Selection of Streamflow and 

Reservoir-Release Models for 

River-Quality Assessment 

By Marshall E. Jennings, James 0. Shearman, 

and Daniel P. Bauer 

RIVER-QUALITY ASSESSMENT OF THE WILLAMETTE RIVER BASIN, OREGON 

GEOLOGICAL SURVEY CIRCULAR 715-E 

1976 
United States Department of the Interior 

THOMAS S. KLEPPE, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Library of Congress Cataloging in Publication Data 

Jennings, Marshall E. 

Selection of streamflow and reservoir-release models for river-quality assessment. (River-quality 

assessment of the Willamette River Basin, Oregon) 

(U.S. Geological Survey Circular 715-E) 

Bibliography: p. 12. 

Supt. of Docs. no.: 119.4/2:715-E 

1. Stream measurements-Mathematical models. 2. Stream measurements-Data processing. 

3. Reservoirs-Mathematical models. 4. Reservoirs-Data processing. 5. Willamette 

Riverwatershed. I. Shearman,JamesO.,jointauthor. II. Bauer,DanielP.,1941-

joint author. III. Title. IV. Series. V. Series: United States Geological Survey Circular 

715-E. 

QE75.C5 no. 715-E[GB1201.72.M35] 557.3'08s [627'.86'097953] 76-608240 

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202 
FOREWORD 

The American public has identified the enhancement and protection of river 

quality as an important national goal, and recent laws have given this commitment considerable force. As a consequence, a considerable investment has been 

made in the past few years to improve the quality of theN at ion's rivers. Further 

improvements will require substantial expenditures and the consumption of 

large amounts of energy. For these reasons, it is important that alternative 

plans for river-quality management be scientifically assessed in terms of their 

relative ability to produce environmental benefits. To aid this endeavor, this 

circular series presents a case history of an intensive river-quality assessment 

in the Willamette River basin, Oregon. 

The series examines approaches to and results of critical aspects of riverquality assessment. The first several circulars describe approaches for providing 

technically sound, timely information for river-basin planning and management. Specific topics include practical approaches to mathematical modeling, 

analysis of river hydrology, analysis of earth resources-river quality relations, 

and development of data-collection programs for assessing specific problems. 

The later circulars describe the application of approaches to existing or potential 

river-quality problems in the Willamette River basin. Specific topics include 

maintenance of high-level dissolved oxygen in the river, effects of reservoir 

release patterns on downstream river quality, algal growth potential, distribution of toxic metals, and the significance of erosion potential to proposed future 

land and water uses. 

Each circular is the product of a study devoted to developing resource information for general use. The circulars are written to be informative and useful to 

informed laymen, resource planners, and resource scientists. This design stems 

from the recognition that the ultimate success of river-quality assessment 

depends on the clarity and utility of approaches and results as well as their basic 

scientific validity. 

Individual circulars will be published in an alphabetical sequence in the 

Geological Survey Circular 715 series entitled uRiver-Quality Assessment of 

the Willamette River Basin, Oregon." 

J. S. Cragwall, Jr. 

Chief Hydrologist 

III 
Cover: Willamette River as it winds through Portland, Oregon. Photograph taken by 

Hugh Ackroyd. 
CONTENTS 

Page 

i\bstract ------------------------------------------------------------------------------ E1 

Introduction -------------------------------------------------------------------------- 1 

Methods of streamflow and reservoir-release modeling------------------------------------ 1 

Streamflow modeling _ _ ____ _ _ _ _ _ _ _ ____ _ ____ __ __ _ _ __ __ _ _ __ __ __ __ _ _ ____ ___ _ _ _ __ __ _ _ __ 2 

Streamflow-routing methods __ _ _ _ _ __ ______ _ _ _ _ ___ _ _ ___ __ __ _ _ __ __ _ _ __ ____ __ _ _ __ __ 2 

Reservoir-release modeling __ __ _ _ _ _ __ __ _ _ __ _____ _ __ __ __ _ _ _ _ __ _ _ _ _ ____ __ _ _ ___ _ _ _ _ _ __ 5 

Reservoir-system models __ _ _ __ __ _ _ ______ ___ _ ____ __ _ _ _ _ __ _ _ _ _ ____ ___ _ _ __ ____ _ __ 6 

HEC-3 model______________________________________________________________ 6 

SIMYLD-II _ _ _ _ __ _ _ _ _ _ _ __ __ _ _ __ ______ _ _ __ __ _ _ _ _ ____ __ _ _ ______ __ _ _ ___ _ __ _ _ __ 8 

Data requirements for modeling applications -------------------------------------------- 8 

Streamflow data _ _ ____ _ _ __ ____ __ _ _ __ ___ _ _ _ _ _ _____ _ _ _ ____ __ _ _ ___ _ __ _ _ __ __ ___ _ _ _ __ __ 8 

