DYNAMICS OF FOOD POLICY IN A CENTRALLY-PLANNED ECONOMY:
The Case of Vietnam
N. L. Bach, K. Saeed, J. E. Lukens
Asian Institute of Technology
Bangkok, Thailand
ABSTRACT
This paper attempts to assess the impact of past and presently contemplated policies to maintain
food self-sufficiency in a centrally-planned economy. The case of Vietnam is used as an illustration.
Experimentation with a system dynamics model of the food production system incorporating
telationships concerning soil ecology and agricultural land management policy serves as a basis
for this assessment. Short-run policies to increase production are detrimental to maintaining food
self-sufficiency in the long-run. A sustainable food production policy must incorporate soil con-
servation and improvement, control of population and, possibly, finding food sources alternative
to grain. Although difficult to implement in a market system, such a policy agenda may be feasible
in a centrally-planned economy.
KEY WORDS: Food Policy, Agricultural System, Centrally-Planned Economies, Soil Ecology,
Population, Agricultural Land Use.
1. INTRODUCTION
Excessive use of land resources has been known to depreciate soil quality. Soil degradation has
occurred in many countries due to erosion, loss of nutrients, loss of texture, water logging and
salinity, which have usually been caused by intensive land use (Bowonder 1981).
Centrally-planned as well as market-economy countries encounter these problems. Chandler (1987)
observes that the environmental failures of centrally-planned nations rank alongside those of free
market economies. The former failed to internalize environmental costs because incentives were
provided to managers to boost production. Moreover, the resources allocated to the managers
teflected no scarcity value, no opportunity cost, no real price; the cost of using the resource was
essentially irrelevant. The absence of prices and competition in the planned economies led to
inefficiency as well as widespread environmental abuse.
With only occasional exceptions, assessments and statements made on environmental degradation
have little effect on policy analysis and decision making. The first reason for this is a compart-
mentalized treatment of a complex system, where feedback between the effect of one subsystem
on the other is not recognized. The second reason is that planning decisions rarely take into account
the cost externalized to the environment, in terms of depleted resources and deteriorated soil con-
ditions, for preparing development agendas (Brown and Wolf 1986). Indeed, the effect of defor-
estation, soil degradation, and cropland abandonment are generally omitted from agricultural
development strategy.
61
2 System Dynamics '90
This paper attempts to assess the impact of the past and presently contemplated policies to maintain
food self-sufficiency in Vietnam, and also illustrates the case of centrally-planned economies. This
assessment is based on experimentation with a system dynamics model of the food production
system that incorporates three subsystems: population, food production, and soil ecology. It is
demonstrated that short-run policies to increase food production to a state-planned level are det-
rimental to maintain food adequacy in the long-run. Designing a sustainable food production policy
must take into account population control, soil conservation and improvement, and in the case of
Vietnam, also finding alternatives to grain.
2. FOOD PRODUCTION SYSTEM IN VIETNAM: An overview
Vietnam is the rugged eastern part of the Indochinese peninsula. Leaning back against the Asian
continent, it looks out on the East China Sea and the Pacific. It is situated entirely in the intertropical
geographical belt, in the center of Southeast Asia.
Vietnam covers an area of 329,600 km’, of which three-fourths are composed of mountains and
plateaus, and only 17 per cent is arable land under cultivated crops. Food crops are mainly planted
in the Red River delta in the North, and the Mekong River delta in the South. Although these plains
are not very large, they are relatively fertile and especially suitable for food crops.
The country has a tropical climate with a humid monsoon season. The year is mainly divided into
two seasons: winter, which is cool and dry, and summer, which is hot and rainy. In the rainy season,
there is enough rainfall for crop planting over the whole year. But without proper water conservation
systems, the fields suffer from drought during the dry part of the year. Also, too much rainfall
causes water logging and floods during the rainy season. In addition, a prolonged winter harms
seed-sowing in the spring, and during heavy rainfall the soil can easily be eroded, and its fertility
decreases very rapidly.
The population of Vietnam is at present basically rural and is concentrated in the two main rice-
growing deltas mentioned above. As a result, agriculture remains fundamental to the Viemamese
economy and provides a direct living for 70% of the country’s labor force, as well as contributing
about 45% of the GDP. A population growthrate of appropriately 2.4% per yearmakes the population
double in about 30 years. Population has risen from 16.5 million in 1930 to 32 million in 1960 and
to 64 million in 1987. A large population, coupled with limited cultivable land, maintains the arable
land per capita at a low value.
