Thermal Analysis of Tiwanaku Raised Field Systems in the Lake Titicaca Basin of Bolivia

Journal of Archaeological Science 1989,16,233-263

Alan L. Kolata’ and Charles Ortloff’

(Received 9 June 1988, revised manuscript accepted 12 October 1988)

Raised field systems of agriculture in seasonally or perennially inundated landscapes have received increasing attention from scholars involved in the analysis of prehistoric agricultural intensification in the New World. This paper discussesthe morphology and function of raised fields associated with the Tiwanaku civilization on the southern rim of Lake Titicaca in Bolivia. The thermal properties, and specifically, the heat storage capacity of raised fields in this high altitude environment are analysed by means of an ANSYS finite element computer model. The analysis concludes that enhanced heat storage capacity was an essential design element of raised field agriculture in the Andean altiplano, and that this thermal effect served to mitigate the chronic hazard of frost damage to maturing crops in this rigorous environment. An experimental verification of this conclusion based on the performance of reconstructed raised fields subjected to severe sub-freezing conditions is briefly described. Keywords: AGRICULTURE, TECHNOLOGY, HYDROLOGY, RAISED FIELDS, SOUTH AMERICA, ANDES, BOLIVIA, COMPUTER MODELING.

Introduction The ability of irrigation agriculture to support large populations and underwrite the agrarian economy of state level society has long been recognized. Most historical and interpretive studies of agricultural reclamation have been concerned with the technologi- cal, sociological, and political implications of hydraulic agriculture in arid lands (Wittfogel, 1938, 1957). The interplay between social institutions, political organization, and agrarian economy based on artificial water distribution has been the subject of a large body of literature (Steward, 1955; Boserup, 1965; Sanders & Price, 1968; Fernea, 1970; Price, 1971; Sanders, 1972; Mitchell, 1973; Downing & Gibson, 1974 and many others). However, the potential of intensive raised field agriculture to provide a similar economic base and to sustain demographic growth is little known. In response to this lacuna, recent interest in the problem of agricultural reclamation in perennially or periodically inundated landscapes has expanded dramatically. The initial product of this emergent interest has been substantial new information on the morphology, functions and implications of raised, drained field agric.ulture in various regions and culture areas of the New World (Denevan, 1980, 1982). A sampling of such studies reflects the geographical “Departmentof Anthropology,The Universityof Chicago,U.S.A. *Central EngineeringLaboratories, FMC Corporation, SantaClara, California, U.S.A. 233 0305-4403/89/030233+31$03.00/0 0 1989Academic Press Limited 234 A. L. KOLATA AND C. ORTLOFF

Figure 1. Location of the study area on the southeastern shore of Lake Titicaca in Bolivia. breadth in the distribution of these paleohydraulic systems, as well as the relatively recent character of scholarly inquiry into them (MAYA-Siemens & Puleston, 1972; Turner, 1974, 1978, 1983; Matheny, 1976, 1978; Harrison, 1977, 1978; Puleston, 1977; Siemens, 1978, 1982, 1983a; Adams, 1980; Scarborough, 1983; VERACRUZ-Siemens, 19836; Wilkerson, 1983; VALLEY OF MEXICO-Armillas, 1971; Calnek, 1972; Parsons, 1976; Parsons et al., 1982; VENEZUELA-Zucchi, 1972; COLUMBIA-West, 1959; Parsons & Bowen, 1966; Broadbent, 1966, 1968; Eidt, 1984; ECUADOR-Parsons, 1969; Batchelor, 1980; Knapp, 1981; Denevan & Mathewson, 1983; Knapp & Ryder, 1983; PERU-Parsons & Denevan, 1967; Smith et al., 1968; Lennon, 1983; Erickson, 1985; BOLIVIA-Pflaker, 1963; Denevan, 1964,1966; Kolata, 1986). In 1986 a binational, interdisciplinary research project directed by one of the authors (A.L.K.) initiated an intensive examination of raised field systems in the near hinterland of the pre-Hispanic city of Tiwanaku (Tiahuanaco) on the southern rim of Lake Titicaca in northwestern Bolivia (Figure 1). This research was centered on a 70 km2 zone referred to TIWANAKU RAISED FIELD SYSTEMS 235 locally as the Pampa Koani (Figure 2). During the 1986 season, excavations in two hydrologically discrete raised field segments were undertaken to examine in greater detail the temporal context, as well as the methods of construction, modification and use of raised field systems within the Tiwanaku hinterland. These excavations, along with correlative investigations of local topography and groundwater, were also intended to form the empirical base line for a computer generated model of raised field function in the Andean altiplano. This paper reports the results of one aspect of that model: the thermal properties of raised field systems in the context of the specific hydrological and climatological conditions characteristic of the littoral and near-shore environment of Lake Titicaca.

Setting Lake Titicaca is located at 16” S, 69” W at 3810 m above sea level in the Andean altiplano. The bedrock of the Titicaca basin is mostly igneous (basalts and andesites) with some sedimentary rocks, mostly shales, sandstones, and isolated pockets of limestone (Newell, 1949). The altiplano and the lake were formed in the Miocene with the rise of the Andes (James, 1971), and attained present form in the Plio-Pleistocene (Lavenu, 1981). Annual rainfall near Lake Titicaca averages 687 f 138 mm (Boulange & Aquize-Jaen, 1981: tables V 8z VI) which falls principally in a wet season between December-March. Annual mean air temperature at lake elevation is c. 9°C with an amplitude of c. 12°C (Boulange & Aquize-Jaen, 198 1). As is characteristic of tropical systems, diurnal temperature variation is greater than seasonal variation. Mean temperature of surfZa1 waters is 1 l-15°C. Average incident radiation is high at 520 cal cmm2 day-‘, and has a subdued seasonality. Evaporative losses account for virtually all of the precipitation in the basin and only a small amount of water leaves Lake Titicaca through its single, sluggish outlet at the Rio Desaguadero, or by deep seepage (Winter, 1981). Over the past 100,000 years the lake has been as much as 100 m above its current level (Servant & Fontes, 1978). Some sedimentological studies suggest a lake-level fluctuation of + 10 to - 50 m for the past 12,000 years (Wirrmann & Oliveira Almeida, 1987). In historical times, the lake level has fluctuated as much as 5 m within a 2 year period [Carmouze & Aquize-Jaen, 1981: figure 3(b)]. The last major fluctuation occurred between September 1985-April1986 when the level of the lake rose nearly 3 m, inundating many square kilometers of the littoral zone in Peru and Bolivia and destroying at least 11,000 ha of agricultural fields planted in potato and other crops. The terrestrial vegetation of the altiplano is characterized by associations of compo- sites, grasses, legumes and solanaceous plants. Dominance of one group over another is primarily dependent upon local precipitation. Edaphic communities occur in areas of sandy, moist soils, or in zones possessing high groundwater (Cabrera, 1968). In general agricultural crops are segregated altitudinally (Bowman, 1916; Galdo Pagaza, 1981). Maize, barley and wheat are grown below 3350 m. Potatoes and quinoa (Chenopodium quinoa) flourish up to 40004200 m, while caiiiwa (Chenopodium pallidicaule) grows to 4300 m. Above 4300 m no agricultural crops are grown, and this high zone supporting natural vegetation such as bunch grasses is given over to pasturage for domestic animals, particularly llama and alpaca. Many indigenous crops of the altiplano are derived from the same families as natural vegetation.

Tiwanaku Raised Field Systems The excavation and analysis of raised fields in the near hinterland of Tiwanaku was initiated with a pilot programme spanning the period from 1979-82 conducted under the aegis of the Instituto National de Arqueologia de Bolivia (INAR). This pilot programme of research, which was directed by Alan Kolata and Oswald0 Rivera of INAR, focused on Figure 2. Map of the Pampa Koani raised field system study area. TIWANAKU RAISED FIELD SYSTEMS 237

Swole with standing water H Excavated section n c 0 IO 30 M Figure 3. Plan and section of raised field segment in the Lacaya sector. This field geometry is used as the basis for the finite element model of field function discussed in the text. large scale raised field systems in the Pampa Koani zone, which Kolata (1987: 188) now designates as the northern component of the Tiwanaku sustaining area. During the pilot programme, test trenches were excavated across raised field segments in the Lakaya sector of the Koani zone in order to determine their chronological context and cultural affiliation, and to examine the technology of construction, modification and use of these massive structures. Diagnostic ceramics within the context of these agricultural structures were extremely rare. However, direct structural association between field segments and habitation mounds containing such diagnostics indicated a clear chronological and cultural affiliation with the Tiwanaku IV-V period (c, AD 400-1200). The possibility of earlier Tiwanaku III (c. AD 10&400) utilization of habitation mounds and raised field segments was also raised by the initial work of the pilot programme, but in the absence of clear, unambiguous ceramic associations this conclusion remains tentative (Kolata, 1986: table 1). Stratigraphic examination of the fields indicated that they were raised earth platforms formed by excavating soil from both sides of the projected field surface and mounding it in the center (Figure 3) This straightforward technique duplicates the process that has been reconstructed for raised field systems elsewhere in the Titicaca basin and in other areas of the ancient Americas. Repeated field surface modifications, and maintenance episodes in the adjoining swales, which averages 3-5 m in width, were recorded in these strata cuts. Together with the zonal settlement analysis undertaken simultaneously during the pilot project, this evidence implied that the Tiwanaku state designed, constructed, and managed a labour and technology intensive agricultural landscape capable of generating sustained surplus production of food crops (see Kolata, 1986 for an elaboration of this hypothesis). A new phase of long-term interdisciplinary research examining the technology and organization of intensive agricultural production in the Tiwanaku state was inaugurated in 1986 near the important regional administrative site of Lukurmata. Aerial photographs indicated that a sector of agricultural fields was incorporated within the site boundaries of 238 A. L. KOLATA AND C. ORTLOFF

Figure 4. Aerial photograph of the site of Lukurmata. This photograph clearly shows the massive artificial canal and agricultural sector that cut access to the principal civic-ceremonial sector of the site. The canalized quebrada in the southern sector of the site can also be seen. (Photograph courtesy of the Institute Geografico Militar, La Paz, Bolivia, Series 54060, Sheet 20520).

