Fate of Chemicals in the Environment - American Chemical Society

7 A New Mathematical Modeling System R. C. JOHANSON

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University of the Pacific, Dept. of Civil Engineering, Stockton, C A 95211

The H y d r o l o g i c a l Simulation Program - F o r t r a n (HSPF) simulates the movement of water, sediment and many a s s o c i a t e d c o n s t i t u e n t s over and under the land surface, and through streams, r i v e r s and shallow lakes. I t is a r e l a t i v e l y new package, funded by the U.S.EPA. Substances such as p e s t i c i d e s are handled by simulating the processes of adsorption and desorption, transport i n the d i s s o l v e d and adsorbed s t a t e s , and degradation through h y d r o l y s i s , photolysis, oxidation by f r e e r a d i c a l s , etc. S p a t i a l v a r i a t i o n is handled by s u b d i v i d i n g the area i n t o "processing u n i t s " , each of which represents a relatively homogeneous segment of pervious or impervious land-surface, or a reach of channel or an impoundment. HSPF operates on the basis of continuous simulation; the user can s e l e c t a time step ranging from 1 minute to 1 day. The software package incorporates many modern features which have made it r e l i a b l e and easy to install on a v a r i e t y of machines. I t is a l s o easy to use and extend.

As environmental c o n t r o l s become more c o s t l y to implement and the p e n a l t i e s of judgment e r r o r s become more severe, water q u a l i t y management requires more e f f i c i e n t a n a l y t i c a l t o o l s based on greater knowledge of the phenomena to be managed. In t h i s connection, the development and a p p l i c a t i o n of mathematical models to simulate the transport and transformation of p o l l u t a n t s through a watershed, and thus to a n t i c i p a t e environmental problems, has been the subject of i n t e n s i v e research by the Environmental Research Laboratory (U.S. EPA) i n Athens, Georgia. HSPF i s one of the most recent products of t h i s work. S t a r t i n g i n 1976, i t was developed from the f o l l o w i n g older models: (1) The Stanford Watershed Model (SWM) developed a t Stanford U n i v e r s i t y (1). I t can simulate the hydrologic behavior of an e n t i r e watershed. 0097-6156/83/0225-0125$06.75/0 © 1983 American Chemical Society

Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

F A T E OF C H E M I C A L S I N T H E E N V I R O N M E N T

126 (2)

(3)

The A g r i c u l t u r a l Runoff Management (ARM) Model, developed by Hydrocomp Inc. f o r the U.S. EPA (_2). I t simulates the hydrology, sediment yield, and nutrient and pesticide behavior of the land phase of the h y d r o l o g i c a l c y c l e . The same o r g a n i z a t i o n s a l s o developed the Non-Point Source (NPS) Model (3) which handles the washoff of miscellaneous p o l l u t a n t s from land s u r f a c e s . The HSP Q u a l i t y Model ( 4 ) . I t simulates a comprehensive set of water q u a l i t y processes i n streams and lakes, but not p e s t i c i d e s and t o x i c substances.

Later some f e a t u r e s of the SERATRA Model, developed by B a t e l l e Northwest L a b o r a t o r i e s (5) were added. T h i s model was designed to simulate the behavior of sediment and a s s o c i a t e d constituents in streams. It i n c l u d e s processes such as h y d r o l y s i s and p h o t o l y s i s and i s thus s u i t a b l e f o r modeling t o x i c substances such as p e s t i c i d e s . Basic P r i n c i p l e s of HSPF A l l of the above models are of the " d e t e r m i n i s t i c conceptual" type. That i s : (1) they do not c o n t a i n random components. A given set of input data w i l l always produce the same set of output. (2) they c o n s i s t of sets of l i n k e d equations which represent, to a c e r t a i n degree, the a c t u a l phenomena being simulated. These models employ continuous, r a t h e r than s i n g l e event, simulation. The advantage i s that continuous output can be analyzed s t a t i s t i c a l l y . The user can o b t a i n answers to questions such as "For what f r a c t i o n of time w i l l the c o n c e n t r a t i o n of X be above Y mg/1 at point Z i n the system?" Or, "What danger does chemical X pose to species A at l o c a t i o n s B and C?". These are the kinds of answers needed i f he i s to make r a t i o n a l d e c i s i o n s regarding the p e r m i s s i b l e uses of chemicals f o r a g r i c u l t u r a l purposes. HSPF represents the temporal v a r i a t i o n s i n a b a s i n by s i m u l a t i n g i t s behavior over an extended period of time, using a constant time step s e l e c t e d by the user. S p a t i a l v a r i a t i o n s are handled by subdividing the basin into several distinct computational elements or Processing U n i t s (PUs) (Figure 1). There are s e v e r a l types of PUs such as: (1) Pervious Land-segments, simulated by the PERLND module. (2) Impervious Land-segments, simulated by the IMPLND module. (3) Free-flowing stream reaches and r e s e r v o i r s , simulated by the RCHRES module. The degree to which the study area i s subdivided i s up to the user; HSPF can handle hundreds of PUs i n a s i n g l e run. For


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A New Mathematical Modeling System

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II Land Segment Number — Land Segment Boundary •—i Stream Reach

Precipitation. Temperature, Etc

Figure 1.

