Terraces and Raised Fields

Terraces and raised fields are earthworks created to improve cultivation conditions by manipulating slope, hydrology, soil fertility, erosion, and microclimate. While both require intense labor, they allow agriculture to flourish in potentially inhospitable topography and environments. Terraces carve out sections of hillsides to improve planting surfaces. Raised fields create dry platforms above wetlands.

Farmers around the world have used terraces since ancient times, from the Hanging Gardens of Babylon to Cajete agroecosystems of Mexico. Terraces are used for defense and aesthetics as well as agriculture. They are critical to growing crops in arid and semiarid climates. Dryland terraces in Yemen enhanced soil water retention, taking maximum advantage of local water sources. The Hopi of the American Southwest irrigated terraces to grow maize. Terraces are also used in moist climates. Wet rice production throughout southeast Asia uses terraces to regulate water flow and grow rice on mountainsides.

Check dams and cross-channel terraces are constructed in hillside hollows where water flows during rainy seasons. They control water runoff and prevent soil erosion. Sloping field or broadbase terraces are used on gently rolling hills or valley floor alluvial fans. Each terrace level is constructed around existing contours, and soil surface is not flattened. These terraces control surface water runoff, thus conserving soil moisture and nutrients and reducing soil erosion. In the United States, farmers use mechanized agriculture on broad-base terraces.

Bench terraces provide a horizontal planting surface and are common on steep hillsides. Examples include those found in Machu Picchu, Peru, and the Ifugao terraces in the Philippines. Bench terraces are constructed by cutting out soil and putting in retaining walls made of stone or other materials. Behind the wall, soil and rock are backfilled in to create a new planting surface. To aid drainage, bottom layers of the new soil profile consist of larger rocks, then smaller cobble, and finally soil. By controlling surface water runoff, terraces reduce erosion, nutrient loss, and flooding risk. Bench terraces modify microclimates and enhance sunlight capture. They often are integrated with irrigation channels. Cultivating on slopes avoids nighttime frosts that settle in valley bottoms. While cultivatable area may be smaller than an unterraced plot, agricultural yield increases significantly.

Raised fields are large platforms of soil constructed in areas with seasonal flooding or permanent wetlands. They are found in Mexican wetlands, coastal India, Indonesian tidal wetlands, New Guinea swamps, Venezuelan and Bolivian savannas, and surrounding Lake Titicaca. Raised fields take many forms, ranging from long ridges to mounds, with flat or ridged surfaces, scattered or in rows. Drainage canals often separate platforms.

Raised fields reduce crop failure risk by altering microclimate, soil nutrients, and hydrology. They allow cultivation in areas otherwise inundated with fresh or saline water, control flooding in rainy seasons, and conserve water during dry seasons. Water in drainage canals may be used for aquaculture. Cleaning muck from canals provides nutrients for raised beds. Around Lake Titicaca, where diurnal temperatures vary up to 30 degrees C, raised fields [Page 1719]reduce frost risk. Solar radiation heats canal water during the day. At night, this stored heat dissipates, heating raised bed soils and air.

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