Agricultural Change As an Incremental Process

William E. Doolittle

Annals of the Association of American Geographers, Vol. 74, No. 1. (Mar., 1984), pp. 124-137.

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Agricultural Change as an Incremental Process

William E. Doolittle

Department of Geography, The University of Texas at Austin, Austin, TX 78712

Abstract. At least two major views of agro-ecosystem change can be recognized-systematic and incremental. Systematic change involves the addition of new fields and associated features that are constructed completely prior to cultivation; incremental change involves gradual trans- formation of fields and features in conjunction with cultivation. The systematic view has been the more dominant of the two, particularly as applied to interpretations of past agro-ecosystems. Using present-day data on temporales or runoff-dependent fields in eastern Sonora, Mexico, this study describes one case of incremental agro-ecosystem change. Small, individual fields are developed into a more complex system by progressive modification resulting from the cumulative actions of individual farmers. In this case, the resulting constructional form of the agro-ecosystem alone does not allow assessment of the process of its development. Interpre- tations of past agro-ecosystems should recognize that both change processes are possible.

Key Words: agricultural change, agro-ecosystems, construction inputs, arroyos, floodwater farming, runoff farming, water harvesting, temporales, Sonora, Mexico.

THE issues of why and how agro-ecosys- and (2) that the final form of ancient agro- tems change have stimulated interdisci- ecosystems involved the periodic addition of

plinary interest-and have been examined by individual fields and associated features such geographers in ancient and modern contexts as terraces and canals (e.g., Woodbury 1961, (e.g., Doolittle 1980; Grossman 1981). Fol- 556). This second assumption has developed lowing theories formalized by Boserup (1965) into the systematic view of agricultural change, and others (e.g., Geertz 1963; Brookfield in which structural transformation is com- 1972), emphasis has been placed on input- pleted before a new field is used for cultiva- output relationships as the key to under- tion. As such, construction normally is thought standing agro-ecosystem change. Studies of to involve inputs applied over short, discrete contemporary agro-ecosystems have been periods of time, and often to include plan- successful i n demonstrat ing short- term ning, engineering expertise, and socially changes because a variety of inputs and out- coordinated effort (e.g., Armillas 1971). In this puts can be recorded, but the time depth is view, maintenance is essential for sustained usually insufficient to show long-term trends. operation of the agro-ecosystem but does not In contrast, studies of ancient agro-ecosys- result in major structural changes in the tems can provide sufficient time depth, but system. For example, systematic change of researchers may not be able to document the cultivation in a swamp might include addi- precise nature of inputs and outputs. tional clearance, excavation of canals, and

To assess the process of long-term change, construction of raised planting surfaces over studies of ancient systems make at least two short periods of time, followed by long-term assumptions: (1) that input-output data from cultivation and maintenance of the entire agro- morphologically similar agro-ecosystems in ecosystem (e.g., Wilken 1981, 14). current use are broadly applicable to ancient An alternative, or incremental, view of agro- systems (e.g., Turner and Harrison 1981,404); ecosystem change is also possible but has

Annals of the Assooatlon of American Geographers, 74111, 1984, pp 124-137 0Copyright 1984 by Assoc~ation of Arnerlcan Geographers

125 Agricultural Change

not received as much attention as the system- atic view, perhaps because studies of extant agro-ecosystems tend to be based upon a time depth that is insufficient to reveal the com- plexity involved (Leach 1959, 6; Siemens 1982, 220). In this view, individual fields and asso- ciated features, and ultimately the entire agro- ecosystem, are created by gradual upgrading through small units of input over long periods of time while cultivation is taking place (e.g., Ruthenberg 1976, 164). New fields and asso- ciated features are not swiftly changed to a final form but are transformed while in use. In this process, acts of construction may not be distinguishable from other categories of agricultural inputs. Rather, construction is an ancillary by-product of cultivation and main- tenance inputs (e.g., Nagata 1970, 128) and need not involve formal planning, engi- neering, or organization beyond that within the domain of the individual farmer or farm unit (e.g., Dodgshon 1980, 24). For example, a swamp may be used for the cultivation of rice, and during its use the farmer may grad- ually extend canals and enlarge bunds and other features until the swamp is transformed into a formal paddy system (e.g., Seavoy 1973).l

