Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 116

Once the IHA program has assessed the statistics, these metrics can then be used to determine
whether there has been some hydrologic alteration to the system or to determine the natural range
of variability approach (RVA) for management. A management strategy for the RVA would ensure
that the annual values of each IHA parameter fall within or outside the selected range of natural
variations for that parameter at the same frequency of occurrence as during the natural low period
(Richter et al. 1996, 1997, 1998). The three methods available in the IHA program for the analysis
of the low parameters are briely described as follows (taken from Swanson 2002):

• IHA Analysis: This analysis provides a preimpact versus a postimpact comparison. A

“scorecard” table and graphs of each IHA parameter are produced to quantify the changes
in each parameter between the preimpact and postimpact low regimes.

• Range of Variability Analysis: In this analysis, the user deines three categories that divide

the range of preimpact data values. Delineation into three equal categories, for example,
would result in parameters less than or equal to the 33rd percentile, parameters between
the 33rd and 67th percentile, and parameters greater than or equal to the 67th percentile.
The program then calculates the expected frequency at which the postimpact values should
occur in each category. A comparison of these two data sets provides a preimpact and a
postimpact measure of the hydrologic alteration between low regimes.

• Trend Analysis: This analysis produces a complete graphical historical analysis of the IHA

parameters, together with a linear regression analysis of the data.

4.6.2.1.5 HAT
The HAT is a primary component of the USGS National Hydrologic Assessment Tool (NATHAT).
Based on daily and peak (optional) streamlow data for a period of record, it is designed to

• Establish a hydrologic baseline (reference time period)

• Establish environmental low standards

• Evaluate past and proposed hydrologic modiications

The suite of software is composed of four components:

1. Hydrologic Index Tool (HIT; Henriksen et al. 2006) is a generic tool (i.e., not developed
for any particular geographic region) to calculate the 171 statistical hydrologic indices
presented in Olden and Poff (2003) based solely on the input of USGS streamlow data for
any gaged site.

2. NATHAT, version 3.0, is a nationwide customization of the HIT based on the six stream
classiications of Olden and Poff (2003). The 6 stream classes were pared down from 10
original classes developed by Poff (1996) based on a study of 420 gages across the contigu-
ous United States. Although the NATHAT performs the full complement of 171 statistical
routines, the default graphical presentation of the results is limited to the 10 nonredundant,
critical indices identiied by Olden and Poff (2003) for each of their 6 national stream
classes.

3. New Jersey Hydrologic Assessment Tool (NJHAT) is a New Jersey-speciic regionaliza-
tion of the NATHAT incorporating the results of the New Jersey Stream Classiication
Tool (NJSCT) and the identiication of 10 primary low indices for each of the 4 stream
classes.

4. NJSCT, version 1.0, is a New Jersey-speciic tool to partition that state's gaged streams into
four stream classes, termed A, B, C, and D, by their relative degree of skewness of daily
lows (high versus low) and by the relative frequency of low-low events (high versus low).
Currently, there is no comparable national stream classiication tool.