 0.33 in
- Run-time multiplier implied by the uniformity
- 1.31 multiplier
- Throat area of one can
- 0.09 ft²
- Cans forming the low quarter
- 6 cans
They open the calculator with your figures already in it
Catch-Can Test Uniformity and Rate Calculator: 1.32 in/h — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1.321 in/h — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Describes the conditions on the day. Wind, supply pressure and a partially blocked nozzle all move the result, and a test run at dawn does not describe a zone that normally waters at midday.
- A grid too sparse to sample the overlap between heads reports a uniformity that is mostly noise, whatever the arithmetic says.
- Measures what lands in the cans, not what enters the soil. Runoff, interception by planting and infiltration limits are separate questions.
The two numbers fail in different directions
Rate and uniformity are not two views of one problem. Rate is how fast the zone delivers depth, and a low rate against the programme is a scheduling error — the grass is short of water because the valve does not run long enough to give it any, and the fix is minutes. Uniformity is how fairly that depth is shared out, and a low figure means some of the ground is short no matter how long the valve runs, because it is short relative to everything else on the same clock. One is arithmetic. The other is hardware.
The scheduling multiplier that falls out of the uniformity — one divided by it — is worth understanding as a price rather than a remedy. Running long enough to bring the driest quarter up to target applies the same multiple everywhere else, so a zone at sixty percent runs two-thirds longer than it needs to across all of it, and that is what turns a distribution fault into a water bill and a waterlogged low corner. It is a legitimate short-term move on a zone that is as good as its layout allows. It is not a legitimate answer to a zone that measures badly because the heads are in the wrong places, and the number to hold in mind is that the industry treats roughly three-quarters as the mark of a well-sorted turf zone and anything under about sixty percent as a layout question rather than a programming one.
Now put the low cans back on the sketch. Where they sit is the actual diagnosis, and it is the step people skip because the headline uniformity number feels like a conclusion. Low cans clustered around one head, low cans strung along one boundary, low cans in a band between two rows and low cans marching toward the far end of a lateral are four different faults with four different bills, and they can all produce the same uniformity figure. The low-quarter measure comes out of the landscape irrigation auditing practice the Irrigation Association's certification programme codified, and the auditing method has always been a map exercise as much as an arithmetic one.
| Pattern in the low cans | The mechanism it fits | What confirms it on site |
|---|---|---|
| Low tightly around one head, normal everywhere else | That head alone is under-throwing: partly blocked nozzle, sunk body, wrong nozzle left after a repair | Pull the nozzle and compare it against the others on the same valve |
| Low along one edge or into one corner | Arc set short, a boundary head throwing onto the fence, or a corner no head reaches | Run the zone and watch that one head through a full rotation |
| Low in a band midway between two rows of heads | Spacing wider than the radius supports at the pressure this zone actually runs | Gauge working pressure at a head, then read the radius off the chart at that row |
| Low progressively toward the far end of the lateral | Friction and elevation loss down the run; the last heads never reach working pressure | Gauge at the first and the last head with the zone running |
| Low everywhere, evenly, but the uniformity is respectable | A uniform zone that is simply slower than the programme assumes | Compare the measured rate against the run time the controller is set to |
When the geometry is at fault
A band of dry ground running between two rows of heads is the classic retrofit signature, and it usually has a history. The zone was laid out against a radius somebody read off the top row of a performance table, at a pressure the site has never held; or the lawn was extended and two heads were added on the end of an existing lateral to cover ground that needed four; or a nozzle set was changed during a repair and the throw quietly shortened by a metre. Whichever it was, the patch sits where the patterns from adjacent heads were supposed to meet and no longer quite do, and the ground under a single pattern always gets less than the ground under two.
Rework the spacing from the radius the nozzle achieves at the pressure you gauged, not the pressure it was designed for. Then take the derate for wind off the manufacturer's chart for that specific nozzle, and take it for the wind at the irrigation hour rather than a daily average — a zone that waters at four in the morning carries a much smaller penalty than the same zone watering at six in the evening, which is one of the two reasons night watering wins. On an existing lawn the layout question is nearly always narrower than a fresh design: you are asking how many heads this rectangle needs at the throw you measured, and comparing that against how many are in the ground.
