-------
VI. APPLICATION AND CALIBRATION
This chapter outlines a general approach for calibrating QUAL-II.
The procedure has evolved from the many applications of earlier versions
of the model and, if followed, can save the user many hours and much
frustration in getting the model to satisfactorily represent his system.
QUANTITY CALIBRATION
Once the system has been discretized into reaches, the next step
is to specify the discharge coefficients a, 3, a and b, or the channel
cross-section for each reach (see HYDRAULIC DATA input specifications).
Care should be taken here to see that the coefficients or channel section
specified are typical for the entire reach. A mistake that is often made,
especially when discharge coefficients are used, is to use the discharge
curves, or cross-section at a gaging station to represent the reach. The
problem with using these data is that the gaging station is generally
located at a control section in the stream and thus is atypical of the
reach.
After the hydraulics data for the reach has been specified, a
run should be made to "rough check" the system. All headwater sources,
and known point source inputs should be specified so that the only inflows
not simulated are incremental inflows. The output from this simulation
can be checked quickly by hand for continuity. If flow continuity is not
satisfied, the system is not properly specified upstream of the point
where the continuity problem occurs. In such a case the input data should
be checked for proper ordering of reaches, tributaries and point source
loads. Examine carefully the Flag Field Data and the Junction Data with
respect to these specifications. Most often the problem is in one of these
two input data types.
67
-------
Once the continuity check has been made the Incremental Inflows
to the reaches can be computed. These inflows are generally groundwater
inflows and/or minor amounts of distributed surface runoff, which is not
accounted for in the point source data. In order to properly specify
these incremental inflows the flow data must be taken from a period(s) of
record when the discharge varied less than l^ percent over a period equal
to the travel time through the stream system. Incremental inflows are
calculated from this data as follows. Starting with the uppermost headwater,
the difference between this flow and the flow at the first gage downstream
is due to point sources and incremental inflow (there may be another
headwater(s) contributing also if the first gage is below a junction(s)).
Once the incremental inflow is calculated, it can be apportioned over the
reaches between the flow gage and the headwater(s) on an inflow per mile
basis, or inflow per square mile of tributary area basis (better) or some
other hydrologically defensible method. Note: To insure that incremental
inflows are properly distributed among the reaches, there is no substitute
for familiarity with the hydrologic characteristics of the basin. Once the
upstream portion of the system has been balanced, the next lower portion
can be calibrated in a similar manner. The process is continued until
the bottom of the stream system is reached. After the incremental inflows
are calculated, a simulation run should be made. The computed flows at
the gage locations should be the same as the recorded values. Make
adjustments as necessary.
When the flow balance is achieved, check the velocity and depth
in every element of every reach. Adjust discharge coefficients or channel
cross sections as appropriate to achieve reasonable values for velocity
and depth. This aspect of the calibration is often ignored with the result
that "strange" water quality responses are simulated on certain river
reaches. Detailed examination of the "strange" reaches reveals simulated
velocities that are high (or low) in reaches that the user thought (or
knows) are slow (fast) in the prototype. Similarly, shallow (deep) depths
may be computed for reaches that are actually deep (shallow).
68
-------
Often a stream system will have a low head dam with a shallow
(10-20 ft.) impoundment behind it. For nonstratified impoundments these
stream segments may be represented as follows. The channel depth is taken
as the average depth of the reservoir and the velocity as the flow divided
by the average cross-sectional area. If discharge coefficients are used,
set 3 and b to 1.0. Calculate a as D/Q. Then compute a as a = V/Q =
(Q/wD)/Q = 1/wD where w is the mean width of the reservoir. Change thes^
coefficients (a and &) every time the input flows (headwaters, point source?,
or incremental inflows) are changed.
If trapezoidal cross sections are used, select a section most
representative of the reservoir. Use the mean depth D that occurs for a
flow Q through the reservoir. Using this value of D and the specifications
of the cross section, compute the cross sectional area A of the flow and
the hydraulic radius R. The energy slope, Se for the reservoir, can then
be computed as
n Q
S
e
1.486 A R2/3
V-l
Use the computed slope Se as the channel slope for the reservoir reach.