Recognition of the incremental process of agro-ecosystem change has at least two sig- nificant implications. First, it provides a means of elaborating the subtleties of change theo- ries. For example, Boserup's (1965) stages of agricultural growth can be interpreted as rep- resenting incremental change; however, each stage apparently emerges in a systematic manner. Second, the incremental process may require some re-evaluation of various socio- economic interpretations given to a particular agro-ecosystem, particularly past systems. For example, does the existence of a particular agro-ecosystem necessarily imply that certain modes of production or types of social organi- zation were necessary for its emergence?

This study describes change in a present- day temporal (runoff-dependent) agro-eco- system in eastern Sonora, Mexico (Figure This agro-ecosystem was first examined by the author from 1977 to 1980, during which time changes in the system were apparent. However, these changes did not seem to follow the systematic process. A formal study of the process was undertaken in 1981 and 1982.At this time, 41 temporales near the town

Figure 1. Sonora, Mexico.

of Baviacora were examined to determine the degree of structural changes in them that had taken place since 1977. In addition, interviews were held with 24 of the temporal farmers to determine field use and cultivation proce- dures during the period in question. This in- formation was used to reconstruct the pro- cess of change in the structure of the tem- porales and to examine the fit of process to the systematic and incremental views of agro- ecosystem change.

Temporal Agro-ecosystem

One solution to the variability of eastern Sonora's rainfall is the practice of cultivating arroyos3 Summertime thunderstorms often produce so much rainfall in such short pe- riods of time that runoff occurs. Excess water eventually flows into arroyos. The effects of rainfall variability are reduced in arroyos with

126 Doolittle

large drainage areas because of the amount of runoff that is collected. Hence, consistently productive temporales are those located in large arroyos.

Farmers have developed numerous ways to use runoff as a water source for agriculture in arid lands (Lawton and Wilke 1979, 3-5). Among the techniques used i n eastern Sonora, the easiest is commonly referred to as "floodwater farming" (Bryan 1929; Nabhan 1979). Floodwater fields are located in usually dry, low-lying areas that are inundated by flash floods when arroyos overflow their banks. These fields are typically small in size and few in number; not all areas in every arroyo are suitable for floodwater farming. The selection of a suitable field site involves an intimate knowledge of local conditions. The field must be floodprone, but the sheet of water must not attain a velocity that will wash out crops or bury them with silt. Frequently, simple brush devices are built perpendicular to the stream flow and used to slow the velocity of runoff so that nutrient-rich silt is deposited on the fields. This new and moist sediment creates a well-watered, fertile surface for planting. Nevertheless, yields are character- istically low in this type of agro-ecosystem because of the poorly controlled water supply.

A more elaborate technique of utilizing runoff is appropriately called "runoff farming" (Nabhan 1981) because agriculture is prac- ticed in areas where crops receive water that runs off adjacent unprepared areas. Situated where they will receive maximum runoff, fields of this type are small and unevenly distrib- uted. Both total and per-hectare yields from this agro-ecosystem tend to be greater than those of the floodwater fields because of a more regular supply of water. In many cases, especially in eastern Sonora, runoff fields have low rock terraces that trap silt, conserve moisture, and distribute water evenly across the planted area (Fogel 1975, 134-39), or they are enclosed by low earthen bunds or borders that retain and conserve moisture. Narrow cropping strips along contour-terraced hill- sides (Fogel 1975, 140-42), and small garden plots behind stone check dams in the beds of intermittent streams with steep grades (e.g., Herold 1970; Donkin 1979) also may be clas- sified as runoff fields. However, neither con- tour-terraces nor check dams are used in Sonora today.

