What Is A Hydrological Map?

A hydrological map serves as an illustration that shows a network of hydrological features, including rivers, streams, and lakes. Some hydrological maps are created for decorative purposes while others are prepared as part of a larger technical report. Engineers and hydrologists often prepare hydrological maps as part of watershed studies, bridge design, and laying out storm drains.

The streams depicted on a hydrological map are generated based on terrain data. 

How To Create Hydrological Maps

Hydrological maps are created by using a Geographic Information System (GIS) software program to determine how water flows across a landscape. The steps for creating a hydrologic map are as follows:

Step 1: Obtain terrain data. A digital elevation model (DEM) is needed to represent the terrain over which water flows. The DEM should be in raster format. An example of DEM is shown below.

Step 2: Using GIS software such as ArcGIS or QGIS, use the Fill Sinks tool to clean up the terrain data by filling in local depressions. A sink is a set of one or more cells which has no downstream cells around it. If these sinks are not filled, flow will not reach portions of the terrain data.

Step 3: Use the Flow Direction tool to determine a Flow Direction raster using the raster data created in Step 2. The image below shows how each raster value corresponds to flow direction.

Step 4: Next, a Flow Accumulation Raster was created used the Flow Direction data created in Step 3. A larger raster value represents a cell that has accumulated more flow from surrounding cells. The image below shows how a Flow Accumulation raster is created.

Step 5: A stream dataset is produced using the Flow Accumulation Raster created in Step 4. A threshold is entered that defines a stream. A larger threshold will result in fewer streams, and a smaller threshold will result in a network of more streams. It may take a few iterations to figure out the appropriate threshold for defining a stream.

Step 6: Finally, the stream data is used to produce a visually appealing hydrological map.

Strahler Stream Order

There are some hydrological maps that show stream lines with differing thickness based on Strahler Stream Order. Stream order is a methodology where a numeric order is assigned to links in a stream network. There are a few methods used to determine stream order, but one of the most common was proposed by Strahler (1957). The image below illustrates how Strahler Stream Order works.

 The Strahler Stream Order methodology assigns all streams that do not have tributaries an order value of one. Two first-order streams combine to make a second-order stream. Similarly, when two second-order streams combine to create a third-order stream. When two streams of different stream order combine, the resulting link will retain the the order of the highest ordered stream. For example, when a first-order tributary and second-order tributary combine, the resulting intersection is a second-order stream rather than a third-order stream.

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