Reservoir characteristics __ _ __ _ ______ _ _ ___ ____ _ _ _ __ __ _ _ _ _ __ __ __ __ _ ___ __ _ _ __ __ _ _ _ _ __ 9 

Water diversion data _________________ _____________ _ _____________ ______________ ____ 9 

Climatological data _ _ ___ _ _ _ _ _ ______ _ _ _ ___ _ _ _ _ __ ____ _ _ ______ _ _ __ __ _ _ _ _ _ ___ _ ___ _ _ ____ 9 

Other data ------------------------------------------------------------------------ 9 

Choice of models _ _ __ __ _ _ __ __ _ _ _ _ _ _ ____ __ __ ____ __ _ _ __ __ _ _ _ _ __ _ _ __ __ _ _ _ _ _ _ ____ _ _ _ _ __ __ _ _ 9 

Summary ---------------------------------------------------------------------------- 11 

References cited ---------------------------------------------------------------------- 12 

ILLUSTRATIONS 

Page 

FIGURE 1. Graph showing simple streamflow routing problem ---------------------------------------------------- E3 

2. Map showing complex streamflow routing problem, upper Kentucky River basin ------------------------ 3 

3. Graph showing typical rule curve for reservoir operation and its relation to computer program HEC-3 ---- 7 

4. Diagram showing model selection process ________________________________________________ -- ______ ------ 10 

TABLE 

Page 

TABLE 1. Example of model study planning-factors for modeling choices ---------------------------------------- Ell 

v 

SELECTION OF STREAMFLOW AND RESERVOIR-RELEASE 

MODELS FOR RIVER-QUALITY ASSESSMENT 

By Marshall E. Jennings, James 0. Shearman, and Daniel P. Bauer 

ABSTRACT 

Sound water-resource planning requires consideration of 

realistic alternatives. Digital models, based on hydraulic and 

hydrologic principles and utilizing adequate input data, can 

be used to simulate the response! s) of a water-resource system 

to various alternatives. Streamflow and reservoir modeling 

methods are reviewed, including a discussion of general data 

requirements. Guidelines for model selection are presented 

with both hypothetical and actual studies used to illustrate 

possible selection procedures. 

INTRODUCTION 

During the last century there has been a steadily increasing need for water and water-oriented 

services. To satisfy these needs, rapid technological advancement is being made to develop 

the necessary tools for water-resource planning. 

These tools, or n1odels, must be effectively applied 

for the timely solution of planning problems connected with increasing development of water resources at the local, regional, and national levels. 

During the next 30 years, the National Water 

Commission (1973J predicts that the Nation will 

decide a number of key issues of water policy. 

Good planning, the Commission states, should be 

based on a range of realistic "alternative futures" 

and should set forth alternative courses of action 

to promote flexible responses. Planning of this 

kind underscores the need for water-resource 

models, based on hydrologic principles, data, and 

related information. Well-verified hydrologic 

models are capable of simulating response(s) to 

"alternative futures" to provide information for 

effective decision making. 

The Willamette River basin was the first basin 

in which the U.S. Geological Survey conducted an 

intensive river-quality assessment study. As 

explained by Rickert and Hines (1975), one objective of that study was to develop and document 

El 

methods for evaluating basin-develop1nent alternatives in terms of potential impacts on river 

quality. One major need of such a study is the 

capability to simulate future streamflow and reservoir releases in the river system. Successful development of this capability depends gr~atly upon 

choosing the most appropriate model(s). To provide an overview of successful model selection, 

this paper ( 1 l reviews the general typ~s of existing models for simulating streamflow and reservoir releases and (2) provides guidelines for model 

selection for a range of study requirements. The 

Willamette River basin study is cited only as one 

specific example of model selection. A complete 

discussion of the Willamette River basin 

reservoir-system model will be presented in a 

subsequent report in this series (Shearman, 

1976). 

METHODS OF STREAMFLOW AND 

RESERVOIR-RELEASE MODELING 

Streamflow modeling and reservoir-release 

modeling are fundamental building blocks in 

modeling surface-water systems. General types of 

models will be reviewed here to familiarize the 

reader with specific flow-modeling concepts 

applicable to river-quality assessment studies. 

The emphasis in this review is on flow modelswater-quality models are discussed and used for 

analyses in other circulars of this series. 

