Modeling Agricultural-Environmental Processes in Crop Production

IIASA COLLABORATIVE PROCEEDINGS SERIES

MODELING AGRICULTURAL- ENVIRONMENTAL PROCESSES

IN CROP PRODUCTION

IIASA COLLABORATIVE PROCEEDINGS SERIES

CP-81 -S1 LARGE-SCALE LINEAR PROGRAMMING Proceedings of an I IASA Workshop, 2-6 June 1980 G.B. Dantzig, M.A.H. Dempster, and M.J. Kallio, Editors CP-81 -S2 THE SHINKANSEN PROGRAM : Transportation, Railway,

Environmental, Regional, and National Development Issues A. Straszak, Editor

CP-82-S1 HUMAN SETTLEMENT SYSTEMS: Spatial Patterns and Trends Selected Papers from an IlASA Conference on the Analysis of Human Settlement Systems

T. Kawashima and P. Korcelli, Editors CP-82-S2 RISK: A Seminar Series

H. Kunreuther. Editor

CP-82-S3 THE OPERATION OF MULTIPLE RESERVOIR SYSTEMS Proceedings of an International Workshop, Jodlowy Dwor, Poland, 28 May-1 June 1979

2 . Kaczmarek and J. Kindler, Editors

CP-82-S4 NONPOINT NITRATE POLLUTION OF MUNICIPAL WATER SUPPLY SOURCES: Issues of Analysis and Control

Proceedings of an 1 IASA Task Force Meeting, 10-12 February 1981 K.-H. Zwirnmann, Editor

C P - 8 2 4 5 MODELING AGRICULTURAL-ENVIRONMENTAL PROCESSES IN CROP PRODUCTION

Proceedings of an IIASA Task Force Meeting, 2-4 June 1980 G. Golubev and I. Shvytov, Editors

MODELING AGRICULTURAL- ENVIRONMENTAL PROCESSES

IN CROP PRODUCTION

Proceedings of a 2 - 4 June 1980 Task Force Meeting

G. Golubev and I. Shvytov, Editors

INTERNATIONAL INSTITUTE FOR APPLIED SYSTEMS ANALYSIS Laxenburg, Austria

1982

lnternational Standard Book Number 3-7045-0033-X

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The views or opinions expressed in this volume do not necessarily represent those of the lnstitute or the National Member Organizations that support it.

Copyright @ 1982 lnternational lnstitute for Applied Systems Analysis A-2361 Laxenburg, Austria

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher.

PREFACE

The International Institute for Applied Systems Analysis began its work on modeling the environmental impacts of crop pro- duction in

1 9 7 8 .

The objective was to clarify what was known about applying mathematical models to the assessment of the envi- ronmental impacts of crop production, and the focus was on the environmental impacts of dry farming. The most important field- scale environmental effects of dry farming--which can potentially lead to such large-scale environmental impacts as eutrophication, water pollution, and cropland losses--were identified as soil erosion, nitrogen leaching, and phosphorus and pesticide losses.

The work in this field was begun by considering the hydro- logical and major natural biogeochemical processes, which, through a chain of events, cause these environmental effects. It became apparent that there are many mathematical models describing single processes such as water percolation, runoff, nitrogen mineraliza- tion, nitrification, denitrification, phosphorus precipitation and adsorption, evapotranspiration, nutrient uptake, pesticide degradation, etc. Moreover, a few complex models (CREAMS,

ARY,

ACTMO, etc.) have been developed. One of these complex models, CREAMS, was transferred to IIASA and used in a number of the In- stitute's National Member Organization countries.

Our experience in collecting and using various mathematical models convinced us not only of the necessity of refining collab- orative efforts in this field, but also of the need to discuss some methodological questions. We pursued these matters at two meetings: a planning workshop in June

1 9 7 8

and an April

1 9 7 9

conference on environmental management of agricultural watersheds.

A third meeting on modeling agricultural-environmental processes

in crop production--which is reported in these proceedings--focused

on:

d i s c u s s i o n s o f t h e s t a t e o f t h e a r t o f d e v e l o p i n g mathe- m a t i c a l models f o r e n v i r o n m e n t a l p r o c e s s e s i n c r o p p r o - d u c t i o n ;

i m p r o v i n g t h e g u i d e l i n e s f o r c o m p l e t i n g t h e IIASA r e s e a r c h o n m a t h e m a t i c a l modeling o f t h e e n v i r o n m e n t a l e f f e c t s o f a g r i c u l t u r e ; a n d

r e f i n i n g t h e I n s t i t u t e ' s c o l l a b o r a t i v e work w i t h o t h e r o r g a n i z a t i o n s .