Lukurmata (Figure 4). The fields themselves were built in a broad, crescent-shaped strip of land that lies between the civic-ceremonial district of the site (the acropolis of Lukurmata) and the steep hillslopes that constitute the site’s southern boundary (Figure 5). This location was particularly appropriate for exploiting drainage from a quebrada cutting into the southern hillslopes of the settlement. This quebrada was artificially canalized in the Tiwanaku IV period through construction of stone revetment walls. These hillslopes are sources of fresh spring water which flows as groundwater northward into a topographic low on which the raised fields of Lukurmata are constructed. The ground water ultimately discharges into Lake Titicaca itself. The agricultural sector at Lukurmata is drained by an artificial canal that surrounds the acropolis escarpment of Lukurmata. Apart from its drainage function, this canal also acts as a moat cutting easy access to the civic-ceremonial architecture that surmounts this escarpment. Water within this canal flows in two directions: to the east and to the west around the peninsula-like acropolis sector of the site into Lake Titicaca. This ununsual bifurcated drainage results from local topographic conditions in combination with direc- tional flow from the canalized quebrada to the south of the canal. Flow from the quebrada TIWANAKU RAISED FIELD SYSTEMS 239

Figure 5. Contour map of Lukurmata with various functional sectors of the site emphasized. The canalized queimdu cutting into the southern hillslopes of the site appears prominently in this map. was directed into a trapezoidal-shaped aqueduct constructed of earth, cobble and gravel fill faced by stone retaining walls. The linked canalized quebrudu-aqueduct system discharged water into the surrounding canal in an easterly direction. The strip of land dedicated to agricultural constructions encompassed approximately 65,000 m*, or 6.5 ha of the site, representing approximately 3% of the total estimated settlement surface. This relatively small agricultural sector could not have supplied the entire subsistence requirements of a settlement as large as Lukurmata. Clearly the bulk of the population at Lukurmata must have derived its sustenance from fields outside of the site boundaries. The massive field systems in the adjoining Pampa Koani zone are the logical candidates for the source of this non-local production. 240 A. L. KOLATA AND C. ORTLOFF

Results of Raised Field Excavations at Lukunnata The morphological and functional properties of high altitude raised field systems modeled in this paper are based on data from excavations in the fields at Lukurmata. A summary of the internal structural characteristics of Tiwanaku raised field systems is essential to understanding the variables relating to field function that have been factored into this model. As indicated above, the raised field system preserved on the surface at Lukurmata consisted of two distint hydrological units demarcated by the breakpoint of bifurcation in the flow of water into the canal which defines the northernmost extension of the agricultural sector at the site. The surface fields and adjoining swales that constitute these two hydrological units are large curvilinear constructions of soil, gravel, and clay that arc around the base of the civic-ceremonial acropolis for a distance of approximately 800 m. The surface fields themselves were exceptionally large forming cultivation platforms that averaged over 10 m in width. The adjacent swales related to these field surfaces were smaller, ranging from 3 to 5 m in width. Two test trenches (operations 1 and 2) that were cut into these surface field constructions encountered distinct sets of stratigraphically separate (that is, vertically stacked) buried raised or ridged fields, representing repeated episodes of field construction confirming that the area had formed part of an ancient landscape of long term agricultural reclamation. As outlined below, these buried fields took several different forms.

Operation I: test trench 1: N2633-N2636 E2948 Buried raised field surfaces were encountered in this trench at a depth of 60 cm below the local datum (Figure 6: stratum 6). Here the soil changed from a dark brown fine sand to a dark grayish-brown loamy clay. This darker, more organic material was mounded to form several agricultural beds (Figure 6: stratum 6). These beds were oriented in the same east-west direction as their counterparts on the surface, although they are substantially smaller in wavelength, approaching 4.5 m when measured from adjacent centers-of-fields. The wavelength of the surface fields taken from the same center-of-field points of reference ranged between 12 and 15 m, or approximately three times that of the buried fields. The overall length of the buried field system was not determined, but as noted above, the surface field constructions both at Lukurmata and on the adjacent Pampa Koani run for several hundred meters. No diagnostic artifacts were encountered directly within this set of buried fields. How- ever, diagnostic Tiwanaku IV and V ceramics were recovered from overlying soil contexts, indicating that these buried fields will date minimally to that period. They may, of course, relate to an earlier phase. The conspicuous absence of post-Tiwanaku and Inca ceramics within these strata, and within the agricultural sector of the site as a whole, strengthens this hypothetical chronology. Furthermore a minimal Tiwanaku IV-V date for these fields is entirely consistent with the similar relative dating of the adjacent Pampa Koani field systems (Kolata, 1986: 7545). Two distinct buried field surfaces appeared in cross-section in the trench profile (Figure 6). As indicated in this profile, the buried fields were roughly 20 cm in height, from top of field surface to the base of the adjacent swale. However, this measurement may not reflect the original morphology of the fields when in production given the probability that processes of swale infilling and surface erosion were active throughout the life of the field system. The homogeneity of the soil matrix forming the agricultural beds indicated that they were the product of a single episode of field construction. The actual planting surface of the buried fields consisted of a loamy clay in which no microstratigraphy was apparent. The agricultural beds presented the appearance of a mixed matrix that was repeatedly disturbed, churned, and homogenized, perhaps through extensive use of a Figure 6. Stratigraphic profile of Tiwanaku raised field segment at Lukurmata: N2633 E2948 to N2636 E2948. Key: l-10 yr 3/3 dark brown loamy clay with some gravel. Sticky when wet, friable when moist, very hard when dry. Contains infrequent carbon fragments (larger than flecks). This is the modem plow zone; 2-10 yr 4/3 brown loamy clay with some gravel and frequent white (salt) inclusions. Sticky when wet, friable when moist, hard when dry. Contains infrequent carbon fragments; 3-10 yr 4/3 brown sandy clay with some gravel and frequent white (salt) inclusions. Sticky when wet, friable when moist. Appears quite similar to no. 2 above, although retains moisture in profile. Contains occasional carbon; 4-10 yr 4/4 dark yellowish-brown, medium sand with very little gravel. Not sticky when wet, not friable when moist. Seen at north end of the trench only, overlies raised fields and associated surfaces; 5-10 yr 4/4 dark yellowish-brown loamy clay, contains lighter concretions. Very sticky when wet, very friable when moist; 610 yr 4/2 dark grayish-brown loamy clay with very little gravel. Very sticky when wet, very friable when moist. Contains carbon. This is an agricultural soil; 7-10 yr 3/3 dark brown clay, with no gravel. Very sticky when wet, very friable when moist. Contains flecks of carbon; 8-10 yr 4/4 dark yellowish-brown fine sand. Not sticky when wet, not friable when moist. Some gravel; 9-10 yr 4/4 dark yellowish-brown sandy clay (very fine sand grains); 10-10 yr 3/2 very dark grayish-brown loamy clay. Sticky when wet, friable when moist. This is a swale soil, somewhat uniform but some mottling present. 242 A. L. KOLATA AND C. ORTLOFF chaqui-t&la-like implement (the characteristic Andean digging stick) during episodes of field preparation and planting. Microstratigraphy within the adjoining swales (Figure 6: strata 4G, 5 & 7) implies that this set of buried fields was in production for several years, and that periodic maintenance in the form of field resurfacing and partial reconstruction was necessary. A layer of sand and gravel overlies these fields indicating that an episode of flooding occurred soon after, or at the time of field abandonment (Figure 6: strata 8 & 8G). Thin bands of microstratigraphy appeared clearly within this stratum in the form of water sorted sands and fine gravel with coarser material deposited at the bottom of the lens. This coarser material was more extensively deposited close to the edge of the adjacent swale, indicating that flooding within the channel with subsequent flow over adjoining field surfaces caused this characteristic pattern of water-borne sand and gravel deposition. Abnormally high rainfall, possibly combined with an elevated lake level may explain this depositional event. Such stratigraphic evidence of ancient flooding events, if confirmed by collateral data drawn from the project’s programme of lake coring and from future field excavations, may provide essential insights into understanding the processes of raised field burial and abandonment in the Lake Titicaca basin.

Operation 2: test trench 2: N2638 E2882 Two distinct types of buried fields were uncovered within this trench. The first type was similar to that encountered in test trench 1: non-complex, homogeneous mounds of dark, loamy clay forming agricultural beds (Figure 7: stratum 2). However, the soil forming these beds differed from that of the trench 1 context in colour and texture: this loamy clay was yellowish-brown in colour and contained a greater admixture of fine sand. We believe these differences relate to minor variations in the distribution of soils within the agricultural sector at Lukurmata, and that the general similarity in field morphology, context, and structure between these two sets of buried fields indicates contemporaneity. Moreover as indicated below, ceramic associations with this first type of buried field in test trench 2 supports a conclusion of contemporaneity with the set of buried fields in trench 1.

