Will the Water Reservoir Last Until the Customer Returns?

Dividing a reservoir's stated capacity by a pump's stated flow rate does not produce a number of days, and the error tends to run in the product's favour. That is what makes it dangerous rather than merely wrong.

Start with the absence period and the plant group

The right starting point is the trip, not the product.

Three things about the absence set the requirement. Its duration, obviously, but also its timing: a week in April and a week in August are different demands on a plant, and the same reservoir answers differently. The plant group, meaning what is being watered, at what size, in what containers, since a tray of seedlings and a mature tomato plant in a pot have entirely different water requirements. And the environment during the absence, which includes whether the plants are under cover, in full sun, in wind, and what the forecast is.

Any answer given before those three are known is a guess. A useful discipline for a retailer is to refuse to give a day count until the customer has described the trip and the planting, and to explain why: the number is a property of the situation, not of the product.

The honest position on the plant side is that this article will not give water requirement figures for plants, because those vary with species, size, container, substrate and weather to a degree that makes a table misleading. The customer's own observation of how often they water by hand, in the same containers, in the same position, is a better input than any published average.

Use the water the system can actually draw

The second step is to establish the usable water, which is smaller than the container's stated capacity and is often considerably smaller.

The usable volume is the water between the actual starting level and the effective stopping level for this installation. Establish that lower limit from the intake or filter position, any required submergence or suction limits, and the controller's cut-off setting; a warning level alone is not necessarily a stopping level. These are overlapping constraints on the same remaining water, not separate volumes to subtract. Use the container's level-to-volume relationship to calculate the amount available.

There is a practical measurement point here that catches people out with irregular containers. The height of water in a butt or a trough is not proportional to its volume unless the sides are vertical and the shape is regular. For an irregular container, a drop measured in centimetres cannot be converted into litres without a level-to-volume relationship for that specific container. Getting one means either using a container with graduations, metering the output instead, or establishing the relationship once by filling it in measured steps and marking the levels. Guessing the conversion is how a comfortable margin turns into a dry reservoir.

There is also a practical deduction that has nothing to do with engineering: filling. A customer who fills a reservoir only to a marked level rather than to the top has a smaller usable volume than the nominal figure, and a customer who fills it with a hose cannot judge the level in an opaque container. The usable water is what the system can draw from the level it is actually filled to.

This is where the temptation to publish a “days of autonomy” figure should be resisted entirely. That figure would have to be computed from usable water divided by consumption, and both terms are situation-specific. A product marketed on a day count is a product whose returns will be driven by the customers whose situation differed.

The public product information from Irrigatia is worth mentioning for a different reason: it states that their systems water every three hours and adjust with the weather, watering longer when hot and less when cool. That is a picture of a system whose delivery is deliberately variable, which is exactly why a fixed day count cannot be derived from it. The same product pages report a survey in which more than 350 UK homeowners took part and over 70 percent said holiday watering was a key reason for choosing an Irrigatia kit. That is a manufacturer's own account of why people buy the category, appearing on the brand's own product pages, with the sample described as UK homeowners and no survey year or methodology stated on those pages. It is useful as an indication of the buyer's motive rather than as independent research or an industry survey, and it says nothing about how long any particular system will last. It should not be cited as if it did, and it should not be given a year that the source does not carry.

Measure delivery across the real watering program

The third step is to replace a flow-rate figure with a measurement, and this is where a retailer can add real value.

Two quantities get confused at this point and they have to be kept apart. What the reservoir gives up is the draw from storage: everything the system takes out, including what leaks, evaporates or drains and never reaches a plant. What arrives at the plants is the delivered water: what actually comes out of the emitters. The second is smaller than the first, sometimes substantially, and it is the second that the plants live on while it is the first that empties the butt.

A pump's quoted flow rate is typically a free-delivery figure: what the pump moves with minimal resistance. In a working system, flow passes through tubing, connectors, filters and drippers, and every one of those removes head. What reaches a single dripper is therefore not comparable to the pump's quoted total, which is a whole-system figure taken at stated conditions. How much lower it runs, and whether adding another outlet raises or lowers what any one outlet receives, depends on the pump's own performance curve and on how the system is arranged, so it has to be measured on the installation rather than predicted from the number of outlets.

Beyond that, a programmed system does not run continuously. It runs at a duty cycle, and the duty cycle is set by the controller, which in weather-responsive systems changes with conditions. The volume delivered per day is therefore the product of delivery rate, run duration and frequency, and all three vary.

