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California’s Water Year 2014 (October 1, 2013 through September 30, 2014) has been one of the driest in decades and follows two consecutive dry years throughout the state. In most years, California receives about half of its precipitation in the months of December, January and February, with much of that precipitation falling as snow in the Sierra. Only a handful of large winter storms can make the difference between a wet year and a dry one.

In normal years, the snowpack stores water during the winter months and releases it through melting in the spring and summer to replenish rivers and reservoirs. However, relatively dry weather conditions this year have reduced the amount of snowpack in California’s mountains. Each of this season’s first four snow surveys – conducted in early January, late January, late February and early April – found a statewide snowpack water equivalent far below average for the dates of the surveys. The water equivalent of the 2014 statewide snowpack began falling in early April after reaching a peak of 10.1 inches and by late May had almost completely melted away, compared to late May’s historic average of about 6 inches.

Rainfall also has been far below normal during this water year as recorded by weather stations throughout the state. Despite a few storms that brought rain in February and March, electronic readings indicate that precipitation at eight Northern California stations has been only about 60 percent of normal for late April. The electronic readings for San Joaquin stations show even drier conditions there – less than 50 percent of normal precipitation from October 1 to late May.

DWR’s late November experimental seasonal forecast for the water year predicted mostly dry conditions for the state, and that prediction has been accurate. Although Mother Nature sometimes surprises us, there’s little expectation of significant rainfall for the foreseeable future. Even a normal precipitation year would not be enough to overcome low soil moisture and water storage conditions. Most water users would require an exceptionally wet year to be made whole, and that’s not likely to be the end result of water year 2014. The drought has no end in sight.

The conditions for the major reservoirs in California:
http://cdec.water.ca.gov/cdecapp/resapp/getResGraphsMain.action

For background on droughts in California and answers to frequently asked questions, see the section below the following graphics.




Processed satellite imagery of a portion of the San Joaquin Valley showing the difference in the NDVI between August 2011 (wet year) and August 2013 (dry year). The Tulare Lake bed is just right of center at the bottom of the image. NDVI is the Normalized Difference Vegetation Index, a measure of vegetation conditions that can also be used to show drought or water shortage impacts in irrigated areas. Image courtesy of Lee Johnson, CSU Monterey Bay.


The US Geological Service has created questions and answers about droughts to help citizens appreciate the weather condition that potentially can affect their lives in profound ways. The USGS’s site can be accessed here. The contents below have been edited to be pertinent to California.

Questions

Answers

What is a drought?

While it is relatively easy to define what a hurricane or earthquake is, defining a drought is more subjective. Droughts do not have the immediate effects of floods, but sustained droughts can cause economic stress throughout an area. The word "drought" has various meanings, depending on a person's perspective. To a farmer, a drought is a period of moisture deficiency that affects the crops under cultivation—even two weeks without rainfall can stress many crops during certain periods of the growing cycle. To a meteorologist, a drought is a prolonged period when precipitation is less than normal. To a water manager, a drought is a deficiency in water supply that affects water availability and water quality. To a hydrologist, a drought is an extended period of decreased precipitation and stream flow.

What causes droughts?

A drought is a period of drier-than-normal conditions that results in water-related problems. Precipitation (rain or snow) falls in uneven patterns across the country. The amount of precipitation at a particular location varies from year to year, but over a period of years, the average amount is fairly constant.

The amount of rain and snow also varies with the seasons. In California, most of the yearly precipitation falls during winter, early spring, and in July. Even if the total amount of rainfall for a year is about average, rainfall shortages can occur during a period when moisture is critically needed for plant growth, such as in the early summer.

When little or no rain falls, soils can dry out and plants can die. When rainfall is less than normal for several weeks, months or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall and the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.

Reference: Moreland, 1993.

When does a drought begin?

The beginning of a drought is difficult to determine. Several weeks, months, or even years may pass before people know that a drought is occurring. The end of a drought can occur as gradually as it began. Dry periods can last for 10 years or more. During the 1930's, most of the United States was much drier than normal. In California, the drought extended from 1928 to 1937. In Missouri, the drought lasted from 1930 to 1941. That extended dry period produced the "Dust Bowl" of the 1930's when dust storms destroyed crops and farms.

