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Climate Science: Investigating Climatic and Environmental Processes
Annual Cycle & Variability (100 year)
Annual Hydrograph & Water Budget
Forcing Factors

Reservoir Management Decision Calendar

Image of Earth's annual cycleThe fact that Earth's 23.5 degree tilt off a vertical axis causes seasonal variations as the planet orbits the sun is not something obvious from everyday experience. In our everyday life, it is hard to tell that the orientation of the axis remains fixed in space, producing changes in the distribution of solar radiation over the course of the year. Nevertheless, because of the tilt of the axis, these changes in the distribution of radiation reaching earth's surface do cause the succession of the seasons, with more extreme seasonal changes generally occurring at high latitudes, and less variation happening closer to the equator.

In the figure above, summer occurs in the northern hemisphere when the northern hemisphere is tilted towards the sun, receiving more direct radiation. (Contrary to the illustration, however, the Earth does not get significantly closer to the sun during any time of year). Winter in the northern hemisphere occurs when there is less direct radiation. The Equinox (when the sun's direct rays hit the equator) occurs around March 21st and September 23rd, give or take a day or two depending on the year.

Learn more about other key cycles of extraterrestrial dynamics such as solar variability (sunspots 11 years.), Obliquity (~41k years. change), Precession (wobble on axis due to gravity of sun and moon in 23k yearrs. cycle) and Eccentricity (96k yrs. change) by visiting the Paleoclimatology Branch online slides series on the Ice Ages and Climate Science 100,000 Years

The hydrograph is an important tool in tracking climate variability over the course of a year and beyond. In watersheds that are not heavily managed with dams and reservoirs, the hydrograph shows an annual climate signature generated by the seasonal runoff pattern of streams and rivers in a region.

Also, the hydrograph is important in developing a water budget (the input and output of water) for a region such as a watershed. See the Decision Calendar below for an overview of how reservoir management decisions are made or click here for more information on a water budget for the Boulder Creek watershed in Colorado, including a look at the role of potential evapotranspiration in a seasonal cycle.

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The tilt of the Earth's axis creates seasonal variability, but there are other factors that may impact climate systems at the annual scale, such as volcanic aerosols which can cool climate and shorten growing seasons.

How Measured
Instruments used to track annual variability and climate patterns include thermometers, rain gauges, and stream gauges. Paleo proxies such as tree rings and cores from corals and ice caps and glaciers also provide information on an annual resolution in terms of precipitation and in some cases extreme events such as fires or volcanic activity.

Reservoir Management Decision Calendar by Andrea Ray, et. al.
The calendar above developed by Andrea Ray (2001) and associates for the Western Water Assessment demonstrates some of the types of decisions that water managers need to consider in planning for water storage and release over the course of a year.

Images from NOAA Paleoclimatology Program, EPA and NGDC.

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