A water harvesting system in coastal Peru?

A few years ago, Mächtle et al. (2009) reported on a possible water harvesting structure in coastal Peru: a khadin-like dam blocking a small valley. Based on radiocarbon dates, they relate this structure to the nearby Late Intermediate Period site of Ciudad Perdida de Huayuri and suggest that climatic conditions must have been different from what we see today. The topic popped up again at the Latin America session of the International Open Workshop 2013 in Kiel a few weeks ago, and I promised to dig out a few numbers.

Satellite image of the proposed khadin (c) GoogleEarth.

Satellite image of the proposed khadin (c) GoogleEarth.

Let’s assume that the structure really served the purpose of water harvesting and agriculture. How does it relate to the size of the nearly settlement? In other words, how many people could be fed, and how many people lived in Ciudad Perdida de Huayuri? The more or less flat area behind the “khadin” dam has an area of roughly 1.7 hectares. Carrying capacity estimates are difficult, in particular if we don’t know which plants were grown and which agricultural techniques were used (e.g. fertilisation, fallow). Assumung that maize was grown as the staple crop, a few numbers available from the literature can help to estimate an upper ceiling for a plausible population size based on the proposed water harvesting structure:

– Engel (1981: 12f.): one family of 5 to 7.5 persons per 1.5–4.5 hectares of farmland
– Wilson (1981: 106): 0.5 persons per hectare based on early maize yield
– Cook (1981: 20f., after Keith 1976 and Romero 1966): 2.1–12.4 persons per hectare

These numbers indicate that the range of carrying capacities could have been somewhere between 0.5 and 12.4 persons per hectare. The area behind the dam could therefore have sustained a population between 1 and 21 persons.

Satellite image of Ciudad Perdida de Huayuri (c) GoogleEarth.

Satellite image of Ciudad Perdida de Huayuri (c) GoogleEarth.

Now, how does tis compare with the nearby settlement? The built-up area of Ciudad Perdida de Huayuri is roughly 6.5 hectares, containing hundreds of individual rooms. Even if not all of the buildings were occupied simultaneously, the settlement is much larger than what could be expected for a population of between one and twenty-one persons.

In comparison to this, the present-day extent of fields irrigated with seasonal river water flow is 173 hectares within a radius of two kilometres around Ciudad Perdida de Huayuri. Using the same per-hectare carrying capacities, that area could have sustained between 90 and 2150 persons – a range which is much more plausible given the size of the settlement.

Ciudad Perida de Huayuri (red), the proposed khadin (dark green) and present-day irrigated fields in the river valley within a radius of two kilometres of Ciudad Perdida. Landsat image in background (c) USGS.

Ciudad Perida de Huayuri (red), the proposed khadin (dark green) and present-day irrigated fields in the river valley within a radius of two kilometres of Ciudad Perdida. Landsat image in background (c) USGS.

Does this mean that this is not a water harvesting structure? From the carrying capacity estimates alone, it’s not possible to answer this question. But if it is a water harvesting structure, it could have at best provided a very minor addition to the agricultural resource base.

Does the interpretation of the structure as a khadin imply climatic changes? No. Even if further investigation would unequivocally prove that it is a water harvesting structure, it could simply reflect the opportunistic use of episodic El Niño rainfall.

References

Cook, N.D., 1981. Demographic collapse: Indian Peru, 1520–1620. Cambridge Latin American Studies 41. Cambridge University Press, Cambridge.

Engel, F.A., 1981. Prehistoric Andean ecology. Man, settlement and environment in the Andes: 2. The deep south. Humanities Press for Hunter College, New York.

Mächtle, B., Eitel, B., Schukraft, G., Ross, K., 2009. Built on sand: climatic oscillation and water harvesting during the Late Intermediate Period. In: Reindel, M., Wagner, G.A. (eds.), New technologies for archaeology. Multidisciplinary investigations in Palpa and Nasca, Peru. Springer, Berlin.

Wilson, D., 1981. Of maize and men: a critique of the maritime hypothesis of state origins on the coast of Peru. American Antiquity: 83, 93–120.

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