The current drought afflicting California is indeed historic, but not because of the low precipitation totals. In fact, in terms of overall precipitation and spring snowpack, the past three years are not record-breakers, according to weather data for the past century.

Similarly, paleoclimate studies show that the current drought is not exceptional given the natural variations in precipitation of the past seven centuries. Nor can it be confidently said that the current drought bears the unequivocal imprint of climate change driven by increasing greenhouse gases, since the low precipitation is well within the bounds of natural variability.

All this being said, it is also clear that this drought is exceptional and should be seen as an historical turning point. Indeed, California is moving into new — and worrisome — territory for three reasons: rising heat, which causes increased evaporation; the continuing depletion of groundwater supplies; and growing water shortages on the Colorado River, the main external source of water for Southern California.

十年前，我第一次写有关加利福尼亚和“完美干旱”的文章。现在，除非采取大胆的步骤应对不断增长的水危机，否则加利福尼亚可能面临着完美干旱的未来。

The heat is rising

首先，有热量。尽管目前的降水量赤字不能归因于全球气候变化，但2014年创纪录的高温可能是。这些升高的温度会增加储层和加剧灌溉需求的蒸发。

The commonly used Palmer Drought Severity Index, which combines both temperature and precipitation, shows that 2014 is indeed off the charts. This combination of low precipitation and high evaporative losses fuels the crisis now being faced.

气候模型对21世纪可能发生的加利福尼亚可能发生的降水变化不提供共识。向前迈进，平均年降水可能没有显着增加或减少。但是，这些模型确实同意温度将继续升高。

Water demands to meet evaporative losses will therefore increase significantly. There is also some evidence that the length and depth of droughts will increase in the later 21st century. As for high temperatures and persistence of extreme conditions, the current drought might well be considered the harbinger of droughts to come.

Heavy reliance on groundwater

Second, increased reliance on groundwater has been an important mechanism by which California coped with past droughts. However, the groundwater resources of the state are displaying clear signs of unsustainability.

Over the past 150 years, agricultural and domestic extraction has caused water table depths to fall by 100 or so feet in some instances, and the deep aquifer water level to decline by even greater depths in parts of the San Joaquin Valley, one of the most productive agricultural areas in the world. In some places the land surface itself has subsided by more than 20 feet. The current drought has led to increased demands on groundwater in regions such as the San Joaquin Valley, where more than 2,400 well permits were issued in 2013 as the drought hit home.

Analysis of such trends and new groundwater storage data collected by NASA’s GRACE satellite has led NASA hydrologist Jay Famiglietti to suggest that the collapse of the San Joaquin groundwater reserves may be only decades away. In 2014, more than 1,400 domestic water supply problems largely related to groundwater were reported in California, with more than half in the San Joaquin Valley.

未来这个世纪,圣华金河谷is projected to experience high degrees of warming, and this will greatly increase agricultural water demands in the region. The strategy of drought relief through increased exploitation of groundwater here and elsewhere in the state has reached its limits.

Colorado River: the third big concern

第三个问题是科罗拉多河。科罗拉多州是南加州最大的单一水源，但主要是由怀俄明州，犹他州和科罗拉多州遥远来源的降水供应。科罗拉多州的水已经减轻了当地干旱的影响。每年，科罗拉多河水的1650万英亩英尺都与科罗拉多盆地和墨西哥的状态分配。加利福尼亚州被分配为河水中最大的份额，每年约440万英亩。在2004年之前，加利福尼亚州能够索取其他“盈余”水域，每年可能会有100万英亩或更多的额外水。

Now, however, several challenges confront this potential source of drought relief. The original apportionment of 15 million acre-feet was devised in the 1920s based on an estimated annual discharge of 17 million acre-feet. Over the 20th century, however, the long-term average discharge of the river has only been 15 million acre-feet. Thus, there is a systemic over-allocation of the water, and in 2003 California agreed to wean itself down to no more than a 4.4 million acre-feet allocation.