Streamflow and reservoir-release models are 

basically deterministic in character, as opposed to 

statistical or stochastic. Deterministic models, 

because they offer a better definition of cause and 

effect relationships, have greater value as waterresource planning tools. They range considerably 

in complexity and level of mathematical sophistication. For example, some of the mcst complex 
models rely on numerical solution of the basic hydraulic equations of flow-the conservation of 

mass and momentum equations-which arise 

from basic. physical laws. On the other extreme, 

some of the simpler hydrologic models utilize a 

''black-box" approach and make minimal attempt 

to recognize and model real-world phenomena in 

a "structure-imitating" way. A "black-box" model 

attempts to reproduce observed system outputs 

using observed system inputs and a simple function, usually linear, which characterizes the unknown physical system. An example of "blackbox'' modeling in hydrology is a simple linear regression model which relates basin runoff(system 

output) to basin rainfall (system input) using 

monthly values. There is much diversity of opinion as to whether highly sophisticated, numerical 

models of explicit physical laws or simpler "hydrologic" models is the better approach. A modeler who is completely enthralled by the divine 

power of mathematics might insist that the 

former is the only valid approach. The other extremist, who feels that the end result justifies any 

means, may insist upon using the simplest 

''black-box" model regardless of its physical insignificance. A sensible modeler will arrive at a 

reasonable choice of a model that lies somewhere 

between these two extremes by balancing recognizable data limitations with ( 1) relative costs 

and problems associated with increasingly complex models and (2) the high improbability of 

achieving full realism with any model. The problem situation should dictate the modeling approach with a proper choice tempered by adherence to Occam's Maxim. 1 Data requirements, 

discussed below, are a definite factor in model 

selection. Another important factor is the time 

scale of events-whether a monthly time interval 

model is satisfactory or whether hourly analysis 

is required. 

STREAMFLOW MODELING 

Streamflow modeling embraces the quantitative description of surface flow in rivers canals 

and estuaries. The objective of streamflo~ model~ ing is to simulate streamflow at any location in a 

river basin where such data are required to 

satisfy a need. This simulation is usually accomplished by using streamflow-routing methods. 

1"Entities are not to be multiplied w1thout necessity" or "It is vain to do with 

more what can be done with fewer," due to W1lham of Occam lea 13001, a western 

philosopher. 

STREAMFLOW-ROUTING METHODS 

Streamflow routing involves calculation of 

flow-rate hydrographs (and sometimes river 

stage) in a river channel system in response to 

streamflow data inputs. The routing nay involve 

low flows, high flows, and mid-range flows on a 

continuous basis for a given time interval (days, 

months, or years). Most water-resource planning 

problems require simulation for as long a period 

as data will allow, in order to facilitate statistical 

analysis of results and calculatiolJ. of flowrecurrence probability. 

The simplest streamflow-routing problem involves routing of flow-rate hydrograJ: hs from an 

upstream to a downstream point in the channel. 

Figure 1 schematically illustrates a very simple 

situation. 

Computation of the flow-rate hydrograph at the 

downstream point could conceivably range from 

(1) a simple multiplication of the ups+,ream flowrate hydrograph by a constant drainage-area 

ratio to (2) the solution of a complex 1:·et of mathematical equations describing the system. Figure 

2 shows the problem setup for a water-resource 

planning problem in the upper Kentucky River 

basin in southeastern Kentucky as reported by 

Shearman and Swisshelm (1973). Tl'~ latter is 

much more typical of the problems a~tually encountered in water-resource planning. Daily 

flows were simulated for a 31-year period using 4 

basic streamflow-routing reaches, 10 long-term 

daily streamflow records, and a linear 

streamflow-routing method. 

There are three basic categories of Etreamflowrouting methods or models: (1) Models based on 

numerical solution of complete or simplified versions of the hydraulic laws of flow-the continuity 

and momentum equations; (2) models based on 

the linear systems approach using convolution solution techniques; and (3) empirical models. As 

listed, the categories of streamflow-routing models are in decreasing mathematical anrl computational complexity. Over the last 30 years, research has produced dozens of streamflow-routing 

models in each category. Most have produced excellent to acceptable results in the particular 

situations for which they were developed. 

The numerical solution of basic hydraulic equations by finite-difference of finite-element 

methods represents the most cc mplex of 

streamflow-routing models. These partial differE2 
RATE FLOW J\ 

TIME 

EXPLANATION 

PLANNED OR 

EXISTING RESERVOIRS 

USGS STREAMGAGING 

STATIONS 

AND 

STREAM FLOW 

ROUTING POINTS 

--.. ~~ DIRECTION OF 

FLOW 

-• •- BASIN BOUNDARY 

B 

FLOW 

RATE J ~ ~"-

TIME 

FIGURE 1.-Simple streamflow routing problem. 

\_ .. -, .. "· . ----... ............. ··--"' ··--.. ---

./· 

~· 

FIGURE 2.-Complex streamflow-routing problem, Upper Kentucky River basin. 

E3 

."'. 

... ..'~/ ./ 
ential equations in one-dimensional form fron1 

Fread (1974) are: 

aA DlAVl -+---q=O Conservation of mass (la) 

at ax 

av 1 av2 (aH n2 IVIV) -+--- +g -r--80+----

Dt 2 D:r ox 2.21 R-±13 

where 

Conservation of 

momentum (lb) 

x = the distance along the river axis, positive in the downstream direction; 

t time~ 

A wetted cross-sectional area; 

V mean velocity in a cross section~ 

H depth of flow in a cross section; 

8 0 channel bottom slope; 

q lateral inflow per unit length along 

the river axis; 

vqx mean velocity of lateral inflow in the 

x-direction~ 

R the hydraulic radius; -

n Manning's roughness coefficient; and 

g acceleration due to gravity. 