Agriculture has been practiced in the country for a very long time. Among the food crops, rice takes
first place, leaving all other crops far behind. Nearly all of the area suitable for rice is under rice
cultivation, Vietnam’s strategy is therefore to promote yield increase rather than to expand rice-
growing areas.
Except for the short-term decrease in food output during the adverse years, food production in
Vietnam has experienced a long upward trend since 1930. This growth was achieved mainly through
increases in both yield per hectare per crop and the intensive use of agricultural land throughout
System Dynamics '90 63
the country. However, the population has risen at a higher rate than the total food production, which
has resulted in a downward trend in food per capita. Vietnam is no longer a grain exporting country
as it was before the 1970's.
Further, intensive use of soil, made possible through high-yield technology, has contributed to both
physical and chemical degradation of soil, especially the loss of soil organic matter. In many places,
especially in hilly areas, the soil has become barren. The most serious soil erosion has occurred in
deforested highlands, such as in the Northwestern Region.
3. A SYSTEM DYNAMICS MODEL OF FOOD PRODUCTION SYSTEM:
The food production system of Vietnam can be characterized by feedback loops shown in Figure
1. The organic relationships underlying the feedbacks loops span population, food production, and
ecological subsystems as discussed below.
Children Total
Jae cose Cultivated
* Land
- « + — am
Births. Food ss
Production
a)
ye Y
_ wy Population ——-» Food/capita -A Yieid/ha/yr
\ Ate
D)o4
* Sette Land
Deaths ating | Improvement
a i
hoy a; V4
/ ‘( Irrigation
Desired Food Land Use
per capita New Varieties G
Land
“y Degradation
Figure 1: Main feedback loops in the food production system
3.1 Population Subsystem
Births and population form a positive feedback loop, which accounts for the observed exponential
population growth. When calculating births per year, we can think in terms of birth fraction per
year, but, when imposing a policy to decrease it, we do not know how this policy would be
System Dynamics ‘90
implemented. Therefore, number of children per couple is included so that a policy for each couple
to reduce number of children becomes more concrete and implementable in reality, especially in a
centrally-planned economy.
Population and deaths form a negative feedback loop which governs population growth. Whereas
the above positive loop generates exploding growth, this negative loop seeks to regulate growth.
Deaths are assumed to be dependent on the self-sufficiency ratio which is food per capita normalized
with respect to a defined value considered adequate. Sustained and increasing food availability
leads to higher life expectancy, thus less deaths; and vice versa. Hence population, food, and deaths
form a self-correcting negative loop.
3.2 Food Production Subsystem
Food supply and yield form several negative loops. As long as food per capita remains below a
sustainable level, agricultural planners have no choice but try to increase production by any possible
method (normally the quickest and least expensive). However, for countries like Vietnam, where
the land suitable for producing food is limited and most arable land has been brought under cul-
tivation, increasing the yield (output per unit of cultivated area and per unit of time) has become a
major objective. Well-known methods to increase yield consist of soil improvement, development
of irrigation to permit multiple cropping, and adopting of high-yield varieties.
So, a decline in food per capita leads to attempts to boost food production through the above methods,
which, in turn, causes food per capita to be increased, ceteris paribus. These self-correcting feedback
loops explain why countries with large population but limited land can still experience a growth in
food production to feed their population, as in Japan and South Korea.
3.3 Ecological Subsystem
Under population pressure and limited arable land, high-yield technology which intensifies land
use and uses new high-yield varieties tends to alter soil properties, unless the system is managed
properly. Under poor management, this will lead to land degradation that, in turn, eventually will
cause crop yield, total food produced, and thus the self-sufficiency ratio to decline in the long-run.
The model assumes land degradation is caused primarily by loss of soil nutrients (through erosion
orintensive cultivation) and the creation of "adverse land" in problem areas. The model also assumes
that the adverse land share is increased with the increase in water-logging and saliniZation in
poorly-managed irrigation of problem soils.
From the above argumentit follows that food production and poor land management form a positive
feedback loop, aggravating both land degradation and food shortage in the long-run.
More details of the flow diagram developed out of the feedback structure outlined above are provided
in Figure 2, while a general description is provided in the appendix. Further technical documentation
and a machine-readable program of the model coded in DYNAMO are available from the authors
on request.
