\u017ddiar 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.<\/p>\n\n\n\n
Before anything else, the system works out where water actually leaves this location \u2014 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 \u017ddiar, every path traced from the village converges on the same outlet, so one catchment covers the whole area \u2014 about 34 square kilometers, more than twenty times the built-up area itself.<\/p>\n\n\n\n