Equation 1a describes the mass balance of fluid 

volumes that must exist within a volume of moving incon1pressible fluid. Equation 1b represents 

a mass balance of forces including those forces 

due to acceleration, potential energy, and friction 

which must be conserved according to basic physical laws, within a volume of moving fluid. Interested readers are referred to well-known texts 

in fluid hydraulics such as Henderson (1966). 

Models based on these equations have a large 

data requirement, are computationally expensive, are difficult for inexperienced hydrologists 

to apply, have a tendency for numerical instability problems, and therefore, may be unsuitable 

for long-term simulation. Nevertheless, these 

models are an attempt at superior realism and 

must be used sometimes to solve problen1s of detailed hydraulic interest, especially those characterized by flow unsteadiness. Recent advances in 

the field of numerical streamflow routing include 

the work of An1ein and Fang (1970), Baltzer and 

Lai (1968), Contractor and Wiggert (1972l, Fread 

(1974), Garrison, Granju, and Price (1969l, and 

U.S. Army Corps of Engineers ( 1973al. Con1puter 

E4 

programs with documentation are available for 

many models of this type. 

Streamflow-routing models based on a linear 

systen1s approach can be summarized in the convolution integral (Schwarz and Friedland, 1965 ), 

Y(.x. _ t) -- fot {'( T) h (X, t-T) dT 

--~ 

I I 

System System input System unitoutput derivative 

function function 

response 

function 

In equation 2, which is used in discrete form in 

actual computation, T is the variable of integration. The convolution integral of equation 2 represents the integration, on a time-varying basis, 

of a systen1 input function (for example, an upstream flow hydrographl and the systen1 unitresponse function, to produce the system output 

function (for example, a downstream flow hydrograph). The systen1 unit-response furdion represents the system output for an instantaneous unit 

input. This type of modeling exhibits the following advantages: ( 1) Linearized solutions of the 

hydraulic equations are an adequatE description 

of the flow process for most streamflow-routing 

problems; (2) linear systems models, employing 

the convolution technique, are easily programmed, are numerically stable, anc are computationally inexpensive; (3) many older linear 

techniques, such as Muskingum routing, can be 

incorporated as options in a basic linear systems 

computer program; and (4) as much detail as 

deemed necessary, in terms of hydraulics, can be 

incorporated into the system unit-reeponse function [that is, the unit-response function can be 

simple, as in the Sauer model (Sauer, 1973), or 

the function can be fairly complicated as in the 

Harley model (Harley, 1967l]. Thus, linear systen1s n1odels are ideal for long-term sin1ulations 

required in many water-resource planning 

studies. Some linear systems streamflow-routing 

n1odels of use in water-resource planning studies 

are those by Sauer (1973 l, Keefer and McQuivey 

(1974), Hayami (1951 ), Harley (1967), and 

Moench, Sauer, and Jennings (1974). The Keefer 

and McQuivey model utilizes a family of unitresponse functions and appears to be especially 

suited for studies which require data input at 

short (for example, hourly) time intervals. The 

Moench model incorporates stream losses and 
gains attributable to interaction with the 

ground-water system into a streamflow-routing 

model. 

Empirical streamflow-routing models include 

statistical methods, successive moving average 

methods, and simple-lag methods. Several 

methods of this type are discussed in Linsley, 

Kohler, and Paulhus ( 1949 I. 

Reverse routing (that is, routing in an upstream direction) is a special case of streamflow 

routing. This type of model is often required in 

water-resource planning studies where an existing downstream gaging record must be moved to 

an upstream computation point. Examples of reverse routing using a numerical model are found 

in Eli, Wiggert, and Contractor (1974). A 

reverse-routing method using a linear-systems 

model was employed by Shearman and Swisshelm (1973). 

Noteworthy among available computer programming packages for streamflow routing, not 

necessarily adapted to simulation, are: ( 1 l the 

HEC-1 package (U.S. Army Corps of Engineers, 

1973bl; (2l the Tennessee Valley Authority package (Garrison and others, 1969l; (3) the Agricultural Research program, HYMO (Williams and 

Hann, 1973l; and (4) the SSARR model (Anderson, 1969). A generalized streamflow-modeling 

package is under development by Shearman 

(written commun., 1976). 

RESERVOIR-RELEASE MODELING 

In addition to being important in itself, the 

quality and quantity of water stored in reservoirs 

is important in controlling downstream river 

quality. Reservoir-release modeling includes 

simulating the operation of single reservoirs, reservoir systems, and water diversions and transfers. Losses of water fron1 reservoir units and systems are included in this type of modeling. 

