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 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 supported by literature for lakes and other freshwater surfaces. For example, in a review of data from water bodies around the world, Mohamed et al. (2012) calculated an average of the ratio between evaporation from vegetated wetlands to evaporation from open water of 0.87, corroborating results of an older analysis (Idso 1981).

Early research conducted in small tanks on land led some investigators to determine that vapor losses from vegetated water surfaces are several times greater than those from open-water surfaces (Timmer and Weldon 1967, Benton et al. 1978). This view is apparently shared by lake and dam managers, yet it is a misconception that has been traced to border effects that were not controlled in experimental studies (Van Der Weert and Kamerling 1974, Idso 1981).

In experiments using very small areas (Fig. 2a), we replicated those experimental systems using water hyacinth Eichhornia crassipes. As determined by daily measurements over 54 days, ET of plants fully-emerged over the water surface was 2.7 times that of the water surface alone, while ET of plants near the surface was only 1.1 times greater. Using taller aquatic plants would likely yield greater differences because border effects would be larger in proportion to vegetation height and smaller in proportion to vegetated area. To further validate these results, we established larger surfaces (5 m2, Fig. 2b) with abundant water hyacinth vegetation or water only, and obtained very similar rates of water loss over 42 days (3.7 vs. 3.6 mm/d, respectively).

Thus, based on known principles, the scientific literature and properly conducted research, a vegetated area of a sufficient size to minimize border effects loses a volume of water as the equivalent area of bare water surface, validating the contention illustrated in Figure 1. In some cases, a water surface covered with vegetation may have decreased evaporation, particularly from losses associated with wind, waves and spray (Idso 1981, Mohamed et al. 2012)

Floating Crop Production Systems

The next question was how to grow crops in a manner that is simple and low-cost, implemented at a large scale and environmentally sound. Towards this end, between late-2012 and mid-2014, we worked in Nicaragua (La Virgen hydroelectric dam and off the coast of Lake Managua, near Leon Viejo) and Costa Rica (Arenal hydroelectric dam and several controlled experiments at the campus of the University of Costa Rica in San Jose). We evaluated land crops in a floating condition (major food/cash crops, including bean, bell pepper, lettuce, maize, rice and tomato; as well as trials with other crops such as potato, basil, radish and ornamentals). We also worked with aquatic plants, particularly water hyacinth and water lettuce Pistia stratiotes, which among floating aquatic plants have a size that allows for individual handling and are fast growing and ubiquitous.

The first approach was to grow floating land crops with their roots directly in water, similar to hydroponic and aquaponic systems. However, although roots grew abundantly (Fig. 3a), indicating an adequate supply of dissolved oxygen, which ranged between 3.3 and 6.2 mg/L, shoot growth was invariably stunted. After several trials, plant nutrient deficiency symptoms (Figs. 3a and b) developed, caused by a shortage of nutrients in the water. For example, nitrate concentration was low in Lake Managua (0.45 mg/L) and Lake Arenal (0.33 mg/L), while ≥ 100 mg/L are recommended for commercial plant hydroponics. After a variety of rather unsuccessful attempts to promote vigorous growth by providing plants with nutrients through foliar spraying and/or dipping roots weekly for several hours in nutrient-rich solutions, we resorted to planting crops in pots containing a growing medium (a commercial potting mix, sometimes including soil and/or sand) that allowed fertilizer additions in a more conventional yet very precise manner to avoid losses. The difference in growth rates obtained was astounding, and an example with lettuce is shown in Figures 3c and 3d.

The initial potting design tested consisted of growing plants in conventional plastic pots placed atop simple 2 m × 2 m floating rafts in Arenal and La Virgen lakes (Fig. 4). Plants were irrigated by daily hand watering or with ‘wicks’ consisting of lengths of wettable rope that conveyed water by capillary action from the water body into the growing medium in each pot. After several trials and testing of available rope materials, three 25-40 cm long wicks were used per pot, which were effective in providing plants with sufficient water. For an experiment conducted in La Virgen with four crops, crops irrigated using wicks yielded more than those that were hand watered (bean and tomato were significantly greater). Although this difference was attributed to inconsistent hand-irrigation (because personnel often preferred not to enter the water), irrigation through wicks was highly efficient and reliable.

Although potted plant growth on floating rafts was adequate, even simple rafts were relatively expensive and heavy. Thus we thought it necessary to try simpler, lower-cost approaches.

Read the rest of this article in the March 2015 issue of World Aquaculture Magazine here