Two structural choices are worth revisiting while you are at it. Offsetting alternate rows overlaps patterns from three directions rather than two and holds up better across open ground, which is why the charts allow a wider spacing for it; aligning the rows sets out more easily against a boundary and is often the honest answer on a small rectangle where an offset row would simply run out of lawn. And arcs have to be paid for in flow — a half-circle nozzle must discharge half of what the full-circle one discharges to lay down the same depth, so a corner head running a full-circle body on a part-circle arc waters its quarter of the lawn several times over while the middle goes short.
The commercial part of this decision is that adding heads means trenching through established turf, and clients feel that far more than they feel a nozzle bill. Be straight about which patches the layout can rescue and which it cannot. Narrow strips beside a drive and long thin verges rarely suit a grid at all, and hanging a strip nozzle off a rotor valve to avoid a second solenoid re-imports exactly the mismatch you are being paid to remove.
The band between two rows is a spacing question, and it is answered by counting what the rectangle needs at the throw you measured on site rather than the throw printed at the top of the table.
The throw the nozzle is rated for, at the pressure the zone will actually run at.
How far apart heads sit, expressed against the throw they are rated for.
The reduction applied for the wind the site sees at the hours you plan to irrigate.
The longer dimension of the rectangle being covered.
The shorter dimension of the rectangle being covered.
Whether alternate rows are aligned or offset by half a spacing.
Heads needed for the zone
12 heads
A rectangle-and-grid count, which is what a first layout needs. It assumes every head is the same nozzle at the same pressure — the moment arcs vary, the flow behind each arc has to vary with them or the corners drift out of step with the middle of the lawn.
- Spacing between heads along each row
- 13.5 ft
- Spacing between rows
- 13.5 ft
- Heads on each row
- 4 heads
- Rows across the zone
- 3 rows
- Ground served by each head
- 84.5 ft²
- Heads actually land at, along each row
- 13 ft
- Rows actually land at, across the zone
- 13 ft
They open the calculator with your figures already in it
Sprinkler Head Spacing and Count Calculator: 12 heads — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Counts a grid on a rectangle. Curved boundaries, planting islands and re-entrant corners all need heads the grid does not know about.
- Assumes a matched nozzle family: a half-circle head must discharge half the flow of a full-circle one to apply the same depth, and mismatched sets fail this quietly.
- Does not check flow. The heads counted here still have to fit within the zone's available flow and the valve's capacity, which is a separate calculation.
When the pressure is at fault
Low cans that get progressively worse toward the end of a run are not a spacing problem and no nozzle change will fix them. Discharge through an orifice varies with the square root of pressure, so pressure that drifts down a lateral is rate drifting down it too, and the far heads throw short into the bargain — the pattern collapses inward and leaves a dry ring around each one. Confirm it with a gauge at the first head and the last one, with the zone running.
There are only three places the pressure went. It was lost to friction in the lateral, which is a function of flow, length, bore and the pipe's roughness, and which grows sharply when someone has extended a run or added heads without upsizing anything. It was lost to elevation, at roughly ten kilopascals for every metre climbed, about 0.43 psi per foot, which is why the crest of a bank is so often the patch and the toe of it is so often waterlogged. Or it never arrived, because a partly closed isolating valve, a clogged filter, a failing regulator or a service pipe that was always marginal is throttling the whole zone — and that one is worth eliminating first because it costs nothing to check.
Work out which by arithmetic before you dig. Put the zone's flow, the run length and the actual internal bore into a friction figure and see whether the loss you calculate accounts for the drop you measured. If it does, the remedy is pipe or fewer heads on that lateral. If the calculated loss is small and the measured drop is large, the loss is somewhere discrete — a valve, a fitting, a filter, a regulator set wrong — and the trench you were about to open would have found nothing.
Before opening a trench, check whether the flow, the length and the bore actually account for the pressure you lost between the first head and the last one — a discrete restriction and an undersized lateral read very differently.
The total length of the pipe run.
The design flow rate through the pipe.
The pipe material's Hazen-Williams roughness coefficient.