Reservoirs are located in stream channels for 

various purposes including flood control, water 

supply, irrigation, recreation, navigation, lowflow augmentation for water-quality or habitat 

enhancement, and electric power generation. A 

broad classification of reservoirs is based on the 

design purpose of the reservoir as follows: ( 1 l 

Single purpose-a reservoir designed for only one 

of the above mentioned purposes; ( 2 l 

multipurpose-a reservoir designed for more 

than one of the purposes listed above; and ( 3) reservoir system-two or more single and (or) mulE5 

tipurpose reservoirs designed to operate interactively. Understanding the purposes of a reservoir 

system and other flow regulation operations is 

fundamental to reservoir-release modeling. 

Several Federal agencies design and construct 

reservoirs. Most notable among these are the U.S. 

Army Corps of Engineers, the U.S. Bureau of Reclamation, and the Tennessee Valley Authority. 

State agencies are becoming more active in reservoir regulation studies as the result of the recent 

trend towards State water-resource planning. 

Reservoir regulation schedules, for existing and 

proposed reservoirs for all reservoir purposes, are 

generally available from such agencies. These are 

necessary for realistic reservoir-release modeling 

and generally form the basis for initial assessnlents aimed at water-resource planning. However, in many cases, departures from preset regulation schedules, either in simulation or in actual 

operation, may be necessary and desircble. 

The mass-balance model of equation 3 is the 

basic type of reservoir release model, 

where 

VOLt, VOLt-l reservoir volumes at times 

t and t-1; 

summation of c ll inflow 

volume contributions for 

the period between t and 

t-1; 

summation of all outflow 

volume contributions for 

the period between t and 

t+1; 

precipitation volume for 

the period between t and 

t-1; and 

evaporation volume for the 

period between t and 

t-1. 

In the simplest case, the Puls solution method 

(Henderson, 1966) or some n1odification is used. 

More complicated methods for sloping reservoir 

surfaces, complex gage operation, or downstream 

control are sometimes necessary. Methois of flood 

control operation are given in Beard (1963). General reservoir-modeling techniques are found in 

U.S. Arn1y Corps of Engineers (1969). A general 

review of reservoir-modeling methods is given in 

Rutter and Engstrom (1964l. The previously cited 
study by Shearman and Swisshelm (1973) is an 

example of reservoir-release modeling for waterresource planning involving a fairly simple 

multipurpose reservoir. 

In addition to definition of the hydraulics and 

hydrology of reservoirs or reservoir systems, the 

solution of reservoir operation problems relies on 

either simulation or mathematical programming 

techniques to arrive at optimal operating policy 

under multiobjective criteria. 

In the simulation approach, the effects of an 

operating rule are simulated on the basis of a sequence of streamflows-either a historical sequence if length of record is sufficient, or a synthetic sequence using methods of synthetic 

streamflow generation described by Fiering and 

Jackson (1971). Trial and error or search procedures are used to find the best operating rule. 

Methodologies for reservoir or reservoir-system 

optimization include the specialized tools of 

mathematical programming, dynamic and linear 

programming, including combinations of these 

methods with stochastic or deterministic constraints. The methods involve solution for storage 

or outflow variables if a system of mass balance 

equations similar to equation 3 describe the balance of water volumes in a more complex reservoir system. Hall and Dracup ( 1970) and 

Hufschmidt and Fiering ( 1966 l have reviewed the 

important methods related to water-resources 

systems engineering. Some recent applications of 

these methods include those by Becker and Y eh 

(1974l and Roefs and Bodin (1970). 

RESERVOIR-SYSTE~I ~IODELS 

Recently, many existing or planned reservoir 

projects have been viewed, designed, or redesigned as reservoir systems. Analyses of such systems can be extremely complex and require the 

use of special computer programming techniques 

and analysis. Two important computer programs 

for reservoir systems analysis are reviewed 

here-the U.S. Army Corps of Engineers (1968) 

HEC-3 program, based on a simulation and 

search procedure, and the Texas Water Development Board ( 1972) program SIMYLD II, based on 

a linear programming procedure. 

HEC-3 MODEL 

This computer program was developed by the 

Corps of Engineers to perform multipurpose, multireservoir routings in a reservoir system. The 

model is capable of providing a variety of information about the response(s) througL':lut a reservoir system when given the system input(s) and 

the system constraint(s). 

HEC-3 is capable of analyzing almost any 

reservoir-system configuration imaginable and is 

fully documented <U.S. Army Corps of Engineers, 

1968). System configuration is described to the 

model by means of control points at reservoirs, 

diversion points, and stream confl.uer~es. At each 

control point, the mass balance may be computed 

for the portion of the system contributing flow to 

that point. System size and record length for 

analysis is limited primarily by available computer core storage. Either uniform or varying 

time periods may be used, with a pre;;:ent limit of 

14 time periods per year. 