‘7 N content "Ss
17 in crop
yy
it NITROGEN (N)
|| ge & \
{| Fertilizer SL ~igeake Ae Le A \
im
1 Leaching Effect of N-—~/ \ Effect of |
a: on Biomass 7 H IP on Bionass),/ /|_% (PP)
1 “ 1 y 7 A !
1) (Norma rae \ 7 \ 7 ‘ |
1 \ Fraction 1 Yi V7 PP he !
\ ae Y Rate 4
1) / = 4\ ao x /
1 \ Normal Effect of H Effect of H vo | Normal 1/
Fraction #\on Leaching J ~--*\ on ead eis St a I Fraction
Xe la oan 7 ca
SY “77 BOO ga op / Effect of \.% //
Gpeiaaa H F a zz ASSR on PP /
Y 7
~ -
OK HUMUS (11) Peo
{ \\ Humi fication,
Oxidatior
n \ nee
i7 va IP rate N
SS
4 IRRIGATION
Hi ‘ Pia 4\S PERCENTAGE
/
\
\
|
|
I
\ ‘. 7 1 \ erection“, /
jormal
\ Effect of Effect of aa J / Effect of ‘porman >
\. \uand use on H _7\uand_use on H) 7 an sue on IP
N 2 2
oes gen <7 Zo : H HPL] POPULATION ios
77 Effect of x
Normal 7 Y f / Births * x Deaths \
Fraction yf n 1 & aan 1
7
\. Degradation at wer 2 id Children ‘Couples | Average ¥
SL ee ~ 5, wi fi y per couple H Life time \
=
» | ADVERSE Zs coop Total Sas Couple 1 i
LAND va LAND Land ~~ Fraction / 1 (Effect of \ t
& Peg Se STL 7 7 \assr on ALT
N en y
<> Enprovement Average Fa
Se1f-Suf ficiency
Normal
Fraction
Figure 2: The flow diagram of the system
b Ratio(ASsR)
Effect of pe
ASSR on Imp/“-—~ he, Sustained
~ ~food/ganite
06, soyureufg, ura3shg
66
System Dynamics ‘90
The important policy assumptions made in the base run are the following:
a) A normal soil fertilization rate is defined such that the system is in equilibrium for the initial
conditions, which may be increased somewhat when the need for fertilizer is perceived. However,
longer term mechanisms affecting soil dynamics, such as immobilization and mineralization, as
suggested by Jones (1984), are ignored.
b) Irrigation, new seed varieties, and soil improvement (conversion of adverse land to good land)
are assumed to develop at specified normal rates unless further modified by other processes. They
remain unchanged as long as the average self-sufficiency ratio (ASSR) is greater than or equal to
1. When this ratio is less than 1, these activities will be increased.
c) Intensity of land use for cultivation represented by the "Land Use Index", for simplicity, is treated
as a linear function of irrigation percentage.”
d) The population is aggregated into a single level only. It is assumed that the couple fraction and
the number of children per couple do not change over time, except through policy intervention.
Average lifetime is a nonlinear asymptotic function of the average self-sufficiency ratio.
e) Total land under food crops (composed of good land and adverse land) is assumed to be constant,
although it can be changed in the model for experimental purposes.
Figure 3 shows a simulation of the model giving the behavior of nitrogen, self-sufficiency ratio,
and yield per hectare per year. Yield per hectare per year experiences an upward trend, slowly
increasing during the early decades of the simulation time, then rapidly increasing due to the use
of methods, especially land use intensity, to boost production. Under the assumed poor management
practices, this results in a downward trend in soil nitrogen stock, a proxy for land fertility. A high
land use index alters the physical and chemical properties of soil, reducing the organic matter
returned to soils. New high-yield varieties adopted extract high amounts of soil nutrient during the
crop growth stage. Coupled with the continuing growth of population, land degradation gives rise
to a decline in self-sufficiency ratio during the course of the simulation, from 1.2 in 1930 to 1 in
1966-1967, then down to nearly 0.8 in 2030. These figures clearly explain why the country, which
used to be a grain-exporting, became a grain-deficit country.
Figure 4 displays the explosion of population and the growth in land use index. Notice that good
land is increased during only the early decades, and then begins to decline,
System Dynamics '90
67
——— Hitregen in ko/ha (18,78. ) ——— ood Land in million ha (1.8,2.8)
—— Self-SufTicieney Ratio (.8,1. —— land Use Index (5,2.5)
.—— Yeld/ra/year in tons (8,18. 2.———— Population in nillion people (8.228.
. 2
1.
\ = |_Seod tend
5. 2.
