Watershed Models

Eight directions pour point model (D8 model)

The eight-direction pour point model (D8 model) is a common method used in hydrology to simulate water flow across a grid model of the ground surface. This model helps understanding how water moves across a landscape, which is crucial in Quadri for producing a stream network, delineation of a watershed, and figuring out how “a single waterdrop” moves along the surface.

When a watershed model is created in Quadri, it internally has 3 key steps: filling sinks, calculating flow directions, and calculating flow accumulations.

Fillsink

Fillsink (aka filling sinks) refers to the process of modifying the terrain grid to remove depressions or pits that do not drain to any other cell. These sinks can be natural or artifacts of the data collection process. In a digital terrain model (DTM), sinks can disrupt the flow of water, causing it to stop or pool in these areas. To ensure continuous flow, these sinks are "filled" by raising the elevation of the sink cells to the level of the lowest neighboring cell that allows water to flow out. In addition to this, Quadri has an extra “fillsink increase” that is used to ensure water really flows out, and no flat areas are created. This process ensures that every cell in the grid can drain to another cell, facilitating proper flow direction and accumulation calculations.