Type 1 buried$elds This first type of buried field was uncovered within the southern end of test trench 2: units N2635 E2882 and N2638 E2882 in the Lukurmata grid map. The buried field was first encountered at 80 cm below local datum. Several Tiwanaku IV sherds were found within the base of the field itself, as well as in overlying strata. In particular, one fragment of a decorated, banded vessel bearing the distinctive polished deep red and black colour scheme of the Tiwanaku IV period (c. AD 400-800) was recovered from the base of the field. These ceramic associations suggest that the field structure dates to this period, although an earlier dating cannot be discounted. No post-Tiwanaku or Inca sherds were encountered in this excavation or in the surrounding surface areas. Unfortunately, there was insufficient organic material in this field construction suitable for a radiocarbon assay. As indicated above, this buried field was a simple mound of soil approximately 1545 cm wide oriented parallel to the modern drainage pattern in the same east-west direction as the surface raised fields. In profile, this field appeared roughly symmetrical in shape, but with a steeper declination along its southern border (Figure 7). At this point, several resurfacings of the original field occur in the profile (Figure 7: stratum 2). Here resurfacing of the raised field appears as arcs of loamy clay that curved over the southern border of the field contour. Each of these arcs was separated from the previous one by a stratum of yellowish-brown loamy clay identical to the field base (Figure 7: strata 2 & 3). The soil constituting the features that we interpret as sequential field resurfacing was Figure 7. Stratigraphic profile of Tiwanaku raised field segment at Lukurmata: N2638 E2882. This protile illustrates buried agricultural features including sequential field resurfacings (stratum 2). Key: l-10 yr 3/3 dark brown loamy clay, sticky when wet, friable when moist; 2-10 yr 3/4 dark yellowish-brown loamy clay, sticky when wet, friable when moist. Occasional carbon flecks; 2’-Same as no. 2 with mottling ofno. 3; 3-10 yr 4/6 dark yellowish-brown loamy clay, sticky when wet, friable when moist; 4-10 yr 4/6 dark yellowish-brown fine sand, not sticky when wet, not friable when moist; 5-10 yr 3/6 dark yellowish medium sand with gravel, pebbles and occasional small cobbles, not sticky when wet, not friable when moist; 6-10 yr 3/4 dark yellowish-brown (mottled) coarse sand with large gravel, pebbles and cobbles, not sticky when wet, not friable when moist; 7-gravel; 8-10 yr 4/6 dark yellowish- brown sand with much gravel, pebbles, and occasional cobbles, not sticky when wet, not friable when moist; !J--Same as no. 2 with gravel and occasional small pebbles, also white (salt) concretions; 10-10 yr 3/3 dark brown loamy clay with gravel and occasional small pebbles, sticky when wet, friable when moist; 1 l-10 yr 4/4 dark yellowish-brown clay with mixture of sand and gravel, slightly sticky when wet, friable when moist; 12- 10 yr 4/6 dark yellowish-brown sandy loam with pockets of sandy clay, coarse gravel and occasional pebbles, stickiness and friability variable; 13- 10 yr 4/2 dark grayish-brown clay, with carbon flecks, very sticky when wet, very friable when moist. 244 A. L. KOLATA AND C. OR-II-OFF darker in cofour, bearing a larger organic content, as might be anticipated from its intended function as “rejuvenated”, organically-enriched planting surfaces. At least five of these resurfacing features were identified in the profile, each capping the other. However, some of these arcs of organic soil are ephemeral, and difficult to distinguish in the highly complex stratigraphy that characterizes these buried agricultural constructions. Accordingly, we cannot state with certainty that this represents the entire sequence of resurfacing. Each of these features was approximately 5 cm thick, and each represents an eroded planting surface. Together they indicate that the field had multiple seasons of use, with regular periods of maintenance and resurfacing. It is difficult to estimate the interval between successive resurfacings of the field, but the relatively substantial soil deposition between these features suggests that this was not an annual event. However, distinct periods of field resurfacing with presumptive replacement of organic rich nutrients from adjacent wale contexts establishes quite clearly that these fields were maintained and in production for a substantial period of time.

Type 2 buriedjelh A second, strikingly different type of buried field was also encountered in test trench 2. This buried field construction is morphologically similar to the raised fields on the surface: both field systems are characterized by massive planting platforms with a wavelength that exceeds 10 m. However, the second type of buried field in test trench 2 possesses a remarkably sophisticated internal composite structure that was not found elsewhere in the agricultural sector at Lukurmata. This second type of raised field was first encountered in the southern end of the trench in Units N2635 E2882 and N263g E2882. This buried field extended south and west, forming a more massive structure than any described above. Futher excavations to the west in Units N2635 E2878.5, N2635 E2877 and N2638 E2880.5 uncovered larger segments of this construction. Excavation of the field revealed a complex, precisely layered sequence of cobbles, clay, sorted gravels, and top soil (Figure 8). The bottom stratum consisted of a packed, level layer of rounded cobbles which functioned as ballast, forming a massive, stable pediment or foundation for the field base (Figure 8). These cobbles were apparently transported to the construction site from the quebrudu that cuts through the southern hillslopes of Lukunnata. The function of the packed cobble pediment seems clear. Since the raised fields in the agricultural sector of Lukurmata were constructed in a topographic low, they would have required some form of stable foundation that provided superior drainage Properties to counteract the excessively marshy conditions of this area. The architects of this field construction then laid down over the cobble pediment a layer of pure, dense clay of apparent lacustrine origin approximately 5-8 cm thick (Figure 8: stratum 13). The presumptive function of this clay stratum, which to our knowledge has never been described for any other raised field construction in the New World, is of special interest and importance for understanding the processes of agricultural reclamation along lake beds in the Titicaca basin. We currently hypothesize that this clay stratum served as an aquaclude which inhibited the percolation of slightly saline water from Lake Titicaca into the main body of the field structure. That is, the clay cap over the cobble pediment acted as an impermeable layer that was designed to mitigate the problem of the migration of salts upward into the crucial zone of root development within the field. Alternatively (or perhaps simultaneously), this clay stratum may have served as a “micro-aquifer” maintaining a thin, evenly distributed lens of fresh water at a stable level into which the root.systems of the food crops could tap. The principal sources of fresh water in this vicinity lie in a series of permanent springs and seasonal streams originating in the mountain slopes to the south of Lukurmata. In effect, both seasonally flowing Figure 8. Stratigraphic profile of Tiwanaku raised field segment at Lukurmata: N2635 E2882 to N2635 E2877. This profile illustrates a series ofcomplex internal structural features of the type 2 buried fields discussed in the text. Key: l-10 yr 3/3 dark brown loamy clay, sticky when wet, friable when moist; 2-10 yr 3/4 dark yellowish-brown loamy clay, sticky when wet, friable when moist. Occasional carbon flecks; 2’-Same as no. 2 with mottling of no. 3; 3-10 yr 4/6 dark yellowish-brown loamy clay, sticky when wet, friable when moist; 4-10 yr 4/6 dark yellowish-brown fine sand, not sticky when wet, not friable when moist; 5-10 yr 3/6 dark yellowish medium sand with gravel, pebbles and occasional small cobbles, not sticky when wet, not friable when moist; 6-10 yr 3/4 dark yellowish-brown (mottled) coarse sand with large gravel, pebbles and cobbles, not sticky when wet, not friable when moist; ‘I-gravel; 8-10 yr 4/6 dark yellowish-brown sand with much gravel, pebbles, and occasional cobbles, not sticky when wet, not friable when moist; !&--Same as no. 2 with gravel and occasional small pebbles, also white (salt) concretions; 10-10 yr 3/3 dark brown loamy clay with gravel and occasional small pebbles, sticky when wet, friable when moist; 1 l-10 yr 4/4 dark yellowish-brown clay with mixture of sand and gravel, slightly sticky when wet, friable when moist; 12-10 yr 4/6 dark yellowish-brown sandy loam with pockets ofsandy clay, coarse gravel and occasional pebbles, stickiness and friability variable; 13-10 yr 4/2 dark grayish-brown clay, with carbon flecks, very sticky when wet, very friable when moist. 246 A. L. KOLATA AND C. ORTLOFF surface water and permanent ground water percolating down from the heads of the mountain springs would have charged the micro-aquifer formed by the clay layer of the fields year round. Surmounting the clay stratum of this buried field were two layers of sorted gravels and sands and a final layer of top soil that capped the entire field surface. The first layer of gravel immediately covering the clay stratum was characterized by a mix of medium to coarse grained sands with relatively large pebble inclusions (c. l&-1.5 cm diameter) (Figure 8: stratum 5). The second layer of gravel consisted of fine grained sands and small pebble inclusions ( < 0.5 cm diameter) with an admixture of sandy clay (Figure 8: stratum 12). These layers of carefully sorted sands and gravel, transported at the cost of substantial labour investment, clearly functioned to enhance drainage within the body of the field structure, thereby preventing oversaturation and subsequent crop damage. The final stratum in this structure was the agricultural topsoil which originally covered the entire domed surface contour of the buried field (Figure 8: stratum 2). This topsoil consisted of composite layers of loamy clay of a dark gray colour reflecting a constitution higher in organic matter than the deeper, structural strata of the field. We may reasonably infer that the immediate source of this topsoil was the rich, organic mud deposits extracted from adjacent swales. As might be anticipated, this original topsoil showed evidence of erosion, particularly toward the field borders that sloped downward toward the adjacent swales. We assume that when this now buried field was in production, the layer of agricultural topsoil was considerably thicker than its current average of approximately 10 cm. Evidence for agricultural resurfacing of this buried field was also detected in cross- section (Figure 8: strata 2 & 4). Multiple planting surfaces were documented in the western portion of the field, as well as along the eastern periphery of the structure as it slopes downward toward an adjacent swale. These soil layers were similar in colour and texture to the resurfacing observed in other contexts described above: dark, yellowish-brown loamy clays. These resurfacing layers were also similar in thickness, roughly 8 cm, and were separated by a loamy clay of a lighter colour. The exceptionally complex stratigraphy evident near the adjacent swale suggests that this was the source of material for the multiple resurfacings (Figure 8). Microstrata formed arc-like patterns near the swale, but the tortuous intercalation of soil deposits at this point prevented us from following these arcing patterns out over the adjoining field surface. However, the general implication of this stratigraphy is clear: soils from swales were periodically dug up and redistributed over field surfaces to provide a new, richly organic planting surface. Chemical analysis of soil samples drawn from different contexts in the excavated raised fields, such as from swales, agricultural topsoils, and field construction fill, supports the interpretation that these sediments represent anthrosols, or human altered soils (Eidt, 1977). In particular, phosphate fractionations of selected soil samples from the Lukurmata fields exhibit high total phosphate values indicative of anthropogenic sediments (Table 1). According to Eidt (in litt. 24/04/87), these values are comparable to results of samples extracted from buried raised fields in Colombia that suggest a land use pattern of long-term mixed residential and associated agricultural activities. Such a conclusion is consonant with the archaeological evidence for the Tiwanaku IV-V phase settlement activities at Lukurmata. A substantial quantity of ceramics were found incorporated within the various layers of structured fill in this buried field construction (approximately 250 m2 in a given level). The bulk of these sherds derived from undecorated utilitarian vessels. However, roughly 10% represented fragments from decorated ceramics, particularly keros, a high status drinking vessel characteristic of Tiwanaku occupations. Preliminary analysis of these sherds indicates that these decorated vessels had a red or orange-red slip, and were painted in Tiwanaku IV colour schemes and motifs. Of particular note was the modelled foot of a Table 1. Phosphate fractionation values for soil samples drawn from multiple contexts in buried raisedfield sediments

