Lake Mead is the largest reservoir in the US, supplying water to Arizona, Southern California, southern Nevada and northern Mexico. As such, its levels can provide a useful indication of the water situation in the US Southwest. And the picture right now is anything but comforting.

At 1073ft above sea level, the July reading at the Hoover Dam was the lowest elevation since the reservoir was filled in the late 1930s, lower even than the first threshold (1075ft) that triggers emergency rationing measures across several states.

However, the crucial measurement is not the current level but the mid-August assessment by the Bureau of Reclamation for 1 January 2017. And if it is below that threshold an official water shortage at Lake Mead will be declared.

As indicated by the box in the graph above, the latest January estimate suggests that cutbacks may be avoided this time around – if only barely. But the bigger picture clearly shows a relentless decline in water levels at Lake Mead since the start of the millennium, with no indication of a reversal.

As less water falls from the sky more has to be pumped from the ground to meet demand. Back in July 2014 a study by NASA and University of California, Irvine, estimated that the Colorado River Basin lost nearly 53 million acre feet of freshwater – almost double the volume of Lake Mead – in just nine years. More than 75 percent of that loss came from underground resources. And because nobody really knows how much groundwater is left, it is impossible to accurately predict when that resource (which in many cases took thousands of years to form) will run out.

What appears much more certain is that things might get even more challenging given the current climatic factors at play. The Western US is just coming off one of the strongest El Niño seasons, which typically brings more wetness to the region. Some major reservoirs in California got a boost as a result (although much more is needed going forward). Those seasons tend to be followed by La Niñas that can generate the opposite effect. So the odds are stacked in favor of less moisture over the foreseeable future, not more (although anything can happen).

Indeed, according to official estimates there’s no letup in sight for the drought afflicting most of the US West, as shown in the graph above. Therefore, the region is going into this period already in a debilitated condition.

What will happen once that first threshold level at Lake Mead is reached?

Immediate water rationing based on recent interstate agreements, hitting Arizona first and with the hardest reduction in its allocation from Lake Mead: 13%. While the impact might be mitigated by accessing some storage facilities, Arizona farmers for one would for sure feel the pinch. Next in line would be Nevada with a 4% reduction. California would not get any cuts at that threshold, but residents that directly depend on this supply, especially in the Los Angeles area, could materially suffer from lower volumes delivered.

That part of the US has gone through massive droughts before and perhaps all of this can be managed over time with some socially acceptable adjustments.

However, as we indicated some months ago this persistent drought might be indicative of an unfolding structural change in weather patterns, as opposed to a much less serious downswing in a cycle. According to archaeological records the 20th century was the wettest in a millennium, and a gradual reversion to the historical mean could really be problematic for the region.

In other words, we could see a substantially drier Southwest interspersed with shorter wet periods taking shape in the decades ahead. And the impact would be felt well beyond the region. The states that depend on the Colorado River for a material proportion of their water supply collectively represent the world’s fifth largest GDP. Northern Mexico would be hit too since it also gets an allotment from the Colorado River.

Are politicians, farmers, investors, real estate owners, economists and millions of other stakeholders properly prepared for what might be coming? Unfortunately it seems we only properly value water once it’s gone.