Input data requirements for the model are extensive. At each control point, the total "natural" 

inflow rates for each time period must be provided 

for each year of analysis. Minimun and maximum flows also may be specified for each control 

point. At each diversion point, the diversion rates 

(or return flow rates) must be specified for each 

time period. Evaporation rates must be specified 

for each time period on a yearly basis. Each reservoir in the system must be described in terms of 

its elevation-area-storage-outlet capacity relationships and its operating rules. Up to eight 

~~storage levels" may be specified for each time 

period to define the operating rules for each reservoir (see fig. 3l. Reservoir releases are computed throughout the system such that all reservoirs are operating at the same relative ~~storage 

level" during each time period. For eyample: during a given time period during a relatively wet 

year, all reservoirs may be operating in the 

seventh Hstorage level", whereas in a very dry 

year all reservoirs may be depleted to the fourth 

''storage level" for the same time period such that 

all desired flows are satisfied. Not only does this 

permit interactive operation of all reE"~rvoirs, but 

storage at one reservoir may be depleted more 

rapidly than the others by assigning a greater 

percentage of its storage to the higher levels. 

Each hydropower plant is described in terms of its 

elevation-capacity efficiency relationships and 

power requirements. Provisions are available to 

permit changes in system operation or demands 

at the end of specified years. 

An iterative procedure is used to maintain 

E6 
--- ~---

JAN FEB MAR APR MAY JUNE JULY AUG SEP OCT NOV DEC 

@ 

@ 

® 

Major Flood Period--reservoir held to minimum elevation (storage) 

for flood control. 

Conservation Storing Period--reservoir filled for conservation 

purposes. 

Conservation Release Period--stored water released as needed. 

Minimum Reservoir Elevation (Storage)--minimum "storage level" 

specified for HEC-3. 

Top of Power Storage--next to lowest "storage level" specified for 

HEC-3. ® Bottom of flood control pool for "dry conditions" (whereas H represents "ideal", "normal", or "average" conditions.) 

@ Intermediate "storage levels" may be used to control the rate of 

depletion below "ideal" for various degrees of "belowaverage" conditions, and among reservoirs in the system. 

@ Bottom of Flood Control Pool--next to highest "storage level" 

specified for HEC-3. 

C!) Maximum Reservoir Elevation (Storage)--maximum "storage level" 

specified for HEC-3. 

FIGURE 3.-Typical rule curve for reservoir operation and its relation to computer program HEG-3. 

E7 
storage balances and required flows throughout 

the system. Initially, reservoir releases are computed such that all flow, power, and diversion 

requirements are satisfied. These releases, combined with evaporation losses, may result in storage imbalances in the system. If so, releases are 

adjusted for storage balancing and, if necessary, 

shortages may be declared for specified power, 

flow, and (or) diversion requirements. 

Model results may be output for every time 

period or limited to yearly output or simply final 

results. Various summaries n1ay be obtained for 

natural and regulated streamflows, reservoir 

data, and shortage information. Summaries may 

be obtained for all years at each control point, or 

all control points for each year. 

In general, HEC--3 is a very useable tool. However, computational costs can be somewhat high 

(especially on a computer system with fairly high 

input/output charges) and input data coding is 

fairly complex. 

Sll\IYLD II 

SIMYLD II is a computer program designed to 

simulate, on a monthly basis, the hydrologic operation of a system of interconnected reservoirs 

within a basin or a multibasin water-resource 

system. The program was developed by the Texas 

Water Development Board, Austin, Tex., and is 

fully documented (Texas Water Development 

Board, 1972). 

Given a specified sequence of demands and 

hydrologic conditions for a multireservoir waterresources system, SIMYLD II may be used to: ( 1 ~ 

sin1ulate the operation of the system, or ( 2) determine the maximum firm yield that can be met 

at a point within the system. 

According to Tischler ( 1973 ), SIMYLD II is designed for extreme flexibility in selecting operating rules for each reservoir. The operating rules 

are formulated as the desired percentage of the 

reservoir capacity (either total or conservation 

capacity) to be held in storage at the end of each 

month. A priority ranking can be assigned to 

storage levels in each reservoir. This ranking is 

then used to determine the allocation of water 

between meeting demands and maintaining storage. The program user may, simply by changing 

the operating rules, vary the desired storage 

levels on a monthly basis and the priority of allocation of water between satisfying immediate 

demands and maintaining storage in the reserEB 

voirs. Constant or time-variable demands and 

any selected hydrologic sequence-historical or 

synthetic-can be used to simulate the operation 

of the system. 

Basic data requireinents are essentially the 

same as for the HEC--3 program. Major exceptions are: ( 1) power-generating reservoirs are not 

considered; <2) only three "storage levels'' (representing average, wet, and dry conditions relative 

to amount of available water) may b~ specified to 

define operating rules; <3) the time periods must 

be calendar months; and (4) certain target flow 

requirements must be specified on a yearly basis 

rather than varied by time periods. 

