Symbology for Mann-Kendall Trend Maps

Upward-pointed triangles indicate statistically significant increasing of the variable in question over time at the station location being analyzed. Likewise, downward-pointed triangles indicate statistically significant decreasing in the variable in question over time at the station location being analyzed. A circle indicates that the trend was not deemed to be statistically significant using the Mann-Kendall trend test (see “Statistical Tests” below). The shading of the triangles and circles shows the magnitude of the Theil-Sen slope estimate at that station (see below), with blue hues indicating an wetting trend and red hues indicating a drying trend.

Symbology for Decadal Trends

For decadal trends, no statistical significance was evaluated, so only circles were used for these maps. Like the Mann-Kendall maps, colors represent the magnitude of the changes, with blue hues indicating a wetting trend and red hues indicating a drying trend.

Statistical Tests

Statistical significance of time series trends was evaluated using the Mann-Kendall test for monotonic trend. For more information on the Mann-Kendall test, please refer to the bottom section of the documentation from NDMC’s Drought Risk Atlas, and to this 1998 paper by authors Hamed and Rao. The Theil-Sen slope estimator was used to detect linear slopes in most time series. Please see the Theil 1950 paper for information on this slope estimator.