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Chapter 5. Are Agroecosystems Sustainable in Semiarid Regions?
B. A. STEWART AND C. A. ROBINSON
billion in 1950,4.7 billion in 1983 (a 1.9% average annual increase), and is projected to reach 6.1 billion in 2000 (a 1.6% annual increase) (World Bank, 1986).
Agricultural production, however, increased even faster, doubling between 1950
and 1980. This increased production resulted in about a 5% increase in per capita
daily calorie consumption. People in developed countries consume approximately 3315 calories per capita compared to only 2180 in developing countries. Approximately 2300 is generally considered the minimum requirement (FAO, 198 1).
From the beginning of agriculture until approximately 1950, increased food
production resulted almost entirely from an expanded cropland base. Since 1950,
however, the yield per unit of land area for major crops has increased dramatically. Much of the increase in yields is due to increased inputs of energy. Between
1950 and 1985, the farm tractor fleet quadrupled, world irrigated area tripled, and
fertilizer use increased ninefold. Between 1950 and 1985, total energy used in
world agriculture increased 6.9 times (a 5.7% average annual increase) (Brown
and Postel, 1987). Some of the agroecosystems that propelled food and fiber production during this period are not being sustained. In addition, there are limited areas, particularly under favored environments, where further development can occur. Consequently, agroecosystems will become increasingly present on marginal
lands in semiarid regions. Semiarid regions are characterized by insufficient precipitation, low soil fertility, and a rapid loss of arable land to soil degradation. An
imbalance between natural resources, population, and basic human needs exists in
these regions. The warm seasonally dry tropics occurring primarily in subSaharan Africa, Southwest and Southeast Asia, Central and South America, and
Northern Australia contain more than 1.5 billion people (Technical Advisory
Committee, 1990). Several of the countries in these regions have population
growth rates among the highest in the world. Kanemasu et al. (1990) estimated
that the semiarid tropics contained 13% of the world’s land and were inhabited by
15% of the world’s people but produced only 11% of the world’s food.
There are growing concerns about the ability of the agricultural community to
continue production of food and fiber at a rate sufficient to meet the demands of a
growing population; this is particularly so if income growth results in dietary
changes that require more grains for meat, eggs, and milk.
Brown (1995) summarized the strategy developed at the 1994 Conference on
Population and Development in Cairo. He concluded that their strategy reflects a
sense of urgency-a feeling that unless population growth can be slowed quickly,
it will push human demands beyond the carrying capacity of the land in many
countries, leading to environmental degradation, economic decline, and social disintegration. Some important trends that Brown emphasized include a significant
decline in the rate of irrigation development, a leveling off of worldwide fertilizer usage, and a slowing of cropland productivity.The world’s irrigated land in 1950
totaled 94 million hectares but increased to 140 million by 1960 (a 4.5% average
annual increase), to 198 million hectares by 1970 (a 3.5% average annual in-
AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS?
crease), and to 271 million hectares in 1985 (a 2.1% average annual increase)
(Rangely, 1985). Since 1985, however, the rate of expansion has slowed to less
than 1% per year (Council for Agricultural Science and Technology, 1988). Worldwide, fertilizer use increased 10-fold between 1950 and 1989, when it peaked and
then began to decline. During the following 4 years it fell 15%, with the decline
concentrated in the former Soviet Union following the withdrawal of subsidies
(Brown, 1995). The rise in rice yield per hectare, which halted in Japan a decade
ago, is now slowing nearly everywhere, edging up only 2% from 1990 to 1994
(Brown, 1995). Another major concern in some countries, particularly China and
India, is the loss of cropland to industrialization.
All these factors are causing the agricultural community to question all agricultural ecosystems regarding their sustainability. The question of sustainability is
especially relevant to agroecosystems in semiarid regions. Are such systems sustainable over the long term? The short answer is yes because there are many examples in which systems have been sustained for centuries. However, there are
also many examples in which the development of agricultural ecosystems has led
to severe soil degradation and a collapse of the agroecosystem. The objective of
this treatise is to examine some of the more important soil-plant-water relationships in semiarid regions so that the steps required to sustain agroecosystems in
these areas will be better understood.
