Square Meters to Square Kilometers

A depth of 25 mm is 0.025 m and the surface is 1,000,000 m², so the gross total is 25,000 m³, or twenty-five megalitres. The building block behind it is that one millimetre over one square kilometre is 1,000 m³, so any depth and any basin size can be worked out in your head from there. Only part of it reaches the outlet: with a coefficient of 0.9 for dense paving expect roughly 22,500 m³, while pasture on free-draining soil might yield 6,000 m³ or less.

There is no rule, only a convention that follows the method in use. Site drainage and attenuation design stay in hectares because the things being sized are individual roofs, car parks and permeable strips. Once a boundary is delineated from terrain data and set against a river gauge, square kilometres take over, and in practice that happens somewhere between 0.5 and 1 km², which is 50 to 100 ha. A safe habit is to keep the raw square-metre value in the model and quote the km² version in the text, so the audit trail retains precision the report does not need.

Multiply it by the basin size: 10 L/s/km² across 3.45 km² gives 34.5 L/s, a flow you could step across. Expressing discharge this way lets a rate measured on a gauged basin be carried onto an ungauged neighbour without pretending the two are the same size. It also converts into a depth over time — 10 L/s/km² sustained for a year comes to roughly 315 mm of runoff, which can be tested against the local rainfall record for plausibility. When a computed value sits far outside the range of nearby gauges, suspect the area figure before you suspect the hydrology.

Neither, in the sense that it depends which arrangement of the formula sits in front of you — and mixing them is a classic way to be out by a factor of a hundred. In the metric form Q = C i A / 3.6, peak flow Q is in m³/s, intensity i is in mm/h and A is in square kilometres. A second common arrangement, Q = C i A / 360, is identical except that A is in hectares. For a 3.45 km² basin with C = 0.35 and i = 50 mm/h the result is 0.35 × 50 × 3.45 ÷ 3.6 = 16.8 m³/s. Most drainage manuals also restrict the method to small areas, often a few square kilometres at most.

Travel time follows the longest flow path, and path length grows roughly with the square root of the surface rather than in step with it. Doubling a basin from 3.45 to 6.9 km² lengthens that path by about 40 %, and empirical expressions such as Kirpich raise length to a power below one, so the resulting time typically rises by only a quarter to a third. The practical consequence is that a larger basin is paired with a longer, gentler design storm at a lower intensity, which is why peak flow per unit of surface falls as basins grow even though the total volume climbs in direct proportion.