The western landscape is varied running from desert to rain forest. However, most of the region tends towards aridness, requiring concentrated reservoirs and irrigation to allow for large scale human settlement. The map and graph below shows the network of reservoirs across the west.
Much of the American West has been in and out of drought for most of the 21st century; leading to lower average water levels than would be typical. For an example, check out reservoir levels in California in 2015 and most of 2016. Click a reservoir to display its levels. Hover over the graph to see the levels for a given month. Data updated monthly; so feel free to check back often.
Percent Full for Month Ending (December 2024), or Most Recently Available Month
21st Century Average Volume: acre feet
Snow is vital to the environmental health of the western United States. Unlike rainfall, it doesn't come as one burst that's here and then quickly gone. Instead, it tends to accrue over the winter and slowly melt as the temperatures rise in the spring. The melt typically continues into the early summer, providing a steady, consistent level of water in mountain watersheds. This is particularly important for a state like California, which receives almost no summer rainfall.
Snow comes in a variety of forms. It can be light and airy, almost dry, or it can heavy and dense. For example, light, fluffy snow that measures four feet deep might actually have the same volume of water as heavier, denser snow that is two feet deep.To account for this variation a measure called snow water equivalence is used to normalize the amount of liquid water in a given level of snow. The graph below uses SNOTEL data; a series of automated weather stations distributed across the west to measure things such as snow levels and temperature. Notice how temperature and snow levels change with elevation. Compare the average snow/water levels by year and elevation. The general trend is towards drier conditions, but wet years are interspersed with drier years.
The Palmer Drought Index is a measure of long term drought. It calculates soil moisture based on recent precipitation and temperature. 0 is considered a normal level of soil moisture, while increasingly negative numbers indicate progressively severe levels of drought. Positive numbers represent increasingly wet soil conditions. Compare the Palmer Index to the temperature and precipitation levels below. Recent droughts have tended to have hotter temperatures than usual. This results in increased levels of evapotranspiration from the soil and plants, leading to more severe droughts and worse Palmer Index numbers.
Note: Hover over a bar to see the value for a particular month.
The United States Drought Monitor defines the effects of various drought levels as follows:
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