SIMYLD II adjusts reservoir content and 

stream reach discharges on a monthly basis to 

compute optimum values to meet the system demand considerations. Results includo. a time history of optimal operation including reservoir 

storages, water transfers and spills, demands 

met, and shortages incurred during the simulation period. 

The program is easier to set up than HEC--3, 

fairly general in application, and relatively inexpensive to use. The program is limited somewhat 

by the exceptions listed above. 

DATA REQUIREMENTS F·'JR 

MODELING APPLICATICNS 

Reliable data are a prerequisite for any waterresources planning activity, and the adequacy of 

the data greatly affects modeling 2 ccuracy and 

the applicability of modeling results. Types of 

data that are required for strean1flow and 

reservoir-release modeling include: ( 1) streamflow, (2) reservoir characteristics, (3) water diversions and transfers, and ( 4) climatological effects. 

STREAMFLOW DATA 

Basic streamflow data, includinE: daily flow 

values at several thousand gaging stations, are 

published yearly by the U.S. Geological Survey 

(for example, U.S. Geological Survey, 1973). Such 

information is also published on a project basis by 

the Corps of Engineers as well as by o~her Federal 

and State agencies. In recent yean· streamflow 

data have been placed on rapid-access computer 

storage devices for ease of use in water-planning 

studies (Show en and Stuthmann, 1973 ). Published streamflow data are not always directly 

applicable to river-quality assessment. Therefore, 
additional studies may be necessary to produce 

the unregulated streamflow data required for 

simulation studies. For most river-quality investigations, daily or monthly streamflow data are 

sufficient. 

RESERVOIR CHARACTERISTICS 

Reservoir characteristics include system configuration, area-capacity curves, outlet capacity 

curves, power generation curves, and channel and 

diversion capacity. These data are available in 

engineering design and planning documents by 

agencies responsible for particular projects. 

WATER DIVERSION DATA 

Water diversion and transfer data are of significant importance for determining the total 

streamflow record and for scaling reservoir system demands. Unfortunately, such data are not 

always based on gaged records and must be estimated. For some studies, future diversion and 

other demands must also be estin1ated. 

CLIMATOLOGICAL DATA 

The most important climatologic factors that 

affect reservoir modeling are usually rainfall and 

evaporation. Such data are available from the 

National Weather Service, National Oceanic and 

Atmospheric Administration, and from project 

offices of construction agencies. Sometimes, particularly in the case of evaporation data, monthly 

estimates, used repetitively for each year of simulation, are the only data available. 

OTHER DATA 

Other data of a specialized nature may be required for some modeling purposes; for example, 

flow-range constraints, channel-capacity constraints, and minimum flows for fish habitats. 

Most reservoir-system models have control points 

where such information is required. In some 

cases, adequate time exists to undertake a shortterm data collection program to provide a 

minimum data base for planning studies. If 

adequate data are available, additional streamflow data may be simulated statistically (Fiering 

and Jackson, 1971) for use as system inputs. 

Synthetic streamflows may also be used to test 

system performance under equally likely, and 

perhaps more severe, hydrologic conditions. 

E9 

CHOICE OF MODELS 

Several choices are presented to the modeler 

who is required to furnish modeling results for 

use in river-quality planning studies. Following a 

definition of modeling objectives and acc·wacy requirements, a thorough study of the exiRting and 

extractable data base must be n1ade. Models 

should be selected and the data base restudied 

along with a review of modeling objectives. Figure 4, which might be called a ~·question-choice" 

diagram, shows the various choices which must 

be made. The questions are not listed in any particular order but, as a group, include the major 

ones confronting the modeler. 

Questions which should be asked at th£ outset of 

the study include: Are streamflow and other data 

adequate to fulfill modeling objectives including 

results based on a long-term simulation? This objective requires continuous data at all important 

system input points. Are such data stored on computer for easy use or must it be keypunched from 

basic data reports? The availability of the data will 

have an impact on the time and cost oft}'~ overall 

study. Are the data reliable and appropriate for 

the study purpose? Is lack of factual climatological 

data a constraint? Should a short-term data base 

be started? 

In considering model selection, additicnal questions to be considered are: Should a complex or a 

simple model be selected in view of study objectives and the data-base adequacy? Can the modeling results attain the necessary accuracy requirement? Does a computer program of the model 

already exist for use? Obviously, these are questions of critical importance. The time interval of 

available data is definitely related to modeling 

sophistication. Obviously the sophistication of a 

streamflow-routing model should not exceed data 

availability. Increased model complex~ty, if required by study objectives, should be tempered by 

review of staff ability and time and money costs. 

The question of whether an optimizaticn method 

based on mathematical programming or a simpler 

simulation method should be used for determining 

system operation is of importance. 

There are no easy answers to the al:ove questions, nor are there ready guidelines for use in 

developing a correct modeling approach for riverquality studies. Each study will undoubtedly possess some unique objectives and data problems. 