The first evidence of drought usually is seen in records of rainfall. Within a short period of time, the amount of moisture in soils can begin to decrease. The effects of a drought on flow in streams and reservoirs may not be noticed for several weeks or months. Water levels in wells may not reflect a shortage of rainfall for a year or more after a drought begins.

Reference: Moreland, 1993.

Does a shortage of rain mean that a drought will occur?

A period of below-normal rainfall does not necessarily result in drought conditions. Some rain returns to the air as water vapor when water evaporates from water surfaces and from moist soil. Plant roots draw some of the moisture from the soil and return it to the air through a process called transpiration. The total amount of water returned to the air by these processes is called evapotranspiration. Sunlight, humidity, temperature, and wind affect the rate of evapotranspiration. When evapotranspiration rates are large, soils can lose moisture and dry conditions can develop. During cool, cloudy weather, evapotranspiration rates may be small enough to offset periods of below-normal precipitation and a drought may be less severe or may not develop at all.

Reference: Moreland, 1993.

Why doesn't a drought go away when it rains?

Rainfall in any form will provide some drought relief. A good analogy might be how medicine and illness relate to each other. A single dose of medicine can alleviate symptoms of illness, but it usually takes a sustained program of medication to cure an illness. Likewise, a single rainstorm will not break the drought, but it may provide temporary relief.

A light to moderate shower will probably only provide cosmetic relief. It might make folks feel better for a while, provide cooling and make the vegetation perk up. During the growing season, most of the rain that falls will be quickly evaporated or used by plants. Its impact is short term.

A thunderstorm will provide some of the same benefits as the shower, but it also may cause loss of life and property if it is severe. Thunderstorms often produce large amounts of precipitation in a very short time, and most of the rain will run off into drainage channels and streams rather than soak into the ground. If the rain happens to fall upstream of a reservoir, much of the runoff will be captured by the reservoir and add to the available water supply. No matter where the rain falls, stream levels will rise quickly and flooding may result. Also, because the rainfall and runoff can be intense, the resulting runoff can carry significant loads of sediment and pollutants that are washed from the land surface.

Soaking rains are the best medicine to alleviate drought. Water that enters the soil recharges ground water, which in turn sustains vegetation and feeds streams during periods when it is not raining. A single soaking rain will provide lasting relief from drought conditions, but multiple such rains over several months may be required to break a drought and return conditions to within the normal range.

Tropical storm rains are usually of the soaking variety, although they may also be intense such as during a thunderstorm and lead to some of the same problems. Tropical storms often produce more total rainfall than a "regular" soaking rain and can provide longer relief than a single soaking rain. However, tropical rains may also be of such intensity that they exceed the capacity of soil to absorb water and often result in significant runoff and flooding. Tropical rains can help to fill water-supply reservoirs and provide long-term drought insurance. If significant rainfall does not occur upstream of reservoirs, the drought relief aspects of tropical storms may be of only little consequence. All things considered, a single tropical storm at the right place, at the right time and with the right amount of rainfall can break a drought.

Considering all of the above, even when a drought has been broken, it may not be truly over. The benefits of substantial rainfall such as from a tropical storm may last for months, but a return to normal rainfall patterns and amounts is necessary for conditions in streams, reservoirs and ground water to also return to normal.

Reference: Moreland, 1993.

Can drought affect the water level in wells?

Ground water, which is found in aquifers below the surface of the Earth, is one of the nation's most important natural resources. Ground water is used to provide a large portion of California’s population with drinking water, it provides business and industries water for their purposes, and is used extensively for irrigation.

The water level in the aquifer that supplies a well does not always stay the same. Droughts, seasonal variations in rainfall and pumping affect the height of the underground water levels. If a well is pumped at a faster rate than the aquifer around it is recharged by precipitation or other underground flow, then water levels in the well can be lowered. This can happen during drought due to the extreme deficit of rain. The water level in a well can also be lowered if other wells near it are withdrawing too much water.

References

Moreland, J.A., 1993, Drought: U.S. Geological Survey Water Fact Sheet, Open-File Report 93-642, 2p.

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