14 \ 2 LAA Fr
1 158 W/
) Sb re,
4] ey 2
ai t. er;
5 ~~ 188 yer
nd
ui wed} --—~ 2 ee \
11 { [}——F € \
ee a 58. -, s ye v
0
Selt-; a
a | ee —_— \
we W
8. 1938. 1958. 1978. 1998. aia. ea, 8. 1938. 1958. 1978. 1998. a8. 2868.
TIME TIME
Figure 3: The Base Run Figure 4: The Base Run
4, SEARCH FOR SUSTAINABLE FOOD POLICY
An advantage of system dynamics computer simulation models over the "real world" is that the
models, when carefully developed, can be used fully as experimental laboratories. The modelers,
as well as the analysts, can devise controlled experiments and well-designed changes in both the
parameters and structure of the systems of interest in order to better understand their endogenous
behaviors and their "natural" responses to the changes induced. Such experimenting could help the
modelers themselves identify the strengths and weaknesses of the model structures, which, if
necessary, makes them modify and improve the system structure until the latter comes to represent
the real system rather well.
Furthermore, careful experimentation with a correctly constructed model helps to quickly identify
which policies are "good" and which ones are "bad". This second advantage can shed much light
into the implications of alternative policies to be adopted so as to improve the system performances
in the future.
The experiments described below, for the sake of testing the model, required changes in the system
parameters which affected almost all of the system’s main parts. The changes are not abstract but
meaningful in the sense that they are related to concrete policies which can be implemented in actual
practice.
System Dynamics '90
4.1 Population Control
As can be seen in Figure 1, the positive feedback loop of births-population causes the population
to explode. So, it is reasonable to think that population control measures to reduce the natural birth
rate through family planning could increase the food per capita, and hence the food self-sufficiency
ratio.
Figure 5 shows that, in order to achieve self-sufficiency at the end of simulation, number of children
per couple should be decreased from 3 to 2 (about one-third) between 1988 and 2030.
Figure 6 shows that if after 1988 children per couple are decreased only 22% (from 3 to 2.34) then
self-sufficiency can be achieved from the year 2015 onward. Thus, strong population control
measures will yield a high outcome. Direct i intervention to implement such a measure might appear
impractical, although there is evidence of its success in Japan and China. There is a need to develop
appropriate policies to implement population control.
Pion 1 in alllera: stir (8. 188.)
Population in niltions: Bese Run (28. , 188.)
Policy (8.,168.) — + Policy (-28.,188.)
Selt-Suiticieny Jato: ase hn (8.12) ——— Self-Sutticieney Ratio: Base Bun (6.1.2)
i, + Policy (.8.1.2) 118. 2 Policy (.8,1.2)
. i A
S f
185) &
1 SS wo
Ns ey oa & f 4
Nie iP, wie
Ne, PK) Toe?
8. << Le
1 nos y aor embolic
Noe a gr
o
&. iB on, - Pras
oe] ieee | pa
' |
"1908, 1958, 1978 1998 aie, oe “tena. 1958, 1978 199 mete, 88
TIN Tie
Figure 5: No. of children from 3 to 2 Figure 6: No. of children from 3 to 2.34
4.2 Land Management Policy
With a high land use index and high-yield seed varieties adopted, soil nutrients tend to be depleted
as shown in the base run. It has been well known that fertilization, when applied properly, and when
System Dynamics ‘90
higher residues are returned to the soil, fertility is sustained. However, the model assumes that the
pressure for quick returns and lack of incentives- and money- for proper management will result in
soil nutrient losses.
Figure 7 shows the behavior of soil nitrogen if fertilizer application is increased by 35% in 1988.
The overshoot shown is due to the fact that nitrogen in the soil is constantly lost through crop uptake
and leaching in proportion with its remaining stock. So, a step increase in fertilization only yields
increases in soil nitrogen temporally.
An annual increase in fertilizer application at the rate of 2.7% can bring nitrogen to the initial level
at the end of simulation as presented in Figure 8. However, self-sufficiency ratio does not return
to its initial level.
——— Hitrogen in kg/ha: Base Run (6.,78.) ® ———— Hittrogen in kg/ha: Base Run (~ 1.)
anne 3 Policy (8.7 meena: : Policy (18.
Self-Sufficiency Ratio: Base Run (.8,1.2) ———— Self-Sufficiency Ratio: Base 2)
3 Policy (.8,1.2) %.——. 3 Policy (1
34.