T h i s volume p r e s e n t s t h e p a p e r s p r e s e n t e d a t t h i s t h i r d meet- i n g i n t h e form i n which t h e y w e r e r e c e i v e d from t h e i r a u t h o r s . The p a p e r on CREAMS i s m i s s i n g , a s i t w i l l be t r e a t e d i n d e t a i l e l s e w h e r e .

The volume c l o s e s w i t h a s h o r t r e v i e w o f t h e main p o i n t s b r o u g h t o u t a t t h e m e e t i n g .

The e d i t o r s would l i k e t o e x p r e s s t h e i r t h a n k s t o a l l t h o s e who c o n t r i b u t e d t o t h i s Task F o r c e M e e t i n g , w h e t h e r by f o r m a l p r e s e n t a t i o n s , o r t h r o u g h p a r t i c i p a t i o n i n t h e d i s c u s s i o n s . The e d i t o r s a r e g r a t e f u l t o t h e c h a i r m e n and r a p p o r t e u r s o f t h e ses- s i o n s (see A p p e n d i x ) , whose r e p o r t s w e r e u s e d t o p r e p a r e t h e f i n a l p a p e r . W e would a l s o l i k e t o e x p r e s s o u r a p p r e c i a t i o n t o Pamela H o t t e n s t e i n and C a r o l i n e G o o d c h i l d f o r t h e i r t e c h n i c a l and o r g a n i z a t i o n a l h e l p .

GENADY N . GOLUBEV I G O R A . SHVYTOV E d i t o r s

CONTENTS

The Modelling of Environmental Impacts of Crop Production

Douglas A . H a i t h

A

Field-Scale Model for Nonpoint Source Pollution Evaluation

K . C. K n i s e l

Review of Simulation Models for Nitrogen Behaviour in Soil in Relation to Plant Uptake and Emission

J o h a n n e s A . v a n V e e n and M a r t i n J . F r i s s e l

A

Critical Evaluation of a Hydrological Layer Model for Forecasting the Redistribution of Unadsorbed .Anions in Cultivated Soils

I a n C. Burns

Modelling Nitrate Movement in Profiles That Contain Soil, Heavy Clay and Chalk

Thomas M . A d d i s c o t t

Retention, Transformations, and Transport of Pesticides in Soil-Water Systems: Model Development and Evaluation P.S.C.

Rao and R . Z . J e s s u p

State of Art of Modeling of the Water Balance Processes in the Agricultural Field and Watershed

J a r o s i a v a a l e k

Model of Surface Runoff From Slope

M i l o s Holy

v i i

A Hierarchical Approach to Agricultural Production Modeling

H . v a n K e u l e n and C.T. d e W i t

Deterministic Models for the Ecologic Simulation of Crop Agricultural Environments

T . C l a r k L y o n s

The Mathematical Model for the Determination of the Optimal Crop Production Structures as Affected by Agroecslogical Conditions

Z s o l t Harnos

Statistical Evaluation of Experts' Estimates

I s t v a n V a l y i

Relations between the Agro-Ecological Potential and Soil

214 K . R a j k a i

A Framework for the Study of the Dynamics of Agricultural

227

Systems

B . R. T r e n b a t h

Overview of the Ideeting

Cenady C o l u b e v and I g o r S h v y t o v

APPENDIXES

Appendix A: Agenda of the Task Force Meeting Appendix B: List of Participants

Appendix C: List of Chairmen and Rapporteurs

THE MODELLING OF

ENVIRONMENTAL IMPACTS OF CROP PRODUCTION Douglas A . H a i t h

Department o f A g r i c u l t u r a l E n g i n e e r i n g Cornel 1 U n i v e r s i t y