Agroecosystems are ecological systems modified by human beings to produce
food, fiber, or other agricultural products (Squires, 1991). In agricultural development, natural ecosystems are transformed for the purpose of food and fiber production. An agroecosystem is a complex of air, water, soil, plants, animals, microorganisms, and everything else in an area that people have modified for agricultural
production. Crops and livestock are the major components of agroecosystems.
An agricultural ecosystem such as a wheat field may be more productive than
the native grass that originally occupied the same land. A greater percentage of the
wheat produced is fit for human consumption than is the grass, but wheat is not
self-sustaining and may be relatively vulnerable to external disturbances. Regular
inputs of energy and nutrients are required to maintain production levels. For example, the soil may be too infertile for continuous crop growth unless it is fortified with fertilizers. Agroecosystems therefore tend to be less sustainable than natural ecosystems unless sustainability is very carefully designed into the system.
Squires (1991) emphasizes that there are three qualities of agroecosystemsproductivity, stability, and sustainability. The productivity of an agroecosystem is
not determined simply by the yield potential of the particular crop or livestock that
B. A. STEWART AND C . A. ROBINSON
is used. The yield that actually occurs depends on the climatic and nutritional (soil)
environment, which is, in part, a consequence of how farmers manage. Productivity is therefore a consequence of the processes in the total interactive agricultural-environmental-social system.
Stability is the reliability or consistency of farm production. It is important because people depend on a certain amount of production year after year. Fluctuation is a normal part of all ecosystems and agroecosystems are no exception. Agricultural production often fluctuates from year to year, and this is particularly true
for agroecosystems in semiarid regions. Cropland in semiarid regions is often
more marginal and precipitation is both lacking and highly sporadic. Although the
risks of partial or complete crop failure are an unavoidable part of farming in semiarid regions, farmers place a high priority on minimizing risk.
Sustainability concerns whether a given level of productivity can be maintained
over the long term. Sustainability is defined as the ability of an agroecosystem to
maintain productivity when subject to a major disturbing force (Squires, 1991).
Salinity, toxicity, erosion, indebtedness, or declining market demand are examples
of such forces. Sustainability thus determines the persistence or durability of an
agroecosystem’sproductivity under known or possible conditions. Sustainability has
a variety of measures associated with various measures of productivity. Some measures of sustainability can be high while others are low for the same agroecosystem.
Although it is desirable for an agroecosystem to be high in all three qualitiesproductivity, stability, and sustainability-the three may conflict. An agroecosystem can often be managed in such a way that the short-term gains are very beneficial, but the long-term consequencesare devastating. Identifying and understanding
the key relationships among these qualities are essential and require that appropriate questions be developed, asked, and answered. The key questions for semiarid
regions are: (i) How can semiarid agriculture be intensified? (ii) Is this possible or
desirable considering possible land degradation and other environmental consequences that threaten its long-term success? and, perhaps even more important, (iii)
How far can agroecosystems be extended into a region of increasing marginality?
Semiarid is a comparative term implying a moisture state intermediate between
truly arid conditions and others that are more humid. Semiarid regions have four
unique characteristics. The first is that no growing season will have nearly the same
amount, kind, or range of precipitation as the previous season; and the temperature average, range, and extremes will also be vastly different. Second, crops cannot be planned or managed the same from season to season. Third, and perhaps
most important from a sustainability standpoint, the soil resource base and water
AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS?
holding capacity does not remain the same for any long period once an agroecosystem is introduced into a semiarid region. Fourth, abundant sunshine and
cloud-free days induce rapid growth when moisture conditions are favorable, but
these conditions cannot be sustained through the season, thereby demanding careful soil water management.
Semiarid regions typically receive substantial precipitation for at least a few
months of the year-enough to replenish soil water content to levels sufficient to
produce amounts of biomass that far exceed those produced in arid regions. Dryland farming systems are common in semiarid regions. Dryland and rain-fed are
often used synonymously, but they are considered vastly different by some workers. They both exclude irrigation, but beyond that they can differ significantly.Dryland agroecosystems in this paper are based on the concept presented by Stewart
and Burnett (1987) that elucidates the differences between dryland and rain-fed
systems. Stewart and Burnett stated that dryland agricultural systems emphasize
water conservation, sustainable crop yields, limited inputs for soil fertility, and
wind and water erosion constraints. Rain-fed systems, although they include dryland systems, can also include systems that emphasize disposal of excess water,
maximum crop yields, and high inputs of fertilizer. Stewart and Burnett (1987)
stressed that dryland farming in semiarid regions has been and will continue to be
a high-risk undertaking and that the key to success is the use of systems and practices that take advantage of the favorable years.
