5.3 Water Movement, Drainage, and Irrigation Decisions

Water Must Be Available, Not Excessive

Roots need water for uptake, transport, and transpirational cooling, and they need oxygen for respiration. So water management is never simply "keep soil wet." The arborist asks whether water enters the soil (infiltration), moves through it (percolation), drains before oxygen is excluded, and is replenished when demand exceeds supply.

Three soil-water benchmarks anchor the topic. Saturation fills every pore; gravity then drains macropores over hours to a day, leaving the soil at field capacity (water held near −0.033 MPa, or about −1/3 bar). As the tree extracts water, tension rises until the permanent wilting point near −1.5 MPa (about −15 bar), where roots can no longer pull water free. Plant-available water is the band between field capacity and wilting point; water held tighter than −1.5 MPa is physically present but unusable.

Sandy soils hold roughly 6–12% available water by volume; loams hold the most, often 15–20%; heavy clays hold a lot of total water but release it slowly.

A site can be too dry and too wet in one season. Compacted soil sheds rain at the surface, then traps water in low spots where micropores hold it and oxygen is excluded. A planting pit in heavy clay acts like a bathtub if surrounding soil drains slowly. A tree in a small paved opening dries fast in heat because roots have little volume to draw from.

The water moves through the soil-plant-atmosphere continuum along a tension gradient measured in negative pressure (water potential). Dry soil and dry air pull water from the leaf, the leaf pulls from the stem, and the stem pulls from the roots — but only if the root-zone water is held loosely enough to release. When soil tension exceeds the leaf's ability to pull (near the wilting point), stomata close, photosynthesis stops, and prolonged closure causes the marginal scorch, premature defoliation, and dieback the exam describes.

The opposite failure, waterlogging, kills roots within days because soil oxygen is consumed and not replaced; the tree then wilts from root death even while standing in wet soil. Recognizing that wilting is ambiguous — it can mean too little or too much water — is one of the most testable ideas in this section.

Soil Water Decision Table

A worked perc test and water budget

To judge drainage, dig a 30 cm (12 in) hole, fill with water, let it drain, refill, and time the drop. A fall of about 2.5–7.5 cm/hr (1–3 in/hr) is good; less than 1 in/hr suggests a drainage problem; very fast loss means droughty soil. For establishment, a defensible rule of thumb is roughly 40–65 L (10–15 gal) per 2.5 cm (1 in) of trunk caliper at each watering, delivered slowly onto the root ball and backfill, every few days during establishment and then tapering. New trees depend on the root ball until roots extend, so wetting only the surrounding turf strands them.

Mulch is a water tool: a 5–10 cm (2–4 in) layer cuts evaporation, moderates soil temperature, and protects structure from raindrop impact. It is not drainage. A volcano of mulch against the trunk holds moisture on bark and hides a buried root collar — keep mulch a hand's width off the trunk in a broad, shallow ring.

Irrigation method matters as much as amount. Turf sprinklers throw a shallow, frequent spray tuned for grass roots in the top few centimeters; they wet the trunk and surface while leaving deeper tree roots dry, and the constant surface moisture invites collar disease. Slow, deep delivery — soaker hose, drip ring, or a slow-release watering bag of about 50–75 L — soaks the root ball and backfill and encourages roots to grow down and out.

On slopes or sealed surfaces where the application rate outruns infiltration, use a cycle-and-soak approach: run water until it begins to run off, pause to let it soak in, then repeat, rather than dumping the full volume at once. The exam consistently favors deeper and less frequent over shallow and daily for established trees, because frequent light watering trains shallow roots and never recharges the storage roots actually draw on.

Drainage correction ranges from redirecting irrigation heads, removing a sealed surface layer, or breaking a perched berm, to engineered solutions — French drains, soil replacement, suspended-pavement/structural-soil systems, or species change. Recognize when a problem exceeds a maintenance fix. The exam may describe a street tree where normal rainfall is "not enough": the answer is usually restricted rooting volume plus high evaporative demand from reflected pavement heat, not a mysterious disease.

The strongest recommendation tells the client how to monitor — probe, hand-feel, or sensor at root depth — and to adjust to weather, which is far more defensible than a fixed calendar schedule.