The residents and ecosystems of central and southern California are no strangers to the threat of drought. Having made itself a regular decadal phenomenon over the better part of the last half-century, drought puts strain not only on urban infrastructure but also the health of residents and the regional natural environment. Often lasting on the order of 3-5 years, the intensity of drought effects greatly depend on previous seasonal rainfalls. As the summer closes in 2015, one the most severe droughts of the last several decades continues to debilitate regions of California.
Extreme Rainfall Shortage
In Los Angeles county alone, the effects have been particularly severe. As we can see in the graph above, rainfall (recorded at Los Angeles International Airport) fluctuates with both annual and semi-annual regularity. Each year, the majority of precipitation occurs in late autumn and winter months (October-February), identified on the graph as sharp upward peaks. The average annual amount of winter precipitation experienced in LA is roughly 21cm over a thirty year period (denoted as a horizontal green line on graph). As we can see, winter rainfall has not passed this threshold for the past four years since 2010-2011. This deficit is more wholly reflected in the 3-year monthly average line seen near the bottom of the graph. This line represents monthly precipitation values averaged over a 3-year period. It is this value which causes the most concern, as a 3-year lag in rainfall is indicative of a major drought event. Currently, the 3-year monthly average for precipitation in Los Angeles is at its lowest level since 1990.
In order to visualize the varying effects of drought over Los Angeles County, we rely on a technique known as “Normalized Difference Vegetation Index” (NDVI). Using imagery acquired from Landsat satellites, this technique allows for the visualization of vegetation density and subsequent health throughout the landscape. The result can be seen above.
Areas in bright green represent dense, healthy vegetation cover. Darker areas, conversely, represent areas with unhealthy or absent vegetation. The left image illustrates vegetation health in an average, non-drought year in Los Angeles County. The strongest concentrations of dense, healthy vegetation are found north of the Los Angeles urbanized area in the San Gabriel Mountains (right side), the Angeles National Forest focused around Castaic Lake (upper left corner) and the Santa Monica Mountains (left-middle). The image on the right represents vegetation health during the summer of 2015. The change is dramatic, showing significant reduction of the extent of healthy vegetation. Nowhere else is this degradation more stark than in the previously mentioned natural areas. In total, vegetation health in the summer of 2015 is roughly 48% below expected levels for any given year.
The drought-induced stress put on vegetation in Los Angeles County and the surrounding region can have devastating effects on both residents and ecosystems alike. Being one of the most densely populated regions in North America, Los Angeles requires a huge amount of resources to maintain the well-being of its citizens. Though rich in many of these essential resources, the region is notoriously parched for water. During episodes of extreme drought, special measures of water extraction and transportation must be taken to ensure clean water is delivered to residents, often at the expense of the regional natural environment.
Additionally, as the regional climate continues to change, temperatures are rising along with the frequency and severity of extreme drought events. Coupled with the steady population growth of southern California, threats like these will put further strain on both environmental and economic resources. Even if the current drought subsides in the near-future, leaders at all levels of government need to recognize the evolving nature of the threat and respond accordingly. Solutions will likely involve some combination of water conservation and expanded ocean desalination operations.
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What other solutions do you think we should be pursuing? Can this threat be reduced without affecting the well-being of residents? Leave your comments below.