The measurement that replaces all of this estimation is straightforward and takes a day: with the system set up as it will be used, record the reservoir level drop across a full cycle, catch the output from a representative outlet of each type over that same cycle, and count the cycles in a day. Total draw from the reservoir, checked against what each outlet delivers, gives two figures that belong side by side: how much the system removes from storage, and how much of it reaches the plants. To get the second figure, either measure all outlets over the same cycle, or measure a representative sample of each outlet type and multiply by the number of outlets of that type, treating the result as an estimate rather than a measurement. Keep both readings over the same interval and note that water already sitting in the lines at the start and end of the test will not be counted the same way twice. Then convert the draw to a daily figure, using litres per day rather than litres per cycle, and divide usable water by that daily figure to give an expected duration for those conditions. The comparison of the two figures then indicates how much of the draw does not arrive at the plants, as an indication rather than a precise audit.

The two readings are not redundant. The level drop tells you what the whole system took from storage, including losses that never reached a plant. The outlet checks tell you whether that volume was distributed where it was intended, and they expose an outlet that is delivering far more or far less than its share. A system can lose a large share of its draw to a leaking connector or a mis-set emitter while the drippers still look adequate one at a time. Remember that the level drop has to be converted into a volume using the container's own level-to-volume relationship, not by assuming depth is proportional to volume.

Three steps make that measurable rather than theoretical. Measure across several days rather than a single one, because the number that matters is the pattern the plants will actually experience, and a system that responds to weather does not control it: hot or dry spells can recur while the customer is away, so the plan has to be checked against the higher-demand conditions that could occur, not only the average. And measure again when the season turns, because a figure taken in cool April weather is not a prediction for August and the whole point of weather-responsive control is that the number moves.

Test the estimate against changing conditions

The fourth step is to stress the estimate, because a single number is a promise and a range is an estimate.

Related reading: solar irrigation performance in low light and winter.

Plant demand generally rises as plants grow and as weather warms, but under a fixed programme the system does not deliver more in response, so the extra demand shows up as a shortfall at the plants rather than as a faster drop in the reservoir. Storage losses occur, through evaporation from an open container, through minor leakage at connectors, and through any drainage at cycle end.

The weather-responsive behaviour deserves separating out, because it works the other way and it is easy to get backwards. In a system that adjusts with the weather, delivery rises in hot weather by design. That is the feature working. It does not follow that a reservoir under a fixed programme always empties sooner because the crop needs more: a fixed programme delivers what it delivers, and higher plant demand does not by itself change how much the system takes from storage over a given period. What higher demand does mean is that the same delivered volume goes less far against the crop's need. The two effects point in different directions, and the practical consequence is that a shortfall can appear without the reservoir emptying any faster than before.

A shortfall against the programme is a third and separate matter: a fixed programme that delivers less than intended, through a clogged filter, a partly closed valve or a pump that is not running its full cycle. That calls for maintenance rather than for more storage.

The reason for treating all of this as a range rather than a number is that a single day's measurement does not describe a season. That is the basis for building in margin rather than computing to the limit.

A workable method for the customer is a short trial before the trip: run the system for two or three days in the real position with the real planting, record how much the level drops per day, and compare the usable volume against the length of the absence with an explicit margin. The trial is a conditional estimate, not a guarantee of unattended operation and not a forecast. It shows what the system did over a few days in the conditions that happened to occur, and conditions can change in the week the customer is away. If the trial suggests it is marginal, it is marginal, and the answer is more storage, a second reservoir where the model supports one and connected as the manufacturer specifies, or a different arrangement, rather than a hopeful departure.

State the refill condition, not an unconditional holiday promise

The last step is about what goes on the product page and in the sales conversation, and it is the difference between a product described accurately and one that generates complaints.

What should be stated: the usable water volume with the deductions named, the reservoir's refill and level-check method, the conditions under which the system is intended to operate, and the fact that duration depends on the planting and the weather, with a recommended trial.

What should never be stated: a number of days the system will keep plants alive, a promise that the customer can leave without arranging anything, or a claim of unattended operation. RHS's guidance on a related product is instructive here: in discussing automatic vent openers, RHS describes them as useful but slow to react, and advises supplementing them with manual control. The same logic applies with more force to watering, because a watering system that stops delivering does not fail loudly.

The refill condition is the specific alternative to a holiday promise, and it is easy to state: this reservoir, filled to this level, with the system set to this programme, supplies roughly this much water per day in these conditions, and the customer should arrange for the level to be checked or the reservoir refilled if the absence is longer than the trial supports, or fit additional storage where the model supports it. That is a sentence a customer can act on, and it is a sentence a supplier can defend.

For the trade, this points to a supply structure rather than a marketing claim. Storage capacity is a modular purchase: additional storage, a linking kit and an inlet arrangement extend the duration further than any promise about a single unit's autonomy, provided the specific model supports the connection and the manufacturer's arrangement is followed. Selling those as a documented addition, with the connection confirmed for the specific system, addresses the customer's actual problem, and it does so without requiring anyone to publish a day count that the first hot week will disprove.

Choosing irrigation kits for customers who need watering during an absence? Share the intended plant group, reservoir arrangement and operating schedule. We can help review available specifications and sourcing options, with actual duration verified for the planned installation.