Soil Properties, Channel Properties, Land Use, Etc

Concentration\ Etc

S u b d i v i s i o n of a basin f o r s i m u l a t i o n using HSPF.


128


every PU the h y d r o l o g i c response i s first simulated. Then calculations of water temperature, sediment transport and chemical behavior are superimposed on the flow c a l c u l a t i o n s . The user s p e c i f i e s how the v a r i o u s PUs are connected, forming the "network" of water, c o n s t i t u e n t and i n f o r m a t i o n flow (Figure 1). There are two c l a s s e s of Operating Module i n HSPF: (1) A p p l i c a t i o n Modules. These simulate the behavior of processes which occur i n the r e a l world (eg. a Pervious Land-segment). (2) U t i l i t y Modules. These perform "housekeeping" operations on time s e r i e s (eg. m u l t i p l y a c o n c e n t r a t i o n time s e r i e s by a flow time s e r i e s , to get a "load" time s e r i e s ) . HSPF i s an expandable system. Operating modules can be added t o , or removed from, the system with r e l a t i v e ease. The software c u r r e n t l y contains three a p p l i c a t i o n modules and s i x u t i l i t y modules (Figure 2 ) . Both c l a s s e s of Operating Module u s u a l l y need one or more input time s e r i e s and produce one or more output time s e r i e s (eg. outflow of water and c o n s t i t u e n t s ) . From experience, the designers of HSPF knew that much of the e f f o r t i n using continuous s i m u l a t i o n models i s a s s o c i a t e d with time series manipulations. Thus, a s o p h i s t i c a t e d Time S e r i e s Management System was i n c l u d e d . I t centers around the Time S e r i e s Store (TSS) (Figure 10), which i s a disk-based f i l e on which any input or output time s e r i e s can be s t o r e d i n d e f i n i t e l y . HSPF can'be run with a time step ranging from 1 minute to 1 day. Data can be stored i n the TSS with a s i m i l a r range of intervals. The system w i l l a u t o m a t i c a l l y convert time s e r i e s from one i n t e r v a l to another, as they are t r a n s f e r r e d between the TSS and the machine memory. T h i s means, f o r example, that a Pervious Land-segment could be run a t an i n t e r v a l of 1 hour, using 15 minute p r e c i p i t a t i o n data and d a i l y evaporation data (stored on the TSS) as i n p u t s . The Pervious Land-segment (PERLND) Module General Comments. The PERLND module simulates a v a r i e t y of processes o c c u r r i n g on and under the surface of a Pervious Land-segment. F i g u r e 3 i s a " s t r u c t u r e c h a r t " (see "Software C o n s i d e r a t i o n s " ) which shows the twelve s e c t i o n s of t h i s module and the f u n c t i o n s they perform. The s e c t i o n s u s u a l l y i n v o l v e d i n s i m u l a t i n g p e s t i c i d e s a r e SNOW and PWATER (hydrology), SEDMNT (sediment), ftSTLAY ( s o l u t e t r a n s p o r t ) and PEST ( p e s t i c i d e s ) . The l a s t 5 s e c t i o n s of the module are of primary importance i n s i m u l a t i n g a g r i c u l t u r a l chemicals. The user s p e c i f i e s which set of s e c t i o n s w i l l be executed i n a given run. For example, he may i n i t i a l l y "switch on" only SNOW and PWATER, to c a l i b r a t e the simulated h y d r o l o g i c a l behavior of a land-segment to observed data. Then he may turn on MSTLAY and


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Application Modules PERLND Snow Water Sediment Quality Pesticide Nitrogen Phosphorus Tracer

IMPLND Snow Water Solids Quality

RCHRES Hydraulics Conservative Temperature Sediment Nonconservative BOD/DO Nitrogen Phosphorus Carbon Plankton

Utility Modules COPY Data transfer DURANL Duration Analysis

Figure

2.

"Operating

PLTGEN Plot data GENER Transform or combine

DISPLY Tabulate, summarize MUTSIN Input sequential Time-series data

M o d u l e s " p r e s e n t l y i n t h e HSPF

software.



130

PERLND Simulate a pervious land segment

4.2(1)

I ATEMP

SNOW

Correct air Simulate temperature snow and ice

-4.2(1)1

4.2(0.2 |4.2(l).2^> MSTLAY

r

Estimate solute transport

4.2(0.8 | 4.2(1)8^>

PWATER I Simulate water budget

4.2(0.3 T |4.2(t).3 y PEST

I

Simulate pesticides

4.2(0.9 3 14 2(0.9 ^>

SEPMNT Simulate sediment

4.2(0.4

PSTEMP I PWTGAS I Estimate Estimate water soil temperature temperature (s) and gas concentrations 4.2(0.6 4.2(0.5

I

Simulate nitrogen

4.2(0.10

1

|4.2(0IQ>

4.2(0.7 |4.2(l).7 >

|4.2(l).4 )> NITR

PQUAL Stimulate general qua/it/ constituents

PHOS

TRACER

Simulate Simulate phosphorus a tracer (conserva five)

4.2(011 f4.20).H^

4.2(012 | 4.2(0.12^

AGkl- CHEMICAL SECTIONS

Figure

3.

Structure chart

f o r t h e P e r v i o u s Land-segment


module.

7.