I t h a c a , N . Y . USA 14853

INTRODUCTION

A g r i c u l t u r a l management p o l i c i e s b a s e d on i n t e n s i v e u s e o f l a n d , w a t e r and c h e m i c a l s have g r e a t l y i n c r e a s e d t h e e f f i c i e n c y o f c r o p pro- d u c t i o n i n t h e t w e n t i e t h c e n t u r y . The p o l i c i e s h a v e a l s o produced d i s - t r i b u t i o n s o f chemical r e s i d u a l s i n t h e environment which may be h a z a r d o u s t o human h e a l t h and n a t u r a l ecosystems. The e n v i r o n m e n t a l p o l l u t i o n problems a s s o c i a t e d w i t h a g r i c u l t u r a l p r o d u c t i o n a r e e x t r e m e l y d i f f i c u l t t o r e s o l v e . The e f f e c t s o f a g r i c u l t u r a l p r a c t i c e s on chemical l o s s e s from c r o p l a n d and t h e u l t i m a t e f a t e o f c h e m i c a l s once t h e y l e a v e c r o p - l a n d a r e p o o r l y u n d e r s t o o d . Even i f t h i s were n o t t h e c a s e , t h e e f f i - c i e n c y o f food and f i b e r p r o d u c t i o n i s s o c r i t i c a l t o t h e w o r l d ' s economy t h a t p o l i c y makers a r e r e l u c t a n t t o impose p o l l u t i o n c o n t r o l p r a c t i c e s which may lower p r o d u c t i o n l e v e l s . The e n v i r o n m e n t a l i m p a c t s o f c r o p p r o d u c t i o n c a n b e managed r a t i o n a l l y o n l y i f two c r i t i c a l i n f o r m a t i o n needs a r e met. F i r s t we must b e a b l e t o q u a n t i t a t i v e l y a s s e s s t h e e n v i r o n - mental damages a s s o c i a t e d with c r o p p r o d u c t i o n p r a c t i c e s . Second, t h e l i k e l y e f f e c t s o f p o l l u t i o n c o n t r o l p r a c t i c e s on c r o p p r o d u c t i o n l e v e l s and farm income must be d e t e r m i n e d and p r a c t i c e s i d e n t i f i e d which w i l l have minimal n e g a t i v e impact on food p r o d u c t i o n . Perhaps t h e most c r i t i c a l c h a l l e n g e f a c i n g a g r i c u l t u r a l and e n v i r o n m e n t a l s c i e n t i s t s

i n t h e r e m a i n d e r o f t h i s c e n t u r y i s t o p r o v i d e t h e i n f o r m a t i o n needed

t o d e v e l o p a g r i c u l t u r a l management p o l i c i e s which f e e d t h e hungry w i t h o u t p o i s o n i n g t h e poor (and t h e a f f l u e n t ) .

Mathematical models have become n e c e s s a r y t o o l s f o r t h e s t u d y o f a g r i c u l t u r a l pollution, m a i n l y b e c a u s e p a s t e m p i r i c a l e x p e r i e n c e h a s p r o v i d e d l i t t l e o f t h e q u a n t i t a t i v e i n f o r m a t i o n n e e d e d . A v a r i e t y o f m o d e l l i n g a p p r o a c h e s have e v o l v e d i n t h e l a s t t e n y e a r s , and t h i s p a p e r i s a b r i e f a t t e m p t t o c a t e g o r i z e models r e l a t e d t o w a t e r q u a l i t y and t o g i v e some examples o f models which h a v e been d e v e l o p e d a t C o r n e l l U n i v e r s i t y o v e r t h e p a s t s e v e r a l y e a r s .

CATEGORIES OF MODELS FOR ANALYZING AGRICULTllRAL NONPOINT SOURCE POLLUTION Water q u a l i t y problems c a u s e d by c r o p p r o d u c t i o n a r e t y p i c a l l y a s s o c i a t e d w i t h n o n p o i n t s o u r c e p o l l u t i o n , which i s t h e c o n t a m i n a t i o n o f w a t e r b o d i e s by c h e m i c a l s and sediment c o n t a i n e d i n d i f f u s e r u n o f f and p e r c o l a t i o n w a t e r f l o w s from l a n d s u r f a c e s . P r o v i s i o n o f t h e i n f o r m a t i o n d i s c u s s e d i n t h e p r e v i o u s s e c t i o n r e q u i r e s e s t i m a t e s o f p o l l u t a n t l o s s e s o r l o a d i n g s from c r o p l a n d t o w a t e r b o d i e s , a s s e s s m e n t o f w a t e r q u a l i t y i m p a c t s o f p o l l u t a n t l o a d i n g s , and d e t e r m i n a t i o n o f economic e f f e c t s o f c o n t r o l p r a c t i c e s . The a u t h o r i s f a m i l i a r w i t h o n l y o n e model which a t t e m p t s t o p r o v i d e a l l o f t h i s i n f o r m a t i o n (Wineman,

e t g . ,

1 9 7 9 ) . More t y p i c a l l y models a r e d e s i g n e d f o r o n l y one o f t h e t h r e e t y p e s o f a n a l y s e s and c a n be c l a s s i f i e d a s

