Bark Beetles of the Southern California Forests


What Defines a Drought?

Specifically, a drought is a period to time when there is less than normal precipitation resulting in hydrologic imbalance, such as water shortages.  Geographic regions vary in both short-term and long-term precipitation patterns and normal precipitation varies among these regions.  What constitutes normal precipitation in one part of the world would constitute a severe drought in another.  Scientists have developed several standardized indices to measure drought.  The Palmer drought indices are water balance indices that include factors for water supply, water demand, and water runoff.  The standardized precipitation index only considers precipitation.  According to the National climactic data center (NCDC) the forests of southern California have experienced moderately dry conditions for the past 24 months.

Precipitation Patterns in Southern California Forests

Southern California climate is Mediterranean; hot, dry summers and mild winters when most of the precipitation occurs.  Precipitation in the southern California forests consists of both rain and snow.  The combined total of rain and snow accounts for the total precipitation received by the forest but the proportion of each in any given year varies.  Unfortunately, little data is available describing the total precipitation received as rain versus snow. 

Precipitation Charts for Three Locations in the San Bernardino National Forest

Big Bear Lake Dam
Mean annual precipitation, 30.78inches; standard deviation, 17.23

Lake Arrowhead
Mean annual precipitation, 31.98inches; standard deviation, 18.33

Mean annual precipitation, 16.78inches; standard deviation, 8.66

San Bernardino County Museum