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131

TRACER so that he can compare the simulated and observed movement of a conservative substance such as c h l o r i d e . F i n a l l y , he may t u r n TRACER o f f and PEST on, to simulate up to 3 p e s t i c i d e s . Hydrologic Simulation i n the PERLND Module. Hydrologic s i m u l a t i o n i s done using the moisture accounting technique f i r s t employed i n the Stanford Watershed Model (Figure 4). That i s , the movement of water i n t o , between, and out o f , a s e t of conceptual storages i s computed using a f i x e d time step. Snow accumulation and melt are simulated i n the SNOW s e c t i o n ( i f i t i s turned on) u s i n g energy balance procedures (6). Rain and snowmelt are subject to i n t e r c e p t i o n . I f that "storage i s f u l l i n f i l t r a t i o n occurs. I n f i l t r a t i o n c a p a c i t y i s a f u n c t i o n of the current moisture storage i n the lower zone and a parameter INFILT which r e f l e c t s the p e r m e a b i l i t y of the s o i l . Infiltrated moisture passes to the lower zone or to groundwater storage. Excess moisture e i t h e r remains on the surface or enters flow paths l e a d i n g to the upper zone or to i n t e r f l o w . Percolation from the upper zone to the lower zone and groundwater i s modeled. The model regards overland flow as equivalent to that along a plane surface of length, slope and roughness s p e c i f i e d by the user. I t uses a kinematic method to c a l c u l a t e the overland flow rate. Other c o n t r i b u t i o n s to streamflow come from i n t e r f l o w and groundwater outflow. E v a p o t r a n s p i r a t i o n (ET) can occur from any of the storages. The model algorithms compute the amount of ET from each storage, based on p o t e n t i a l ET data s u p p l i e d by the user. Sediment Simulation i n the PERLND Module. The processes modeled i n the SEDMNT s e c t i o n are shown i n F i g u r e 5. I t a l s o shows the simple equations used, which are based on one of the f i r s t continuous sediment s i m u l a t i o n models (_7). The r a t e of detachment by r a i n f a l l i s a power f u n c t i o n o f r a i n f a l l i n t e n s i t y , modified to account f o r p r o t e c t i v e cover (C) and any s p e c i a l management p r a c t i c e s (SMPF) (e.g. t e r r a c i n g , contouring). SMPF corresponds to the f a c t o r P i n the U n i v e r s a l S o i l Loss Equation. Washoff (WS) i s the removal, by overland flow, of detached material. I t i s modeled as a power f u n c t i o n of overland flow, which i s computed by the hydrology s e c t i o n (PWATER), but washoff i s l i m i t e d by the supply of detached m a t e r i a l . This supply can be a l t e r e d by the user a t any time, to simulate the e f f e c t of soil tillage. Scour (SCR) i s a l s o modeled as a power f u n c t i o n of overland ( s u r f a c e ) outflow. This simulates d i r e c t e r o s i o n by surface outflow, such as g u l l y formation. F o r scour, the model considers the supply of parent material unlimited. The c o e f f i c i e n t s and exponents (KRER, JRER, e t c ) must be determined by experience and/or c a l i b r a t i o n . The sediment s e c t i o n a l s o accounts for soil compaction (using a f i r s t - o r d e r process) and d e p o s i t i o n or removal o f detached sediment (e.g. by wind).


132

FATE OF CHEMICALS I N T H E E N V I R O N M E N T

Potential ET Precipitation Temperature Radiation Wind, Dewpoint z

Actual

ET

(Subroutine) (

U-{ET>

% Decision ET-Evapotranspiration

Interception Surface Runoff Interflow

Lower Zone Storage

)

1 Storage"}

V/---T Snowmeh Interception Storage

Input )

(Output

r M

V

Upper Zone Storage

Overland Flow

Interflow

3_

Groundwater Storage Deep or Inactive Groundwater

F i g u r e 4. R e p r e s e n t a t i o n o f t h e h y d r o l o g i c a l P e r v i o u s Land-segment.

To Stream processes


in a

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133

Detachment:

DET = ( l - O * SMPF * K R E R * R A I N

Washoff:

WS = KSER * S U R O

Scour:

SCR = K6ER * S U R O

F i g u r e 5. S e d i m e n t - r e l a t e d p r o c e s s e s segment, a s m o d e l e d i n HSPF.

