Andenes and acequias — the terrace-and-canal engineering of the Inca fields
The stepped terraces that still contour the Andes and the long canals that fed them are described in engineering detail in book V of Garcilaso's Comentarios reales: retaining walls built three deep and battered inward, hillsides climbed terrace by terrace "á manera de escalera", canals carried fifteen and twenty leagues to water a few fanegas, and a named corps of "ingenieros de acequias de agua" whose works were still visible in his day. Read from the page.
Why this wins its question: Inca terraces and canals are a stock image of Andean tourism but are rarely quoted from a period text; the usual English account gives a photograph of Machu Picchu and a sentence about clever farmers. This object quotes book V of Garcilaso from the page: the named "ingenieros de acequias de agua" whose works still stood, the three battered retaining walls of each terrace, the hillside climbed "á manera de escalera", the canal carried twenty leagues for a few fanegas, the pasture-canals whose traces survived the conquest, and the three-way division of the enlarged land — and it marks plainly that its one witness is a partisan of the state he describes, so the walls carry more weight than the harmony.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
Book V of Garcilaso's Comentarios reales (read here in the Madrid 1829 reissue of the 1609 original) opens by stating that on conquering a province the Inca ordered the arable land increased, "para lo cual mandaba traer los ingenieros de acequias de agua, que los hubo famosisimos como lo muestran hoy sus obras" — a named class of canal engineers whose works, both the ruined ones whose traces remained and the ones still in use, he offers as proof.
The construction of the terraces is described step by step. To level the hillsides they "hacian andenes para allanarlas, como hoy se ven en el Cozco y en todo el Perú": for each terrace they threw up three walls of strong masonry, one in front and two at the sides, battered inward "(como son todas las paredes que labran) para que puedan sufrir el peso de la tierra que les arriman hasta emparejar con lo alto de las paredes". Above the first terrace they made a smaller one, and above that a smaller still, gaining the whole hill little by little, levelling it by its terraces "á manera de escalera", and where there were rocky places they cleared the stone and carried in soil from elsewhere so the site should not be wasted.
The terraces were graded in size. The first were large, "anchos y largos de ciento y de docientas y trecientas mas y menos hanegas de sembradura", the next smaller, diminishing as they rose until the topmost came to be of only two or three rows of maize; "tan aplicados como esto fueron los Incas en lo que era aumentar tierras para sembrar el maiz". Garcilaso adds that in many places they carried a canal fifteen and twenty leagues to irrigate very few fanegas of bread-land "porque no se perdiesen".
The canals served a documented water regime. The masters, Garcilaso writes, drew the necessary acequias according to the land worth using, "porque es de saber que por la mayor parte toda aquella tierra es pobre de tierras de pan", and being under the Torrid Zone it needed irrigation, so "no sembraban grano de maiz sin agua de riego"; they also opened canals to water the pastures when autumn withheld its rains, and those pasture-canals "se perdieron luego que los españoles entraron en la tierra, pero viven hoy los rastros dellas". In lands short of water it was rationed "por su órden y medida" so that there should be no quarrel over taking it, the measuring done in the dry years when need was greatest — the subject of the chapter Garcilaso heads "Cómo repartian el agua para regar: castigaban á los flojos y descuidados".
The improved land was divided by rule. Once the terraces and canals had enlarged the fields, all the land of each province was measured, town by town, and split three ways, "la una para el sol, y la otra para el rey, y la otra para los naturales", always with attention that the natives had enough to sow, "que antes les sobrase que les faltase"; when a town or province grew in number, land was taken from the share of the sun and of the Inca for the vassals, "de manera que no tomaba el rey para sí ni para el sol sino las tierras que habian de quedar desiertas sin dueño". The terraces themselves were mostly assigned to the sun and the Inca, "porque las habia él mandado hacer".
The single source read here is an interested one, and its confidence is set accordingly. Garcilaso was a defender of Inca governance writing in Spain half a century after the conquest, and the orderly, provident state his terraces and water-quotas describe is exactly the picture his book was written to give; the physical works — the stepped retaining walls, the long canals, the surviving "rastros" — are far better attested by their remains than the administrative harmony he reports around them. No independent witness is cited here for the land-division rule, which is stated at below 0.9 for that reason.