Geoscience Reference
In-Depth Information
8.1 Introduction
With the narrowing gap between water supply and demand in many of the world's
river basins has come an increased interest in the susceptibility of agriculture, indus-
try, hydropower, and municipal development to climatic fluctuations. Paleoclimatic
data have played a major role in convincing hydrologists and water resources plan-
ners that the snapshot of climatic variation provided by the short instrumental record
may not be sufficient to capture all modes of variability important to the planning
horizon. An obvious contribution of dendroclimatology is augmentation of hydro-
logic records—precipitation and streamflow—on which water resources planning
relies. Tree-ring data are ideally suited for this purpose. Tree growth and natural
runoff respond similarly to changes in net precipitation, or the residual of precip-
itation and evapotranspiration. Moisture-sensitive trees are widely distributed over
large portions of watersheds, especially in the temperate latitudes, and frequently are
most plentiful in mountainous areas, which contribute most of the runoff in semiarid
watersheds.
Dendrohydrology has been defined as 'a subfield of dendroecology which utilizes
dated tree rings to study and date hydrologic phenomena, such as river flow, lake
some applications of tree rings to hydrology address ecology only peripherally,
it is perhaps also reasonable to regard dendrohydrology as a subfield of den-
drochronology on equal footing with dendroecology. We restrict our treatment in
this chapter mainly to streamflow reconstruction, a particular subfield of dendrohy-
drology relevant to water resources planning on the river-basin scale. Streamflow
reconstruction—the statistical augmentation of streamflow time series using tree-
ring data—has had a particularly rich history of application in the semiarid western
United States. In keeping with the 'sharpening the focus' theme of this topic,
we confine the presentation to selected aspects of streamflow reconstruction we
consider novel in the time frame of the last 30 years.
It is perhaps useful at the outset to distinguish streamflow reconstruction from
other types of dendroclimatic reconstruction, which are thoroughly covered in other
chapters. The distinction is indeed blurred, as climatic variability is a central issue in
streamflow reconstruction. Droughts are commonly the focus of streamflow recon-
structions and of dendroclimatic reconstructions of precipitation, drought indices,
and atmospheric circulation patterns. Streamflow reconstruction is most often con-
cerned with hydrologic drought, as manifested by unusually low streamflow over
some time interval. A hydrologic drought would be mirrored by a meteorological
drought, perhaps manifested in anomalous precipitation or patterns of atmospheric
circulation responsible for moisture delivery. The relative severity of a particular
drought by hydrologic and meteorological measures would likely differ, however,
depending on many factors, including the exact drought metrics used, the watershed
initial conditions, and the spatial scale of the basin.
A streamflow reconstruction study also usually includes the tailoring of analy-
sis to some specific interest of water users, and considerable feedback from water
resources planners or the agency or entity requiring the long-term information.