J R E R

J S E R

J G E R

i na

Pervious

Land-


134


P e s t i c i d e S i m u l a t i o n i n the PERLND Module. The procedures used to simulate p e s t i c i d e s were d e r i v e d from those used i n the Agricultural Runoff Management (ARM) Model C2). HSPF can simulate up to 3 p e s t i c i d e s i n one run. The s o i l i s viewed as having four l a y e r s ( F i g u r e 6), corresponding to the s u r f a c e , upper, lower and groundwater storages used i n the hydrology section (Figure 4). Although i n nature the t r a n s p o r t and r e a c t i o n s of p e s t i c i d e s occur simultaneously, the model t r e a t s these processes s e r i a l l y , i n each time step. Transport r a t e s f o r d i s s o l v e d m a t e r i a l are based on the i n t e r n a l and e x t e r n a l f l u x e s (flows) computed i n the hydrology s e c t i o n of the module. Soluble chemicals are transported down through the s o i l p r o f i l e and are washed out i n t o streams with surface runoff, interflow and groundwater flow. Sediment associated pesticides (and n u t r i e n t s ) are removed from the s u r f a c e l a y e r whenever sediment washoff occurs. The two p e s t i c i d e r e a c t i o n s simulated by HSPF are: (1) Adsorption and d e s o r p t i o n . The user can choose to handle t h i s u s i n g e i t h e r temperature-corrected f i r s t order r e a c t i o n k i n e t i c s , i n which case the c o n c e n t r a t i o n s are always moving towards e q u i l i b r i u m but never q u i t e reach i t , or he can use a F r e u n d l i c h isotherm, i n which instantaneous e q u i l i b r i u m i s assumed. With the F r e u n d l i c h method, he can e l e c t e i t h e r to use a s i n g l e - v a l u e d isotherm or a non-single-valued one. T h i s was i n c l u d e d i n the model because there i s experimental evidence which suggests that p e s t i c i d e s do not always f o l l o w the same curve on d e s o r p t i o n as they do on a d s o r p t i o n . (2) Degradation. Although the a c t u a l mechanisms of degradation are many and complex, HSPF uses a simple first-order r e l a t i o n s h i p to approximate t h i s process. Adsorption, d e s o r p t i o n and degradation are simulated i n each of the four s o i l l a y e r s ( F i g u r e 6). D i f f e r e n t parameters can be used i n each l a y e r . Note that t h i s s e c t i o n of the PERLND module could be used to simulate substances other than p e s t i c i d e s , provided the processes i n c l u d e d i n the model adequately represent the behavior of the compound i n q u e s t i o n . The

Impervious

Land-segment (IMPLND) Module

T h i s module i s designed to simulate processes i n areas where the ground i s t o t a l l y impervious; u s u a l l y i t i s used on p a r t s of urban areas. I t i s not designed to handle p e s t i c i d e s . The Reach/reservoir (RCHRES) Module General Comments. As the s t r u c t u r e c h a r t f o r t h i s module shows (Figure 7), i t i s designed to simulate the t r a n s p o r t and


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JOHANSON

Application

Outflow, to Stream with: Sediment, Surface

135


Degradation

A

Layer: WML Surface

runof.L^^

Interflow

Ground water

A = Adsorption

D= Desorption

F i g u r e 6. P e s t i c i d e r e l a t e d p r o c e s s e s i n a P e r v i o u s segment, a s m o d e l e d i n HSPF.


Land-

136

FATE OF CHEMICALS IN THE ENVIRONMENT

RCHRES Simulate a reach or mixed reservoir 4.2(3)

HYDR Simulate hydraulic behavior

ADCALC Prepare to simulate advection

4.2(3)1

4.2(3).2

SEDTRN Simulate inorganic sediment 4.2(3).5

Figure 7.

HTRCH Simulate water

CONS Simulate conservative constituents

GQUAL Simulate general quality constituents 4.2(3).6

temperature

4.2(3).3

4.2(3).4

RQUAL Simulate biochemical constituents 4.2(3).7

Structure chart f o r the Reach/reservoir

module.


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Mathematical Modeling System

137

r e a c t i o n s of a wide v a r i e t y of c o n s t i t u e n t s i n streams and l a k e s . Like the PERLND module, each s e c t i o n of the RCHRES module simulates a d i f f e r e n t set of processes, and the user can switch on that combination of s e c t i o n s which i s best s u i t e d to simulate the c o n s t i t u e n t s which he i s studying. S e c t i o n HYDR simulates the movement of water ( h y d r a u l i c routing). Section HTRCH evaluates the exchange of heat between a reach and the atmosphere and, thus, simulates water temperature. These sections are important because transport and temperature g r e a t l y i n f l u e n c e almost a l l the other processes simulated by the module. Sections SEDTRN and GQUAL simulate the movement of sediment and "generalized" quality constituents (e.g. pesticides). Section RQUAL simulates the "traditional" biochemical c o n s t i t u e n t s , such as oxygen, biochemical oxygen demand, n u t r i e n t s , phytoplankton, zooplankton, carbon dioxide and r e f r a c t o r y organic products of b i o l o g i c a l death and r e s p i r a t i o n . One s i g n i f i c a n t l i m i t a t i o n of the RCHRES module i s that i t assumes t o t a l mixing i n the water body; thus i t does not simulate s t r a t i f i e d impoundments. H y d r a u l i c Routing i n the RCHRES Module. HSPF uses a simple technique f o r flow r o u t i n g . The catchment stream network i s subdivided i n t o "reaches" (Figure 1) and c a l c u l a t i o n s s t a r t with the upstream ones. Each reach may have s e v e r a l outflows and each outflow rate may be a f u n c t i o n of storage i n the reach (storage r o u t i n g ) or a f u n c t i o n of time (e.g. to supply demands of i r r i g a t o r s ) , or a combination of both. HSPF can handle a reach network of any complexity; i t can even handle s i t u a t i o n s where flows are s p l i t ( d i v e r t e d ) and l a t e r recombined further downstream (e.g. through hydro-power diversion tunnels). A l s o , i t makes no assumptions regarding the shape of the water body. For example, streams do not have to be represented with t r a p e z o i d a l cross s e c t i o n s . Sediment Routing i n the RCHRES Module. The sediment routing method has been adapted from that used i n the SERATRA model (_5). Each reach i s viewed as c o n t a i n i n g one " l a y e r " of suspended, or entrained, sediment and one l a y e r of bed sediment. Three c l a s s e s of sediment are handled sand, s i l t and clay. Each i s separately routed through the reach and i t s d e p o s i t i o n or e r o s i o n rate i s calculated. For sand, the transport c a p a c i t y i s f i r s t c a l c u l a t e d using e i t h e r the Colby (9), or T o f f a l e t i (10) method, or a user s u p p l i e d power f u n c t i o n of v e l o c i t y . I f the c a l c u l a t e d transport c a p a c i t y exceeds the load present scour i s simulated and i f the opposite i s true d e p o s i t i o n i s simulated. For s i l t and c l a y , the c r i t i c a l shear s t r e s s concept i s used. I f the c r i t i c a l shear s t r e s s f o r scour i s exceeded, scour takes place. On the other hand, i f the a c t u a l shear s t r e s s i s l e s s than the c r i t i c a l value f o r d e p o s i t i o n , d e p o s i t i o n occurs.