The United Nations Conference on Desertification (UNESCO, 1977) defined
bioclimatic zones based on the climatic aridity index: PIETP, where P is precipitation and ETP is potential evapotranspiration calculated by the method of Penman
(Doorenbos and Pruitt, 1977),taking into account atmospheric humidity, wind, and
solar radiation. The zones established by the conference were as follows:
The hyperarid zone (PIETP = <0.03)-consisting of areas largely void of
vegetation except for ephemerals and shrubs in riverbeds and that are virtually
The arid zone (0.03 < PETP < 0.20)&comprising dryland areas with sparse
perennial and annual vegetation utilized mainly in pastoral systems.
The semiarid zone (0.20 < PIETP < OSO)-including steppe or tropical
shrubland with a discontinuous herbaceous layer and increased frequency of
perennials where dry land farming is widely practiced.
The subhumid zone (0.50 < P E T P < 0,75)--characterized by more dense
vegetation, where rain-fed farming is widely practiced with crops adapted to
B. A. STEWART AND C . A. ROBINSON
B. LENGTHOF GROWINGPERIOD
The Food and Agriculture Organization of the United Nations (FAO, 1978) used
the growing period as the basis for assessing climatic resources in developing
countries. The growing period is the number of days during a year when precipitation exceeds half the potential evapotranspiration, plus a period required to use
an assumed 100 mm of water from excess precipitation (or less, if not available)
stored in the soil profile. Any time interval during the period when water is available is excluded if the temperature is too low for crop growth (mean temperature
below 6.5OC). Areas where precipitation never exceeds half the potential evapotranspiration are classified as dry with no growing period. An area having a growing period between l and 74 days is classified as arid, and areas with growing
periods between 75 and 119 days are considered semiarid.
The average monthly precipitation, potential evapotranspiration, and half POtential evapotranspiration for three locations are presented in Fig. 1. All three locations are classified as semiarid by the aridity index (0.20 < P E T P < 0.50).
However, by the FA0 growing period classification, only the Rajkot, India, location is classified as semiarid. The Amman, Jordan, location has a growing period
in excess of 119 days, so this would be considered subhumid. Bushland, Texas, is
classified as dry, with a 0-day growing period, because the average monthly precipitation never exceeds 0.5 ETP.
These examples illustrate that there is still a lack of precision in defining and classifying climatic zones. Each classification scheme presented, as well as others in
the literature, has advantages for specific purposes and locations; however, subjective judgment is required for their interpretation. The most important point, however, is to recognize and understand that there are very major differences in specific climates among semiarid regions and that each one must be addressed differently.
Average precipitation values in semiarid regions can also be very misleading.
For example, the average annual precipitation for Bushland, Texas, is 470 mm, but
rainfall for any given year has been as low as 240 mm and as high as 830 mm. For
a month, the variation can be even much greater. The average monthly precipitation for June is 76 mm, but the range has been from 1 to 248 mm, and less than 62
mm has been received in one-half of the years. In semiarid regions, there are more
years below the average value than above because the high-rainfall years raise the
average more than the low-rainfall years lower the average; thus, there is a degree
of skewness inversely related to the amount of rainfall. The degree of skewness
generally increases as the amount of annual precipitation for an area decreases.
Mageed (1986) stated that in areas where annual precipitation amounts are
200-300 mm, the amount received in a given year ranges from 40% of the aver-
AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS?
Figure 1 Agroclimatic characteristicsof three representativesemiarid locations. (From Stewart et
age to 200%; for areas where the average is 100 mm per year, the range is from
30% of the average to 350%. This makes it imperative that more emphasis be
placed on probabilistic analysis, with less attention given to average values when
designing agroecosystems in semiarid regions.
Kanemasu et al. ( 1990) have extensively reviewed the agroecological features
B. A. STEWART AND C . A. ROBINSON
of the semiarid tropics and presented strategies for identifying agroclimatic constraints for increased agricultural production. They concluded that the rainfall variability in the semiarid regions of the world is on the increase. They further concluded that the productivity of traditional agriculture in these areas is declining,
and that the threat of serious food shortages is rising. At the same time, they proposed that by properly evaluating the agroclimatic resources and transferring and
adapting improved technologies, the yet underexploited potential of semiarid tropical regions can be realized for sustained and improved agriculture.