In contrast to fields that conserve runoff water, a third technique, "water harvesting" (Frasier 1975), relies on runoff collected from prepared catchment areas or altered wa- tersheds and diverts this runoff onto fields. In some cases, slopes are cleared of vegetation and loose rocks may be compacted, removed, or stacked into piles to induce runoff (Tad- more et al. 1958). Although water harvesting involves greater labor inputs than the other systems, the better water control makes it more rewarding in terms of total yields and yields per hectare. The best examples of water harvesting techniques are unquestionably the ancient fields that have been reconstructed in the Negev Desert of Israel (Evenari, Shanan, and Tadmore 1971). However, similar agri- cultural systems are found in other arid and semi-arid regions around the world (Nir 1974; Hall, Cannell, and Lawton 1979). Although in- volving no manipulation of the watersheds per se, some temporales in eastern Sonora, espe- cially those in use for several years, have water-harvesting properties in that water is diverted from the arroyo channels by use of brush weirs through canals onto the fields.

Most agro-ecosystems, including those of eastern Sonora, do not fit neatly into one of these three types. Most runoff-dependent ag- riculture involves a combination of field types and agro-ecosystems, contingent largely on local climatic, geomorphic, and edaphic con- ditions (Nabhan 1979, 246-47; Lawton and Wilke 1979, 5). Though various agro-ecosys- tems have been viewed traditionally as spe- cific adaptations to localized environs (e.g., Woosley 1980), they may also be envisioned as stages on a continuum of agro-technolog- ical development (Spencer and Hale 1961; Glassow 1980), as is the case with temporales in eastern Sonora. Most temporales are true floodwater fields during their first few years of existence. They gradually evolve into var- ious types of runoff fields and then into mod- ified versions of water-harvesting systems. Changes like these make construction activ- ities difficult to distinguish from those usually thought of as maintenance and cultivation activities.

The shift from floodwater to runoff farming and then to water harvesting requires capital

127 Agricultural Change

(physical) improvements. Although the actual improvement of temporales through con- struction is difficult if not impossible to doc- ument, development can be envisioned as a two-part process. The selection of a field site and the initial clearing and cultivation com- prise one part of development in which the area under cult ivation is intentionally in- creased up to a legislated m a ~ i m u m . ~ This part of the process involves individual fields and the decisions and actions of independent farmers. Another part of the development process, in which the sizes of the individual fields are not increased per se but rather are maintained, involves the relocation of fences, the trading of fragmented parcels of land, and the communal use of certain agricultural fea- tures. Because a number of farmers are in- volved, the agro-ecosystem as a whole un- dergoes modification and reorganization in conjunction with the second part of temporal development.

Field Establishment

One part of the temporal development pro- cess is defined principally on the basis of land clearance, an activity that has been described as the most difficult agricultural task in parts of Sonora with similar natural vegetation (Holden et al. 1936, 117). Farmers claim that clearing, including the removal of stumps, usually takes about ten years. However, clearing in as few years as three and as many as 45 has been reported.

This difference in clearance rates is largely due to variations in farmers' wealth or amount of available time. The few farmers who have time or money can clear their fields rather quickly or pay to have the job done. Most have other fields that must be worked or other jobs that severely limit the amount of time they can invest in clearing new fields. The temporal farmers of eastern Sonora are not financially secure and, with the possible exception of those who inheri t f ie lds, no one begins farming without some other source of income or means of subsistence. Most farmers take several years to clear their temporales com- pletely. As a result of time constraints, only a small amount of land, averaging approxi- mately 0.25 hectare, is cleared each year; the percentage of the fenced area being culti- vated therefore increases annually.

Although land clearance is the criterion that defines the first part of temporal develop- ment, the removal of trees actually begins as ancillary land improvement. The first im- provement is fencing. Enclosure is usually begun as soon as a site is chosen for culti- vation. Cutting fence posts from mesquite trees growing on the chosen site results au- tomatically in the clearing of a portion of the field. On the average, a fence enclosing a typ- ical 2.5 hectare field requires approximately 2,100 meters of barbed wire and 385 posts. Because as many as 1,000 trees may be found growing on a typical field site, the percentage of trees used for posts and the size of the area cleared initially are both small. Although some planting may be done between trees in un- cleared portions of the enclosure, the cleared portion comprises the principal part of the cultivated area during the early years.