138


P e s t i c i d e Simulation i n the RCHRES Module. P e s t i c i d e s and many other t o x i c substances are subject to a v a r i e t y of processes i n the aquatic environment. In the RCHRES module, such compounds are c a l l e d "generalized q u a l i t y c o n s t i t u e n t s " and they are simulated using module s e c t i o n GQUAL (Figure 7). The algorithms used by module s e c t i o n GQUAL are, again, based on those i n c o r p o r a t e d i n the SERATRA model. The chemical forms which i t can handle and the processes i n c l u d e d are shown s c h e m a t i c a l l y i n F i g u r e 8. In t h i s s e c t i o n of the module i t i s assumed that a l l chemicals e x i s t i n s o l u t i o n and are, thus, p o t e n t i a l l y subject to the processes shown on the l e f t side of the f i g u r e . These i n c l u d e : (1) movement with the water ( a d v e c t i o n ) . (2) h y d r o l y s i s . A f i r s t order pH-dependent equation i s used. (3) o x i d a t i o n by agents such as s i n g l e t oxygen and a l k y l p e r o x y r a d i c a l s . A second order equation i s used. (4) v o l a t i l i z a t i o n . T h i s i s l i n k e d to the oxygen r e a e r a t i o n r a t e which can be computed u s i n g a v a r i e t y of equations. (5) biodegradation. A second order equation i s used. (6) "other" methods of decay. A f i r s t order equation i s used. (7) formation of "daughter" products by decay of "parent" compounds. The user decides which of the above processes w i l l be simulated ( a c t i v e ) f o r each chemical. He need only supply input f o r those processes that are a c t i v e . In t h i s connection, note that: (1) a l l of the above decay rates can be adjusted for temperature. (2) much of the supplementary input r e q u i r e d f o r these processes (e.g. biomass c o n c e n t r a t i o n s , f r e e r a d i c a l concentration) can be s u p p l i e d e i t h e r as time s e r i e s , or as monthly c y c l i c data, or s i n g l e f i x e d v a l u e s . If the user specifies that the chemical i s sediment a s s o c i a t e d , then a l l the processes shown on the r i g h t of F i g u r e 8 a l s o become a c t i v e : (1) Adsorption and d e s o r p t i o n between the s o l u t i o n phase and sand, s i l t and c l a y i n suspension and on the bed. First order r e a c t i o n k i n e t i c s are used. (2) Transport of adsorbed m a t e r i a l with che sediment. This i n c l u d e s advection, scour and d e p o s i t i o n . (3) Decay of adsorbed chemical, modeled as a first-order process.


7.

JOHANSON

139


inflow in so/n.

Constituent on susp. sediment

inflow on clay

adsorption desorption

outflow in so/n.

On susp. clay

outflow _^ on clay

On susp. silt input from decay of "parents"

On susp. sand Deposition and scour with sediment

Decay Fluxes hydrolysis On bed sand

oxidation photolysis volatilization

On bed silt

biodegradation general (other)

Constituent on bed sediment Figure 8. i n HSPF.

adsorption desorption

Simulation of a " g e n e r a l i z e d

quality

On bed cloy

constituent"