P in ppm % P Sample Depth below no. Unit Profile datum (cm) Context Fr. Ia Fr. Ib Fr. II Fr. III Total Ia+Ib II III

949 N2633 East 13 Agricultural Soil 6.5 199.6 122.2 689.6 1017.9 20 12 68 E2948 stratum 6 953 N2636 East 73 Agricultural soil 9.9 113.0 94.6 451.2 668.7 18 14 68 E2948 stratum 6 954 N2636 East 42 Above field surface 2.6 69.1 142.9 355.6 570.2 13 25 62 E2948 stratum 2 956 N2636 East 78 Swale 5.5 17.9 164.7 388.0 636.1 13 26 61 E2948 stratum 7 951 N2636 East 83 Swale 8.1 84.7 115.1 391.2 599.9 16 19 65 E2948 stratum 10 966 N2635 East 17 Agricultural soil 4.9 180.1 124.6 459.7 169.3 24 16 60 E2882 stratum 2 967 N2635 East 94 Agricultural soil 4.4 182.3 188.6 482.1 857.4 22 22 56 E2882 stratum 2 969 N2635 East 94 Agricultural soil 3.5 208.6 162.8 461.1 836.0 25 19 55 E2882 stratum 2 915 N2635 South 52 Agricultural soil 6.4 35.2 72.5 321.6 435.1 10 17 14 E2878.5 stratum 2 978 N2635 South 49 Agricultural soil 3.9 204.4 251.9 605.4 1065.5 20 24 57 E2878.5 stratum 2 248 A. L. KOLATA AND C. ORTLOFF classic Tiwanaku IV puma incensario, or incense burner. This material indicates that the fill of the field structure was scraped up from surfaces bearing cultural deposits and artificially transported to the construction site. Furthermore, the diagnostic ceramics suggest a chronological assocation with the Tiwanaku IV period, although a later Tiwanaku V association cannot be discounted until completion of the ceramic analysis from this context. At some point in the history of agricultural reclamation at Lukurmata, the swales of this complex field structure were filled in and the old field surfaces covered over to provide the base for the overlying, surface raised field system (Figure 8: strata 8 & 10). No natural deposits, such as the fine, water-sorted sands overlying the buried field structures in test trench 1, were uncovered here. The lack of such natural water-borne, or aeolian deposits on the surface of the type 2 buried fields in trench 2 indicates that: (1) relatively little time elapsed between the cessation of production on this buried field and the period of reconstruction represented by development of the surface fields; (2) the episode of flooding that clearly affected the buried field system uncovered in trench 1 did not play a role in the history of this field system; and (3) therefore there is a high probability that the two types of buried fields uncovered in both test trenches represent temporally distinct phases of agricultural reclamation in the agricultural sector of Lukurmata.

Modeling Raised Field Function: Introduction and Analysis The representative field systems excavated within the Lukurmata and Lakaya areas reveal significant diversity in terms of ridge/swale geometry, orientation and stratigraphy. Despite this diversity, typical patterns can be discerned and analysed that relate field morphology and internal structure to specific function in the demanding environmental context of the Andean altiplano. To gauge the types of analysis that need to be performed and to understand the work- ings and efficiencies of raised field systems, a number of factors need to be considered. These include: (I) sulinization of field growing zones by incursion of saline Lake Titicaca waters [(for cases for which the lake level (temporarily) exceeds the local (fresh) water table height or the water table recedes due to drought conditions leading to saline water intrusion)]; (2) drainage of (fresh water) saturated soils into raised field swales when the lake level is below the local water table height; (3) agricultural land area that depends upon the lake level (since the ground slope of lakeside land is low, a rise in lake level can reduce this area considerably); (4) heat storage capability of a raised field system; and (5) soilfertility (which may continually decrease due to nutrient leaching, monocultural cropping, and lack of replenishment of organic matter). All of these parameters depend on the variability of Lake Titicaca water level. This level undergoes episodic and long-term fluctuations that are not currently well-understood, although potentially associated with global changes in climatic and precipitation patterns. The relation between these five parameters and lake level h,, measured from a given reference plane, is shown in Figure 9 in schematic form. In this figure ht,,,, -C/Z,,, where h,, and h,, represent maximum and minimum possible Lake Titicaca levels. The quantity h,, represents the extended drought period lake level while h,, represents the flood condition lake level. The “normal” lake level fluctuation range is defined to be h,, -Ch < hN1, where h,, and hN1, represent the normal seasonal lake level variation. Other lake level fluctuations may occur due to long or short term climate cycles involv- ing varying rainfall amounts into the altiplano drainage or watershed zone that channels groundwater and runoff into the lake. As,lake level increases, salinization, which is a function of (/z~-/z,)-~ (Figure 9) also increases. This reflects that fact that saturation by other than (fresh) ground water is increasing as h2-/z, decreases. The quantity h, is defined (Figure 9) to be the groundwater average height above a reference plane. Similarly, TIWANAKU RAISED FIELD SYSTEMS 249

Lake Titicaco hm “f t Lake level level change t G l Ridged fields hwi hl L&e Ridged fields

\\ Solinizqtion /

Drainage

(Stole) Heat storage, Dromage, Land area, Solinization

Figure 9. Survivability polygon for raised field agricultural systems. drainage of groundwater varies with h-h,: that is, the higher the groundwater level with respect to lake level the greater the drainage capability of the raised field. This factor greatly affects the start and finish times for the growing season. The arrival of maximum groundwater flow to field systems near the lake edge occurs past the peak of the rainy season. This is due to the generally low flow speeds of ground water deposited at remote locations within the collection basin of the lake. The (local) low ground permeability constants and flow pressure gradients are then the cause of the continuous arrival of ground water to the lake edge, including significant groundwater flows in the dry season. Thus the swales can serve as channels to help drain ridges in dry (as well as wet) months provided h, > ht. The swales present alternate drainage paths for saturated soils in the ridges. Water may either flow into the swales and then to the lake (provided h,>h,) or through the aquifer to the lake. The residence time for water particles for the latter path is much greater than for the swale path, therefore the swale paths are better drainage routes to the lake. The land area available for agriculture is likewise dependent upon the lake height as previously discussed. For cases in which the ridges protrude above the lake level (for near shoreline agriculture) and h, is approximately equal to h,, then artificial mounding and/or buried impervious clay layers, such as that found in the field system at Lukurmata, may be used to regulate the ridge surface to water table height differences to protect crop root systems from waterlogging. The factor of field system heat storage capability depends upon the saturation state of the raised field soils. The heat storage from solar flux heat input plays the role of modulating diurnal ground temperature. As soil saturation increases as a function of groundwater height with respect to the field system surface, the heat storage capability of 250 A. L. KOLATA AND C. ORTLOFF the soil expands. Expansion of the heat storage capacity enhances the survival of root crops during cold altiplano nights and determines an effective “end of the growing season” as such effects limit ground freezing and concomitant destruction of root crops. The degree to which these parameters are effective in determining the success of the agricultural system depends additionally on soil fertility. As soil fertility declines, we expect that crop toleration to salinization, to waterlogging, or to decreased heat storage capacity will similarly decline. As the range of toleration of crops to these conditions contracts, a new set of constraints affecting the type, sequence, and fallow periods of crops grown in given field segments will be established with a probable correlative reduction in plant productivity. Based on the general effects of these five main parameters and their qualitative variation with lake level (Figure 9), a “normal operating range” of lakeside raised field agriculture can be determined for a given soil fertility level. If the normal height fluctuation of the lake is h,,