The factor limiting the amount of cultiva- tion is, of course, the time spent in the com- bined clearing and fence building. This ac- tivity takes an average of 56 person-days to complete. Though this figure may seem small, it is a significant amount of time to invest in a field that for some time will serve as only a supplementary source of food or income. It is, in fact, the equivalent of working more than one day extra each week for a year in order to receive a benefit that will not be fully avail- able for several years.

The area under cultivation increases as more land is cleared during subsequent years. As a result, relatively more time is spent in activities other than clearing. During this time it becomes increasingly difficult to differen- tiate between construction, cultivation, and maintenance activities. For example, each year one or two days have to be spent cutting and replacing posts and repairing sections of fence that have been washed away by floods. Is the cutting of new posts a construction (clearance) activity or a maintenance activity (Grossman 1979, 313)? Similarly, are the un- derbrush and small branches that are woven between the wire strands and stacked against the fence intended as reinforcement for aging, weakening enclosures (maintenance) or as a device to slow the velocity and spread of floodwaters (construction)? They are both.

As the cultivated area and yields increase, so too does the need to insure against crop failures. Yields during the first few years of

128 Doolittle

cult ivation are small. Crop losses are ex- pected during this time and are perceived as being unimportant. However, the farmer's de- sire to minimize losses and maximize returns increases with investment. Water control takes on progressively greater importance as the cleared and cultivated area is expanded.

The simplest but least effective way of con- trolling water is to pile cleared brush along the fences parallel to the arroyo channel and along the fences on the upstream side of the field in an attempt to slow the velocity of water flooding the field. The brush eventually de- cays and needs to be replaced regularly; it does not take root and retard erosion per- manently as do the "living fence rows" found along major Sonora streams (e.g., Nabhan and Sheridan 1977).

Because the amount of available brush de- creases as a field becomes older, more effi- cient use is necessary as the supply of brush diminishes. The most common method of uti- lizing small amounts of brush to control large amounts of water in eastern Sonora is the building of water spreaders (Figure 2). These features are found most frequently in fields that are more than 50 percent cleared. Water

spreaders vary in length from five to 40 me- ters, range in number up to seven, and av- erage a total of 19 linear meters per field. Al- though they may be found scattered across a field or in groups, water spreaders are always located in gullies where surface flow is con- centrated. By slowing the velocity of runoff and spreading it over the field, these devices facilitate the deposition of silt and organic materials, thereby providing nutrients and finer materials to the otherwise coarse, sandy soils (Nabhan, et al. 1980, 73). In some cases, water spreaders are built along the down- stream edge of fields in order to halt the ad- vancement of headward erosion. They per- form this function most effectively.

Brush devices require regular maintenance because of decay, and in some cases total replacement because of floodwater destruc- tion. At a construction rate of 18 linear meters per person per day, an average field can have nearly all of its water spreaders replaced (constructed-maintained) in one day each year. Although newer fields require more time for the repair of water spreaders than older fields, maintenance problems tend to be ex- acerbated as the field is increasingly cleared

Figure 2. Brush water spreaders

129 Agricultural Change

and the supply of brush is depleted. Rocks become increasingly more prominent in the construction of water control devices as the percentage of the area under cultivation be- comes larger. Terraces that begin as merely a single row of cobbles laid perpendicular to the flow of water and parallel to the plowed furrows are most frequently encountered in fields that are more than 75 percent cleared. In fact, most of the fields with terraces fall into this category and only a few fields with any amount of terracing are less than 50 per- cent cleared.

The rocks used in the construction of ter- races are not brought to the fields from an outside source, but are uncovered during cul- tivation. Rocks that are unearthed have to be deposited in some convenient, yet out-of-the- way place. The most commonly used locales are along fences, in piles within the enclosed area, and on terraces (Figure 3). The first lo- cale is chosen most often when rocks are unearthed near the edges of the fields. Drop- ping the rocks along fences also has ancillary implications because such rock alignments ameliorate erosion problems where fields border arroyo channels. Rocks that are en- countered in areas not bordering the edges

of fields may be piled within the fields, but most frequently they are aligned across rills, often at regular intervals. Perhaps not supris- ingly, combination water spreader-terraces, or brush-and-rock devices, are not un-common. Of course, water spreaders tend to be increasingly replaced by terraces as fields get older.

