5.7 Clay pot and porous capsule irrigation in Brazil

Subsistence farmers in the tropical, semi-arid parts of Brazil depend on rainfall to provide water for the growth of most of their crops. In years of low rainfall, agricultural production is severely affected. To make matters worse, most agricultural plots in this region cannot be served by conventional systems of irrigation because of the huge volume of water that would be required. To overcome these difficulties, Empresa Brasileira de Pesquisa Agropecuaria (EMBRAPA) introduced the use of porous or clay pot irrigation systems in Brazil in 1978. These systems have contributed to ensuring steady or even higher agricultural output owing to the highly efficient and economical use of water. A system of this kind was used centuries ago by the Romans, and initial experience in Brazil suggests that different varieties of plants can thrive in normal, saline, and saline-sodic soils on small amounts of water using the clay pot technology. Water use is roughly equivalent to 17 mm/ha/800 pots over a period of 70 days (Modal, 1978). This technology is described in Part B, Chapter 4, "Water Conservation."

Technical Description

· Clay Pot Irrigation

The clay pot system of irrigation, which consists of individual pots or a series of pots connected with plastic tubing, is easy to install, operate, and maintain. The main components of the system are shown in Figure 37.

A main supply line connects the raw water source (a reservoir, tank, pit, dam, etc.), by way of a storage tank and sand filter, to the series of clay pots, which are joined together with ½" diameter polyethylene tubing. The water level in the pots is kept steady by a system of float valves. The pots, similar to the tanks used to store drinking water in the home, are generally conical in shape and can hold 10 to 121 of water. They are typically made of clay and baked in home ovens by individual craftsmen. Because each is handmade, they will not all have the same volume. These pots, in turn, may be connected to a row of secondary storage pots which are connected to load pots in the main row. The secondary pots are installed in curved lines and are used to grow different crops.

Before the pots are set up, the soil must be prepared enough to mark out the curved lines where the pots in the secondary line of supply are to be placed. Holes are dug at the desired distance apart to accommodate the pots in the main and secondary lines. Manure from the farm is added at this time, if necessary. Then one pot is placed in each hole and the tubing is attached with an epoxy glue. The tubing is aligned in a small furrow, about 8 cm deep, so that it can be fully covered with soil. It is essential that the pots in the second row be aligned parallel to the gradient so that the water in each pot is kept at the same level.

The clay pot method of irrigation should only be used on small plots of up to one hectare because the pots do not usually release the same volume of water. The system is recommended for home vegetable gardens (10 to 20 pots) and for small orchards in rural communities. The steps involved in setting up a family vegetable garden include:

· Choosing an area with regular to clayey soils to a depth of more than 1 m situated near a water source with a good supply of clean water (without clay particles in suspension).

· Marking out one or two 10 m x 1 m beds in the area selected and digging circular holes 80 cm in diameter and 60 cm deep, about 1 m apart from center point to center point. This will be large enough for 10 clay pots to be placed in each bed. The soil removed should be left beside the hole. Likewise, the soil in the 40 cm strip running from the edge of one hole to the edge of the next and in a 20 cm strip around the borders of the beds should be removed to a depth of 30 cm to take the "wet bulbs" of the clay pots, and left beside the bed.

· Breaking up the soil that has been dug up into pieces of less than 1 cm in diameter, and mixing it with at least 50 kg of manure for each bed.

· Placing the soil-manure mixture in the bottom of each hole to a depth of 30 cm.

· Inserting a porous clay pot 30 cm in diameter and 50 cm high, with a 10 to 12 l capacity, into the center of each hole and filling the rest of the hole with the soil-manure mixture, leaving just the neck of the pot protruding. In the case of heavy clay soils, a fine layer of sand should be placed around the pot.

· Filling the pots with clean water; muddy water or water with clay particles should never be used as the silt particles will interfere with the porosity of the clay vessels.

· Planting vegetable seeds in the bed in the same way as in traditional vegetable gardens. The seeds should be irrigated two or three times a day until germination, which usually takes about 6 days, depending on climatic conditions.

Three days after the pots are initially filled with water, six to eight holes are dug about 2 m to 4 m from the side of each pot for the final planting of the seedlings. The hole should be covered with dry soil and irrigated daily. The pots are refilled with water every day until the seeds or seedlings are able to survive on their own using just the regular release from the clay pots. The same procedure is used for seedlings that are planted directly.

The clay pots should release, on average, at least 3.5 l/day of water each day, although, to start with, as much as 20 l of water may be released. (The important thing is for the pots to be able to release a minimum of 3.5 l/day during the period when the need for water is greatest.) In areas where the clay pots are not baked in closed ovens, sand should be added to the clay mixture to make them more porous.

· Porous Capsule Irrigation

This method is technologically a little more sophisticated than the previous method. It has the advantage of a standard volume for each capsule. Each capsule is also more porous and releases more water. As in the clay pot technology, the capsules are made from a clay mixture. They are reddish brown and conical, with sides about 60 mm thick.

The Center for Research in Tropical Semi-Arid Regions (CPATSA) of Brazil has conducted comparative studies of the capsules used in this technology and has found that Mexican pots have four openings: two at the top and two at the bottom. They are made of pure, nonexpandable clay and are baked in ovens at 850°C. They can hold 600 cm³ of water and have a porosity of 18% (Santos, 1977). In contrast, the capsules currently in use in Brazil have a 700 cm³ capacity with a mechanical resistance of 5 kg/cm², a porosity of 21%, and two connector spouts at the top. They are commercially made from a mixture of plastic and elastic materials, and baked in closed tunnel ovens at 1120°C. These units can accommodate higher volumes of water and release an average of 5 l/day. They are set in 100 m rows, making them easy to join together. This makes the system economical to install and eliminates the need for a hose (Silva et al., 1981).

