With the “D” word a headline in just about every Bay Area newspaper this morning it bears some examination of "drought" definitions. The word drought does not have a universal definition and it’s much more complicated more than just a deficit of rain or snow in a particular place. The condition is also largely dependent on the number and type of water-users, and whether only local water is used or if it is imported from other regions.
Consequently, extended moisture deficiency can be thought of in terms of meteorological, hydrological, agricultural or socioeconomic drought. Choose your favorite.
Below are some links to resources on Drought.
U.S. Drought Monitorhttp://drought.unl.edu/dm/monitor.html has been widely featured the last couple of days in news reports but it should be used with caution as it is primarily an index of agricultural drought. It does not factor in such important things like existing water supplies, population and usage patterns. This is even stated within the website as it notes on each page “The Drought Monitor focuses on broad-scale conditions. Local conditions may vary.” Further buried in the text is “It should be noted that the relationship between indicators and impacts varies, sometimes markedly, with location and season. This is particularly true of water supplies, which are additionally dependent on the source (or sources) tapped, management practices, and legal mandates. Exercise caution when attempting to relate these maps to specific impact implications for a particular location and time of year. The blend-to-impact correlation is not always direct, and will vary spatially and temporally.”
The definition from the American Meteorological Society Glossary of Meteorology is: drought A period of abnormally dry weather sufficiently long enough to cause a serious hydrological imbalance. Drought is a relative term, therefore any discussion in terms of precipitation deficit must refer to the particular precipitation-related activity that is under discussion. For example, there may be a shortage of precipitation during the growing season resulting in crop damage (agricultural drought), or during the winter runoff and percolation season affecting water supplies (hydrological drought). Compare dry spell; see absolute drought, partial drought.
A drought is a period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives below average precipitation over an extended period, usually ranging from several months to several years.
Meteorological droughts are usually defined by the departure from normal precipitation on a monthly or seasonal time scale. Hydrological drought is primarily a function of the effect of precipitation on surface and subsurface water supply, but is also influenced by such factors as local water usage and storage.
Probably the most localized and fastest responding variety of drought is agricultural drought, which can vary from crop to crop and is measured on time scales as short as a week or two. How all of the above interact with human activity is reflected in a catch-all term, socioeconomic drought. That term is essentially a balance sheet of the supply and demand of water on residential, industrial and agricultural usage as well as its impact on hydroelectric power and energy conservation.
California Climate Tracker at http://www.wrcc.dri.edu/monitor/cal-mon/index.html
California DWR Hydrologic Conditions: http://cdec.water.ca.gov/cgi-progs/current/EXECSUM
Summary for California’s Condition:
This year is not unlike some years in the 90’s where we’ve seen dry Jan-March periods, with more precipitation in April and May that gets us closer to being on track. Don’t panic yet.
The bottom line is that coming off several very wet years that topped off most reservoirs and groundwater tables that it is premature to say universally that California is in a drought. However, there are local and even some region interests that are facing deficits and would at least qualify as “drought impacted”.
Thanks to Jan Null, former lead forecaster for the National Weather Service for much of the information above.