The removal of rocks continues as long as the area under cultivation is being increased, so that the rock alignments tend to become longer and higher, growing into small walls that trap sediment while distributing runoff evenly over the planting area (e.g., Netting 1968,58; Sanders, Parsons, and Santley 1979, 248-49). The construction of these terraces cannot be separated from cultivation activi- ties. On many occasions farmers have been seen to interrupt their plowing long enough to carry a rock from a furrow to a terrace. Is this time spent in cultivation or construction? In some cases it could be considered partially as a maintenance activity. Terraces, however crude, do need some periodic repair, but probably not more than one day each year. Terrace building in this fashion is an on-going, never-ending activity of which land filling and leveling are actually ancillary by-products.

Figure 3. A low rock terrace that functions primarily to distribute sheetflow evenly across a field.

130 Doolittle

As with brush, rocks tend to become less abundant as cultivation increases. Although the supply of rocks is never completely ex- hausted, the availability declines markedly after repeated plowing and continuous de- position of silt. However, the need for more and better water control increases because of the need to protect the farmer's invest- ment. New materials must be sought.

The one material that is always abundant is fine sediment. Its use for water control is more common on established fields than on new fields for two reasons. First, and most obvious, sediment is available when other materials are not. Second, because of the high volumes of rapidly flowing floodwater, earthen features would be destroyed easily during the early years of field use. Only in the later years, when other methods of water control have altered runoff characteristics, can bunds be used effectively.

The building of earthen features is usually considered construction. Several studies have focused on the amount of earth that can be moved in a given period of time (Ashbee and Cornwall 1961 ; United Nations 1961 ; Erasmus 1965; Gomez-Pompa, et al. 1982) in order to

complete a new agricultural feature. Con- struction of earthen features in temporales is not so clean cut, however. Bunds, for ex- ample, are partially the product of sediment accumulation. Annual plowing combined with natural sheet flow over the field results in the movement of soil downslope. After extended periods of time, this material accumulates be- hind rock terraces at regular but wide inter- vals across fields, expecially along the down- stream edges. Here it is easily shoveled into small ridges on top of rock terraces (Figure 4) that collect excess runoff, thereby con-serving scarce moisture and retaining soil that can be used to heighten the bund perhaps indefinitely. At least one case is known where the field has been heightened and leveled more than 1.5 meters. Although bunds can involve several cubic meters of material per field, their creation is the result of cultivation and terrace maintenance. On the average, 12 cubic meters of earth extending 33 linear me- ters can be maintained-constructed per person per day. Of course, larger and estab- lished fields with many bunds involve more work than new fields with few bunds.

The transfer of water from natural channels

Figure 4. An earthen bund, showing an exceptionally high amount of maintenance, intended to retain water on a field.

131 Agricultural Change

to fields is one of the most intensive aspects of agriculture (Turner and Doolittle 1978, 300). The construction of diversion and water-car- rying features may, however, be the most mis- understood of all agricultural construction activities. In the case of temporales, water harvesting involves brush-and-stake diver- sion weirs and canals (Figure 5). Construc- tion of weirs is basically the same as that of the water spreaders, only the scale and level of inputs are different. Diversion weirs vary considerably in size, but average between five and ten t imes larger than typical water spreaders. Like the smaller devices, the weirs have to be rebuilt at least annually and some- t imes more often, depending on the fre- quency and intensity of streamflow follow- i n g t h u n d e r s t o r m s i n t h e u p p e r pa r t s of the drainage basin. Construction usually takes a little more than one day per person, but two days may be required for exceptionally large weirs.