140

F A T E OF CHEMICALS I N T H E E N V I R O N M E N T

The U t i l i t y Modules HSPF's u t i l i t y modules (Figure 2) are designed to give the user maximum f l e x i b i l i t y i n managing s i m u l a t i o n input and output. COPY i s used to manipulate time s e r i e s , such as the t r a n s f e r of data from tape to the TSS. The user can change the form of the time s e r i e s during the COPY o p e r a t i o n . A 5-minute rainfall record may be aggregated to an hourly time i n t e r v a l , f o r example. The PLTGEN and DISPLY modules are discussed later, under "Output". The GENER module i s used to transform a time s e r i e s (A) to produce a new s e r i e s (C) or to combine two time s e r i e s (A&B) to create a new one (C). For example, t h i s module i s u s e f u l i f one wants to compute the mass outflow of a c o n s t i t u e n t from the two time s e r i e s of flow and c o n c e n t r a t i o n . DURANL performs a d u r a t i o n and excursion a n a l y s i s on a time series and also computes some s t a t i s t i c s . It can answer questions l i k e : "How o f t e n does the DO concentration stay below 4 mg/1 f o r 4 consecutive hours?" This module i n c l u d e s a f e a t u r e , d e r i v e d from O n i s h i et a l ( I I ) , f o r a s s e s s i n g the e f f e c t of the l i k e l y exposure of a s p e c i f i e d species to a given chemical. It i s presumed that the organism w i l l s u f f e r no damage i f the chemical i s always present at levels below the "maximum acceptable t o x i c a n t c o n c e n t r a t i o n " (MATC). But i f t h i s l e v e l i s exceeded the organism w i l l s u f f e r e i t h e r acute or chronic damage, depending on the c o n c e n t r a t i o n and the time f o r which i t p e r s i s t s (Figure 9). For example, the b o r d e r l i n e f o r 7-day continuous exposure might be 1 ppm and the corresponding 1-day value might be 10 ppm. To perform t h i s type of a n a l y s i s using HSPF, the user s u p p l i e s module DURANL with the data necessary to compose F i g u r e 9, and the time s e r i e s of chemical concentration values. Then, i n a d d i t i o n to performing the usual s t a t i s t i c a l analyses, DURANL determines the percentage of time that acute, chronic and s u b - l e t h a l c o n d i t i o n s would e x i s t . The time s e r i e s can be e i t h e r simulated or observed data. Software Design Considerations Development of HSPF i n v o l v e d the merging of most of the c a p a b i l i t i e s of a set of e x i s t i n g models. This k i n d of p r o j e c t can be accomplished i n at l e a s t two ways. On the one hand, the e x i s t i n g software can be l e f t l a r g e l y i n t a c t ; modules can be merged using i n t e r f a c e s which r e q u i r e a minimum of new code and alterations to e x i s t i n g programs. On the other hand, the f u n c t i o n s performed by the o l d e r models can be incorporated i n a totally new package. The first approach i n v o l v e s a lower investment but the shortcomings of the e x i s t i n g models, and any i n c o n s i s t e n c i e s between them, remain. The second approach i s c o s t l y but overcomes these problems; a design can be adopted which draws on the experience gained i n working with the o l d e r models and a l s o i n c o r p o r a t e s modern program design technology. For intermediate values of shear s t r e s s , the bed i s s t a b l e .


7.

JOHANSON


141

SUBLETHAL REGION

EXPOSURE DURATION F i g u r e 9.

L e t h a l i t y a n a l y s i s of chemical

• concentration


data.

F A T E OF CHEMICALS I N T H E

142

ENVIRONMENT

In t h i s way a system can be b u i l t which i s i n t e r n a l l y c o n s i s t e n t , r e l i a b l e and r e l a t i v e l y easy to use, maintain and expand. The designers (and EPA) chose the second route. Developing, modifying, or even t r y i n g to understand, a l a r g e computer program can be a very f r u s t r a t i n g a c t i v i t y . To a l a r g e extent, the problems can be a l l e v i a t e d by using Structured Programming Technology. Because of i t s obvious suitability, extensive use was made of t h i s technology on t h i s p r o j e c t . The e n t i r e set of software was arranged i n h i e r a r c h i c a l order, shown on a " s t r u c t u r e c h a r t " (eg. Figures 3 and 7). The general idea i s that the e n t i r e system should form a t r e e , branching out from the MAIN sub-program. "Continuation f l a g s " point to subordinate s t r u c t u r e c h a r t s , so that the e n t i r e HSPF program can be viewed by studying the 80 s t r u c t u r e charts needed to completely describe it. These charts t o t a l l y supplant the t r a d i t i o n a l "flowcharts". Within each sub-program, i n s t r u c t i o n s were f i r s t coded i n "pseudo code," s i m i l a r to A l g o l . In accordance with the tenets of s t r u c t u r e d programming, as developed by D i j k s t r a and others during the 1960's, the pseudo code included only the f o l l o w i n g five basic "structure figures:" SEQUENCE, IF-THEN-ELSE, DO-UNTIL, WHILE-DO and CASE. The pseudo code was then t r a n s l a t e d to standard Fortran. The b e n e f i t s of w r i t i n g i n s t r u c t u r e d pseudo code, compared to F o r t r a n , became very obvious as the p r o j e c t progressed. The

entire

HSPF

system

is

documented

in

a

User's

Manual

(12). Operation

of the Model

Overview. Figure 10 shows, i n s i m p l i f i e d form, the a c t i v i t i e s , inputs and outputs i n v o l v e d i n running HSPF, from a t y p i c a l user's point of view. The f i r s t phase i n v o l v e s copying input time s e r i e s , such as m e t e o r o l o g i c a l data, from s e q u e n t i a l f i l e s (cards, tape, d i s c ) to the Time S e r i e s Store (TSS). This i s sometimes done i n a s i n g l e run but i n most p r a c t i c a l s i t u a t i o n s , where data have to be gathered from d i v e r s e sources and gaps have to be patched, s e v e r a l runs are made. In the second phase the input time s e r i e s f o r s i m u l a t i o n , data d i s p l a y and a n a l y s i s runs u s u a l l y come from the TSS, although they can sometimes be obtained d i r e c t l y from s e q u e n t i a l f i l e s , thus bypassing the f i r s t phase described above. The other type of input, r e q u i r e d i n a l l HSPF runs, i s c a l l e d the User's C o n t r o l Input. The User's C o n t r o l Input. The HSPF system has been made as " i n t e l l i g e n t " as p o s s i b l e . For example: (1) I t checks that u s e r - s u p p l i e d values f a l l w i t h i n a reasonable range, where p o s s i b l e . (2) I f a user omits some input, HSPF w i l l supply d e f a u l t values i f they e x i s t , or report an e r r o r i f they do not.


