Ždiar sits in the Tatras, a village of about a thousand people on a slope above a stream called Biela. Search for it on Meratch and you get a report: the land that drains toward the village, the shape of the ground inside it, and how fast rain would reach the point where the water leaves. Behind it are six steps, each built on a different kind of evidence.
Step one: finding the one way out
Before anything else, the system works out where water actually leaves this location — the outlet. Sometimes there’s a single outlet; a location can also drain toward several at once, each one traced on its own. From every outlet, the calculation works backward, uphill, marking every patch of land whose rain eventually flows past it. Together, these patches make up the location’s catchment. In Ždiar, every path traced from the village converges on the same outlet, so one catchment covers the whole area — about 34 square kilometers, more than twenty times the built-up area itself.
Step two: choosing a terrain model
A catchment is only as good as the terrain model beneath it — a digital map of how high the ground sits at every point. So the system draws the same boundary five times, on five different terrain models: one from a radar mission flown on a space shuttle in 2000, two from newer satellites, a European elevation dataset, and a national model for Slovakia that records one height for every 10-by-10-meter square of ground. In Ždiar, the five outlines come out within about one percent of each other in area. Elsewhere the match isn’t always this close: in some catchments, a small difference between models leaves out an entire side valley. A person compares the five outlines side by side and picks the one the rest of the report will use.
Step three: reading the shape of the ground
The chosen terrain model gives up more than elevation. Every point in it has a slope and a shape — ridge, hollow, open slope, valley floor — and each shape has a general flood tendency. Hollows and open slopes tend to concentrate fast-moving water during short, heavy rain; valley floors tend to collect the most water overall. On its own, the shape of the ground can’t say whether a place will flood on a given day.
Step four: what covers the ground
Bare ground sheds water fast; a forest floor holds it back. So the report also needs to know what covers the catchment, not just its shape. It draws on two separate satellite records of this land cover: one that tracks how forest cover has shifted since 2001, another that gives a detailed snapshot of today’s surface, sorted into classes such as forest, pasture and built-up land. It’s that snapshot, not the long-term record, that tells the next step how much the ground slows water running downhill.
Step five: how long water takes to arrive
With a shape and a surface type for every point in the catchment, the system can ask the same question any traveler asks about a trip: how long will it take? It calculates how fast water could move from each point and how long it would take to reach the outlet. Lines of equal travel time spread out from the outlet like contour lines on a hiking map, showing which parts of the catchment would send water there within minutes, and which would take longer.
Step six: putting it together as a wave
Travel times from every point, added up across the whole catchment, give something closer to what people picture when they hear “flood”: a wave arriving at the outlet over time. For a short, heavy storm over Ždiar, that wave peaks roughly 45 to 60 minutes after the rain starts — a shape, not a volume. The report doesn’t estimate how much water such a storm would carry. It shows how the catchment would spread that water out in time.
Ždiar already has its report. Do you have one for your location yet?