Construction of canals is not straightfor- ward and is much more complex than may be apparent at f irst glance. It has long been thought that the principal means by which ca- nals or canal networks were constructed was by periodic systematic extensions (Leach

1961, 16-18, 65; Woodbury 1961, 556; Mo- seley 1978, 16). Canals associated with tem- porales, however, are constructed incremen- tally in a manner that is more complicated but actually requires fewer inputs than simple ex- tension. Construction usually begins during the first, but no later than the second, year of c learance-cul t ivat ion. Sometime dur ing ground preparation, farmers take a few min- utes to plow a short furrow (usually less than 25 meters) from the arroyo channel to the field. These small ditches have the effect of helping to direct the floodwater to selected places. Surprisingly, this simple plowing comprises the bulk of the actual construction inputs for canals because the velocity of the floodwater flowing through the ditches is sufficient to result in degradation. Scouring widens and deepens the ditch. Approximately 25 meters of additional ditch are usually plowed each year until what appears to be a completed canal exists. The process of completing a canal takes an average of about ten years.

Of course, siltation can be a problem any time during the construct ion sequence. An av- erage canal usually silts up less than 25 per- cent of its volume during any flood with a slow discharge, although siltation of 100 per-

Figure 5. A diversion weir and canal that direct water from the arroyo channel onto a field

132 Doolittle

cent was witnessed during the course of this study. Whatever the amount, farmers must re- move sediment periodically, usually after each flood, if the field is to remain in use. Most temporal farmers claim that only one or two days are required to clean their canals after each flood. For the most part, inputs associ- ated with canals are related to maintenance rather than to construction. Only infrequently is excavation involved in construction.

In summary, once fence construction is completed, an activity that results in the initial clearance of a portion of the field, an average of only 8 to 12 working days of combined maintenance-construction are needed each year to keep a temporal operating and ex- panding in cultivated area.5 More time, of course, may be needed if heavy volumes of sheetwash and stream discharge occur more than once each year. Moreover, the time spent maintaining-constructing a temporal tends to remain stable throughout the course of de- velopment. Some features disappear and do not require upkeep as others take their place through incremental construction. All fea- tures result from increased cultivation and in- tensified land use."

F loodp la ,n A r r o y o Bo t t om

Established F l e d 8 Unde r C u t v a t l o o

For More Than 10 Years

Fields Under C u l t ~ v a t f o n For 10 Years Or Less

Date Indicates F ~ r s fYear 01 Use

Agro-ecosystem Reorganization

The second part of the temporal develop- ment process usually begins a year or two after a field is fenced and clearing and culti- vation has begun. Although it is difficult to record reorganizational changes as they occur in the fields, it is possible to observe the re- sult of those changes after several years or decades.

Fields in cultivation for ten years or less tend to be either isolated upstream, or adja- cent and connected to the upstream ends of fields that have existed for more than ten years (Figure 6). This distribution is the result of new farmers choosing locales suitable for floodwater farming. Optimal sites for such ac- tivities are characterized by their proximity to shallow, braided arroyo channels. In such areas, sheetwash through the fields is common because water is neither contained nor directed rapidly downstream through a well-defined channel. For example, channel depths near fields that are less than ten years old range from 0.3 to 0.6 meters (Figure 5), whereas channels near established fields av- erage 2.0 meters deep (Figure 7). The channel

K~ lo rne te r s

Figure 6. Temporales in Arroyo Rancho, near the town of Baviacora.

133 Agricultural Change

Figure 7. The bifurcation of the arroyo channel and a canal rendered useless, and hence abandoned, because of channel downcutting.

depth near the established fields was once as shallow as it is near the newer fields today. Increasing water control through the con- struction of canals and especially, terraces and bunds probably facilitated the restriction of water to a single channel, thereby in- creasing velocity of flow and downcutting. That downcutting has occured since cultiva- tion of arroyos began is evident by the pres- ence of a few relic canals that are higher than the channel bottoms at their points of bifur- cation (Figure 7). Such canals, once impor- tant for directing water to the fields, had to be abandoned in favor of other canals as the arroyo channel was downcut.