This will produce a representation of the reservoir as a trapezoidal channel
with a cross section as specified and a depth D at discharge Q.
Be sure to check the simulated depth and discharge in each reservoir
for reasonableness. Make adjustments as necessary.
QUALITY CALIBRATION
It is assumed here that the user has 1) a working knowledge of
water quality relationships in streams and 2) reads and understands
Chapters II and Ill—General Model Formulation, and Constituent Reactions
and Interrelationships, respectively. The user should also be familiar
with the air-water interface energy transfer relationships in Chapter IV,
69
-------
if temperature is to be simulated. Familiarity with the computation of
radiation (solar, atmospheric, and longwave back radiation) is desirable
but not mandatory.
There is no required order in which the water quality parameters
must be calibrated; however, if the order suggested is followed, a much
faster calibration of the whole model can generally be achieved. The
suggested order of constituent calibration is:
1. Conservative Constituents
2. Temperature
3. BOD, Coliforms, and the Arbitrary Nonconservative Constituent
4. Algae, NH3, N02, NOa, and P04
5. DO
The rational for the order shown is that we wish to calibrate those constituents
whose concentrations are independent of other constituent concentrations
first. The last constituents to be simulated are the most interdependent
constituents. The following sections provide some pointers on calibration
of these parameter groups.
ConservativeConstituents
These parameters should be calibrated first, especially if
constituents such as chlorides, total dissolved solids, or heavy metal(s)
are included. Since there is no decay rate, calibration consists only of
adjusting input loads in headwaters, from point sources and from incremental
inflows. Thus this calibration becomes an excellent tool for locating
previously unidentified waste sources.
Temperature
This parameter is calibrated next because once the point waste
sources are all identified the only factor that affects temperature is
70
-------
the heat transfer through the air-water interface. Temperature, on the
other hand, affects the value of nearly all the rate constants.
To calibrate temperature, first make sure that the Climatological
Data (Type 12 data) is specified as accurately as possible. Wihdspeed
along the river—channeled by a canyon or sheltered by high vegetation
growths on the banks—may be significantly different than the value
measured at the Class A weather station. Take this into account as much
as possible. Secondly, cloud cover should be carefully estimated and
adjustments should be made if necessary between cloud cover at the
observation point and cloud cover over the river. Furthermore, a tree-
lined river flowing in a North-South direction will be partially or
wholly shaded. The only way to effectively account for this condition
is to increase the cloud cover as appropriate.
Once the Climatological data have been adjusted to the user's
satisfaction, the remaining calibration parameters are the dust attenuation
coefficient and the evaporation coefficients a^ and b^. Of these three
parameters, temperature is most sensitive to the coefficient b_. It should
be possible to calibrate the temperature by varying b_ over acceptable
ranges with the dust attenuation coefficient and the evaporation coefficient
a^ set at their recommended values. If this is not possible within accepted
ranges of coefficient b_, then recheck the meteorological input data,
i.e. the data on the last four Type 1 Data Cards. If these data are
correct, then adjust the dust attenuation coefficient and lastly the
evaporation coefficient a^.
BOD, Coliforms, and the Arbitrary Nonconservative Constituent
Coliforms and the arbitrary nonconservative constituent should
be calibrated next since they are affected only by waste input strength
and decay rate. Coliform inputs will be headwaters, point sources, and
incremental inflows if it is surface runoff. The trick here is to get
71
-------
the concentration of the incremental inflow correct. A high incremental
inflow load plus a high decay rate will produce the same in-stream
concentration as a low incremental inflow rate and a low decay rate.
This relationship applies to all three constituents.
In the case of BOD, another parameter that affects the concentration
is the sedimentation rate, 1(3. When calibrating BOD, keep in mind that
d(BOD)/dt = K-BOD where K = K] + £3. Thus for an observed in-stream loss
rate, K and an assumed BOD decay rate, KI , the value of KS is K - KI .