Recognition of the major factors and choices is 
r----

1 

I 

I 

I 

I 

I L __ 

Modeling objectives 

Accuracy requirements 

Streamflow models 

-, 

I 

I 

I 

I 

I 

I ., 

I 

_J 

• complex or simple models? 

• streamflow and other data adequate? • computer processing capability? 

• time interval required? • relation to other models? 

• long-term simulation? • staff ability? 

• computer data storage? • time and money costs? 

• optimization or simulation? 

r------- --------, 

1 I 

Reservoir-release models •,.....~---! I 

I 

I 

I 

I 

I I 

L ______________ _j 

FIGURE 4.-Model selection process by use of a question-choice diagram. 

ElO 
important in arriving at cost-effective modeling 

decisions. These factors for many modeling studies 

are: (1) study requirements, (2) study resources, 

and (3) study choices. Shown in table 1 in 

generalized form are suggested categories of 

choices associated with each of the three factors. 

Table 1 expands upon the selection criteria introduced in figure 4. A table similar to table 1 but 

structured to the unique study situation is recommended as part of the study planning phase. Such 

a table would serve to focus attention on alternatives and aid in making correct study choices. 

The objective of the "choice" process might be to 

keep the categories as consistent as possible. For 

example, suppose a hypothetical set of modeling 

study requirements are: (1) 20 percent error, (2) 

short-term study, (3) transient (daily) riverquality analysis, and (4) a single reservoir. The 

study resources are: ( 1) Research staff, (2) flexible 

funding, (3) available computer program extendably applicable, and (4 large-core digital computer 

a vail able. Assuming table 1 applied to the 

hypothetical study, choices might follow as: (1) 

Accept the study, (2) choose rather complex 

streamflow and reservoir release models, ( 3) 

choose a daily modeling time interval, and ( 4) 

choose a simulation period of 1-5 years for hydrologic detail but no statistical comparisons. 

As a further example, suppose modeling study 

requirements are: (1) 20 percent error, (2) moderate-term study, (3) steady-state river-quality 

analysis, and ( 4) reservoir-system configuration. Study resources include: ( 1) strong staffhydrologic modeling background, (2) flexible funding, (3) available computer programs extendably 

applicable, (4) large-core digital computer available, and (5) fair data availability and adequacy. A 

planning analysis similar to table 1 might lead to 

project choices as follows: (1) Accept the study, (2) 

choose complex modeling for reservoir-system 

analyses, (3) choose monthly time interval of modeling, and (4) choose 25-year or more simulation 

period in order to produce statistical comparisons. 

The latter example follows essentially the modeling choices made in connection with the Geological Survey's case study of the Willamette River 

basin (Shearman, 1976). 

SUMMARY 

Sound water-resource planning for the future 

will rely heavily on hydrologic models as an aid in 

TABLE 1.-Example of modeling study planning-factors for 

modeling choices 

Factors Categones 

I. Study requirements A Required accuracy loverall error in percent of 

mean value of system 

parameter estimated I-- 20 percent 30 percent 50 percent 

B. Study time frame _ short-term moderate- hng-term 

term 

C. Nature of river-quality 

analyRis very transient transient steady-state 

(hourly I 1daily1 I monthly I 

D. Reservoir system config- uratton ___ single reservoir streamreservoirs system multireservoir 

II. Study resources A. Staff ability - research strong hcks 

hydrologic computer 

modeling programmmg 

background ability 

B. Funding marginally adequate flexible 

adequate 

C. AYailable computer programs -- directly extendably not available 

applicable apphcable 

D. Computational capa- bility ___ large-core desk-top slide rule 

digital computer 

computer 

E. Data avatlab1lity, attainabihty, and adequacy-- good fa1r poor 

III. Studv chozces A. Modelmg study accept- tance ___ refused accepted recommend 

redesign 

B. Model sophist1cat10n complex s1mplified simpleempirical 

C. Modelmg time intervaL_ hourly da1ly monthly 

D. Simulation period_ 1-5 years 10 years 25 years or 

more 

decision-making. Recent water-quality studies by 

the U.S. Geological Survey, oriented toward the 

needs for planning, require the use of streamflow 

and reservoir-release models. This report has presented a review of the major types of su~h models 

including streamflow routing models, based on 

numerical solution of hydraulic laws offl ow and on 

the linear systems approach, and reservoir-release 

models based on simulation and on mathematical 

programming concepts. Data requirements for 

modeling applications are also discussed. Suggestions are offered on the problems of modeling 

choices and examples are given. Spe~ifically a 

diagrammatic approach which lists the questions 

and choices connected with a given study is presented. Also presented is a table showing an itemization of the major factors of study requirements, 

study resources, and study choices. Such tools are 

intended to assist the streamflow and reservoirrelease modeler in constructing guie.elines for 

making cost-effective modeling choice~. The approach has been used in selecting streamflow and 

reservoir-release models for a case study in the 

Willamette River basin, Oreg., but it is also generally applicable to other studies. 

Ell 
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E12