Replacement canals are constructed in much the same way as were the original ca- nals-by plowing followed by scouring. How- ever, replacement canals cannot always con- nect the individual f ields and the arroyo channel directly. In order to maintain a gra- dient that would facilitate a sufficient flow of water, each replacement canal would have to be longer than its predecessor. Presumably, a time would come when greater inputs would be expended on canals than on fields them- selves and when a greater amount of arroyo

land would be traversed by canals than would be covered by crops. Instead of each farmer excavating increasingly longer individual ca- nals, a short ditch is plowed between the ter- minal of the canal built by the farmer using the field farthest upstream and the canal of the farmer using the adjacent f ield down- stream (Figure 8). This work involves some cooperation between farmers because a por- tion of the field used by the farmer farthest upstream has to be taken out of cultivation so that his downstream neighbors will have access to water. The sequence of canal re-locations and extensions has been going on for some time and will continue as long as downcutting occurs.

Paralleling the incremental change from in- dividual to communal canals is a gradual shift from private to common fences. Flooding and channel meandering often destroy portions of fences. During reconstruction, fences are rarely rebuilt in their original locations, but are built parallel to the new stream course. Fences other than those destroyed are also moved as the farmer attempts to maintain the cultivated area allotted by law. These fence relocations result in changes in field shapes

134 Doolittle

N e w Weir

0 Abandoned

N e w C a n a l

- -R e l o c a t e d

.... Abandoned

Figure 8. Schematic of sequential canal reloca-tions and extensions.

and locations. Furthermore, the movement of fences tends to eliminate the isolation of in-dividual fields so evident in their earlier years of use. Adjacent fields with fences shared by two farmers become commonplace as more land is brought into use and as portions of fields are exchanged in order to prevent frag-mentation of usership. Whereas braided channels, tracts of uncleared areas, and iso-lated fields with individual canals are common during the early years of use, incised chan-nels, few uncultivated areas, and common fences with connected fields and canals abound in later years. Reorganization of ag-ricultural space is a significant characteristic of the temporal landscape.

Conclusion

The description of structural changes in the temporal agro-ecosystems of eastern Sonora

conforms to that expected in the incremental view. The process of change has been gradual, resulting from small capital improvements made largely by individual farmers during cul t ivat ion (Green 1980, 340, 344). Even though farmers operate within the social, cul-tural, and economic milieu of the usufruct system of the ejido (Mexican agricultural community), their actions are, with the ex-ception of a few activities such as the linking of canals, independent of each other. Com-munal and large-scale, short-term construc-tion inputs are not incorporated. Similarly, only minimal formal or long-range planning is involved. In many respects, development of the temporal agro-ecosystem is the product of in-the-field trial and error; engineering ex-pertise is minimal and initial high-cost tech-nology would not be of much value in the continuously changing arroyo environs.

The temporales of eastern Sonora admit-tedly are neither numerous nor of great im-portance to the regional or national economy. Nevertheless, the broad implications of their development are significant to theories of ag-ricultural change and may provide insight into the origins of agro-ecosystems. The study of temporales indicates that a thorough under-standing of the nature and t iming of con-struction activities is critical for assessing how agro-ecosystems change.

The incremental process described here suggests that studies of past or relic agro-ecosystems must be cautious of inferring process from pattern. Sychronic or cross-cul-tural investigations of past agro-ecosystems relying on contemporary construction ana-logs may not reveal much in terms of how a field complex was developed (Boserup 1965, 59). The temporales are an example of a field system constructed with rather low levels of inputs. These levels are low because con-struction is essentially the result of cultivation and maintenance activities. In contrast, the view of agricultural change as systematic im-plies a distinctive sequence of construction, cultivation, and maintenance, each with its own scale of inputs. Relic agro-ecosystems that are thought to have been developed sys-tematically may have developed incremen-tally. If this is the case, then previous views of the role of agriculture in the development of past cultures may have to be revised. For example, if i t can be demonstrated that the

Agricultural Change

irrigation system of the ancient Hohokam cul- ture in what is today southern Arizona was constructed incrementally rather than syste- matically, then explanations based on inde- pendent development (e.g., Doyel 1980) may be more viable than the currently more pop- ular theory that irrigation technology was in- troduced by people migrating from Mesoam- erica (e.g., Haury 1976).