1 . Chemical and Sediment Loading Models

2 . Water Qua1 i t y Impact Models

3 . P l a n n l n g and Management Models

Water q u a l i t y models a r e n o t u n i q u e t o n o n p o i n t s o u r c e s s i n c e t h e y a r e i n g e n e r a l d e s i g n e d t o p r e d i c t t h e r e s p o n s e of a w a t e r body t o b o t h

point and nonpoint sources. The l i t e r a t u r e c o n t a i n s hundreds of examples of such models and t h e y a r e omitted from t h i s d i s c u s s i o n . Sediment load- ing models a r e a l s o o m i t t e d , p a r t l y i n t h e i n t e r e s t o f b r e v i t y , but a l s o a s a r e f l e c t i o n o f t h e f a c t t h a t sediment p e r s e i s seldom a c r i t i c a l o r manageable water q u a l i t y problem. Rather, sediment i s important mainly a s a c a r r i e r of chemicals, and sediment loading models a r e i n t e g r a l com- ponents o f many chemical loading models.

Chemical Loading Models

Chemical loading models have been c o n s t r u c t e d t o p r e d i c t t h e f o l - lowing l o s s e s from croplands: d i s s o l v e d and solid-phase n u t r i e n t s , s a l t s , and p e s t i c i d e s i n r u n o f f , and d i s s o l v e d n u t r i e n t s , s a l t s and p e s t i - c i d e s i n p e r c o l a t i o n o r watershed base flows. The models a r e developed f o r e i t h e r f i e l d o r watershed s c a l e and f a l l i n t o t h r e e d i s t i n c t groups.

Continuous simulation models a r e t h e most a n a l y t i c a l models and a r e based on systems o f d i f f e r e n t i a l e q u a t i o n s f o r s o l u t e movement.

E s s e n t i a l l y a l l o f t h e models apply t o groundwater problems, and most focus on n i t r a t e o r phosphate movement. Examples a r e given by Davidson e t a l . (1978). Czyzewski

g.

(1980), van Veen (1977) and Shah

g.

- -

(1975). Continuous simulation models r e q u i r e c a l i b r a t i o n and have seen l i m i t e d f i e l d t e s t i n g .

D i s c r e t e simulation models s o l v e chemical t r a n s p o r t problems by r e p e t i t i v e mass balance c a l c u l a t i o n s f o r d i s c r e t e time s t e p s and a r e g e n e r a l l y more o p e r a t i o n a l than t h e continuous models. D i s c r e t e simula- t i o n models a r e o f t e n based on previously developed hydrologic and s e d i - ment t r a n s p o r t models. Examples include models f o r n i t r o g e n i n p e r c o l a t i o n

by A d d i s c o t t (1977) and Saxton

st.

(1977), watershed models f o r n u t r i e n t s by Williams and Hann (1978) and Tseng (1979), and f i e l d - s c a l e models f o r n u t r i e n t s and p e s t i c i d e s developed by Donigian

st.

(1977) and Knisel

5 &.

(1979).

A f i n a l group o f models a r e f u n c t i o n a l models which do n o t a t t e m p t t o s i m u l a t e t h e fundamental p r o c e s s e s which a f f e c t chemical l o s s e s . Rather t h e y a r e s i m p l e ' p r e d i c t i v e e q u a t i o n s , o f t e n e m p i r i c a l , which can p r o v i d e rough e s t i m a t e s o f t h e q u a n t i t i e s o f chemical l o s s e s . F u n c t i o n a l models a r e d e s i g n e d t o p r o v i d e i n f o r m a t i o n r a p i d l y w i t h r e l a t i v e l y l i t t l e d a t a i n p u t . Examples a r e t h e n i t r a t e l e a c h i n g model o f Burns (1974, 1975) and t h e g e n e r a l " l o a d i n g f u n c t i o n s " proposed by

McElroy

5 s.

^(1976).

Planning and Management Models

Planning and management models a r e i n p r i n c i p l e t h e most u s e f u l models f o r p o l i c y making s i n c e t h e y d e t e r m i n e economic impacts o f

p o t e n t i a l p o l l u t i o n c o n t r o l p r a c t i c e s . In t h e o r y , t h e models can p r o v i d e e s t i m a t e s o f t r a d e - o f f s between a g r i c u l t u r a l p r o d u c t i o n and environmental q u a l i t y o b j e c t i v e s . However, t h e economic components o f t h e models a r e much b e t t e r developed t h a n components f o r p r e d i c t i o n o f p o l l u t i o n , which a r e commonly l i m i t e d t o sediment l o s s e s e s t i m a t e d by t h e U n i v e r s a l S o i l Loss Equation. A l l p l a n n i n g and management models a r e based on budget- t i n g approaches and a r e u s u a l l y s o l v e d by l i n e a r programming.