The pipe's actual internal (bore) diameter.
Friction head loss
17.96 ft
They open the calculator with your figures already in it
Domestic Water Pipe Friction Loss Calculator (Hazen-Williams): 17.96 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Hazen-Williams is an empirical fit to cold water in turbulent flow, roughly 40 to 75 °F (4 to 24 °C) at velocities in the 2 to 10 ft/s (0.6 to 3 m/s) band. Outside that it drifts: glycol-charged loops, hot recirculating mains and low-flow trickles are Darcy-Weisbach problems, where viscosity enters the calculation instead of being absorbed into a single C value. The formula still returns a number for any of them; it is just not the loss you will measure.
- Head loss is one term of the pressure the fixture actually sees. What is available is the incoming main, less the static lift to the outlet, less this friction, less the fittings, and less every device in the line — a meter, a backflow preventer, a filter and a softener can take 20 to 30 psi (207 kPa) between them. A run whose friction loss looks comfortable here can still open at a shower with nothing left.
- Velocity is not reported, and velocity is often what sizes the pipe rather than head loss. Copper is normally held near 8 ft/s cold and 5 ft/s or less on hot recirculating lines, because water moving faster erodes the inside of elbows and tees over the years and makes the pipe audible in the wall. A small pipe on a short run can pass a friction check comfortably and still be the wrong size.
When the water arrives and the soil turns it away
The hardest version of this job is the zone that audits well. Rate on target, uniformity in the high seventies, cans over the patch reading the same as cans over the green grass — and the patch is still there. That result is not a failed audit; it is the audit doing its job, because it has just eliminated the entire irrigation system and sent you back to the plug. What lands on that ground is fine. What the ground does with it is not.
Repellency is treated by getting water past the coated grains long enough for the profile to rewet, and by breaking the physical layer that is deflecting it. Hollow-tine coring lifts plugs out and gives water a route down that does not depend on the surface admitting it; solid tining relieves compaction but can glaze the walls of the hole on a heavy soil, so coring is the safer default on a patch that has been dry for several seasons. A soil surfactant applied over the top makes the rewetting stick rather than running through once and reverting, and the rate, the interval and whether it has to be watered in are set by the product label and nothing else — this is one of the few places on an irrigation job where the manufacturer's instruction genuinely is the specification.
If you want a number for how fast the ground can take water rather than an impression, measure it. A double-ring infiltrometer test to ASTM D3385 gives a field infiltration rate for that patch, and comparing it against the delivered rate you measured with the cans answers the runoff question directly instead of by argument. More work than most domestic jobs justify — but on a slope, a heavy clay, or a job where the client has already paid twice for a fix that did not hold, it is the measurement that ends the dispute.
Then dress the holes back. Coring leaves a patch that is open, uneven and will scalp on the first mow if it is left, so it gets topdressed and brushed in — a sandy dressing on a sandy rootzone, a compatible topsoil where the profile is loam, and never a layer of something the existing soil will not knit to, because a texture change buried at ten millimetres becomes next year's perched water table. ASTM D5268 is the specification to hold a topsoil supplier against for landscaping material; on turf the dressing also has to be dry enough to brush, which no specification covers and which decides whether the job looks finished on the day.
A cored patch dressed back level is an area multiplied by a depth, and ordering it as a volume rather than as a guessed number of bags is the difference between one delivery and a second trip.
The length of the garden bed or area.
The width of the garden bed or area.
6-12 in (15-30 cm) is typical for a new garden bed; shallower for topping up an existing lawn.
Estimated topsoil needed
0.9877 cubic yards
- Area
- 40 ft²
They open the calculator with your figures already in it
Topsoil Calculator: 0.9877 cubic yards — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The answer is a loose volume and nothing turns it into weight, because no bulk density is asked for anywhere in the form — topsoil is very often quoted, delivered and tipped by the ton or tonne, and how much a cubic yard of it weighs moves with how wet it is, how much organic matter it carries and how finely it has been screened.
- No allowance for consolidation is folded in: the number is a bare length x width x depth, so soil tipped to the depth you typed will read shallower once it has been rained on and walked over, and making the level good again is a second, smaller order.