Although an alternative perspective on ag- ricultural change, the incremental process does not suggest that systematic change does not occur in many cases, or that the input- output postulate on which systematic change is based is wrong. It may appear, for example, that the temporal evidence contradicts many studies of agricultural change (e.g., Brown and Podolefsky 1976; Turner, Hanham, and Por- tararo 1977) because an agro-ecosystem was constructed without the application of many inputs in a short period of time. On the con- trary, the scale of temporal inputs is deceiv- ingly low because construction, cultivation, and maintenance are inextricably mixed ac- tivities. Large and technologically sophisti- cated agro-ecosystems generally require more inputs for their continued use than do small and technologically simple ones. Many studies of past agro-ecosystem changes may have erred in assuming that high levels of inputs were expended in initial construction and that these inputs were dist inct f rom, and ex- pended in addition to, those related to culti- vation and maintenance.

Acknowledgements

I thank 6.L. Turner II, Ian R. Manners, C. S. Davies, Robin Doughty, J. Richard Jones, Kent Mathewson, Gary Paul Nabhan, Thomas Sheridan, Larry Grossman, and the anonymous reviewers for their comments and input; and William McCary, Stephen C. Lebo, Ricardo Loera, and numerous Mexican farmers and government officials for their assis- tance. This material is based upon work supported by the National Science Foundation under Grant No. SES-8200546, and by grants from the Biolog- ical and Physical Sciences Research Institute and the Vice President for Graduate Studies and Re- search, Mississippi State University. Earlier ver- sions of this paper were read at a colloquium in the Department of Anthropology, The University of Texas at Austin, and at the 1983 Annual Meeting of the Association of American Geographers, Denver, Colorado.

Notes

1. A third, revolutionary scheme in which large and complex agro-ecosystems are entirely con- structed in a brief period of time is also pos- sible. However, the necessarily high inputs, levels of technology, and ancillary social re- quirements needed for such monumental change are unavailable in many cases, partic- ularly in times past (Wilken 1976, 419-20).

2. The Spanish term temporal, meaning tempo- rary, weather, storm, and a spell of rainy weather, is applied to fields that rely on direct rainfall, runoff, or a combination of both water sources. The focus of this study is on small fields that rely principally on runoff but do receive some direct rainfall. These fields are typically planted in near-subsistence or locally market- able crops, principally maize. Large fields that are "dry farmed" (Lawton and Wilke 1979, 3- 4) exclusively are not considered. Such fields are planted in commercially important crops such as sorghum and cotton; hence, they re- ceive much assistance from the Mexican gov- ernment.

3. Another solution, and one that is far more im- portant in terms of total cultivated area, is the irrigation of the floodplains of major rivers.

4, Individual farmers in the Ejido Baviacora are allowed to cultivate a maximum of three hec- tares under the current laws of the usufruct system.

5. A typical work day begins around 6:00 a.m. and ends when working under the summer sun be- comes unbearable, around noon. Some agri- cultural work is done in the winter months. Most, however, is done during the principal maize- growing season, July through September.

6. For reference and comparative purposes, a fully established 2.5 hectare temporal involves an average of 52 person-days of cultivation inputs annually: plowing, 6; planting, 6; weeding, 12; harvesting, 28. Proportionately less time is spent on fields with smaller cleared areas.

7. If land is available, farmers will move fences in order to retain as much of their allotted three hectare area as is possible whenever portions of the temporales are destroyed.

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Correction to "Arctic Ocean Ice and Climate: Perspectives on a Century of Polar Research" by R. G. Barry

In the paper "Arctic Ocean Ice and Climate: Perspectives on a Century of Polar Research" by R. G. Barry (Annals o f the Associat ion o f Amer ican Geographers, 73(4), 1983, pp. 485-501) there are two mistakes in the text. On

page 494, first column, line 26 should read "is weakly related to winds or summer tem- peratures. . . . " On page 495, first column, line 26 should read " . . . to Arctic tempera- tures, from 4 out to 18 months. . . . "