USGS Washington Water Science Center
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WA345 - Phase I Evaluation of Hydrologic Variable Trends in Washington - Completed FY1991
Problem - Determination of the amounts of water present in streams and reservoirs, and in the groundwater system monthly and annually, especially during periods of drought, has traditionally been a primary goal of the USGS. The USGS has developed a new method that can be used to uniformly evaluate and compare the trends inherent in records of hydrologic data. These trends may very likely be related to short- and long-term changes in global climate such as El Nino and global warming. The proposed method of trend analysis very likely could provide a new and reliable means of determining the amounts of water available in different areas, most importantly during drought periods.
Objectives - The primary objective of Phase I of this investigation, which is the subject of this proposal, is to develop information for water suppliers and water-resource managers about regional trends in precipitation, streamflow, lake levels, and other hydrologic variables in Washington. The objective of Phase II, when it is implemented, would be to develop methods for estimating the average annual amounts of water supply potentially available in the regions.
Approach - The proposed method of trend analysis is based on time-series calculations of the cumulative sum of departures of sequential values of hydrologic variables from base-period mean values of the variables. The departures of the individual variable values are normalized by dividing them by the standard deviation of that variable for the base period. The application of normalization and the use of a common base period for all hydrologic variables allows direct numerical or graphical comparison of the trend patterns for variables at the same site or at different sites. The types of hydrologic records that will be evaluated are monthly mean values of streamflow, rainfall, lake levels, and air temperature. Graphs of the normalized cumulative sum of departures will be prepared for the entirety of all historical records of monthly values that are available in computer files when the investigation begins, and these trend graphs will be compared visually to ascertain regions of the state where similar trends have been experienced.