The basic components of this system are shown in Figure 38. The storage area of the system consists of a receptacle (a home-made clay pot will do) that can hold 10 to 12 l. A float keeps the water level inside the pot constant. This level then creates the pressure head, which is the difference between the surface level of the water in the reservoir and the average level in each porous pot. The main supply line, consisting of 1" polyethylene tubing, connects the porous pots to this storage reservoir. The porous pots are placed in a series joined together in a curve parallel to the contour, or at a slight incline when the lines exceed 100 m in length, and are connected to the main supply line. This method of irrigation does not require a conventional motor to pump the water; it is distributed automatically and continuously, in direct proportion to the difference in potential between the water level in the pot and the soil surface and inversely proportional to the resistance created by the porosity of the pot.

Extent of Use

Given their limitations in terms of area served and volume of water, irrigation systems of this kind ought to be used mainly on small family farms. The technology is used at present for irrigating small farm plots, small orchards, and small-scale horticultural operations.

Operation and Maintenance

Water is automatically and continuously released owing to the difference in potential (head) between the water level in each unit and the dry soil. As the plants take in water from the soil, the potential between the soil and the irrigation pots increases, causing water to flow directly to the soil and supply the needs of the crop. This system is easy to operate and maintain.

If the required volume of water is not released, this can be corrected by drilling four small (1.5 mm-diameter) drainage holes at regular intervals in the side of the pot, about 10 cm to 15 cm below the soil level. In any case, the pot gradually loses its original capacity to release water after long periods of use. When this occurs, the user has two alternatives: the pot's original capacity to release water can be restored by baking it once more in the oven, or it can be replaced with a new one.

Level of Involvement

The government of the state of Pernambuco recently established a porous pot manufacturing plant, and capsule set up irrigation units on a number of small farms using this system to irrigate the main food crops such as maize and beans. Most clay pot/porous capsule irrigation systems are constructed privately by individual landowners.

Costs

The average cost of an irrigation system is approximately $1 300/ha for an orchard and $1 800/ha for a vegetable garden. Representative costs are shown in Tables 24 and 25 for the two technologies.

Effectiveness of the Technology

Water is released automatically from both the clay pots and porous capsules, as the process of evapotranspiration occurs. As a result, water is not lost through percolation or surface runoff as is the case with conventional irrigation systems. Hence, the system is extremely effective. However, it is limited to small-scale operations at present.

Suitability

This system can be used on agricultural plots that do not have access to water for conventional irrigation methods. It has been well accepted in the semi-arid regions of northeastern Brazil. EMBRAPA reports that the systems have also been well received in other parts of Latin America.

Table 24 Installation Costs of a Clay Pot Irrigation System on a 0.2 ha Plot

Disadvantages

· With the clay pot system, water may not be released at the same rate from all the pots; since they are handmade by individual craftsmen, there is little control over the proportions of materials used.

· In the case of porous capsules, even though the proportions of materials used can be better monitored, the amount of water released gradually diminishes over time. This problem can be minimized by ensuring that clean water is used at all times, so that water with particles in suspension does not pass through the sides of the pot.

Further Development of the Technology

Research is being carried out to increase the useful life of the system for producers in rural areas, and to develop economical variations of this technology that can be used commercially.

Information Sources

Contacts

Everaldo Rocha Porto, Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA), Centro de Pesquisa Agropecuaria do Trópico Semi-Arido (CPATSA), BR-428 km 152, Zona Rural, Caixa Postal 23, 56300-000 Petrolina, PE, Brasil. Tel. (55-81)862-1711. Fax (55-81)862-1744. E-mail: [email protected].

Aderaldo de Souza Silva, Empresa Brasileira de Pesquisa Agropecuaria (EMBRAPA), Centro Nacional de Pesquisa, Monitoramento e Avaliação do Impacto Ambiental (CNPMA), Rodovia SP-340 km 127.5, Tanquinho Velho, Caixa Postal 69,13820-000 Jaguariuna, Sao Paulo, Brasil. Tel. (55-198)67-5633. Fax (55-198)67-5225. Telex (55-19)2655.

Luiza Teixeira de Lima Brito, Empresa Brasileira de Pesquisa Agropecuaria (EMBRAPA), Centro de Pesquisa Agropecuaria do Tropico Semi-Arido (CPATSA), BR-428 km 152, Zona Rural, Caixa Postal 23, 56300-000 Petrolina, PE, Brasil. Tel. (55-81)862-1711. Fax (55-81)862-1744. E-mail: [email protected].

Bibliography

Díaz Santos, Ebis. 1977. Determinación de la Evapotranspiración en Trigo Mediante Riego por Succión. Chapingo, México, Colegio de Postgraduados de Chapingo.

Modal, R.C. 1978. "Pitcher Farming is Economical," World Crops, 30(3), p. 124.

Silva, A. de S., and A.A. Magalhães. 1978. Efeito da Irrigação Mínima na Produtividade de Milho e Eficiência no Uso de Água. Petrolina, PE, Brasil, EMBRAPA-CPATSA.

Silva, A. de S. and E.R. Porto. 1982. Utilização e Conservação dos Recursos Hídricos em Áreas Rurais do Trópico Semi-árido do Brasil, Petrolina, PE, Brasil, EMBRAPA-CPATSA. (Documentos, 14)

Silva, D.A. da, A. de S. Silva, and H.R. Gheyi. 1981. "Irrigação por Cápsulas Porosas. III: Avaliação Técnica do Método por Pressão Hidrostática." In Pequena Irrigação para o Trópico Semi-Arido: Vazantes e Cápsulas Porosas. Petrolina, PE, Brasil, EMBRAPA-CPATSA. pp. 20-42. (Boletim de Pesquisa, 3)