Heat Storage ia High Altitude Raised Field Systems The present paper examines only one of the important parameters delimiting successful operation of raised field systems: the heat storage capability. In order to analyse this TIWANAKU RAISED FIELD SYSTEMS 251

p q density T = temperature c = specific heat t = time k = thermal conductivity K = thermal diffusivity = k/pc

Solution : x/2&i

T = (2T,,/&) ems2 ds q Toerf[X/2&?] I J 0

T=O .X=0

- -- T=ToZO - ---.-- ---.---___ -- t-o

A Air 0.187 Rock 0*0188 Soil (41/l 0.0046 Soil (sandy, dry) O-0028 Soil (sandy, moist) 0.0033 Water O-0014 Figure 10. Heat conduction equation and solution with table of thermal diffusivity values for raised field systems. effect on crop survival under cold seasonal temperature variations, consider first a model problem to illustrate the physical phenomena involved in the heat storage effect. With reference to Figure 10, consider a one-dimensional unsteady heat conduction problem governed by the parabolic heat conduction equation. Variables and parameters are defined below the governing equation in this figure. The solution to the partial differential equation can be written in analytic form in terms of the error function (erf) of argument (x/2(JKt)) where t is time, Kis the thermal diffusivity and x is the depth into a medium from its surface plane. Suppose next that the initial-boundary value problem to be solved is one of a medium initially at a temperature To at t =0 for 0 0, the x =0 boundary is maintained at T= 0. The solution to the heat conduction problem then gives the ensuing T(x, t) temperature distribution in the medium for t 2 0,O

LIST IIATERIALS 1 TO ODY 1 wwEafinm DATA TEnPERnnJRt D41n PROPERTY- RLL e.eeeeeE~ee c1.78700E-es 2381.8 0.787eet -es PROPERTY TABLE KXX MAT- 1 NM. POINTS. 2 PROPERTY TABLE DENS MT- 4 Nun. POINTS- 2 TEnPERAfURE DATA TEMPERATURE DATA TEI’IPERATURE DATA TEflPERnTURE own e.eeeeeE+ee e.356eeE-e6 2399.8 @.35608E-06 e.eeeeeEtee e.36eeeE-ei 23ee.e 6.36BBBE-61 PROPERTY TABLE KYY HAT* 1 Null. POINTS= 2 PROPERTY TABLE C MAT- 4 NUR. POINTS- i! TEI’IPERATURE DATA TErWERAlURE TEMPERATURE DATA TEAPERATURE ofm e.eeeeeE*ee 8.356eeE-86 23eo.e 0 . 356eeE-e6DATA e.eeeeeE+ee i.eeee e3ee.e l.oBBB PROPERTY TABLE DENS HAT= 1 NM. POINTS- 2 PROPERTY TADLE KXX RAT- 5 NM. POINIS- 2 TEfWERAlURE DATA TEIIPERATURE DRTA TEFIPERATURE DATA TERPERATURE DATA e.eeeeeEtee 0.4188eE-84 2388.8 e.41eeeE-e4 e.oeeeeEtee iee.ee 23ae.a iee.ee PROPERTY TABLE C AAt= 1 WM. POINTS. 2 PROPERTY TABLE DENS MT- 5 NM. POINTS- L TEFIPERATURE DATA TENPERATURE DATA TEIWERATURE DATA TEIIPLRATURE DATA e.eeeeeEiee 9.244teo 2388.0 e.24eee e.eeeeoc+ee 0.3eeee 23ee.e e .3eeee PROPERTY TABLE KXX na1* 2 NlJll. POINTS. 2 PROPERTY TABLE C RAT* 5 NlJDl. POINTS= 2 TEnPERATURE DATA TEMPERATURE TEIU’ERATURE DATA TEAPERATURE DATA e.eeeeeE+ee e.meeE-e4 z3ee.e 0 . 278eeE - e.eeeeeEtee e.iaeer 23ee.e 0.12888 PROPERTY TABLE KW tlAu)I= 2 NM. POINTS- 2 PROPERTY TABLE HF MT- 6 MM. POINTS* 2 tEr\PERATURE DATA TEIIPERATURE DATA TEWERATURE DATA TMPERATURE DATA e.eeeeeEtee e.278eef-e4 23ee. 0 0.278eeE-04 o.eeeew+oe 0.386eeE-es 23ee.e e.386eeE-es PROPERTY TABLE DENS MAT- 2 NM. POINTS= 2 PROPERTY TtiBLE HF FIAT- 7 NM. POINTS- 2 TEnPERnTURE DATA TERPERATLIRE DATA TEM‘ERATURE DATA TERPERnTURE DATA e.eeeeeE+ee 8.58eeoE-01 23ee.e e.seeoeE-et e.eeeeeE+ee e.i3seeE-e4 23ee.o e.i35eeE-e4 PROPERTY TADLE C MT- 2 NIJR. POINTS= 2 PROPERTY TABLE KXX tIAT- 8 Nlllt. POINTS. 2 TEnPERATURE DATA TERPERATURE TEtU’ERATURE DATA TERPERATURE DATA e.eeeeeE+ee e.7eeee 23ee. e e.7eeae e.eeeeectee e.374eeE-e4 23w.o e.374eeE-e4 PROPERTY TABLE KXY FIAT= 3 HLJR. POJNTS. 2 PROPERTY TABLE KYY MT* 8 NlHl. POINTS. 2 TEnPERATURE DATA TERPERATURE DATA TEf’WERATURE DATA TEIIPERATURE DnTA e.eeeeeE+ee e.347eeE-e4 2389.0 e.347eeE-84 o.oeeeoE4ee 0.374eec-e4 23ee.o e.374eeE-84 PROPERTY TABLE KYY MT* 3 NM. POINTS= 2 PROPERTY TABLE DENS RAT- 8 nun. POINTS= 2 TEmPERATtiRE DATA TERPERATURE TEFWERATURE DATA TEWERATURE DATA e.eeeeeE+ee e,347eeE-84 2388.0 e . 347eeE - eYTA e.eeeeoE*ee e.eleeec-ei 23ee.e e.eIeeec-81 PROPERTY TABLE DENS MT* 3 NUII. POINTS. 2 PROPERTY TABLE C FIAT= 8 NW. POINTS- 2 TEnPERATURE DATA TEI’IPERATURE DATA TEI’V’ERATURE DATA TEIIPERATURE DATA e.eeeeeE+ee e.eioeeE-el 23ee. 0 e.eieeeE-et e.eeeeeftee e.9wee 23ee. e e. weee PREP7 -INP- PROPERTY TABLE C mu- 3 NM. POINTS- 2 TEfIPERATURE DATA TEMPERATURE DllTA e.eeeeeE+ee e.99898 23ee. e e. 9eeee PROPERTY TABLE KXX’ MT= 4 Nun. POINTS. 2 TERPERATURE DATA TEIlPERATURE DATA e.eeeeeE+ee e.787eeE-es 2381.0 e.787eeE-es

PROPERTY TABLE KW I1At= 4 NM. POINTS= 2 254 A. L. KOLATA AND C. ORTLOFF

Table 3. Material and real consonants for finite element model: reference 4 formal

LIST tlATERIALS 10 TO OBY 1 PROPERTY- ML PROPERTY TABLE KXX RAT- 10 NM. POINTS. 2 TERPERATURE DATA TEtlPERATURE . DATA e.QeeeeE+ee B.leeeBE-82 2388.0 e.ieeeeE-02 PROPERTY TABLE KVY RAT- 1. Null. POINTS- 2 TERPERATUKE DATA TEtlPERATURE DATA e.eeeeeftee e.waeeE-e2 2399.8 e.ieeeeE-e2

PROPERTY TABLE DENS RAT- 10 NUN. POINTS- 2 TERPERATLIRE DATA TEflPERATURE DATA e.eeeeeE+ee e.i19eeE-86 2368.8 e.lIQeeE-86

PROPLHTY TABLE C MAT- 18 NM. POINTS- 2 TEflPERATlJRE DATA TEMPERATURE DATA e.eeeeeEtee 92.730 2389.0 92.739 PREP7 -INP- RLIST.1.20.1

LIST REAL SETS 1 TO 18BY 1

REAL CONSTANT SET 1 ITEllS IT0 6 1. eeee e.eeeeeEte(r e.eeeeeEtee e.eeeeeEtee e.eeeeeEtee e.eeeeeE+ee

REAL COtISTAtiT SET 2 ITEMS IT0 6 l.eeeo e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee

REAL CONSTANT SET 3 ITEM IT0 6 1 .eeee e.eeeeeEtee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeEtee Q.eeeeeEiee

REAL CONSTANT SET 4 ITEMS 1TO 6 1 .eeee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeE+ee e.eeeeeEtee