Ay
B
a8
1938, 1958. 1978. 1998, 2818.. 2888. , 1938. 1558. 1978, 1998, 2818, 8.
TINE TINE
Figure 7: 35% increase in fertilizer Figure 8: Fertilizer at the rate of 2.7%
Crop residues returned into the soil after harvest, plus animal manure application, does improve
the nitrogen stock in the short-term but not in the long-term, as seen in Figure 9. There is a 50%
increase in residues returned from 1988 and 2030, but the self-sufficiency ratio is not improved.
Another policy option is to increase investment to improve adverse land conditions. Figure 10
shows a simulation incorporating land improvement effected at the rate of 10%. It results in an
upward trend of "good" land share, but a downward trend in the self-sufficiency ratio in the long-run,
though the latter exhibits an upward trend in the short-run.
System Dynamics '90
However, all of these policies aimed at improving land management cannot bring the food self-
sufficiency ratio above 1 in the long-term, mainly because population is still increasing too rapidly.
——— Hlitrogen in kg/ha: Base Run (6.,78.) ———— Good Land in sillion ha: Base Run (8.,3.)
— + Policy (6.,78.) — + Policy (@.,3.)
——— Self-Sufficiency Ratio: Base Tan (.8,1.2) Self-Sutficieney Ratio: Base Bin (.8,1.2)
%.—— 2 Policy (.8,1.2) 3—— + Kolicy (.8,1.2)
1. 1.
7 N T | de
m \ Good Land
54 \. 29 as ee
i S: tee AN
hs Daa
* Ny ‘, S
ac ‘ PN
3 Soy, Potiay 15 kz
t] °S t S 1
Vy, | 2886 Rig “es oe
%q, Ss oa
2. fi f| & 4
one
fase a 2a
58 Fi
6 ~ 8 r"
1908, 1958, 1978. 1398 7B. 2828. 108, 1958 197, (198 m8.
TIME 1M
Figure 9: Increase in residues Figure 10: Land improvement
4.3 Water Management Policy
A different strategy to boost food output is to increase the land use index through irrigation. As
shown in Figure 11, an 80% increase in irrigated land, which is only made at very high investment,
steps up food self-sufficiency ratio in the short-run, but may also lead to water-logging and sali-
nization which, coupled with loss of soil nutrients, leads to a decline in productivity. Thus, a more
intensive land use policy (with poor management of the irrigation system) is assumed to boost food
output at the expense of land degradation in the long-run.
In the all previous experiments, the total land under cultivation was assumed to be constant.
Consequently, when population continues to grow (and so the demand for food), the limited land
is under great pressure. As an experiment, it was assumed that new land could be opened to raise
the total land planted to food crops. Figure 12, where a 20% increase in new land development from
1988 to 2030 is made, shows that adverse land goes up, while self-sufficiency ratio is slightly
increased, then decreased again.
System Dynamics ‘90
71
Tin Land in nillion ha: Base Run (.8,2.8) ———— Total Cultivated Lend in million ha: Base Run (2.2,5.2)
icy (.8,2.8) — + Policy (2.2.5.2)
—salt-utticierey fat: Base in (8.1.6) Adverse Land in nillion ha Base un (.5,4.5)
+ Policy (.8,1.8) + Policy (5.4.5)
Irrigation Percentage: Base fun (~.2,1.) f-Sufficiency Ratio: Base fun (.8,1.2)
2.g—— + Polley (~.2,t. + Policy (.8,1.2)
i
rH I “|
2 Good Land a
i
J ‘Total Cult. Land: T
{ Base we
5 37
t P a rae
cw rf Zi
7 ne eal ; ae
1 1 solic}
oe 4 ——s
4 | ea ees ten | ad ee Rr
-2 1938, 1858. 1978. 1998. ata. 8. 2 1938. 1958. 1978. 1998. 2818, p<
TINE THE
Figure 11: Increase in irrigation Figure 12: New land development
The reason for this perhaps unexpected result is that since the best land for food production has
already been brought into cultivation, only marginal lands remain to be developed, which causes
an increase in adverse land, and thus could not bring about the desired outcome (without a large
initial investment in land improvement, which the government of Vietnam cannot afford).
5.4 A Combination of Policy Alternatives
The previous policies were considered separately, with each related to only one sector of the complex
system structure. Now, a combination of the most effective among these policies is examined as
an illustration of the promising results that could be obtained.
