In an open body of water such as a bay, water quality may vary considerably in different regions, and water quality also changes continually over time. Tidal influence, inflow from tributaries, seasonal temperature changes, and rainfall are the most important factors driving these differences. A Water Quality Contour Map (WQCM) shows the water quality in different parts of the bay at a single point in time. By creating and comparing multiple contour maps, it is possible to detect patterns in water quality changes from month to month or from season to season, and to identify water quality trends over a longer period.
How are Water Quality Contour Maps Generated?
Water Quality Contour Maps are generated using water quality sample results for a single parameter (nitrogen, phosphorus, etc.), taken from sample sites spread across an open area of water like a bay or inlet, where the sampling events occur at regular intervals and reasonably close to each other in time.
The use of the word "contour" is a hold-over from the original maps which used lines (called isolines) to demarcate zones with similar water quality parameter values. Instead, the current method employs a "raster" or spatial surface that uses colors to indicate incremental changes in a water quality parameter over space. The geographic area to be represented is divided into equal-sized cells. Colors are assigned to subsets of the entire anticipated range of sample values.
Each cell receives a color value corresponding to the parameter value at that location. Cells that contain sample sites are colored using actual data. For all other cells, parameter values are calculated using a geographic information system (GIS) interpolation method called Inverse Distance Weighting (IDW) that is described below, under "Calculations." Once parameter values are calculated for each cell and each has been assigned a corresponding color, the resulting continuous color field creates a visual approximation of spatial water quality differences over the selected geographic area.
Viewed individually, Water Quality Contour Maps do not reveal temporal patterns in water quality. For this reason, maps are produced at regular intervals for each period during which water quality samples are available. When viewed chronologically, the maps can be used to detect seasonal patterns and long-term trends in water quality.