Keep in mind that K-j-BOD is the oxygen uptake rate by suspended BOD.
K3-BOD is a loss of BOD to the stream, but it does not exert an oxygen
demand per se.
Algae, NHs, N02. N03, and P04
Each of these variables are related to one or more of the others,
thus the calibration of this set of constituents can be time consuming.
The following pointers will save some time. First, calibrate P04 as a
conservative constituent. Do not spend too much time on this initial
calibration; keep in mind that algae takes up P04 for growth and produces
it through respiration. Remember benthos deposits as well as wasteloads
and incremental inflows may also be a source of P04-
Next calibrate NH3 and N03. Don't worry about calibrating N02;
use a high decay rate that will keep concentrations low. This is the
phenomenon that is observed in natural waters. Nitrite serves only as
the intermediate product of the NH3-+N03 nitrification process and its
oxidation to N03 is rapid. The process which actually controls the rate
of NH3+N03 oxidation process is the ammonia decay rate. Calibrate NH3
and N03 initially ignoring algae. Calibrate NH3 on the low side to account
for algae respiration and N03 a little high to provide for N03 uptake by
algae growth (keep in mind that one mg of algae growth or respiration uses
or produces only 0.12 mg of NOs or NH3 as N). Note that the only loss of
72
-------
N03 from the system is by algae growth. If simulated values of N03 are
too high it is probably because the ammonia decay rate is too high.
Recall that benthos deposits can be a source of ammonia to the system.
In some river systems it may happen that simulated values of
NH3 and/or N03 and P04 are too high and cannot be reduced to observed
values through realistic adjustment of rate and uptake coefficients.
In such cases, one might suspect the presence of macrophytes and benthic
algae. These plants can take up significant quantities of NH3 and P04
from the stream. If such a situation appears probable a stream visit
is probably in order.
Once P04 and the nitrogen series have been initially calibrated,
then algae may be simulated. The primary calibration parameter here is
the algae specific growth rate which can vary from 1.0 to 3.5 per day. The
recommended values for the nitrogen and phosphorus content of algae and
the half saturation constants (see the test problem) should not be changed
unless the user has a defendable argument for doing so. The algae
respiration rate can also be varied. Another factor that affects the
algae concentration is the light extinction coefficient. The value can
be initially estimated as follows. The 10 percent light level is about
three times the Secchi disc reading (call it Zs). Thus we can say at the
10 percent light depth:
•j- = 0.10 = e"X(3Zs) (V-2a)
or
Ln 0.10 = -2.3 = - X3ZS (V-2b)
or
^» ^
(V-2c)
73
-------
where A is the light extinction coefficient. In clear water Zs could be
ten feet or more; in turbid water it would be quite small. If algae
concentrations are around 1.0 mg/1, Zs will probably be one to two feet.
Algae can also be lost to the system by settling. This might
occur in a small reservoir and possibly in very sluggish river reaches.
For velocities over 1.5 ft/sec, algae settling will probably not occur.
Dissolved Oxygen
If everything else has been calibrated satisfactorily, the
dissolved oxygen (DO) calibration should be relatively straightforward.
Do not change the algae 02 uptake or respiration constants unless there
is a biologically defensible reason. This leaves basically only the
reaeration constants and the benthic oxygen demand to adjust.
If the user has no preference with respect to the reaeration
option to use, we suggest Tsivoglou's formulation be selected. The
reaeration constant can be adjusted reach by reach.
In some reaches the river may be fast flowing possibly with
white water resulting in high reaeration rates. The user may wish to
specify option 1 for such reaches and enter a reaeration rate for that
reach as input data.
Finally, the spillway on many low head dams serves as an "in-
stfeam" aeration device causing DO levels just downstream of the spillway
to be much higher (probably near saturation) than those in the reservoir
just upstream of the spillway. For these cases the user may want to
define a reach, just below the spillway, that is one computational
element long and which has a high K2 value specified as input data.
74
US GOVERNMENT PRINTING OFFICE: 1*1.757-009/8008
-------