The Frequently Asked Questions listed here are
specifically related to the SPARROW Decision Support System
(SPARROW DSS). FAQs related to SPARROW models and how
they work can be found
SPARROW models are created by hydrologists to model a
river system. Those models can then be submitted to
the SPARROW DSS to be viewed and explored online.
FAQ Questions and Answers
1: What is 'DSS'? How did you come up with that weird
Answer: 'DSS' is short for Decision Support System. The
SPARROW Decision Support System (DSS) provides access to
national, regional, and basin-wide SPARROW models for water
managers, researchers, and the general public. Decision
support is provided by generating predictions of long-term,
average water-quality conditions based on existing or
hypothetical source contributions. The prediction results
of existing conditions or source-reduction scenarios can be
mapped over variety of detailed map layers, tracked
downstream to receiving waters, exported and compared.
2: Why do the predicted values in the DSS not match the
prediction values in some of the published reports?
Answer: SPARROW models are expressed in the form of a
mass balance. The law of conservation of mass implies that
certain basic accounting rules must apply to a mass balance
water-quality model, such as: (1) the sum of fluxes
entering the confluence of two streams equals the flux
leaving the confluence; (2) the sum of the fluxes
attributable to each source must equal total flux; and (3)
a doubling of all sources in the model results in an exact
doubling of the predicted flux at each location. Mass
balance provides a basis for flux accounting, whereby flux
can be allocated to its various sources, both spatially and
topically (that is, according to the location and type of
source—for example, fertilizer, atmospheric
deposition, etc.). For example, mass balance makes it
possible to attribute nutrients discharged to the Gulf of
Mexico to specific sources within the Mississippi basin,
thereby providing guidance in managing the reduction of
this discharge. Predictions from a SPARROW model employing
mass balance are referred to as
unadjusted predictions. When mass balance is
employed, predictions of flux in monitored reaches (those
with calibration sites) are unlikely to exactly match the
measured flux in monitored reaches.
It is possible in SPARROW to condition predictions on
measured flux in monitored reaches, so that predictions of
flux in monitored reaches exactly match the measured flux
in monitored reaches. This capability implies that
predictions for all reaches at or below the monitored reach
will be adjusted to be consistent with the measured flux.
It is important to understand, however, that this
capability does not necessarily imply that predicted flux
immediately downstream of a monitored reach will be close
in value to the measured flux, because of the assumed error
structure of the model. Predictions that are conditioned on
measured flux in monitored reaches are referred to as
adjusted predictions. Adjusted predictions do not
preserve mass balance, and thus they do not provide the
ability to trace predicted in-stream flux in a given stream
reach to the individual sources in each of the upstream
The use of unadjusted versus adjusted predictions
depends upon the objective. More accurate predictions may
be obtained through the use of adjusted predictions, but
identification of important upstream sources can only be
obtained through the use of unadjusted predictions.
Additionally, it is not appropriate to use adjusted
predictions when the SPARROW model is run in prediction
mode for purposes of simulating water-quality conditions
under alternative management scenarios. The measured flux
is consistent with management actions actually taken. As
such, the conditioning of predictions on measured flux
would not incorporate the changes in water quality
associated with the alternative management scenario.
Because one of the main functionalities of the DSS is
simulating alternative management scenarios, all
predictions available in the DSS are unadjusted.
Some published reports may include adjusted predictions.
These will not match the unadjusted predictions in the
3: Why does it seem like the New England / Mid-Atlantic
Regional models run slower than the other SPARROW
Answer: The New England / Mid-Atlantic Regional models
are based on the National Hydrography Dataset Plus
(NHDPlus) stream network, which includes a stream network
based on the medium resolution NHD (1:100,000 scale). The
other SPARROW models are based on the Enhanced River Reach
File (ERF1_2), which has a 1:500,000 scale. Because the New
England / Mid-Atlantic models are based on a stream network
with higher resolution, there are many more reaches to
model and to display on the map, making them run slower in
4: Are loads coming from streams in Canada, which drain
into stream in the U.S., included in the SPARROW model load
Answer: There are various assumptions involved in
load predictions that may be affected by streams in Canada
and it varies from model to model. See documentation on the
individual SPARROW models for more information.
5: I see reach lines drawn in unexpected places: Across
lakes, along the sides of rivers, along the the ocean
shore, straight lines in the desert, and some reaches don't
even seem to connect to anything. Are these errors?
Answer: The stream networks include lines and catchment
areas to account for non-stream features in the SPARROW
model. Lines across lakes, or inter-lake reaches, transport
flow from the inlet of a lake to the outlet of a lake.
Lines alongside lakes and oceans are shore reaches, which
represent flow entering directly into a lake or ocean from
the immediate catchment area alongside the waterbody.
Straight lines represent network pipes, pseudo-reaches, and
other oddities that redirect flow from the natural stream
6: Why are the DSS predicted values for Total Yield
different then the published reports?
Answer: Total yield is derived from Total Load divided
by Watershed Area. The watershed area can have some
variability in how it is calculated, see the _Watershed
areas_ Q & A.
7: Why are the DSS reported Watershed areas (the total
drainage area upstream of a reach) different than published
Answer: Several potential issues may be involved with
the calculation of the watershed area. First, it is
important to understand how the watershed area in SPARROW
actually is calculated - See the Watershed Area section of
[this help page |
_(this needs to be updated to point to prod docs when
available)_ for a complete explanation. The key point is
that the watershed area used by SPARROW DSS is *not* the
same as the geographical upstream area of a reach and
comparisons to other published drainage areas would not be
expected to match.
Second, in some cases. the modelers have split the flow
in a reach using the fraction value in order to
simulate flow leaving the reach (perhaps a water utility
pulling water from the river). This modification affects
the DSS watershed area calculation as described above.
However, the modelers may have reported the original
watershed areas prior to the split in a published
Finally, in some cases, the modelers have made specific
changes to certain watershed areas by intentionally
ignoring the river topology in the reach network to provide
a more accurate watershed area. The DSS currently cannot
incorporate these changes. If you believe this issue is
affecting your data, please contact the system
administrator and we may be able to resolve it.
8: Why do the DSS predictions for some shore reaches differ
from the published reports?
Answer: An error was discovered in some of the models
where some shore reaches were accumulating loads from other
connected reaches. Shore reaches, by definition, have no
upstream reaches and exist only to account for load that
directly enters a waterbody without entering a river reach.
A good example of this would be a beach, where load goes
directly into a lake, but never enters any type of river.
This error existed at the time the MRB models were
published, but was later discovered and fixed in the DSS.
Modelers may have since published an errata or corrected
9: How do I match the reach ID's in the New England /
Mid-Atlantic models from the DSS to the reach ID's in
NHDPlus? How do I match the reach ID's of other models to
the reach ID's in the appropriate stream network?
Answer: Reach ID's for the reaches in the DSS come from the
stream network on which the SPARROW model is
For models based on the NHDPlus stream network,
including the New England / Mid-Atlantic models, reach ID's
assigned to streams in the DSS match up to the NHDPlus
GRIDCODE not the COMID (see
For models based on the Enhanced River Reach File
(ERF1_2) stream network, including the National models,
reach ID's assigned to the streams in the DSS match up to
the E2RF1 identifier (see
For models based on the MRB Enhanced River Reach File
(MRB_ERF\!_2) stream network, including the MRB regional
models, reach ID's assigned to the streams in the DSS match
up to the MRB_E2RF1 identifier (see