Three d i f f e r e n t s c a l e s o f models a r e a p p a r e n t . Regional impact models a r e used f o r macro-scale s t u d i e s o f farm and consumer income

[Heady and Vocke, 1979; T a y l o r and Frohberg, 1977). Watershed p l a n n i n g

models such as those of Onishi and Swanson (1974). Casler and Jacobs (1975) and Scherer (1977) are applied to specific water quality problems and evaluate impacts of management practices, subsidies and taxes on pollution and farm income.

Farm

management models estimate the effects of pollution control on the activities of individual farmers. Examples are given by Smith

s g .

(1979), Coote

&s.

(1976) and Miller and Gill (1976).

EXAMPLES

The remainder of this paper is a description of four operational models which have been developed at Cornell University for the analysis of agricultural nonpoint source pollution. Using the terminology of the previous section, three of the models are chemical loading models, including two discrete simulation models and one functional model. The fourth model is a farm management model. The purpose of the examples is to illustrate some general characteristics of models used to evaluate environmental impacts of crop production and also to provide a progress report on a modelling research program which the author has been involved in for several years. The discussion is limited to the general structures of the models and some results of their applications. Mathematical details are provided in the cited references.

Watershed Loading Functions

The estimation of pollutant export in streamflow from large agri- cultural watersheds is difficult. The basic modelling problem is how to recognize the great spatial variability of a watershed's land surface without resorting to a model which is so complex that data and computer

r e q u i r e m e n t s r e n d e r i t i m p r a c t i c a l . The approach used i n t h e p r e s e n t c a s e was t h e a p p l i c a t i o n o f a f u n c t i o n a l model t o each of a w a t e r s h e d ' s s p a t i a l u n i t s and t h e n a g g r e g a t i n g r e s u l t s from a l l u n i t s i n t h e water- shed. The model i s d e s c r i b e d i n H a i t h and Tubbs (1980). E a r l i e r v e r - s i o n s a r e i n H a i t h and Tubbs (1979) and H a i t h and Dougherty (1976).

The s t r u c t u r e o f t h e l o a d i n g f u n c t i o n s i s shown i n F i g u r e 1 . Unit s o u r c e a r e a s o r f i e l d s which a r e homogeneous w i t h r e s p e c t t o s o i l s and c r o p s a r e i d e n t i f i e d u s i n g a random sampling p r o c e d u r e . S e p a r a t e e s t i - mates a r e made f o r d i s s o l v e d and s o l i d - p h a s e l o s s e s o f t h e chemical o f

i n t e r e s t . Dissolved chemicals a r e c a r r i e d i n r u n o f f a s p r e d i c t e d by t h e U.S. S o i l C o n s e r v a t i o n S e r v i c e ' s Curve Number Equation (CNE). Snowmelt r u n o f f i s based on a degree-day melt e q u a t i o n and t h e CNE. S o l i d - p h a s e chemicals move w i t h sediment l o s s e s a s p r e d i c t e d by t h e U n i v e r s a l S o i l Loss Equation (USLE). Both t h e CNE and USLE a r e a p p l i e d on an e v e n t b a s i s and hence p r e d i c t i o n s can be made f o r any t i m e p e r i o d o f i n t e r e s t . S o l i d - p h a s e chemical c o n c e n t r a t i o n s a r e based on c o n c e n t r a t i o n s i n t h e s o i l and d i s s o l v e d c o n c e n t r a t i o n s a r e e x t r a p o l a t e d from f i e l d e x p e r i - mental s t u d i e s . C o n c e n t r a t i o n s a r e m u l t i p l i e d by r u n o f f o r s o i l l o s s t o produce e d g e - o f - f i c l d chemical l o a d i n g s . These l o a d i n g s a r e con- v e r t e d t o watershed e x p o r t by m u l t i p l i c a t i o n by t r a n s p o r t and a t t e n u a t i o n

f a c t o r s . The f a c t o r s a r e assumed t o be e q u a l t o one f o r d i s s o l v e d l o a d i n g s S o l i d - p h a s e a t t e n u a t i o n f a c t o r s a r e g i v e n by t h e w a t e r s h e d ' s sediment d e l i v e r y r a t i o .

