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CONCEPT – RICE AND WATER
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- Rice is special: Rice is the only cereal that can stand water submergence, and this helps to explain the long and diversified linkages between rice and water. For hundreds of years, natural selection pressures such as drought, submergence, flooding, and nutrient and biotic stresses led to a great diversity in rice ecosystems.
- Adaptation: The plant’s adaptation strategies include surviving submerged conditions without damage, elongating its stems to escape oxygen deficiency when water tables rise, and withstanding severe drought periods. But rice is not an aquatic crop: it has great ability to tolerate submergence. Water creates unfavourable conditions for weeds, by cutting off sunlight and aeration to the ground. So, rice farmers (esp. in India) adopted the practice of submerging rice in water to check weeds. Over the years, it has almost become a prerequisite for rice cultivation.
- Five rice plants: Ecologists have distinguished five water-related categories of rice plant: rainfed lowland, deep water, tidal wetland, rainfed upland and irrigated rice. Historically, rice cultivation has been a collective enterprise. The investment and shaping of the landscape that are needed for the ponding system (terraces) require collective organization within the community. Water management also relies on collective interest: crop and water calendars must be organized for large blocks of fields in order to manage water efficiently and organize such work as land preparation, transplantation and drying for harvesting.
- Why water: Rice systems need water for three main purposes: i) evapotranspiration; ii) seepage and percolation; and iii) specific water management practices such as land preparation and drainage prior to tillering. The actual water demand of farmers is often much higher than what theoretically is assumed, because conventional application techniques are often less than 50 percent efficient.
- The Punjab case: In many rice-based systems, a great proportion of water enters the field as precipitation, surface irrigation or spillage/percolation from adjacent fields. But in the fields of Punjab, the water table has depleted sharply as most of the water used is extracted from wells, tube wells and so on. Replenishment has suffered while rice cultivation regions have expanded.
- The India situation: Rice as grown in India has become a water-guzzler, because farmers use on an average 5000 to 10,000 litres to produce one kg of paddy, though water technologists say no more than 600 litres is needed if proper water management techniques are followed. Given that 45 per cent of the country's total irrigation water is used solely for rice cultivation, the need to improve farming methods is imperative.
- Besides being wasteful, excessive use of water results in lower yields and adverse environmental effects such as soil salinity and waterlogging. Paddy yields in irrigated regions of Tamil Nadu, Punjab and Haryana range from five to six tonnes/ha, whereas in the high-rainfall areas of eastern UP, Bihar, West Bengal and Orissa, the yields are about 1.8 tonnes/ha. The main reasons for the poor yields are improper irrigation management and waterlogging.
- Farmers widely believe that water should be stored on paddy fields to prevent weed growth. But studies in West Bengal and Orissa show that standing water of more than 10 cm height leads to heavy leaching of soil nutrients and percolation losses. In all seasons, a two to five cm water depth and intermittent irrigation saves 10 to 50 per cent of water without adversely affecting yields.
- New science: During recent decades, international and national rice institutes have tested various new techniques for growing rice – aerobic, alternate wet and dry system, rice intensification – which partially or totally suppress the need for ponding at the field level. These new techniques are revolutionizing the age-old idea that rice is an aquatic crop.
- Summary:
- Rice is the only cereal crop that can survive periods of submergence in water, thanks to the adaptation strategies that rice plants have evolved over the centuries.
- Paddy rice consumes more water than any other crop, but much of this water is recycled and put to other uses.
- Rice needs water for evapotranspiration, seepage and percolation, as well as for management practices such as land preparation and drainage.
- Submerged rice cultivation practices help to promote water percolation and groundwater recharge, control flooding during heavy rains, and prevent weed growth in rice fields.
- Scientists are working to develop rice cultivation techniques that require less water. However, the benefits of these new techniques will have to be weighed against the advantages of the present relationship between water and rice, most of which would be lost if that relationship were to change.
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