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New Jersey Water Science Center
Great Falls of the Passaic River at Paterson, N.J.
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Ecological Relevant Hydrologic Indices for a Baseline Period of Record for Selected Stream Gages
The Hydroecological Integrity Assessment Process for New Jersey (NJHIP)
Source: Rachel A. Esralew, 2008
Background Information for the HIP
A number of ecologically important streamflow characteristics constitute the natural flow regime, which include magnitude, frequency, duration, timing, and rate of change of low, average, and peak streamflow (Poff et al., 1997). Understanding the effect of natural and altered flow regimes on aquatic ecosystems can serve as an important tool in protecting ecological integrity in the river basin. In order to assess natural and changing variability of multiple elements of the flow regime and its effect on biological resources, researchers have developed, investigated, and applied a number of hydrologic indices in order to characterize different components of the flow regimes.
These ecologically relevant hydrologic indices (ERHI's) have been developed to characterize elements of the flow regime in terms of biologically relevant flow variables, qualify short term and long term variability in patterns of the flow regime, and quantify characteristics of streamflow that may be sensitive to anthropogenic alterations in the drainage basin. Examples of ERHI's that have been investigated as to their role in ecosystem functionality include indices of average flow conditions, variations in mean daily flow, predictability of high and low flow events, skewness in flow and peak discharges, flood frequency and frequency curve slopes, seasonal distributions of monthly flows, duration of high and low flows, and rates of change of patterns in annual discharges (Olden and Poff, 2003).
Olden and Poff, using principle component analysis, examined 171 ERHI's from 13 published papers using 420 sites from across the continental US, to highlight patterns of redundancy in these indices in order to provide a number of statistically and ecologically based recommendations for selection of a reduced set of ERHI's that can best characterize the most relevant and non-redundant hydrologic indices for different classifications of streams based on typical natural flow patterns specific to those streams. Six stream types based on these flow patterns were identified by Poff using the method outlined in "A hydrogeography of unregulated streams in the United States and an examination of scale-dependence in some hydrological descriptors" (Poff, 1996). Following this, a series of principal components analyses were conducted to identify the most significant ERHIs that are associated with 10 sub-components of the flow regime (magnitude – low, average, high; frequency – low, high; duration – low, high; timing – low, high; rate of change – average) for each of the six stream types.
A matrix was produced by identifying, for each stream type, the indices that are most significant for each of the 10 sub-components of the flow regime (magnitude – low, average, high; frequency – low, high; duration – low, high; timing – low, high; rate of change – average). Significant indices were derived using principle components analysis (PCA) (Kennen and others 2007) . Loadings of the hydroecological indices on each significant principal component were used to identify indices that explain dominant patterns of hydrologic variation provided by the indices (Olden and Poff, 2003). Surrogate indices for each primary index for each type of flow were also identified that are exhibit patterns of variation and non-redundancy that are similar to the primary index (Olden and Poff, 2003).
Using these methods, USGS researchers have developed the Hydroloecological Integrity Assessment Process (HIP) which can be applied at a state or other large geographical area scale but is applied at the stream reach level). USGS has developed the National Hydrologic Assessment Tool (NATHAT), which is available as a windows-based software tool, in order to assist environmental managers in the identification of 10 non-redudant ERHI's based on stream type classification using the HIP process. This analysis software can be used to determine streamflow characteristics and variability based on a daily hydrograph. NATHAT requires users to make a determination of a general stream classification for a river or stream reach of interest based on a national stream classification using six stream types. The results of this method at the national level may not yield a specific enough resolution to adequately classify streams, and may result in a less relevant selection of critical ERHI's for a specific stream reach.
NJHIP and Classification of New Jersey Streams
To make NATHAT more applicable to specific regions or basins, USGS has been working with several states, including New Jersey, to re-classify streams using the HIP which more adequately reflects streamflow conditions at the local stream reach. The results of this application constitute the NJHIP. Under the original study, seven streams in New Jersey were examined and fell into only two of the perennial stream types (Olden and Poff 2003). USGS researchers (Kennen and others 2007; Henriksen and others, 2006) expanded the HIP process to conduct a more relevant analysis of New Jersey streams . Reclassification of local streams, using 95 gaged stations in New Jersey with 10 or more years of continuous record, has resulted in four stream types that can be characterized by the relative degree of skewness of daily flows (low skewness is considered more stable flow, while higher skewness reflects a more "flashy" streamflow response) and frequency of low-flow events.
It was observed that streams belonging to stream class A tend to be semi-flashy with moderately low baseflow, class B streams tend to be stable with high base flow, class C streams tend to be moderately stable with a moderately high base flow, and class D streams tend to be flashy with a low base flow (Kennen and others 2007). In some cases a stream's classification changes from upstream to downstream. The map (figure 1) shows the spatial distribution of each stream type at selected streamflow-gaging stations throughout New Jersey. PCA was run for these four stream types to identify the most significant ERHIs that are associated with 10 sub-components of the flow regime. Surrogate indices were also identified, i.e., other indices within each sub-component that are collinear with the indices of interest (Henriksen and others, 2006).
Kennen, J.G.; Henriksen, J.A.; Nieswand, S.P., Development of the Hydroecological Integrity Assessment Process for Determining Environmental Flows for New Jersey Streams: U.S. Geological Survey Scientific Investigations Report 2007-5206, 56 p.
Henriksen, J.A, J. Heasley, J.G. Kennen, and S. Nieswand. 2006. Users' manual for the Hydroecological Integrity Assessment Process software (including the New Jersey Assessment Tools): U.S. Geological Survey, Open-File Report 2006-1093, 71 p.
Olden, J.D., and N.L. Poff. 2003. Redundancy and the choice of hydrologic indices for characterizing streamflow regimes. River Research and Applications 19:101-121.
Poff, N.L., Allan, J.D., Bain, M.B., Karr, J.R., Prestegaard, K.L., Richter, B.D., Sparks, R.E., and Stromberg, J.C., 1997, The natural flow regime: A new paradigm for riverine conservation and restoration: BioScience, v. 47, p. 769-784.
Poff, N.L. 1996. A hydrogeography of unregulated streams in the United States and an examination of scale-dependence in some hydrological descriptors. Freshwater Biology 36:71-91.
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