When completing these analyses, there are some clear methodological outcomes. ArcHydro Flow Accumulation and RDI methods of analyzing and visualizing data are both practical but provide differing landscape views. In a relatively flat valley such as Casma, flow accumulation works best to visualize large rivers and for stream delineation. RDI instead provides a more nuanced perspective of surface hydrology. The base layer gives a view of runoff accumulation and water pooling more generally, showing a spectrum of low to high angles. These angles can then be isolated to portray specific drainage channels or ridges. Isolating these low angles shows areas generally wetter and likely to accumulate runoff rather than being restricted to primary drainage channels. Primary drainage channels can be isolated as well but require additional post-processing. Ultimately, these two methods complement each other well. An additional consideration is that the flow accumulation method is relatively user-friendly for those with access to ESRI software, while the RDI program is also relatively simple given a basic understanding of programming and does not require a license.
The presented low-cost thermal approaches were practical and provided useful results, but there is room for improvement. The presented method of terrain modeling using RGB imagery and overlaying this with thermal textures worked well, but resulted in a slightly larger time investment than would typically be necessary. One limitation is a requirement for daytime flights, as RGB imagery requires light. This problem can be bypassed by doing identical flights day and night to obtain the RGB and thermal components. Overall, the specific thermal sensor used in this project was not ideal and is not recommended, as it does not allow for direct temperature measurement, instead providing RGB visualization, which severely limits the post-processing and analyses that can be completed.
Water and Hydrology
When looking at the entire Casma Valley, the primary drainage route follows the Casma River down from the eastern mountains to the coast, periodically branching into minor tributaries. Just before this drainage route meets the Pacific Ocean, it branches southward to the area of the raised fields (Figure 11). This secondary drainage line follows the southern edge of
the raised field complex before branching off in multiple directions. Additionally, multiple minor drainage routes move along the northern edge of the field complex toward the river, with drainage coming down from the hills to the southwest. RDI results indicate significant water pooling within the Casma floodplain, with heavy pooling at Laguna de San Diego. There is less water pooling in the desert immediately outside the floodplain and essentially none on the intrusive hills to the south. In the lower valley, four additional drainage routes travel north-south between the raised fields and the Casma Valley, with no significant east-west drainage aside from Acequia La Muenga at the south side of the fields.
The raised fields are at a low point in the landscape, a focal point of drainage from the north and south. Water drains out of the field system to the east into Laguna de San Diego, surrounded by higher-elevation landscapes. To the west, the canal is mostly level in elevation, meeting up with the Casma River before it reaches the ocean. The route of this canal to the west of the fields is relatively level in elevation at 0 – 2 masl. Though the canal is interrupted by the modern Puerto Casma, these level elevations make it unlikely that this was a significant drainage route from the raised fields. These fields were passively irrigated by groundwater flow from the north and limited precipitation but only had minimal drainage from the east and possibly the west. Additionally, intrusive sedimentary hills to the west, south, and east cut into the Casma aquifer and would prevent significant groundwater drainage.
Based on these hydrologic models and satellite imagery, four levels of water control within the raised field complex were defined: 1) Major canals, extensive waterways (40 – 100 m wide) that move water within the region and connect to nearby sources of water; 2) Minor canals, smaller bodies of water (3 – 10 m wide) that connect sub-areas of the field system; 3) Field canals (1 – 3 m wide), which move water within or between specific fields but do not typically extend beyond two or three field lengths; and 4) small irrigation furrows (1 – 3 m wide) cutting across existing field surfaces. A significant number of other furrows or small drainage inlets, though not visible on typical satellite imagery, were visible in the UAV elevation data. These furrows are curious as they do not fit the shape or style of the original raised field layout, and there is no clear pattern to their placement. They typically connect two canals or cross an unused raised field. While these furrows may have been part of the original field construction, they appear to be cut into existing fields and do not fit the size or shape of surrounding canals. More likely, these were post-construction modifications, cut into the fields later when certain areas were not moving sufficient water to support crop growth and wash out salts.
Salinization
These field systems have severe issues with salinization in the modern day, to the point that crop cultivation would be nearly impossible without significant landscape modifications. Several lines of evidence support the argument that salinization would have also been a significant issue for past cultivation. The overall climate and weather patterns are hyperarid, with extremely low annual precipitation and high evapotranspiration. In this climate, irrigation water, deposited primarily via groundwater infiltration and passive irrigation from canals, would evaporate quickly, leaving behind soluble salt buildup. There would be insufficient rainfall to wash these salts out, and the only realistic solutions would be the creation of additional irrigation systems or consistent applications of fresh water to the field surface. Additionally, the area immediately around the raised fields has inherently poor water transportation and drainage. The only viable drainage routes are east toward Laguna de San Diego and west toward the coast. As the laguna is a low point in the landscape with no outlet, only a limited amount of water could be drained in this direction. The modern town of Puerto Casma blocks the drainage route to the west, but it is unlikely that this was a drainage route with significant water transport potential even before construction. This route is mostly level in elevation and is close to sea level, so there would be limited gravity-aided drainage.