The l o a d i n g f u n c t i o n s were used t o e s t i m a t e n i t r o g e n ( N ) and phos- phorus ( P ) l o s s e s from t h e 391 km2 Pequea Creek watershed i n P e n n s y l v a n i a . Water q u a l i t y sampling d a t a provided measurements o f d i s s o l v e d and

LOADINGS FROM OTHER UNIT SOURCE AREAS

-

~. -.. TRANSPORT AND CONCENTRATIONS ATTENUATION

\ J

UNlT SOURCE AREA Crop --I'

1

man EDGE -OF-FIELD DELIVERY TO POLLUTANT LOSSES WATERSHED

WATERSHED EXPORT weal her rolld- phase pollutants LOADINGS FROM OTHER UNIT SOURCE AREAS Figure 1. General Methodology for Watershed Loading Functions

s o l i d - p h a s e N and t o t a l P i n r u n o f f from t h e watershed f o r a 16-mo p e r i o d i n 1977 and 1978. P r e d i c t i o n s a r e compared w i t h o b s e r v a t i o n s i n T a b l e 1 . Nitrogen p r e d i c t i o n s were r e l a t i v e l y a c c u r a t e , b u t t o t a l P p r e d i c t i o n s ( d i s s o l v e d & s o l i d - p h a s e ) were l e s s s o . S i n c e d a t a was n o t a v a i l a b l e t o t e s t t h e model's p r e d i c t i v e a b i l i t y f o r t h e d i s s o l v e d and s o l i d - p h a s e P f r a c t i o n s , f u r t h e r t e s t i n g w i l l be n e c e s s a r y .

P e s t i c i d e Runoff Model

T h i s model s i m u l a t e s t h e b e h a v i o r o f p e s t i c i d e s i n t h e s o i l and e s t i m a t e s d i s s o l v e d and s o l i d - p h a s e l o s s e s i n r u n o f f . The model i s d e s c r i b e d i n H a i t h and Tubbs (1980) and H a i t h (1980).

The g e n e r a l components o f t h e model a r e shown i n F i g u r e 2. Runoff l o s s e s a r e based on t h e t o t a l p e s t i c i d e i n t h e s u r f a c e c e n t i m e t e r o f s o i l . T h i s q u a n t i t y i s assumed t o decay e x p o n e n t i a l l y w i t h t i m e and when a p r e c i p i t a t i o n event o c c u r s , i s p a r t i t i o n e d i n t o adsorbed and d i s s o l v e d c o n s t i t u e n t s based on a s i n g l e p a r a m e t e r l i n e a r a d s o r p t i o n i s o t h e r m . Runoff l o s s e s a r e p r e d i c t e d by t h e CNE and s o i l l o s s e s a r e determined by a n e v e n t - b a s e d v e r s i o n o f t h e USLE. The model was t e s t e d u s i n g d a t a f o r a t r a z i n e l o s s e s from two small catchments (PZ and P4) i n W a t k i n s v i l l e , Geoergia. P r e d i c t i o n s a r e compared w i t h o b s e r v a t i o n s f o r 17 major p r e c i p i t a t i o n e v e n t s i n 1973-1975 i n T a b l e 2. The model's a c c u r a c y c l e a r l y v a r i e s among storms b u t t h e magnitudes o f measured and p r e d i c t e d t o t a l l o s s e s f o r t h e t h r e e - y e a r p e r i o d compare f a v o r a b l y . C o r r e l a t i o n c o e f f i c i e n t s between measured and p r e d i c t e d a t r a z i n e l o s s e s f o r t h e 17 e v e n t s a r e 0 . 9 5 , 0.92 and 0.94 f o r s o l i d - p h a s e , d i s s o l v e d and t o t a l m n o f f l o s s e s .

Predicted by Loading Functions

Measured i n Direct Runoff

Dissolved Solid-Phase Total

Nitrogen Nitrogen Phosphorus

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^{(103 kg)}

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Table 1 . Comparison o f Predicted and Observed Nutrient Export from Pequea Creek, Feb., 1977

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May, 1978.

PESTICIDE I SURFACE '(50 1 L \ \ \ I RUNOFF - ERODED, SOIL

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/v

I

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TIME \

t

PERCOLATION FIGURE

2.

COMPONENTS OF PESTICIDE RUNOFF MODEL