Tree Watering Calculator

Average Temperature moves Weekly Water Need in tiered steps rather than a smooth curve — the calculator applies a 0.7x multiplier below 70F, 1.0x from 70-85F, 1.2x from 85-95F, and 1.5x above 95F, so crossing one of those thresholds jumps the water requirement rather than raising it gradually. Weekly Rainfall works the opposite direction: it is converted into gallons collected across the tree's estimated drip-zone area and subtracted from the raw need, so more rainfall always reduces or holds steady the calculated Weekly Water Need, never raises it, and the result is floored at zero rather than going negative. Trunk Diameter's effect is more complex, because it drives two competing terms at once: it raises the base water demand linearly, but it also raises the estimated drip-zone area — and therefore the rainfall credit subtracted from that demand — by roughly the square of the diameter.

For small-to-moderate trunks with light rainfall this means bigger trees need more water, as expected; but for very large, mature trunk diameters even a modest rain week can generate a large enough rainfall credit to offset most or all of the calculated need. The drip-zone radius itself is estimated directly from Trunk Diameter (trunk diameter x 0.75 feet) as a simplified proxy — that multiplier is this calculator's own approximation, not a figure drawn from a published arboriculture standard, since real drip-line radius tracks canopy spread, which can vary independently of trunk diameter. (The "10 gallons per inch of trunk diameter per week" baseline and the 0.623 gallons-per-square-foot-per-inch-of-rain conversion, by contrast, are both commonly-published extension-service figures.) Tree Age Category and Soil Type both apply fixed multipliers (newly planted trees need more water than established ones; sandy soil drains faster than clay) but neither changes the underlying temperature- or rainfall- driven mechanics described above.