Use of Dendrochronology and Climate Information to Reveal Decadal and Larger Scale Cycles of Drought, and Connections to Water Demand by Plants

Recent findings demonstrate there are significant decadal-scale cycles of precipitation and temperature in the Intermountain West region of the US. These oscillations also exhibit multiple frequencies of wet and dry events. The variations in climatic features here that are critical to surface water balance are not random, but display coherent patterns. Some of these have been connected to variations in ocean temperature and upper atmospheric features. However, others still await a proper documentation and explanation. The usual approach to averaging climatic variables and their effects on critical biophysical processes is not appropriate, and possibly misleading in these situations.

Most of the above findings have been determined using the instrument record of a little over a century, which limits knowledge of low frequency dry and wet events. In order to understand and then simulate the effect of any future climate changes on water resources, these patterns must be quantified over longer periods.

Recent dendrochronology data obtained in northern Utah offer the ability to capture information about precipitation and seasonal temperature back about 400 – 500 years. Preliminary analyses already show certain locations have good sensitivity to annual changes in precipitation and growing season temperature. These are two key properties that govern the water resources and water balance of ecosystems in the region. There is also a distinct correlation between both annual and seasonal precipitation, and summer saturation deficit in this region. This offers some ability to reconstruct both annual precipitation and summer saturation deficit from dendrochronology data.

We hypothesize that a combination of tree ring index and climatic data can be used to make quantitative estimates of quasi-decadal scale cycles of both precipitation and summer saturation deficit values. In this region of dry summers, the saturation deficit is a key factor that governs evapotranspiration (ET) and water balance of various ecosystems, including irrigated agriculture. So knowledge of any cycles of the combination of precipitation and saturation deficit, will allow the affects of climatic variations on ET and water resource issues.

The goals of the project are:

Quantify the coherence of time series of tree ring index values for a juniper site in northern Utah, with the measured precipitation at a nearby location for the 100+ year instrument record. Repeat the analyses for several seasonal totals of precipitation. Identify the significant cycles and their frequencies. Include distribution of duration and severity of droughts.

From the above relationships, extend the climatic estimates back to the reliable dendrochronology of the tree group, in this case about 500 years.

Use recent measured summer temperature and dew point data, as well as extracted values from other data sets, to estimate saturation deficit values for the warm season. Examine the coherence between these values and the tree ring indices.

Use above findings to extend both precipitation and saturation deficit data back for the entire 400 – 500 year record. Make estimates of the subsequent variations in reference values of ET, and imposed stress by the atmosphere on plants.

The results will document the coherent variations in wet and dry periods, as well as hot summer periods, for a longer time scale. These can then be used to estimate the changes in amount of water used if available (demand), in concert with the amount of water delivered by the climate, in response to the significant climatic events. Such information, even in a probabilistic sense, is currently missing in the enterprise of examining future changes in water resources and the effects of climate.