In the last module, you learned about considerations around managing for water deficits in agroecosystems, as well as the use of irrigation to supply water in times of need for specific crops. In this module, you’ll explore what happens when there is too much water in a system and how to manage agroecologically. Too much water in a system often causes “Water-logging”, when excess water is present in an agroecosystem for an extended period of time, or the movement of excess water out of the system is impeded. Water-logging and its associated changes can be caused by high rainfall, poor irrigation management, unfavorable topography, and poor surface drainage. Recently, costly damming and draining infrastructures have been developed to deal with excess water, but simpler and traditional techniques work with the condition of excess water rather than attempting to get rid of it. For example, in many wet areas of the world, rice is cultivated as it is ideally suited to wetland agriculture.
Negative Effects of Excess Water
Excess water can stop the diffusion of O2 in the soil, which can lead to the build up of anaerobic microorganisms, limit the respiration of root cells, establish conditions to reduce nutrient availability, and lead to the buildup of CO2 and other gases. In each of these conditions, plants are weakened and become more susceptible to diseases, especially in the root zone.
Drainage systems are used to remove excess water from the root zone of crops and to prevent the flooding of farmland. Records of drainage systems exist in Roman and Chinese farming areas, from 2000 years ago. Many areas in high production today, like the Yangtze River Valley of China, the lowlands of the Netherlands and the Delta Region of California would not be farmable today without complex drainage systems.
Drainage systems involve the construction of levees, canals and ditch systems that either keep low-lying areas from being flooded or lower the water table so cropping can take place. In areas with highly saturated soils, mounded or raised beds are used.
There are disadvantages to drainage systems- often there are economic costs of installation and maintenance, while there are ecological costs as the removed water carries with it nutrient and sediments that are lost from the system and must be replaced from external sources. The disposal of drainage water can be a problem as it can carry pesticide residues and high salt loads that can damage nearby natural ecosystems.
There are many crops that are adapted to tolerate wetland soils, and by selecting these crops you can adapt to the particularly ecosystem without having to transform it. The best known example is rice (Oryza sativa) which is an aquatic/swamp-adapted plant that flourishes in wet habitats. It has special adaptations, such as air-space tissue in the stems that diffuses oxygen to the roots, roots that are adapted to grow under low oxygen concentrations, and seeds that germinate underwater due to an extremely low oxygen requirement.
Other crops that have adaptations that allow them to tolerate periodic flooding include taro (Colocasia esculenta) which has the ability to store oxygen in the base of its leaves.
Agroecosystem Design for Excess Soil Water
By understanding the wetland ecosystem, an agroecological approach can be used to adapt planting to that system. Instead of trying to change the system by eliminating or diverting the water, or restricting production to monocultures of wetland-adapted crops, agroecosystems can be designed to work with the excess water. These systems, in areas with a high water table or periods of rainfall inundation, soil levels are built up and varied. Soil is dug up to build raised beds, and in the process, canals or ditches are formed, which can serve for drainage if too much water builds up. These ditches serve as a catchment for eroding sediments and organic matter. In some cases, they can even make fish production possible. Instead of changing the level of the water table, the cropping areas are raised above the water table. Instead of changing the ecosystem, farmers adapt their growing systems. During an extended dry season, capillary movement of water from the water table can be sufficient to maintain crops, or irrigation water can be drawn from the canals into the farming systems.
Examples of this include the Chinampas of central Mexico, the pond-dike systems of the Pearl River Delta of Southern China, and the canal-field systems of the Netherlands. All of these agroecosystems have a long history of successful management.
Irrigation Canals between tulips in the Netherlands:
Pond-dike systems of Pearl River Delta (mulberry fields with fish ponds)
Excerpt 1: Prehispanic Raised Field Systems in the Quintana Roo, Mexico
by Steve Gliessman | from Agroecology.org
Overview Remnants of ancient canals and raised beds in the lowlands of Quintana Roo, Mexico demonstrate techniques of wetland farming that were sustained for more than 1000 years.
From 800 BC to 200 AD, a vast wetland-based agroecosystem was managed in southeastern Mexico using canals, raised beds (platforms), and other water management structures that occupied nearly 20,000 hectares. These structures evidence several strategies used by prehispanic Maya of the Yucatan Peninsula to cultivate in conditions of excess water, caused by annual precipitation averaging 2,000 mm of which 76% occurs between May and October. By analyzing the remains of these canal and platform systems, scientists have deduced strategies for their use. Mayans excavated canals down to the bottom of the topsoil layers and formed platforms by mounding the removed soil to create planting surfaces between the canals. Soils eroded from platforms included organic materials that slowly filled canals. It is not known exactly what types of crops were planted in this system due to quick plant decomposition in the tropics, but work in other regions suggests maize and cotton were important crops.
Many agroecosystems are situated in wetlands or regions that are periodically flooded during a wet season. Management of these wetland systems requires limiting water in times of inundation and extending the cropping season in times of drought. Much can be learned from prehistoric wetland based agroecosystem management and the current indigenous farming techniques in wetland areas of Mexico that were most likely handed down from Mayan ancestors. Periodic cleaning of canals returns rich soils to the planting platforms, and soil studies demonstrate that nitrogen levels were highest on the platforms and dropped off rapidly with increased depth. Maize yields are over four times greater in these traditional indigenous farming systems compared to adjacent fields that have been cleared, drained and farmed using modern techniques.