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Background

Annual growth rings in trees have been used to reconstruct Southwest climate back to AD 1000.Drought is a common feature of the Southwestern United States and presents challenges for people living in the region. Ensuring an adequate water supply through prolonged dry periods requires advanced planning for the distribution and storage of water for later use and effective management of this valuable resource.

Winter precipitation is crucial to water managers, farmers, ranchers, and urban areas throughout the Southwest. Precipitation during the cool winter months usually falls from slow moving storms that enable water to percolate into the soil, to recharge aquifers, or collect in reservoirs. Moreover, snow falling during the winter months stores water until later in the season when temperatures rise and is an important part of the water budget. In contrast, summer precipitation often results from strong convective thunderstorms and is often intense—running off the land's surface before being captured for use by plants, and is subject to the high evaporation rates that accompany the Southwest's intense summer heat.

The relationship between tree growth and cool-season precipitation. The blue line shows the tree-ring growth index for El Malpais National Monument, New Mexico and the red line depicts precipitation recorded by rain gauges in New Mexico Climate Division 4. Notice that while the tree-rings do a good job of matching dry winters, they do not quite match the wet years. Above a certain threshold, precipitation is no longer limiting to tree growth. Also note the very dry conditions during the 1950s and the post-1976 wet period. Source: Tree-ring chronology developed by Dr. Henri Grissino-Mayer. Chronology data available from the International Tree-Ring Data Bank.In the Southwest region, the longest instrumental records date back about 100 years at some locations. While these records are crucial to understanding variations in precipitation over the last 100 years on annual and decadal timescales, the full range of variability in Southwestern climate may not have been experienced within this time frame.  The extension of the record to earlier times can provide additional information on the length and severity of past droughts and can be used to provide water managers with information about the range of pre-industrial climate variability.

Tree rings provide a useful tool to help extend winter precipitation records further back in time than the instrumental (rain gauge) record. The growth of many Southwestern tree species can be linked directly to the total amount of precipitation that falls during the extended winter, or cool season (November-April). Tree rings act as a surrogates for direct observations, providing information on pre-historic precipitation variability before rain gauges were in service. Statistical models can be developed to relate tree growth to precipitation during periods when instrumental records are available. These relationships are then applied to periods prior to the instrumental record to reconstruct past precipitation back a thousand years.