REAL CONSTANT SET S ITEM 1TO 6 15.880 e.eeeeeEt68 e.eeeeeEtee e.eeeeef+ee e.eeeeeE+ee e.eeeeeEtee

REAL CONSTANT SET 6 ITEM IT0 6 is. eee 1. eeee 8.48888 e.33ieeE-14 e.eeeeeEtee e.eeeeeE+ee

REAL CONSTANT SET 7 ITEM I TO is.868 e.eeeeeEtee’ e.eeeeeE+ee6 e.eeeeeE+ee e.eeeeeftee e.eeeeeEtee

REAL CONSTANT SET 8 ITEM IT0 6 i .eeee e.eeeeeE*ee e.eeeeeE+ee e.eeeeef+ee e.eeeeef+ee e.eeeeeE+ee

REAL CONSTANT SET 18 ITEM 1TO 6 1.8068 e.eeeeeE+ee e.eeeeeE+ee e.eeeo6f+ee e.eeeeef+ee e.eeeeeEtee PREP? -1NP. used as input to the heat transfer problem. These temperature values are estimated from observations taken on a 24-h basis in a typical altiplano winter month. The finite element method for heat transfer problem solution starts with the Poisson Equation with temperature as the dependent variable. This equation, in matrix form, is: where [c] = specific heat matrix (including the appropriate mass effects), [k] = thermal conductivity matrix (including equivalent face convection terms), {T} = nodal temperature vector, {Q} = heat flow rate vector (including the applied heat flow, internal heat generation and convection). This equation is solved by an implicit direct integration scheme based on a modified Houbolt method (Swanson & DeSalvo, 1985) and uses a quadratic temperature function TIWANAKU RAISED FIELD SYSTEMS 255

VOI. 0~008000

0.007200 -

0.006400 -

0-005600 -

0~004000 - Solar flux 0~004000 - (BTWsec-in’)

0.003200 -

0.002400 --/

0.001600 -

0*000800 - 0’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 14.80 19.60 114.4 119.2 124.02 2.40 7.20 12.0 16.8 21.6 Step

Figure Il. Graphic input for the solar heat flux diurnal cycle in the Pampa Koani zone.

VOl 525.00, 1

Degrees ronkme

485.00 - b Midnlght 475.001 I I I I I I I I I 0 14.80 19.60 114.4 119.2 124-O 2.40 7.20 12-o 16.8 21.6 Step Figure 12. Diurnal air temperature variation in the Pampa Koani raised field zone.

T, = a+ bt + c?. This function is substituted into the governing equation to yield an equation in three (spatial) unknowns: a, b and c. If At is the step between iterations, a set of three equations may be defined at t, t - A t and t - 2 A t and solved simultaneously to give the integration equation 256 A. L. KOLATA AND C. ORTLOFF

Phreotic Zone Water Stone Loyer

Soturoted Soil

Figure 13. ANSYS finite element model based on empirically determined raised field geometries. which may be solved for T, since solutions at previous times are known. For the present problem, the deep ground water temperature is fixed at a given value on the lower boundary (Figure 13). A value of 50”F, determined by test measurements in the field, was utilized. The diurnal air temperature variations are prescribed as discussed previously. Initially, typical raised field material constants are set (density, specific heat and conductivity) for the four material types (Table 3). Additional constants are set for convection and radiative heat transfer loss terms at the ridged field and swale water surfaces. Time dependent solar flux and air temperature values are input and the heat transfer problem solved for t > 0. Results are in the form of temperature profile contours within the raised field model at various times during the diurnal cycle.

Results of the Calculations Shown in Figures 14-15 are typical temperature profiles within the raised field system (Figure 3) on the Pampa Koani at times during the day and night. Contour letters in the plots can be identified with the RHS temperature values. At a time corresponding to noon-to-early-afternoon when solar flux and air temperature peaks, the soil temperature exhibits maxima on the soil surface (Figure 14). As solar flux and air temperature decrease toward late afternoon and early evening, surface soil temperate decreases while internal soil temperatures remain high due to the low thermal diffusivity of the soil and water compared to the adjacent air elements. Toward evening and night (Figure 15) the temperature maximum is clearly internal, within the soil ridges, while the outer soil layers experience temperature minimums. The presence of groundwater at near uniform temperature above that of the ambient air temperature effectively serves to direct heat flows upward at night adding to the heat storage effect of the high thermal diffusivity soils. During the day, the low groundwater temperature insures a thermal gradient thereby improving the input heat transfer. Although the groundwater temperature is low, it is still higher than ambient air temperature at night. This thermal differential leads to an upward heat flow. The presence of the swale water is also effective in maintaining higher than ambient air temperatures within the interior of the ridges. The zone of interest to root crops is that between soil surface and water height in the swales (one or two finite element TIWANAKU RAISED FIELD SYSTEMS

1111111111111111111~ ------1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1 I I I I I IV,, Hr,------_------.__--_------__------d-mI I I I I I I I I I I I I I I I I I I I I I I I I I I I’,,-- II II I I I I I I I I I I I I I I I I I I I I I I I I I 1-l-l-l-l-l-~-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-_____------‘I-1 --__------I I I I I III I I I I I I I I I I I I I I I -----I I I I ‘I-1 I I I I I I I I ------m-wI I I I I I I I I I I I I I I I I I -- I ‘l-l-l-l-l-l-l-l-l-l------I i I I I I I I I I I I I I I I I I I i-1 ------e--w-I I I I I I I I I I I I I I I I I I I I I I I I I I I I-I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1-1-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l-______------.I-l-l-l-l-l-l-l-l-l-l-l-l-l-l-l ---__-__-___-_--se--I I I I I I I I I I I I I j~l~l~l~l~l~l~l~l~l ,I *I ,I cI *I J J *I J ?I ?I J Cl pl *I -1 PI J ?I c .------_--_-_------me-- ,wbmarn r:cLuWsIcn I- awL”~*‘

Figure 14. Raised field soil temperature profile: modeled in late morning. MX denotes maximum values; MN denotes minimum values. heights from the ridge top surface). Observation of the temperatures below the surface then indicates the diurnal temperature variation of the soil containing the root crops. Clearly, the heat storage effect is effective in keeping internal soil temperatures somewhat warmer than air temperature, with the consequence of preventing, or ameliorating root damage during subfreezing altiplano nights. Although only a few degrees Rankine (T(“F) +460”F) separate internal from external temperatures during the night, this differ- ence is instrumental in keeping the internal ground temperature above the freezing limit at depths where root systems exist. As freezing temperatures are encountered within the interior of the raised fields, a certain amount of time is required to extract the latent heat of solidification from the soils (and root crops) before total freezing destroys the crop. The presence of this auxiliary heat storage capability is therefore important to root crop survival. The freezing of plant parts at the soil surface level is likewise delayed by the heat absorption effect which helps to keep these temperatures slightly higher than ambient air temperature. It should also be noted that native aquatic macrophytes, such as Miriophillum-Elodea, Potamogeton, Ruppia, and Schoenoplectus totora, were most likely dredged from the swales between field platforms and used as green manure to replenish macronutrients (N,P,K) in the soil. The decomposition of this plant material, if mixed into the soil, may generate a further source of heat during the planting season. The calculations presented here are for typical air temperatures, solar fluxes and soil material constant values as found in the Pampa Koani area for given field geometrical 258 A. L. KOLATA AND C. ORTLOFF

Figure 15. Raised field soil temperature profile: modeled in late evening. MX denotes maximum values; MN denotes minimum values. patterns. A summary of the material and real properties in the Swanson & DeSalvo format are given in Tables 2 & 3. Although these field system patterns change locally on the pampa (and exhibit different orientations and thus different received flux values) with respect to the sun’s path, none the less, the results presented in this summary are typical of qualitative trends in heat storage capability of raised field systems. Soil temperature values may change in time with different soil types, ground cover solar flux, and air temperature levels, as well as with diurnal wind speed changes over the fields. However, the qualitative trends discussed here are expected to apply in a general sense. Field measurements of temperature were made for air, swale water, and ground temperature at various times during the diurnal cycle to check the qualitative predictions of the theory. Generally, the heat storage effect was confirmed. The model assumed clear water absorption/reflection/transmittance radiation properties. However, the presence of darker organic material within the swales provided a further solar radiation trap that yielded higher swale water temperatures than predicted by the model. It was observed, for example, that actual swale water temperature can exceed ambient air temperature by as much as lO-15°F during peak solar flux times. The presence of this augmented thermal effect through higher (than predicted) swale water temperatures clearly enhances the heat storage capability of raised field agricultural systems. In combination with higher vadose zone ground temperatures, this effect provides an additional source of heat storage for raised field agricultural systems in high altitude environments. TIWANAKU RAISED FIELD SYSTEMS 259