- The footprint is one rectangle carried at a single depth, so a curved border reads as its bounding box, ground that falls away across the bed is not picked up, and the room already filled by shrub root balls, stepping stones or the thickness of edging boards is never deducted.
- Fill depth is capped at 24 in (0.61 m) and each side at 50 m (about 164 ft), which puts berms, deep planting pits and field-scale regrading outside the form, and inside that band the depth entered is simply taken as correct — nothing checks that it gives enough rooting depth for what you intend to grow.
- Only soil going in is counted, with nothing coming out, so where turf has to be stripped or tired ground dug down before filling, neither the spoil to be carted away nor the level already occupied by what is there is netted off the delivery.
Run time is the last lever, not the first
By now the programme is the only thing left to adjust, and it should be adjusted against measurements rather than against the look of the grass. The delivered rate from the cans and the depth the turf needs give a run time directly, and the useful discipline is to write both of those numbers down before touching a station, because a controller edited by feel drifts upward every August and never drifts back. Where the measured rate outruns what the soil will take — which the plug or the infiltrometer has already told you — the same total depth gets split into shorter starts with soak periods between them, and that is a change of shape rather than a change of quantity.
Deeper and less frequent beats shallow and daily on established turf, for the same reason the dry patch appeared in the first place: light frequent applications wet the thatch and the top thirty millimetres, evaporate off, and never build the moisture at depth that carries grass through a hot fortnight. The exception is the patch you have just cored and dressed, which needs the opposite treatment for a few weeks until it knits, and that is a temporary programme with a date on it rather than a new normal.
Sensors and weather-based control belong here rather than earlier, because they scale a programme that is already correct and do nothing whatsoever for one that is not. A controller that irrigates to local evapotranspiration will hold a well-audited zone at the right depth all season with no further attention; put the same controller on a zone with a sixty percent uniformity and it will scale a bad distribution up and down very precisely. The US EPA's WaterSense specification for weather-based irrigation controllers is the reference to check a product against if the client is choosing one, and many water purveyors require rain or soil-moisture sensing regardless of what anybody prefers.
Leave a record that settles it next August
This job comes back. The patch either returns or it does not, and twelve months later nobody remembers what was measured, what was changed, or in which order — which is how a client ends up paying for the same diagnosis three times. Leave the audit behind: the sketch with the can positions and the individual readings, the delivered rate, the low-quarter figure, the gauged pressure at the first and last head, and the run times as they were set on the day. Inside the controller door is the right place for the short version, because that is the only document that is guaranteed to still be on site.
Say plainly which findings the work addressed and which it did not. A guide like this ends with a fix; a real lawn often ends with two causes, one of them solved and one of them structural — the strip over the mainline backfill that will always dry first, the bank crest that will always be the driest ground on the plot, the hedge that is going to keep drinking. Writing that down is not a disclaimer. It is the difference between a client who understands they have a good system with one honest limitation and a client who believes they have been sold a repair that failed.
What goes in the van for a dry-patch call
None of this is a material list. It is the kit and the recorded figures that make a diagnosis stand up, ordered the way the tests are run: ground first, water second, programme last.
- Soil probe or corer, plus a long screwdriver — Two plugs per patch — one from the dead ground, one from healthy grass alongside — cut after a full cycle, not in the dry.
- Identical catch cans, enough for a regular grid over the whole zone — Same throat, same height, seated level, and that throat measured once for the set — without its area the volumes never become a depth. A grid too sparse to sample the overlap between heads returns noise with a decimal point on it.
- Graduated cylinder and a written sketch of can positions — Every can measured and recorded individually; the positions are what turn a uniformity number into a diagnosis.
- Pressure gauge that fits a head or a riser — Read at the first and the last head with the zone running. A static reading at the tap describes a system doing nothing.
- The manufacturer's performance chart for the nozzles actually in the ground — Radius, discharge and permitted spacing at the pressure you gauged, read off that row rather than the top of the table.
- Hollow tines, a soil surfactant and a compatible dressing — Rate and interval for the surfactant come off the product label; the dressing has to knit to the existing profile, not sit on it.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.