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The IndicatorThis indicator has two aspects: (1) the percentage of streams with at least one day of no flow (also referred to as zero flow) in a year and (2) for sites with at least one day of zero flow, the duration of zero flow events, compared to a long-term average (50 years in this case). Together, these two variables help describe both the frequency and duration of zero-flow events. Changes in either of these could have significant effects on aquatic and riparian species. Relatively intact/undisturbed watersheds (including their upland, riparian, and wetland components) are capable of maintaining the maximum duration of streamflow their climates will support. When soil conditions and the kinds and proportions of vegetation promote the infiltration of moisture falling in the watershed, and when evapotranspiration and groundwater recharge are in balance, rapid loss of moisture to overland flow is minimized and long-duration, frequently perennial (i.e., yearround) flow is maintained. Intact riparian areas and wetlands are capable of retaining water during high-flow periods and metering out stored moisture during periods of low flow—further supporting longer duration, or perennial flow. Conversely, the increase of impervious surfaces through soil compaction or development and/or the loss of protective vegetation result in increased overland flow and rapid runoff events—depleting moisture storage to maintain long-duration flows. Improved management of grazing that promotes stream-side vegetation can lead to increased stream flow. In contrast, moisture loss from excessive evapotranspiration caused by plant community imbalances can also reduce the amount and duration of stream flow—this is attributed to encroachment of pinyon-juniper woodlands, Western juniper, and other species that are not actively managed. Changes in annual weather patterns or long-term climatic changes also influence streamflow quantity and flow duration. The DataData Source: Data reported here are from the U.S. Geological Survey (USGS) stream gauge network. USGS has placed stream gauges and maintained flow rate records throughout the United States since the end of the 19th century. Data Collection Methodology: Stream gauging data are collected using standard USGS protocols. Data Manipulation: The goal of the initial data manipulation was to identify stream gauges in watersheds where more than 50 percent of the land cover is grassland or shrubland. Each site was referenced to a watershed cataloguing unit (known as a 4-digit Hydrologic Unit Code, or HUC4) using latitude and longitude. Grassland and shrubland were defined using the National Land Cover Dataset (see http://www.epa.gov/mrlc/nlcd.html) using land cover categories 51 (shrubland), 71 (grassland/herbaceous), and 31 (bare rock, sand, clay) (see http://landcover.usgs.gov/classes.asp and the technical note for the national extent indicator. The HUC4s were also paired with their corresponding ecoregions (see below for description of the ecoregions used). Only sites with greater than 50% grass/shrub cover were used in the analysis. The number of sites with at least one no-flow day in a year was determined for each water year from 1950 to 1999. The corresponding percentage value for that year was also calculated as 100 x (number of sites/total sites). The percentage values were then averaged over each decade (i.e., 1950s, 1960s, 1970s, 1980s, and 1990s). This procedure was followed for all sites with greater than 50% grassland/shrubland cover as well as for each ecoregion. For the analysis of duration of zero-flow, only sites with at least one no-flow day in each decade between October 1, 1949, and September 31, 1999, were considered. The analysis determined whether there was a substantial increase, substantial decrease, or minimal change in the number of no-flow days, compared to the long-term (50-year) average for each stream. These categories are defined by the percent change in average zero-flow days, as compared to the long-term average, on a stream-by-stream basis. Thus, a “substantial increase” is defined as an increase of more than 100 percent in the duration of zero flow, or a change from perennial (no zero-flow) to intermittent. Likewise, a “substantial decrease” is defined as a decrease of at last 50 percent in the duration of zero flow, or a change from intermittent to perennial. “Minimal change” is defined as anything between a 100% increase and a 50% decrease. Data analysis was conducted by David Raff, Department of Civil Engineering, Colorado State University, and N. LeRoy Poff, Department of Biology, Colorado State University. Ecoregions: This indicator is reported using an ecoregional approach developed by the USDA Forest Service (Bailey 1995). The Bailey system has several levels into which the United States may be divided, based on dominant biological and physical attributes. The scheme has three domains, 13 divisions, and 52 provinces. We have chosen to report this indicator on the basis of divisions. We selected three major suites of Bailey’s divisions:
Data Access: The data records used in this study are available on the Internet in the form of daily streamflow values reported as the average volume of water per second over a 24-hour period (http://waterdata.usgs.gov/nwis/discharge/). ReferencesBailey, R.G. 1995. Description of the ecoregions of the United States. 2nd ed. rev. and expanded (1st ed. 1980). Misc. Publ. No. 1391 (rev). Washington, DC: USDA Forest Service. Chaney, E., W. Elmore, and W.S. Platts. 1990. Managing change: Livestock grazing on western riparian areas. Washington, DC: U.S. Environmental Protection Agency, General Accounting Office. General Accounting Office. 1988. Public rangelands: Some riparian areas restored but widespread improvement will be slow. RCED-88-105. Washington, DC: GAO. |
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