Why plan a greywater irrigation schedule?

Homeowners in drought-prone regions increasingly install laundry-to-landscape systems or divert shower drains into surge tanks. Reusing that lightly soapy water can trim potable irrigation demand by hundreds of gallons each week, but timing matters. Discharging greywater too soon wastes moisture during cool mornings, while storing it too long triggers odors and health concerns. Municipal codes often mandate subsurface dispersal within 24 to 48 hours. Garden beds also have distinct water needs based on crop type, soil texture, and sun exposure. This scheduler bridges the gap by taking household water production, regulatory storage limits, irrigation efficiency, and rainfall skips to generate a balanced seven-day plan. Instead of guessing whether Tuesday’s laundry loads can cover Saturday’s fruit trees, you see the numbers.

Greywater planning also protects soil health. Over-watering with nutrient-rich laundry discharge can leach detergents or create anaerobic pockets. Under-watering wastes the infrastructure investment because the backup municipal system must fill the deficit. The calculator tallies daily inflows and outflows from your holding tank, highlighting when overflow purges occur or when you must top up from potable water. By simulating storage ages, it ensures your plan respects public health rules that limit how long greywater may sit before discharge. The output table doubles as a maintenance log you can print and keep near the diverter valve to guide family members or landscapers.

Finally, the scheduler contextualizes how much area you can support. New gardeners often assume a family of four produces enough water for sprawling beds, but the math depends on capture efficiency and plant species. A well-designed system uses all captured water with minimal overflow during dry weeks yet avoids chronic deficits. The summary metrics quantify weekly supply, irrigation demand, reuse ratio, and municipal supplements so you can right-size storage tanks or adjust crop layout.

How the calculations work

The planner begins by estimating weekly greywater supply. If your household produces $g$ gallons per person per day, has $n$ occupants, and captures a fraction $c$ of that flow, the available greywater in a week is $S=7\times n\times g\times c$. Plant demand follows the classic landscape formula: required depth in inches multiplied by area in square feet and converted to gallons using the factor 0.623. Because irrigation is rarely perfect, the tool divides by efficiency to determine how much greywater must be applied. In MathML form, $D=\frac{0.623}{e}\times A\times h$, where $A$ is area, $h$ is water depth, and $e$ is efficiency expressed as a decimal. Dividing the weekly demand by the number of irrigation events yields the target volume per application.

Storage dynamics complicate the picture. The script tracks greywater age in daily buckets. On each simulated day, it advances the age of water already in the tank, discards anything that exceeds the maximum storage window, and adds the new daily capture. If the tank would exceed its physical capacity, the excess becomes overflow that should be routed to a mulch basin or sewer. When an irrigation event occurs, the planner draws from the oldest bucket first, which mimics emptying a surge tank from the bottom. This approach both minimizes stagnation and respects health rules by never holding water beyond the specified days.

Rainy days matter as well. The schedule builder spreads the specified number of skip days across the week and reduces irrigation events accordingly. If rainfall cancels too many events, the script flags the resulting deficit. You can use the scenario table to see how adding a small auxiliary cistern or reducing the irrigated area changes the coverage ratio.

Worked example: edible landscape in a semi-arid climate

Consider a family of four in Tucson diverting laundry and shower water. They estimate 25 gallons per person per day and capture 60% of it, yielding 420 gallons per week. Their 600-square-foot edible garden requires 1.2 inches of water weekly. With a 75% irrigation efficiency, the plants need roughly 598 gallons applied. The family schedules three irrigation events per week, stores up to 200 gallons in a surge tank, and must discharge within two days per county code. Plugging these numbers into the calculator shows that each irrigation should deliver about 199 gallons. Because the captured supply falls short of demand, the municipal top-up column reveals deficits on two of the three irrigation days. The weekly summary notes a coverage ratio of 0.70, meaning greywater covers 70% of the landscape’s needs. Overflow stays minimal because the tank rarely fills; the real constraint is supply, not storage.

If the family installs low-flow showerheads reducing production to 18 gallons per person but adds a 300-gallon storage tank, the schedule shifts. The larger tank captures more weekend laundry surges, but the reduced supply drops the coverage ratio to 0.50. The scenario table quantifies how many gallons of municipal water must supplement irrigation. Conversely, upgrading capture efficiency to 80% by diverting bathroom sinks increases weekly supply to 560 gallons, cutting the deficit to just one irrigation day. The table immediately shows overflow rising slightly because the tank hits capacity after rainy-day skips; a mulch basin is now advisable.

Comparison table: crop water demand benchmarks

To translate the schedule into planting decisions, compare your crops against typical water needs. The table below summarizes weekly requirements for common drought-tolerant and thirstier species.

Approximate weekly water depth for popular plantings

Plant type	Typical depth (inches/week)	Notes
Native xeriscape shrubs	0.5	Often satisfied with alternate-week greywater cycles
Fruit trees with mulch basins	1.0	Beneficial to combine with rainwater harvesting during monsoon season
Raised-bed vegetables	1.5	Prefer frequent smaller applications to avoid leaching nutrients
Lawn or turf alternative	2.0	Greywater alone rarely meets demand; plan for municipal backup

Seeing these benchmarks helps you allocate limited greywater. Pair thirsty crops with rainwater catchment or drip emitters, while native perennials can survive on occasional overflow routed to a mulch basin.

Limitations and assumptions

The scheduler assumes steady daily water production, though real households produce more greywater on laundry days and less while traveling. You can mimic variability by adjusting the per-person figure and rerunning the model. The irrigation efficiency input captures distribution losses but cannot model clogged emitters or sloped terrain. The tool also assumes immediate infiltration; clay soils might require slower application rates to prevent pooling. Finally, always consult local regulations—many jurisdictions restrict surface application or mandate specific soaps. Use the generated schedule as a starting point and refine it with soil moisture sensors or periodic infiltration tests.