Immediately south of the fields and east of the laguna, the Casma Aquifer is blocked by intrusive sedimentary hills that prevent groundwater drainage. The primary source of raised field irrigation water is from the north, draining down from the higher elevation Casma River. While there may have been some limited drainage to the east and west, this would be severely limited by topography and elevation, likely resulting in alternating under-irrigated and waterlogged areas. Ongoing soil geochemical investigations in these raised fields are assessing whether the soil stratigraphy supports this conclusion. Additionally, the severity of modern soil salinization suggests systemic drainage problems that would have also been present in the past, at least to some degree. Given the coincidence of these climatic, weather, environmental, and soil conditions, these ancient, raised fields are unlikely to have escaped salinization problems. However, it is difficult to assess precisely how severe this salinization would have been.
Field Form Differences
When comparing different field types, some patterns are evident from the flow modeling results. The most visible patterns are differences in water transport capacity and direction depending on field size and shape. The large serpentine fields exhibit the highest water transport capacity, as evidenced by single drainage lines with extremely high flow accumulation and low RDI values (Figure 12). On the other end of the spectrum, small circular fields have significantly lower accumulation values but with many branching flow routes. This data suggests that a smaller volume of water would move through the circular systems but with much more multidirectional movement and water pooling. Small hooked, E-shaped, and serpentine fields have similar patterns as circular fields, with low water transport and higher water pooling. However, these fields have less spread of flow accumulation, suggesting that the multiple water inlets in these systems would improve drainage. Smaller, straight fields fall between the two extremes, with higher water transport and some branching drainage routes.
Thermal imagery results show some significant differences in temperature patterns between field types. The canals appear hotter in the morning than field surfaces; in the afternoon, they are much closer in heat levels. Water conducts heat much more effectively than air, so the wet canal soils heat up faster than the drier fields soon after sunrise, contrasting with later in the day when many fields reach a higher maximum temperature than the canals. Aside from this, perhaps the most noticeable difference is based on size. Compared to smaller fields, the large serpentine fields heat up slowly but reach significantly higher maximum temperatures midday. This pattern is evident in many locations, with long, wide fields being higher in temperature. Salinization also seems to be a factor, as the most heavily salinized surfaces are lower in temperature. Another variable is field connectivity. Completely isolated fields, mainly circular or straight forms, heat up relatively slowly and have the lowest maximum temperatures. The lack of connection with surrounding fields would reduce the ability to conduct heat efficiently. Overall, more extensive, wider fields reach higher maximum temperatures but do not heat up efficiently in the morning (Figure 13). Smaller serpentine, E-shaped, and hooked fields often heat up quickly but have lower maximum temperatures. Circular and small straight fields are the slowest to heat up and the lowest in maximum temperature. This differential heating would have several effects on cultivation. A field with high maximum heat levels would typically be more effective at conserving and maintaining heat after sundown to prevent crop damage from frosts. On the other hand, when fields heat up quickly in the morning, this suggests that they would also cool down quickly at night. When looking at midday thermal signatures, fields with higher maximum temperatures would typically be better for seed germination. By planting in overall hotter fields, seed germination may be improved or sped up.
These field-specific analyses support the following interpretations of field layout. In terms of water flow, large serpentine and straight fields have the most efficient water transport and irrigation systems. Higher volumes of water are transported more directly between areas, and large serpentine fields would serve to irrigate larger planting surfaces with a limited water supply. Smaller, less linear fields, primarily E-shaped, serpentine, circular, and hooked in shape, would transport water less efficiently. Flow is less direct, and there would be significant water pooling. There may have been a choice between water and heat requirements for these fields. For instance, circular fields do not heat up efficiently but have significant water pooling around them, which could be ideal for certain water-intensive crops. However, these fields would be the least efficient when considering frost reduction and seed germination. The larger straight and serpentine fields may have been “safer” for general cultivation in areas not directly adjacent to large canals. They would have more efficient irrigation, frost reduction, and good seed germination conditions. This aligns with the general field layout, as the larger serpentine fields are more commonly found towards the center of the field system, away from the primary canals. These larger fields were likely developed to irrigate large parcels of land most efficiently with minimal water usage. The greater water flow volume could also help reduce salinization in areas that receive less passive irrigation from large canals. The smaller fields, mainly circular, hooked, and E-shaped forms, are instead more common on the edges of the field system. With their proximity to primary canals and passive irrigation from these sources, these fields would not need the most efficient irrigation systems, as water needs would be met regardless. It is also possible that a broader range of crops could be cultivated in these areas, as water availability would be plentiful. Based on this research, the most likely scenario is one in which raised field forms were designed differently and purposefully, based primarily on hydrologic needs and proximity to large canals. In areas with less access to water, fields were designed to make the best possible use of limited water resources.