62 MARCH 2015 • WORLD AQUACULTURE • WWW.WAS.ORG Water is the most limiting factor to achieving the large increases in food production needed to satisfy the requirements of a growing and more-demanding population. Crop irrigation uses more than 70 percent of available fresh water worldwide (Madramootoo and Fyles 2010), 73 percent in Nicaragua and 77 percent in Costa Rica (CIA 2014). Rainfed crop production is greatly affected by rainfall variability. Moreover, irrigated and rainfed agriculture are both affected by climate change as it increases the frequency and intensity of extreme events, like droughts and floods, and increases water demand from evapotranspiration. On the other hand, many tropical countries are endowed with extensive lakes and other freshwater bodies. Just three lakes (Victoria, Tanganyika, Malawi) shared by several African countries occupy over 100,000 km2, lakes in Nicaragua (Lk. Managua) and Bolivia/ Peru (Lk. Titicaca) occupy about 17,000 km2 and seasonal floods inundate extensive areas of Bangladesh every year. In all, lake areas in tropical developing countries occupy well over 300,000 km2 (CIA 2014). Traditionally, in terms of food production, these water bodies are used only for fishing (and usually suffer from overfishing), some caged fed-fish aquaculture and extraction of water for irrigated agriculture during dry seasons, when irrigation is needed most yet water levels and replenishment are at their lowest. Although seaweed farming for food is an established activity in the sea, growing plant crops on freshwater surfaces is at most barely developed (Irfanullah et al. 2011, Castine et al. 2013). Marine seaweed farming can save massive amounts of fresh water if implemented at a large scale (Radulovich 2011). The untapped AQUATIC AGRICULTURE: Cultivating Floating Crops on Lakes Ricardo Radulovich, Schery Umanzor, Rebeca Mata and Desiree Elizondo opportunity of existing freshwater bodies led us to explore the possibility of direct use of the surfaces of lakes, dams and floodplains for food production. Evaporation and Evapotranspiration Although water bodies provide a variety of ecosystem services, to occupy a fraction of the water surface with floating crops should not disrupt services, particularly considering that evaporation will occur at a similar rate as crop evapotranspiration. Lake evaporation cannot be controlled economically over large surface areas and is also not easy to measure directly (Finch and Calver 2008, Mohamed et al. 2012). Evaporation is about 5 mm/day during dry seasons (Finch and Calver 2008), equivalent to a daily loss of water vapor from lake surfaces of about 50 m3/ha. This rate compounds into hundreds of cubic kilometers of lost water from freshwater bodies of the tropical world. Given that only a small fraction of water is fixed within vegetation, the key contention that must be met is that crops grown floating on the surface of water bodies should consume water as evapotranspiration (ET) at a similar rate to what is normally evaporated (E) from the free surface (Fig. 1). This contention is thermodynamically sound because evapotranspirational demand is determined mostly by environmental factors and less by characteristics of the evaporating surface, as long as the surface is well supplied with water. This is the essence of irrigation technology, whereby even the tallest growing crops consume at most 1.20 (maize, banana) to 1.25 (sugar cane) times as much water as a surface of short grass (FAO 1998). This is also well FIGURE 1. Schematic representation of the basic contention that supports this work, the similarity between evapotranspiration rates from vegetated water surfaces, whether covered by land crops or aquatic plants, and evaporation rate from a free water surface. FIGURE 2. Research set-up to emulate border effects: a) Comparison of evaporation and evapotranspiration rates between a free-water surface (right), non-emergent vegetated surface (center) and emergent vegetated surface (left); b) Pools used to evaluate if the evapotranspiration rate of a vegetated surface is similar to free-water evaporation. a. b.
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