As can be seen from the above experiments, radical policy changes in each separated area were not
successful in achieving food self-sufficiency. In addition, a strong change in each separate policy
may be impractical and unfeasible. However, a plausible combination of policies can yield a very
encouraging outcome.
Combined policies, currently implemented from the year 1988, consist of:
a) 10% decrease in number of children per couple;
b) an annual 2% increase in fertilizer application to replenish soil nutrients;
72
System Dynamics ’90
c) 10% increase in residues returned into soil after harvest; and
d) an annual 5% increase in land improvement effort in terms of conversion of adverse to good
land.
Such moderate changes in each system part may prove to be implementable in practice. The key is
to introduce them together.
The long-term result generated by this policy combinations are better than before, as seen from
Figure 13. Nitrogen stock could attain the same level as initially. Good land does not decline.
Population, of course, still increases, but at a rate lower than in the base run. All these improvements
account for the upward trend in the food self-sufficiency ratio, which nearly reaches the initial
level.
———— Nitrogen in kg/ha: Base Run (6..78.)
_—_ + Policy (6.,78.)
Good Land in million has Base Rin (8.,2.6)
— 1 Policy (8.,2.6)
Self-Sufficieney Ratio: Sase Ban (.8,2.)
.—— 1 Policy (.8,2.)
2 —
aS Sedtne Policy.
fd‘ ak.
; ‘. RR
Mite, |
2. ~Nee, Policy
a [|
1.
. 885 Rg —|
|, |
: elas Folic!’
Aufl Wie,
6. j-———Base Run
8
318, 1958. 1978, 1998, 1, 8,
TIME
Figure 13: A combination of policy alternatives
5. CONCLUSION
This paper has characterized the food production system in Vietnam at an aggregate level which °
represents a centrally-planned economy. A model for this system has been developed to incorporate
relationships concerning population, food production, soil conditions, and agricultural land man-
agement policy.
System Dynamics '90 73
It turns out that there are policies which, when adopted to keep food production apace with the
population explosion, can have very detrimental consequences over the long-term. Intensifying land
utilization, expanding cultivation into marginal areas, using “miracle varieties" without adequate
fertilization all might bring premature hope and lessen concerns over an exploding population. Such
over-optimistic thingking could lead to policies in which the long-term/short-term trade-off are not
carefully considered.
The model developed in this study has also demonstrated that food production is basically an
extractive activity, which is likely to lead to land degradation. Population growth can be seen as
the fundamental driving force behind this process.
It has now become obvious through experimenting with the model that wise land use management
and proper population control measures can bring the system into a balance one, which can be
sustained for a rather long time. This requires realistic and long-term planning, since degraded land
takes a long time to recover and population also takes a long time to control.
The model developed in this paper, however, is a simple and aggregate representation of the real
food production system for the centrally-planned economy of Vietnam. In order for a better food
policy to be formulated and successfully implemented, the system boundary must be expanded.
REFERENCES
Bowonder, B. 1981. The Myth and Reality of High Yield Varieties in Indian Agriculture.
Development and Change. 12(2).
Brown, L.R. and E.C. Wolf. 1986. Assessing Ecological Decline. In State of the World 1986.
Brown, L.R. et al. Worldwatch Institute. W.W. Norton & company.
Chandler, W.U. 1987. Designing Sustainable Economies. In State of the World 1987. Brown,
L.R. et al. Worldwatch Institute. W.E. Norton & Company.
Jones, D.K. 1984. Soil Nitrogen Dynamics. Master’s Thesis, Thayer School of Engineering,
Darmouth College, Hanover, NH.
74 System Dynamics ‘90
APPENDIX: GENERAL MODEL DESCRIPTION
1. d(N)idt =NFAR +f,(B)*f,(LUD —f,(8) -RA)* 6); f>0; K<0; f>0; K<0; f<0;
N = Soil Nitrogen, NFAR = N Fertilizer Application Rate, B = Biomass,
LUI = Land Use Index, H = Humus.
2. d(H dt = f(LUI*f(B)- LUD *Z(H); <0; H>0; fe>0; > 05
3. d(IP dt = f(ASSR); fio <0;
IP = Irrigation Percentage; ASSR = Average Self-Sufficiency Ratio.
4. d(PFydt = f,(ASSR); fi, <0;
PF = Product Percentage.
5. d(P)dt = fio(P)—PIfs(ASSR); fi2>0; frs>0;
P = Population.
6. d(GLydt = f,(ASSR)*AL —f,(IP)*GL; fig <0; fis>0
GL = Good Land; AL = Adverse Land.