Conclusions The intensification of agricultural production through reclamation of seasonally or periodically inundated land in the Titicaca basin represented a prime economic strategy of the Tiwanaku state. The technology of land reclamation entailed construction of massive raised field systems in the near shore environment of Lake Titicaca. These raised fields, characterized by a complex internal structure not reported for such systems elsewhere in the ancient Americas, were linked to sources of fresh water by near-surface groundwater flow, and by spring fed, open-channel aqueduct and canalized quebrada delivery networks. ANSYS finite element modelling of the thermal properties of excavated Tiwanaku raised fields, utilizing empirically established field geometries, internal structure, and hydrological conditions, indicates that heat conservation was an essential design element of these agricultural features. Although the general hypothesis of frost mitigation in high altitude (Brookfield 1961; Denevan, 1970; Wadell, 1972; Erickson, 1985) and in middle latitude (Riley & Freimuth, 1979; Riley er al., 1980) raised field systems has been suggested previously, the ANSYS finite element model presented in this paper represents the first formal consideration of specific heat storage pathways and potentials within these systems. Graphic experimental confirmation of the heat conservation effects described here was obtained during the 1987-8 growing season in the Lakaya sector of the Pampa Koani. A 1.5 ha plot of land with well-preserved Tiwanaku raised fields was reconstructed by local Aymara communities between August and September, 1987 and planted in a variety of indigenous (principally potato) and introduced crops. No commerical fertilizers were applied to the experimentally rehabilitated fields, and cultivation and weeding proceeded in a traditional manner. On the nights of February 28-29, 1988, the Bolivian altiplano in the Pampa Koani region suffered a killing frost with temperatures in the Lakaya sector dropping to - 5°C in some areas. Substantial zones of potato and quinoa cultivation on plains and hillslopes along thesouthern rim of Lake Titicaca were severely damaged by this heavy frost. Many traditional potato fields within a few hundred meters of the experimental raised field plots experienced crop losses as high as 70-90%. In dramatic contrast, losses in the experimental raised field of Lakaya I were limited to superficial frost “burning” of leaves on potato plants. Barley, broad beans, quinoa, caiiiwa, onions, and lettuce were equally substantially unaffected. Only 10 experimentally placed maize plants were lost in this hard freeze. We attribute this remarkable differential in plant survivability to the heat storage benefits of the saturated raised fields. Subsequently, several 100 sm2 test plots of potato in the reconstructed raised fields in Lakaya were harvested with an average yield of 33 metric tons/ha-’ (a figure that is consistent with similar yields from an experimental raised field in the Huatta district in Puno, Peru reported by Erickson, 1985). This elevated yield contrasts with an average potato harvest of 3-8 metric tons/ha obtained from traditional shallow furrow, dry farm- ing techniques practiced in the area (MACA, 1985). Although this tremendous differential between traditional and experimental raised field systems may be attributed in part to the fact that the experimental fields were reconstructed in areas not in intensive cultivation for over 800 years, and one would therefore expect higher than average production, the radically different impact of subfreezing air temperatures on these two forms of cultivation must play a significant role in explaining this yield differential. At present, the empirical evidence supporting the heat conservation hypothesis can be characterized as anecdotal. Experiments explicitly directed toward testing the thermal properties of ridged fields in middle latitude, temperate geographical zones in the United States (specifically, central Illinois) by Riley et al. (1979, 1980) tend to confirm this hypothesis, even though the suggested pathways of frost mitigation in this experiment differ from those proposed here. Long-term experimental research in reconstructed raised 260 A. L. KOLATA AND C. ORTLOFF field systems at high altitude will be required for confirmation of the heat storage capacity of these systems. We anticipate that current plans for continuing experimental crop production on the rehabilitated fields at Lakaya will contribute to verification and quantitative refinement of the model of the thermal properties of these systems presented here. Nevertheless the broader historical implications of the model are evident. If the heat conservation effects in raised fields that we describe here are further verified, then archaeological perception of the subsistence base of the Tiwanaku civilization will be radically altered. We must consider the possibility that Tiwanaku agriculture in the circum-Titicaca basin routinely entailed a regime of double cropping. Demonstrating the viability of such a regime of intensification would, or course, have strong ramifications for reconstruction of the limits of surplus agricultural production in the heartland of the Tiwanaku state. This research will result in a new and more sophisticated understanding of the agricultural and demographic potentials of the Andean high plateau, which has long, and improperly, been regarded as a marginal environment for sustaining large and concentrated human populations.

Acknowledgements The results of the research discussed here are based on the project entitled “The Tech- nology and Organization of Agricultural Production in the Tiwanaku State”. This project is supported by research grants from the National Science Foundation (BNS 8607541) and the National Endowment for the Humanities (RO 21368-86), Alan L. Kolata, Principal Investigator. The United States Government has certain rights in this material. Additional funding was provided by grants from the Inter-American Foundation (BO-252,1987) and the Pittsburgh Foundation. We gratefully acknowledge support for computation from FMC Corporation, Santa Clara, CA. The research was authorized in Bolivia by the Instituto National de Arqueologia (INAR), La Paz, Dr Carlos Urquiso Sossa, Director. We are particularly indebted to Dr Carlos Ponce Sanginis, Director Emeritus of INAR and to Oswald0 Rivera Sundt (INAR), Co-Director of the project for facilitating research activites in Bolivia. Gray Graffam, Department of Anthropology, University of Toronto supervised the excavation and stratigraphic recording of raised fields reported here. Michael Binford, Graduate School of Design, Harvard University provided valuable data and references on the ecological setting of Lake Titicaca incorporated in this paper. Michael Moseley and Geoffrey Conrad offered valuable comments on a previous draft of this manuscript. Soil chemistry analyses were performed at the State Soils Laboratory of the University of Wisconsin-Milwaukee. A.L.K. wishes to acknowledge the regrettably brief, but productive collaboration of the late Robert Eidt in the analysis of the soil samples.

References Abramowitz, M. & Stegun, I. (1964). Handbook of Mathematical Functions. National Bureau of Standards, Applied Mathematics Series,No. 55. Adams, R. E. W. (1980). Swamps, canals, and the locations of ancient Maya cities. Antiquity 54, 206214. Armillas, P. (197 1). Gardens on swamps. Science S74,653-66 1. Batchelor, B. (1980). Los camellones de caymhe elk1: $ ierra de Ecuador. America Indigena 40, 671689. Boserup, E. (1965). The Conditions of Agricultural Growth: T?w Economcis of Agrarian Change Under Population Pressure. Chicago: Aldine Atherton. Boulange, B. & Aquize-Jaen, E. (1981). Morphologie, hydrographic et climatologie du lac Titicaca et de son bassin versant. Revue d’Hydrobiologie Tropicale 14,26!?-287. TIWANAKU RAISED FIELD SYSTEMS 261

Bowman, I. (1916). The Andes of Southern Peru: Geographical Reconnaissance Along the Seventy- third Meridien. New York: Henry Holt. Broadbent, S. (1966). The site of Chibcha Bogota. #awpa Pacha 4,135-147. Broadbent, S. (1968). A prehistoric field system in Chibcha territory, Columbia. sawpa Pacha 6, l-13. Brookfield, H. (1961). The highland people of New Guinea: a study of distribution and localization. Geographical Journal 127,436448. Cabrera, A. L. (1968). Ecologia vegetal de la puna. In (C. Troll, Ed) Geo-ecology of the Mountainous Regions of the Tropical Americas. Berlin: Ferd. Dummlers, pp. 91-l 16. Calnek, E. (1972). Settlement pattern and chinampa agriculture. American Antiquity 37, 104-l 15. Carmouze, J. P. & Aquize-Jaen, E. (1981). La regulation hydrique du lac Titicaca et l’hydrologie de sestributaires. Revue d’Hydrobiologie Tropicale 14,31 l-328. Denevan, W. (1964). Pre-Spanish earthworks in the Llanos de Mojos of northeastern Bolivia. Revista Geographica 32,17-25. Denevan, W. (1966). The Aboriginal Cultural Geography of the Llanos de Mojos of Bolivia. Zbero-Americana, University of California Press, Berkeley. Denevan, W. (1970). Aboriginal drained-field cultivation in the Americas. Science 169,647-654. Denevan, W. (1980). Tipologia de configuraciones agricolas prehispanica. America Indigena 40, 619-652. Denevan, W. (1982). Hydraulic agriculture in the American tropics: forms, measures, and recent research. In (K. V. Flannery, Ed.) Maya Subsistence: Essays in Honor of Dennis E. Puleston. New York: Academic Press, pp. 18l-203. Denevan, W. & Mathewson, K. (1983). Preliminary results of the Samborondon Raised Field Project, Guayas Basin, Ecuador. In (J. Darth, Ed.) Drained Field Agriculture in Central and South America. B. A. R. International Series 189,167-182. Downing, T. E. & Gibson, M. (Eds) (1974). Irrigation’s Impact on Society. Tucson: University of Arizona Press. Duffie, J. A. & Beckman, W. A. (1974). Solar Energy Thermal Processes. New York: Wiley- Interscience. Eidt, R. C. (1977). Detection and examination of anthrosols by phosphate analysis. Science 197, 1327-1333. Erickson, C. L. (1985). Applications of prehistoric Andean technology: experiments in raised field agriculture, Huatta, Lake Titicaca: 1981-1982. In (I. S. Farrington, Ed.) Prehistoric Intensive Agriculture in the Tropics. B. A. R. International Series232(i), 209-232. Fernea, R. A. (1970). Shaykh and Effendi: Changing ,Patterns of Authority Among the El Shabana ofsouthern Iraq. Cambridge: Harvard University Press. Gallagher, R. H. (1975). Finite Element Analysis Fundamentals. Englewood Cliffs, N. J.: Prentice Hall. Galdo Pagaza, R. (1981). Artesanias y pequenas industrias en el area colindante con el lago Titicaca. In Acerca de la Historia y el Universo Aymara. Lima: Centro de Information Estudios y Documentation, pp. 77-106. Harrison, P. D. (1977). The rise of the bajos and the fall of the Maya. In (N. Hammond, Ed.) Social Process in Maya Prehistory: Studies in Honour of Sir Eric Thompson. New York: Academic Press,pp. 47&508. Harrison, P. D. (1978). Bajos revisited: visual evidence for one systemof agriculture. In (P. D. Harrison & B. L. Turner II, Eds) Pre-hispanic Maya Agriculture. Albuquerque: University of New Mexico Press,pp. 247-254. James, D. E. (1971). Plate tectonic models for the evolution of the central Andes. Geofogical Society of America Bulletin 82,332553346. Knapp, G. (198 1). El niche ecologico llanura humeda en la economia prehistorica de 10sAndes de altura: evidencia etnohistorica, geographica y arqueologica. Sarance (Ecuador) 9,83-95. Knapp, G. & Ryder, R. (1983). Aspects of the origin, morphology and function of ridged fields in the Quito altiplano, Ecuador. In (J. Darth, Ed.) Drained Field Agriculture in Central and South America. B. A. R. International Series 189,201-221. Kolata, A. L. (1986). The agricultural foundations of the Tiwanaku state: a view from the heartland. American Antiquity 51,748-762. 262 A. L. KOLATA AND C. ORTLOFF

