Catching the rains

 Edward Pulver, Santiago Jaramillo, Sara Moreira, and Gonzalo Zorrilla   |  
Small-scale water harvesting and storage facilities capture excess rainwater during the period of high precipitation and supplements irrigation for dry season production or during droughts. (Photos: Santiago Jaramillo)

Small-scale water harvesting and storage facilities capture excess rainwater during the period of high precipitation and supplements irrigation for dry season production or during droughts. (Photos: Santiago Jaramillo)

The successive articles Catching the rains, Pressure in the South, and Shift to rice?  focus on rice production in Latin America.

Although rice production in the Latin America and Caribbean (LAC) region is minor within the global context, rice is the most important food crop in the region. Annual production is approximately 26 million metric tons of paddy from a harvested area of more than 5.6 million hectares. The LAC region (including Mexico), however, has a net deficit of nearly 2 million tons. Production consists of a mixture of irrigated and rainfed ecologies, with irrigated rice occupying approximately 50% of the area but accounting for more than 70% of total production. The areas of major concern are Central America and Mexico, which have a net annual rice deficit of nearly 1.5 million tons of paddy. These countries, which usually incur deficits, depend on rainfed rice, similar to Africa and parts of Asia.

National programs in the region have released many varieties but yields remain low and unstable; hence, imports continue to soar. All countries in Central America have a rainy season that starts in May and extends until November, with a marked dry season during December to May. The dry season has very high solar radiation, often exceeding 20.9 megajoules per square meter per day, allowing yield potentials of 8–12 tons per hectare. In contrast, solar radiation during the rainy season varies between 13.8 and 15.5 megajoules per square meter per day, limiting yield to about 4 tons per hectare even with irrigation or improved crop management.

Most on-farm demonstration plots from the Latin American Fund for Irrigated Rice (FLAR) agronomy program showed that yields of 8–10 tons per hectare are feasible, but only during the dry season—provided there is irrigation. This limits the technology to only a few farmers who have irrigation and excludes a vast number of small rainfed rice growers who depend on rice and other food crops for income. FLAR, with its partners, the Mexican Rice Council, the Nicaraguan Rice Growers Association, SENUMISA Company in Costa Rica, and the International Center for Tropical Agriculture, with support from the Common Fund for Commodities (CFC), innovatively converted rainfed agriculture to irrigated production and took advantage of the vast hydrological resources. The transformation of rainfed agriculture to irrigated agriculture is based on water harvesting, in which excess rainwater during the period of high precipitation is captured and stored in reservoirs and then used for irrigation during the highly productive dry season.

Rainwater harvesting

This pilot project focuses on southern Mexico, Nicaragua, and Costa Rica, which represent the larger rainfed ecology in Latin America. Most countries in this region have vast quantities of renewable water, but failure to catch and maximize the use of excess rainwater for irrigation inhibits development and increases dependence on high-risk rainfed agriculture. Simple water-capturing techniques plus adequate storage can provide sufficient water resources for irrigation during the dry season. Catching and making use of rainwater in situ1 is termed “rainwater harvesting.” The captured water—stored in reservoirs—can be used to supplement irrigation in areas that suffer from periodic droughts or for production during the dry season when climatic conditions are more favorable for high yields.

Water harvesting is not new. The Americas have approximately 1 million hectares of irrigated rice grown per year in the temperate areas of southern Brazil and Uruguay using simple on-farm water catchments. Small-scale water harvesting and storage facilities should not be confused with large-scale irrigation schemes based on constructed river dams that form large lakes that often cannot be maintained and, as a result,harm the environment. Despite being simple small catchments, building these facilities needs attention to a location, which requires hydrogeological analysis and also topography, climate, and soil chemical and physical data.

Constructing catchments

The process starts with proper site selection that considers soil type, geology, topography, source of water supply, and public safety. The suitability of a catchment site depends on the ability of the soil in the area to hold water. Soil made of clay or silty clay is excellent for catchments and must contain at least 20% clay by weight to prevent excess seepage.

Land topography is the single most significant factor that influences the costs of constructing catchments. For simple economic reasons, catchments should be located where the largest storage volume can be obtained and where there will be less soil movement to build a small dam. A dam built between two ridges crossing a narrow section of a valley allows a relatively large area for the catchment to be constructed with minimal soil movement (Fig.1). The size of the reservoir should be relative to the size of the watershed (drainage area). This allows runoff of excess water to the site. The information on hydrological balance, amount of runoff, and rate of infiltration is important for estimating the area required to fill the reservoir. Poor attention to water control structures is often the main cause of a dam failure. Dam construction begins with the construction of a cutoff trench immediately under the dam site that is filled with heavy clay and compacted to prevent excess seepage under the dam structure.

Gravity irrigation is facilitated by installing pipes (reinforced polyvinyl chloride pipes), which are embedded in concrete with reinforced metal bars, at the base of the dam. The opening of the tubing within the reservoir is fitted with a small filter structure to prevent the entry of debris that can clog the pipe. After installing the tubing, soil is compacted around the concrete structure and the dam is completed by continuous layers of soil and compacting.

Technology for dry and wet seasons

This FLAR-CFC project aims to introduce proven water capturing technologies, train local staff to identify suitable sites for catchment facilities, and demonstrate to small farmers—who are currently confined to high-risk, low-income, upland rice—the economic benefits of a diversified rice-based production system under irrigation. In Mexico, the main research and development organization of the government has financed the construction of four reservoirs—three have been completed and irrigated crops have already been planted. In Nicaragua, the main supporters of this project are the local governments (mayors) that have helped in the construction of 12 pilot reservoirs, six of which have been completed, while six are still under construction. In Costa Rica, the project is supported by the Ministry of Agriculture and the national water management agency. Fifteen sites have been identified and designs for reservoirs have been prepared for all sites.

Access to irrigation opens many opportunities, including more competitive rice production and diversification into several food crops and other income-generating enterprises, such as fish (Fig. 2). Many crops can be incorporated into a rice-based system, provided irrigation is available. Preliminary data from Nicaragua and Mexico show that bean yields 1.5–2.5 tons per hectare under irrigation—two to five times more than what farmers get from rainfed production.Maize is also highly productive under irrigation during the dry season and, based on initial data acquired from Nicaragua, it can yield 9 tons per hectare or more than triple the yields on the same farm under rainfed conditions. In addition to crops, small catchments offer a chance for fish production during the rainy season when reservoirs are being filled with rainwater.

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