7.

JOHANSON


143

Meteorological Data

Time Series Input Run(s) UCI

UCI

Run Interpreter Output^

Regular Printed Summaries^

Figure 10. A c t i v i t i e s user's viewpoint.

Simulation, Data Display and Analysis Runts)

Special Summaries

Statistical Analyses

Plotter Output

involved i n running HSPF, from the


144 (3)

(4)

FATE

OF

CHEMICALS

IN

THE

ENVIRONMENT

I t w i l l ignore unnecessary input and blank or "comment" lines. Thus, i f a user has been simulating p e s t i c i d e s and then turns that s e c t i o n o f f , p o s s i b l y to r e - c a l i b r a t e the hydrology, he does not have to delete the p e s t i c i d e - r e l a t e d input. HSPF w i l l ignore i t , u n t i l he once again turns the PEST s e c t i o n on. I t can accept input i n M e t r i c or E n g l i s h u n i t s , (e.g. p e s t i c i d e a p p l i c a t i o n i n kg/ha or l b / a c r e , r a i n f a l l and runoff i n mm or i n c h e s ) .

Output. HSPF produces s e v e r a l c l a s s e s of output: (1) Continuous time s e r i e s . These data are e i t h e r passed as input to operations f u r t h e r "downstream" i n the network or they are recorded on disk i n the Time Series Store, or both things may be done (Figure 10). (2) Run I n t e r p r e t e r output. This i s produced as the User's C o n t r o l Input i s scanned and checked, d e f a u l t s are s u p p l i e d , etc. I t i s , roughly, an echo of the input, plus d e f a u l t values supplied by HSPF. (3) Regular p r i n t e d summaries. Once the simulation time loop i s commenced, data are accumulated f o r d i s p l a y at an i n t e r v a l s p e c i f i e d by the user. The frequency of t h i s output can be v a r i e d from once per time step (say 1 hour) to once per year. Regardless of the r e p o r t i n g period, the format of the report i s the same. F i r s t , the values of a l l s i g n i f i c a n t s t a t e v a r i a b l e s (eg. storages), at the end of the r e p o r t i n g period, are given. Then, the fluxes (eg. flows), accumulated since the preceding r e p o r t , are summarized. The user can s p e c i f y whether p r i n t o u t i s to be given i n E n g l i s h or Metric u n i t s (regardless of the u n i t s used f o r i n p u t ) . Or he may request output i n both systems. (4) S p e c i a l summaries. By using module DISPLY, the user may s e l e c t any time s e r i e s f o r s p e c i a l d i s p l a y . For example, he may wish to p r i n t out d a i l y average values of the t o t a l amount of A l a c h l o r i n the Upper Layer. In t h i s case, the values (simulated or observed) would automatically be averaged over each day and a year's worth of d a i l y values would appear i n a n e a t l y formatted table on a s i n g l e page (suitable for direct inclusion i n a report). (5) S t a t i s t i c a l analyses. Again, any time s e r i e s can be analyzed, by p o i n t i n g i t to module DURANL. The functions of t h i s module have already been described i n some d e t a i l . (6) P l o t data. The PLTGEN module can be used to route a set of time s e r i e s to a f i l e , so that they can l a t e r be d i s p l a y e d i n g r a p h i c a l form, e i t h e r together or on separate graphs. In t h i s connection, Figures 11 and 12 show t y p i c a l p e s t i c i d e simulation r e s u l t s f o r a small watershed i n Iowa, as reported by Donigian et a l (13).


7.

JOHANSON

O O


2.5 -

Total \

T

(75 o -E o o o CO

145

Simulated

—

Observed

I

Upper Layer

1.5-

Simulated

—

Observed

J

0.5-

15

-i

25

1

5

May

1 15

1

1 25

June

1976

F i g u r e 11. V e r i f i c a t i o n o f s i m u l a t e d s o i l s t o r a g e o f a l a c h l o r i n F o u r M i l e C r e e k w a t e r s h e d , Iowa. ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 13.)

O

Observed

:

Note trib. area JSC

2.5-

2

50.5(km)

-Simulated

a o ^

1.5

o o TD

0.5

o 15

25

25

May

1976 F i g u r e 12. V e r i f i c a t i o n of simulated flow of alachlor i n F o u r M i l e C r e e k a t T r a e r , Iowa. ( R e p r o d u c e d w i t h p e r m i s s i o n f r o m R e f . 13.)


