RAINMAKER

06 · THE SCIENCE (SIMPLIFIED)

1. Sunlight evaporates standing water and the atmosphere’s circulation carries the vapor. It condenses into clouds, falls back as rain, and gravity pulls the water downhill. USGS Water Science School — The Water Cycle
2. Hills and mountains push moist air upward; the rising air cools and drops its moisture as precipitation. In this toy, clouds simply rain sooner over high ground. NOAA National Weather Service — glossary: orographic
3. Fast-moving water picks up and carries sediment, even larger grains, more easily than slow water; where it slows down, the load settles out. USGS Water Science School — Sediment and Suspended Sediment
4. A river slows as it nears its mouth and drops its sediment, building new land — a delta. National Geographic Education — Delta
5. Water flow, erosion, deposition and evaporation follow the virtual-pipe model of Mei, Decaudin & Hu (2007). Mei, Decaudin & Hu — Fast Hydraulic Erosion Simulation and Visualization on GPU (PDF)

This is a toy, not a forecast. The map is not to scale, time and volumes are model units, and clouds follow a simple “rain when too full” rule. Clouds also wrap around the map edges: moisture that drifts off one side comes back on the opposite side, so even a wind blowing out to sea brings rain to the far side of the map. Our additions to the paper’s model — flow friction, a minimum speed before water can lift soil, depth-limited load and evaporation, sediment that rides with the water, a slope-slump step and an open ocean edge — are simplifications for speed and stability.