The potential effect of climate change on continental water resources is of critical societal importance. Of direct relevance to the management of water resources are questions regarding how different climatic regimes affect the spatial and temporal distribution of hydrologic extremes and the distribution of seasonal water balances. By studying such hydrologic characteristics under known past meteorologic conditions, we can better anticipate the effects of postulated climate changes. Conversely, records of the past occurrence of hydrologic phenomena can indicate when a change in the pattern of the prevailing meteorologic fluxes and hence, a change in climate, may have occurred.
It has long been known that surface-water conditions are generally correlated with fluctuations in meteorologic variables such as precipitation and temperature. (For example, see discussion by Newell, 1891.) However, the dynamics of streamflow are not just a simple first-order response to existing atmospheric conditions. "Streamflow", the surface-water discharge measured in a natural channel, differs from "runoff", the fraction of precipitation that appears in surface streams. (Definitions taken from Langbein and Iseri: 1960.) There is a complex, interactive relationship between hydrologic conditions and meteorologic fluxes. Inputs from diverse specific local precipitation events are collected over the surface of the watershed, so that the meteorologic fluxes are spatially integrated by the watershed. Meteorologic events and conditions are also temporally integrated because the watershed retains moisture both on and below its surface. Storage on the land surface occurs primarily in lakes, ponds and wetlands and in the seasonal or even multiyear accumulations of snow, as well as in the biomass covering the surface. Storage in the subsurface occurs in both the unsaturated and the saturated zones. Furthermore, water can enter or leave the watershed through the subsurface. Losses of water to the atmosphere (through evaporative and transpirative processes) occur throughout the watershed. This moisture may return again to the watershed as precipitation or it may leave the watershed entirely by advection. Thus, the watershed acts to dampen the noisy signal of specific instantaneous and local meteorologic events. Records of streamflow can provide a filtered account of meteorologic fluctuations over the watershed i.e., of prevailing climatic conditions.
The ability of streamflow records to reflect variations in the prevailing climate is conditioned on the absence of any other major causes that would radically alter streamflow patterns during the period of record. Such confounding processes would generally be anthropogenic in origin, that is, induced by human activity, either intentionally or unintentionally. Some human actions, such as the removal of water from a stream for consumptive use or the regulation of extremes by a control structure, have a direct effect on streamflow patterns. For other activities, the consequences may be indirect but can be equally significant, such as the effects of a change in the watershed storage capacity due to major land-use changes during the period of record. In either case, the pattern of past climate variation to be discerned in the streamflow record would be confounded by changes induced by anthropogenic activity. An exception might be made, however, if the non-climatic forcing factor was consistent in its nature and degree over the period of record. In this case, the effects are present as a constant background level within the record. In this case, the record of streamflow measured at a surface-water gaging station in the watershed could still be useful for the study of past climate variation.
A significant potential effect of a change in prevailing climatic conditions would be a shift in the seasonal patterns of precipitation and temperature that could induce a seasonal shift in the timing of hydrologic events as well. Such a change in the hydrologic patterns could only be discerned in a record whose values represent the average streamflow conditions over a sufficiently small time span, one that is shorter than the interval of the induced effect. In the case of a seasonal shift, the time step within the hydrologic record should be at least a month. Thus, the criteria for the suitability of a streamflow record for the study of potential effects of climate variation is that the discharge values be representative of at most monthly mean conditions and that the effects of anthropogenic controls, either intentional or unintentional, should be negligible for the average discharge over the time step of the record.
In order to characterize the variation in streamflow over the land area of the United States during the last past century, a period of great temporal and spatial climatic variation on the North American continent (for example, see discussion by Diaz, 1986), and to contrast this variation with that of concurrent meteorological conditions, it is necessary to assemble a large data set of suitable and long streamflow records. The primary source of such information for the United States is the U.S. Geological Survey (USGS). Currently, streamflow data are collected by nationally standardized procedures through the activities of the USGS network of District offices. (Except for three multi-State Districts, each District office corresponds administratively to a single State.) A discussion of these national procedures for the measurement and computation of streamflow can be found in the report of Rantz and others (1982).
The USGS National Water Storage and Retrieval System (WATSTORE) data base contains streamflow records for the United States and its trust territories going back into the 19th century. (See Hutchinson and others, 1975, for a description of the structure of WATSTORE.) The data base even contains data that predate the establishment of the initial USGS gaging station on the Colorado River at Embudo, New Mexico, in 1888 (Frazier and Heckler, 1972) that were deemed to be of appropriate quality and to satisfy appropriate standard practices. Streamflow data are available in published form, such as in the annual State Water Data Reports, and in electronic form from WATSTORE through the USGS National Water Data Exchange (NAWDEX) Office. (For contact information, see Blackwell, 1991, or Dodd and others, 1989.) However, given the development that has occurred throughout the United States over the last century, many of the records in WATSTORE are affected, entirely or after some date, by confounding anthropogenic activities, such as discussed above. Thus, many records are not appropriate for the study of climate variation Consequently, an effort was undertaken to identify and assemble the long records of daily mean discharge held by the USGS that are relatively free from confounding anthropogenic effects, as judged according to a single set of consistent criteria.
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