The California drought continues.

While we do not know yet what the rest of the wet season will bring – and while we hope for the major storms needed to recharge our rivers, groundwater and reservoirs – it seems increasingly likely that California will not see enough precipitation to get out of the very deep deficit that three years of drought (so far) have produced.

There is, however, some misleading and confusing information out there. Some are already arguing that California’s rainfall is nearly back to normal or that because there may have been more serious droughts in the past we needn’t worry anymore. Most of these claims are based on misunderstandings of California’s hydrology, water systems, or current conditions, and on very narrow definitions of “drought.”

First, to understand the data, it is vital to realize that California’s “water year” runs from October 1 to September 30. This is not the “calendar year” (January to December). This distinction is important, because mixing data from different water years produces inaccurate analyses.

Here is a great example. If we look at the 2014 “calendar” year, it appears that California received a decent amount of water (Figure 1) – still dry, but not abnormally so.

Figure 1. California's 2014 "calendar year" precipitation seems just slightly dry compared to the past 120 years. But this is a misleading graph. The State's precipitation is measured by "water year" (Oct-Sept). See Figure 2. (Source: NOAA) Figure 1. California's 2014 "calendar year" precipitation seems just slightly dry compared to the past 120 years. But this is a misleading graph. The State's precipitation is measured by "water year" (Oct-Sept). See Figure 2. (Source: NOAA)

But this is grossly distorted by the heavy rains received in December 2014 – which is actually part of the 2015 water year. If we look at the 2014 water year (October 2013 to September 2014) we can see that last year was critically dry (Figure 2): in fact, only two previous years out of the past 120 were drier (1923-24 and 1976-77).

Figure 2. Precipitation in California's 2014 "water year" (Oct-Sept) was extraordinarily dry -- one of the three driest years in the 120-year record. (Source: NOAA) Figure 2. Precipitation in California's 2014 "water year" (Oct-Sept) was extraordinarily dry -- one of the three driest years in the 120-year record. (Source: NOAA)

Even more appropriate is to look at the past three years of persistent, cumulative drought. And when the last three water years are evaluated (October 1, 2011 to September 30, 2014), we see that the current drought (measured only by precipitation levels) is by far the most severe in the entire instrumental record (Figure 3).

Figure 3: The past three water years (2012 to 2014) are the driest in the entire instrumental record for California. Figure 3: The past three water years (2012 to 2014) are the driest in the entire instrumental record for California.

Second, it is important to understand that “drought” means – from a practical perspective – far more than just “precipitation deficit.” California’s drought is the result of several factors: how much precipitation we receive in rain and snow; how much water is available after taking into account reservoir storage, soil moisture, and groundwater; additional losses of water due to higher than normal temperatures (the past three years have been by far the hottest in California's record); and the human demand for water. If all of these factors are included, the current drought in California can be considered the worst in recorded history.

And it isn’t over yet.

The current status of the drought – some key indicators.

As noted above, the rains received in December are counted as part of the 2015 “water year” – October 1, 2014 to September 30, 2015. Yet even these rains were not especially heavy. When we put all the data together (and a regular update of these data can be found at the Pacific Institute's California Drought Update page), here is what we see:

Soil Moisture: One key indicator of the severity of the current drought is a standard measure of soil moisture conditions, called the Palmer Drought Severity Index (PDSI). This index is used to prepare the drought maps published at the US Drought Monitor. As the most recent version shows, the entire state of California is still in severe drought, despite the December rains (Figure 4).

Figure 4: The California Drought Monitor as of January 6, 2015 shows that 100% of the state remains in drought -- much of it extreme. Figure 4: The California Drought Monitor as of January 6, 2015 shows that 100% of the state remains in drought -- much of it extreme.

Precipitation: And what did those rains actually do? Not much. As Figures 5 and 6 show, precipitation to date for Northern California is barely at average; and for Southern California it is already below average. Not a great start.

Figure 5: Precipitation to date for Northern California is barely at average, despite the December storms. Far more is needed to fill the current deficit in soil moisture, reservoirs, and snowpack. (Source: DWR) Figure 5: Precipitation to date for Northern California is barely at average, despite the December storms. Far more is needed to fill the current deficit in soil moisture, reservoirs, and snowpack. (Source: DWR)

Figure 6: Precipitation to date for Southern/Central California is already far below average for this date. (Source: DWR) Figure 6: Precipitation to date for Southern/Central California is already far below average for this date. (Source: DWR)

Reservoir Storage: Even worse, we are starting the water year with critically dry reservoirs. Figure 7 shows the current status of California’s major reservoirs, all of which are remain well below normal even with the storms last month.

Figure 7: California's reservoirs are still far below normal for this date. Without water in storage, deliveries to farmers and cities will almost certainly be cut back again in 2015 -- a classic indicator of drought. (Source: DWR) Figure 7: California's reservoirs are still far below normal for this date. Without water in storage, deliveries to farmers and cities will almost certainly be cut back again in 2015 -- a classic indicator of drought. (Source: DWR)

Snowpack: Finally, one of the most important measures is how much snow is stored in the mountains. This snow provides water that is used throughout the rest of the year. And as Figure 8 shows, three and a half months into the 2015 water year, California’s snowpack is far below normal. This is very bad.

Figure 8: California snowpack is well below normal for this date. This indicator is particularly important for water supply. (Source: DWR) Figure 8: California snowpack is well below normal for this date. This indicator is particularly important for water supply. (Source: DWR)

California will not dry up and blow away: drought means less water than normal, not zero water. But if the drought continues, increasingly difficult and costly decisions will have to be made, and the ecological, economic, and human impacts will grow. But this is no time to be a Pollyanna – we had better continue to prepare for the worst, since there is no indication that nature will bail us out in the near future.

Peter Gleick