Iv. THE ISSUE OF SUSTAINABILITY
Until the past few years, sustainability was seldom mentioned in agricultural
literature. Now, it is one of the most widely used words. High costs of irrigation
development, escalating energy costs during the 1970%public concern over potential negative impacts of fertilizers and pesticides on water supplies, soil erosion, soil compaction, and salinity, as well as other concerns, have caused many
people to question whether many of the current agricultural systems can be sustained.
There are many different concepts of sustainability, but none is generally accepted. Sustainability, to many, conveys the idea of a balance between human
needs and environmental concerns. The United States Congress defined sustainable agriculture in the 1990 farm bill as
an integrated system of plant and animal practices having a site-specific application that will over the long term: satisfy human food and fiber needs; enhance
environmental quality and the natural resource base upon which the agriculture
economy depends; make the most efficient use of nonrenewable resources and
on-farm resources and integrate, where appropriate, natural biological cycles
and controls; sustain the economic viability of farm operations; and enhance the
quality of life for farmers and society as a whole.
Ruttan (1989) proposed, as a guide to research, that the definition of sustainability should include (i) the development of technology and practices that maintain and/or enhance the quality of land and water resources, and (ii) the improvement in plants and animals and the advances in production practices that
will facilitate the substitution of biological technology for chemical technology.
The first part of the Ruttan definition is similar to that of the United States Congress, but the key to the second part of the Ruttan definition is the emphasis on
more reliance on biological versus chemical technology. Sustainability should be
considered dynamic because, ultimately, it will reflect the changing needs of an
increasing global population. The common thread among all definitions of sustainability, however, is that quality of the resource base should be enhanced.
AGROECOSYSTEMS SUSTAINABLE IN SEMIARID REGIONS?
Francis and Youngberg (1990) stated that sustainable agriculture is a philosophy based on human goals and on understanding the long-term impact of human
activities on the environment and on other species. Use of this philosophy guides
our application of prior experience and the latest scientific advances to create integrated, resource-conserving, equitable farming systems. These systems reduce
environmental degradation, maintain agricultural productivity, promote economic viability in both the short and long term, and maintain stable rural communities
and quality of life. Despite the appeal of the philosophy for sustainability, there is
often controversy about how best to achieve it. It is not known what methods and
systems in diverse locations will really lead to sustainability, and this lack of
knowledge often leads to conflict.
Rodale ( 1988) listed seven reasons that sustainability has risen to importance
on the farm policy agenda. The first is that there is currently enough food in the
world. Surpluses and their timely distribution at an affordable price are more of
a problem than shortages per se. Although many people are going hungry, it is
not because of the inability to produce. Some policymakers are, however, wondering if this level of production can be sustained. The second reason is that nonrenewable resources are vitally important to the operation of the conventional
system of agriculture in the United States and most other developed countries.
Without adequate and low-cost supplies of oil, gas, phosphate rock, and other
similar resources, U.S. agriculture would not be able to turn out large amounts
of low-cost food and fiber. Third, high levels of production are having negative
impacts on some systems. Soil erosion, degradation, and deforestation are widespread. This is particularly true in some developing countries where millions of
people are literally trading soil for food. Even in the U.S. corn belt, there are approximately 2 tons of soil lost for every ton of corn produced and the loss ratio
is even greater in wheat-producing areas (Larson et al., 1981). The fourth reason is that pollution problems can be traced to agriculture. Increasingly, residues
of fertilizers and pesticides are being found in surface and underground water
supplies. Fifth, world population continues to grow-in some places very rapidly. Although there is enough food in total today, there will be a much larger population to feed and clothe in future years. Sixth, society will possibly need agricultural production for energy and chemical feedstocks as well as for food. As
fossil fuel reserves become more expensive, agriculture may have to meet new
needs. Lastly, Rodale listed the concern for the family farm. He questioned
whether the good life in rural areas can be sustained without a vibrant family
Sustainability clearly involves technical, economic, and social conditions. All
three conditions are highly important and interrelated. It may be entirely possible
to have an agroecosystem that is technically sustainable that is neither economically feasible nor socially acceptable. Herein, however, we will deal primarily with
sustainability in regard to how agricultural practices and systems affect the sustainability of the soil resource base.