Kolata, A. L. (1987). Research objectives and strategies: the 1987 field season.In (A. Kolata, C. Stanish 8~ 0. Rivera, Eds) The Technology and Organization of Agricultural Production in the Tiwanaku State. First Annual Research Report submitted to the National ScienceFoundation, the National Endowment for the Humanities, and the Instituto National de Arqueologia, La Paz, Bolivia, pp. 187-194. Kreith, F. (1973). Principles of Heat Transfer. New York: Intext Educational. Lavenu, A. (1981). Origine et evolution ntotectonique du lac Titicaca. Revue d’Hydrobiologie Tropicale 14,289298. Lennon, T. K. (1983). Pattern analysis of prehispanic raised fields of Lake Titicaca, Peru. In (J. Darth, Ed.) Drained Field Agriculture in Central and South America. B. A. R. International Series189,183-200. MACA, (Ministerio de Asuntos Campcsinos y Agropecuarios) (1974). Diagnostic0 del Sector Agropecuario, 2 Vols. La Paz, Bolivia: Ministerio de Asuntos Campesinos y Agropecuarios, Oficina de Planeamiento Sectorial, La Paz. Matheny, R. (1976). Maya lowland hydraulic systems. Science 193,639-646. Matheny, R. (1978). Northern Maya lowland water-control systems. In (P. D. Harrison & B. L. Turner, Eds) Pre-Hispanic Maya Agriculture. Albuquerque: University of New Mexico Press, pp- 185210. Mitchell, W. P. (1973). The hydraulic hypothesis: a reappraisal. Current Anthropology 14,(5), 532-534. Newell, N. D. (1949). Geology of the Lake Titicaca Region, Peru and Bolivia. Geological Society of America Memoirs 36, l-l 11. Parsons, J. J. (1969). Ridged fields in the Rio Guayas valley, Ecuador. American Antiquity 34, 7680. Parsons, J. J. & Bowen, W. (1966). Ancient ridged fields of the San Jorge River floodplain, Columbia. The Geographical Review 56,3 17-343. Parsons, J. J. & Denevan, W. (1967). Pre-Columbian ridged fields. Scientific American 217, 93-100. Parsons, J. R. (1976). The role of chinampa agriculture in the food supply of Aztec Tenochtitlan. In (C. D. Cleland, Ed.) Cultural Change and Continuity: Essays in Honor of James Bennett Grz#in. New York: Academic Press,pp. 233-262. Parsons, J. R., Parsons, M., Popper, V&Taft, M. (1982). Late Prehistoric Chinampa Agriculture on Lake Chalco-Xochimilco, Mexico. Preliminary report, Dec., 1982. Mimeographed manuscript submitted to the National ScienceFoundation and the Instituto National de Anthropologia e Historia, Mexico, D. F. PlIaker, G. (1963). Observations on archaeological remains in northeastern Bolivia. American Antiquity 28,372-378. Price, B. (1971). Prehistoric irrigation agriculture in nuclear America. Latin America Research Review VI(3), 3-60. Puleston, D. E. (1977). The art and archaeology of hydraulic agriculture in the Maya lowlands. In (N. Hammond, Ed.) Social Process in Maya Prehistory: Essays in Honour of Sir Eric Thompson. New York: Academic Press,pp. 449467. Riley, T. J. & Freimuth, G. (1979). Field systemsand frost drainage in the prehistoric agriculture of the upper great lakes. American Antiquity 44,271-285. Riley, T. J., Moffat, C. & Friemuth, G. (1980). Campos elevados prehistoricos en el medio-oeste superior de 10sEstados Unidos. America Indigena 40,797-8 15. Sagerlind, L. J. (1976). Applied Finite Analysis. New York: Wiley. Sanders, W. T. (1972). Population, agricultural history, and societal evolution in Mesoamerica. In (B. Spooner, Ed.) Population Growth: Anthropological Implications. Cambridge: Massachusetts Institute of Technology Press,pp. 101-153. Sanders, W. T. & Price, B. (1968). Mesoamerica: The Evolution of a Civilization. New York: Random House. Scarborough, V. (1983). Raised field detection at Cerros, northern Belize. In (J. Darth, Ed.) Drained Field Agriculture in Central and South America. B. A. R. International Series 189, 123-136. TIWANAKU RAISED FIELD SYSTEMS 263

Servant, M. & Fontes, J. C. (1978). Les lacs quatemaires des hauts plateaux des Andes Boliviennes, Premieres interpretations paleoclimatiques. Cahiers O.R.S.T.O.M., Serie Geologie 10,9-24. Siemens, A. H. (1978). Karst and the pre-Hispanic Maya in the southern lowlands. In (P. D. Harrison & B. L. Turner II, Eds) Pre-Hispanic Maya Agriculture. Albuquerque: University of New Mexico Press, pp. 117-144. Siemens, A. H. (1982). Prehispanic agricultural use of the wetlands of northern Belize. In (K. V. Flannery, Ed.) Maya Subsistence: Essays in Honor of Dennis E. Puleston. New York: Academic Press,pp. 205225. Siemens, A. H. (1983a). Wetland agriculture in pre-Hispanic Mesoamerica. The Geographical Review 73, 166181. Siemens, A. H. (19836). Modelling prehispanic hydroagriculture on levee backslopes in northern Veracruz, Mexico. In (J. Darth, Ed.) Drained Field Agriculture in Central and Southern America. B. A. R. International Series189, pp 27-54. Siemens, A. H. & Puleston, D. E. (1972). Ridged fields and associated features in southern Campeche: new perspectiveson the lowland Maya. American Antiquity 37,228-239. Smith, C. T., Denevan, W. & Hamilton, P. (1968). Ancient ridged fields in the region of Lake Titicaca. Geographical Journal 134,353-367. Steward, J. (Ed.) (1955). Irrigation Civilizations: A Comparative Study. Washington D. C.: Pan American Union. Swanson, J. A. & DeSalvo, G. J. (1985). ANSYS Engineering Analysis System UserS Manual, Vols I & II. Rev. 4.2B. Houston, PA.: Swanson Analysis Systems. Turner, B. L., II (1974). Prehistoric intensive agriculture in the Maya lowlands. Science 185, 118-124. Turner, B. L., II (1978). Ancient agricultural land use in the central Maya lowlands. In (P. D. Harrison & B. L. Turner, Eds) Pre-Hispanic Maya Agriculture. Albuquerque: University of New Mexico Press,pp. 163-184. Turner, B. L., II (1983). Constructional systems for major agrosystems of the ancient Maya. In (J. Darth, Ed.) Drained Field Agriculture in Central and South America. B. A. R. International Series189,1 l-26. Wadell, E. (1972) The Mound Builders. American Ethnological Society Monographs. Seattle: The University of Washington Press. West, R. (1959). Ridge or era agriculture in the Columbian Andes. Proceedings of the 33rd International Congress of Americanists 33,279282. Wilkerson, S. J. K. (1983). So green and like a garden: intensive agriculture in ancient Veracruz. In (J. Darth, Ed.) Drained Field Agriculture in Central and South America. B. A. R. International Series189,5>90. Winter, T. C. (1981). Uncertainties in estimating the water balance of lakes. Water Research Bulletin 17,82-l 15. Wirrmann, D. & Oliveira Almeida, L. F. (1987). Low Holocene level (7700-3650 years ago) of Lake Titicaca (Bolivia). Paleogeography, Paleoclimatology and Paleoecology 59,3 15-323. Wittfogel, K. (1938). Die theorie der orientalischen gesellschaft. In Zeitschrtftfur Socialforschung, VII: 9&l 12. Translated by the Far Eastern and Russian Institute of the University of Washington (1959): The theory of Oriental society. In (M. Fried, Ed.) Readings in Anthropology, Vol. II. New York: Thomas Crowell, pp. 94113. Wittfogel, K. (1957). Oriental Despotism: A Comparative Study in Total Power. New Haven: Yale University Press. Zienkiewicz, 0. C. (1971). The Finite Element Method in Engineering Science. New York: McGraw-Hill. Zucchi, A. (1972). Aboriginal earth structures of the western Venezuelan Llanos. Caribbean Journal of Science 12,95-99.