146

F A T E OF CHEMICALS I N T H E

ENVIRONMENT

Concluding Comments 1

Is HSPF l i v i n g up to the d e s i g n e r s expectations? The author's answer i s i n the a f f i r m a t i v e , f o r the f o l l o w i n g reasons: (1) As p r e d i c t e d , programming e r r o r s have been q u i t e r a r e and easy to l o c a t e and f i x . (2) Users who have studied the code have found i t easy to f o l l o w - I b e l i e v e t h i s i s an important t e s t of program c l a r i t y and consistency. (3) Operating modules and i n d i v i d u a l subprograms have been added or modified with r e l a t i v e ease. (4) Adaptation of HSPF to computers with widely d i f f e r i n g memory c o n f i g u r a t i o n s and word lengths has been r e l a t i v e l y simple. (5) The program has been used i n a v a r i e t y of s i t u a t i o n s ranging from simple s i n g l e land-segment simulations to studies i n v o l v i n g complex networks. The c o n s t i t u e n t s handled have ranged from water (only) to p e s t i c i d e s , n u t r i e n t s and certain biota. This demonstrates the v e r s a t i l i t y of the program. In summary, HSPF i s an advanced software framework which can accommodate a wide v a r i e t y of time series-based simulation modules and a s s o c i a t e d u t i l i t y f u n c t i o n s . As b e t t e r algorithms f o r simulating the various n a t u r a l processes become a v a i l a b l e , they can be incorporated i n t o the system. Plans are already under way to i n c l u d e i n the PERLND module an improved technique f o r s o l v i n g the equations governing the advection and r e a c t i o n s of p e s t i c i d e s and n u t r i e n t s . The c o n s t r u c t i o n of an i n t e r a c t i v e input preparer, c u r r e n t l y under way, i s being g r e a t l y f a c i l i t a t e d by the h i g h l y s t r u c t u r e d design of the User's C o n t r o l Input. T h i s new software w i l l make the package even e a s i e r to use. HSPF can grow with the s t a t e of the a r t and should have a long and useful l i f e . I t i s being maintained, d i s t r i b u t e d and a c t i v e l y supported by the EPA Center f o r Water Q u a l i t y Modeling, i n Athens, Georgia. Literature Cited 1.

2.

3.

4.

Crawford, N.H., and R.K. Linsley. D i g i t a l Simulation in Hydrology: Stanford Watershed Model IV. Dept. of C i v . Eng., Stanford Univ., Stanford, C a l i f . Tech. Rep.39. 1966. 210 pages. Donigian, A.S., J r . , D.C. B e y e r l e i n , H.H. Davis, J r . , and N.H. Crawford. A g r i c u l t u r a l Runoff Management (ARM) Model V e r s i o n I I : Refinement and T e s t i n g . Env. Res. Lab., Athens, Georgia. 1977. EPA600/3-77-098. 294 pages. Donigian, A.S. J r . , and N.H. Crawford. Modeling Nonpoint P o l l u t i o n from the Land Surface. Env. Res. Lab., Athens, Georgia. 1976. EPA 600/3-76-083. 280 pages. Hydrocomp Inc. Hydrocomp Water Q u a l i t y Operations Manual.


7.

5.

6.

7.

8.

9. 10. 11.

12.

13.

JOHANSON

A New

Mathematical Modeling System

147

Hydrocomp Inc., 201 San Antonio C i r c l e , Mountain View, CA 94040. 1977. 192 pages. O n i s h i , Y,. and S.E. Wise. Mathematical Model, SERATRA, f o r Sediment-Contaminant Transport i n Rivers and its A p p l i c a t i o n to P e s t i c i d e Transport i n Four M i l e and Wolf Creeks in Iowa. B a t e l l e P a c i f i c NW Labs, R i c h l a n d , WA. 1979. U.S. Army Corps of Engineers. Snow Hydrology, Summary Report of the Snow Investigations. North Pacific Division, P o r t l a n d , OR. 1956. 437 pages. Negev, M. A Sediment Model on a Digital Computer. Dept. of C i v . Eng., Stanford Univ., Stanford, CA. Tech. Rep.76. 1967. 109 pages. Crawford, N.H. and A.S. Donigian, J r . P e s t i c i d e Transport and Runoff Model f o r A g r i c u l t u r a l Lands. O f f i c e of R and D, U.S. EPA, Wash. DC, 1973. EPA 660/2-74-013. 211 pages. Colby, B.R. P r a c t i c a l Computation of Bed-Material Discharge. J . Hyd. Div., ASCE. 1964, 90(HY2),217-246. Toffaleti, F.B. D e f i n i t i v e Computations of Sand Discharge in R i v e r s . J . Hyd. Div., ASCE. 1969, 95(HY1), 225-248. O n i s h i , Y., S.M. Brown, A.R. Olsen, M.A. Parkhurst, S.E. Wise and W.H. Walters. Methodology f o r Overland and Instream M i g r a t i o n and Risk Assessment of P e s t i c i d e s . B a t e l l e P a c i f i c NW Labs., Richland, WA. Prepared f o r U.S. EPA, Athens, GA. 1979. Johanson, R.C, J.C. Imhoff and H.H. Davis, Jr. Users Manual f o r H y d r o l o g i c a l Simulation Program - F o r t r a n (HSPF). Env. Res. Lab., Ath ens, GA. 1980. EPA 600/9-80-015. 678 pages. Donigian, A.S., J r . , J.C. Imhoff and B.R. Bricknell. Modeling Water Q u a l i t y and the E f f e c t s of Best Management P r a c t i c e s i n Four M i l e Creek, Iowa. D r a f t report on c o n t r a c t 68-03-2895, f o r U.S. EPA, Athens, GA. 1981.

R E C E I V E D April 15, 1983.

American Chemical Society Library '1155 16th St. N. W. Washington, D. C. 20036 Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Fate of Chemicals in the Environment - American Chemical Society

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