Like most of the Southwest, the Colorado River basin has also experienced generally hot and arid conditions over the early 21st century. The flow of the Colorado River has declined and the water stored in its massive reservoir system has dropped precipitously. Lake Mead, the largest reservoir in the U.S., now stands at 37% of its maximum capacity.

填海局最近预测，到2017年1月，米德湖的地面海拔将降至海平面以上1,075英尺以下。这将援引联邦水短缺声明，并将对内华达州的水拨款减少4.3％，亚利桑那州的水分拨款增加了11.4％。尽管加利福尼亚州具有高级权利不会削减，但可以想象，就其高级权利而言，加利福尼亚的政治和公共压力将会受到政治和公共压力。在米德湖（Lake Mead）和科罗拉多河管理层的历史上，从来没有发表过短缺的声明。

Recent research by the Bureau of Reclamation estimates that future climate warming alone will lead to a 10% increase in evaporation in Lake Mead as the 21st century progresses. Just as in the case of California’s groundwater, the Colorado River has been oversubscribed, and the drought lifeline afforded by the river is further shrinking as the climate warms.

The road ahead

Ten years ago, my colleagues and I framed this situation as a perfect drought that affects local Southern California precipitation, extra-regional supplies from Northern California, and the external supplies from the Colorado River for periods greater than one or two years. What we are experiencing today is indeed a perfect drought, but it is also something beyond that.

We looked at Southern California’s perfect droughts as discrete events. Although hydrologically droughts are indeed discrete events, and the current one will come to a close sooner or later, this drought should focus our attention on the fact that things have changed in terms of the context in which these droughts occur. The rising temperatures will, year by year, increase the demands for water, particularly in our agricultural sector, which accounts for about 80 percent of the applied water in the state. Due to the ever-increasing rates of evaporation, each future drought will have a deeper bite than the previous one.

节水工作

So, what is to be done? At the household level, we can continue to change our landscaping mix from lawns and other water-intensive plants to increased use of water-sipping native plants. At the municipal level, we can expand the use of recycled water and desalination, which will likely lead to higher water costs. Stormwater capture will also help on domestic and citywide scales.

但是，当然，大奖是农业。在许多情况下，已经安装了节水灌溉技术。现在，将需要对最佳农作物在水压力环境中生长的最佳决定做出艰难的决定。这些选择提出了严重的经济，农村环境正义和粮食安全问题。可以通过额外的基础设施来实现一些收益，以进行水捕获，存储和分配。但是，在某些情况下，这些基础设施策略不受重要的环境问题或竞争利益的影响，例如圣华金河谷和南加州的水需求之间的萨克拉曼多三角洲冲突，与当地农民的水权以及濒危濒危的水权鱼类。

No matter what we decide to do, we will not, as we have done in the past, be able to depend upon either groundwater or external water supplies to see us through these hot droughts of the future. Should Lake Mead fall below the turbine intakes and lowest outlets of Hoover Dam, at 895 feet in elevation — as some have predicted — the fact that California has senior water rights will be meaningless. With groundwater, we face an agricultural cataclysm if the aquifers in the San Joaquin and other parts of the state should indeed fail.

We need to look at the situation today as representing Tomorrow’s Drought — a view into the hydrological future of California and the West. There is no question we will see similar climatological droughts over the next century. The question is: Will we have the foresight to learn all we can from the current drought and the will to put in place the changes in infrastructure, policy, and public attitudes that will be needed to cope next time around? Whether it is dry again next winter or rains like mad, the hydrological clock is ticking toward an increasingly difficult 21st century. The time to tackle our longterm water challenges is right now.

格伦·麦克唐纳（Glen MacDonald）是约翰·缪尔（John Muir）地理纪念主席，也是加利福尼亚大学洛杉矶分校的杰出教授。本专栏最初发布在Yale Environment 360and is reprinted here with permission.