Comparing Precipitation in States

The parched earth I see in televised news reports from California seems a universe removed from the lingering snow drifts outside my window near Boston. Here in the Northeast it feels like we have had more than our average share of rain and snowfall over the past decade.  That perception illustrates a larger reality.  Massachusetts has seen about five inches more precipitation on average since 1970 compared to levels dating back to the turn of the twentieth century.


No such good news seems to be in the cards for California.  Its average precipitation level runs about half that of Massachusetts, and unlike the Bay State, statistical tests show no trends or significant differences between older and more recent data.  The current drought appears driven by a succession of abnormally low years starting around 2007 and some historic temperature highs over the past three years. California reached its historical low for rainfall in 2013, a miniscule 7.93 inches.  In 2014, the temperature for California averaged four degrees (Fahrenheit) above the state’s 1901-2000 baseline. Temperatures in California have been running above the historical baseline for many years now.

Inches of precipitation give us the vertical dimension of a state’s total volume of rainfall; the other needed dimensions are measured by its area.  We can estimate the total volume of precipitation that fell on a state each year by multiplying together its inches of precipitation and its total area. Americans use the acre-foot as the standard unit of measure for large volumes of water.  An acre-foot represents the amount of water required to cover an acre-sized flat surface to a depth of one foot.  That works out to 43,560 cubic feet or about 326,000 gallons.

To estimate the total volume of precipitation for an average year in California, I’ll start with NOAA’s “baseline,” California’s average for the years 1901-2000 of 22.9 inches.  I divide that by twelve to convert to feet, then multiply the result by the area of California in acres to get

22.9 inches / (12 inches/foot) (158,648 sq mi)  (640 acres/sq mi) = 2.27 billion acre-feet

of precipitation in an average year.  For much smaller but wetter Massachusetts the comparable figure works out to about a tenth that for California

44.6 inches / (12 inches/foot) (8,262 sq mi)  (640 acres/sq mi) = 236 million acre-feet

Massachusetts is one-twentieth the size of California but sees about twice as much precipitation.  These factors offset to make the total volume for Massachusetts about a tenth that for the Golden State.

This calculation gives us a rough estimate of the “supply” of water available from precipitation within a state. Normalizing this total supply by the size of each state’s population gives us a metric we can use to compare water resources across states, their acre-feet of annual precipitation per capita. If we now divide that volume by the total number of people living in a state, we can compare these values across states and time like this:


At the beginning of the twentieth century Florida had the most abundant precipitation volumes per capita and Massachusetts the smallest.  Massachusetts also had a larger population (2.8 million) than any of the other states in the chart except Texas (3.0 million).  The graph portrays how the rapid growth of the Sunbelt destinations like California, Florida, and Texas reduced the per-capita supply of precipitation.  The other three states show slower declines in the availability of water because of their lower rates of population growth.  Nebraska’s combination of a large surface area and small population gives it ample supplies of water to fulfill its citizens’ needs and those of agriculture.  Massachusetts shows that even a small state with consequently limited water resources can manage if population grows slowly enough.  The state that clearly stands out the most is California.

Per-capita precipitation in the Golden State has now declined to Massachusetts levels, but California’s entire economy and society is geared to high-demand activities from almond groves to swimming pools.  Forty percent of California’s water is devoted to agriculture.  In Massachusetts that figure is less than ten percent, and most of that goes to irrigate cranberry bogs.  On its current path, California faces some stunningly difficult policy choices in the years ahead to adapt its economy to precipitation levels like those seen in Massachusetts.