Fish Farming Guide
Weeds are controlled by irrigation, because many kinds are not adapted to submergence. No other crop is planted to flooded soil except rice. Besides, there are some additional merits in irrigated rice farming. Cattle can be fed with the weeds on levees. Fish culture in canals or swamps provides protein resources. The nutritional balance of rice proteins is one of the best among the staple cereals. Farms in arid regions are desalinized by a regular rotation of rice planting.
There are nearly 50 000 dams in the world with heights above 15 m- defined as large dams - and an almost innumerable number of small dams built for farm ponds and other tiny impoundments. These dams can retain 6500 km3 of water, which represents 15 of the annual global runoff (Figure 1). The area of former terrestrial habitat inundated by all large ( 108 m3) reservoirs in the world is comparable to the area of California or France. The environmental values that were lost as a result of this inundation are only known sporadically. It is not even known how many people were forced to move because of the reservoirs. The estimated number is 40-80 million people. Given that California has a population of approximately 37 million and the French population is 64 million, the magnitude seems accurate.
In Taiwan, Chin, Komamura, and Takasu (1987) developed a model for the estimation of effective rainfall in order to use rainfall more effectively. The basis of the model is the equilibrium equation of the water balance in a paddy field. An irrigation area of a farm pond in northwest Taiwan was chosen to test the model's accuracy because of its simple cropping and single-rotation irrigation block, where inflow and outflow could be easily measured. The average measured and computed values were in close agreement, and the effective rainfall rate for this area was 40 to 65 percent.
There is also considerable scope for implementing technologies that improve soil moisture without irrigation, such as conservation tillage and rainwater harvesting (Ngigi et al. 2005 Hobbs et al. 2008). The latter includes techniques such as farm ponds and zai pits, and may be increasingly relevant if rainfall becomes more episodic and intense, as suggested by many climate models (Chapter 3).
The effects on specific social and economic groups need to be further documented. For example, development of hydroelectricity may reduce water availability for fish farming and irrigation of home gardens, potentially adversely affecting the food security of women and children (Andah et al., 2004 Hirsch and Wyatt, 2004). Linking carbon sequestration and community development could generate new opportunities for women and marginal socio-economic groups, but this will depend on many local factors and needs to be evaluated with empirical research.
Figure 8 Relationship between the surface area of farm ponds and the annual average precipitation in several political units (after Downing and coworkers). The line is a least squares regression (r2 0.80, n 13) where the area of farm ponds expressed as a percentage of the area of farm land (FP) rises with annual average precipitation (P mm) as FP 0.019 e0 0036P. Figure 8 Relationship between the surface area of farm ponds and the annual average precipitation in several political units (after Downing and coworkers). The line is a least squares regression (r2 0.80, n 13) where the area of farm ponds expressed as a percentage of the area of farm land (FP) rises with annual average precipitation (P mm) as FP 0.019 e0 0036P. 0.22 of the land surface. The area covered by small, low-tech farm ponds is about the same as that covered by the world's three largest impoundments. Impoundment area is generally underestimated, however, by poor availability of data on small and moderately sized...
Most analyses of impoundment size distributions have ignored small, low-tech impoundments created using small-scale technologies. Farm and agricultural ponds are growing in abundance, world-wide, and are built as sources of water for livestock, irrigation, fish culture, recreation, sedimentation, and water quality control. Figure 8 shows that the area of water impounded by agricultural ponds in several political units expressed as a fraction of farm land varies systematically with climate. In dry regions, farm ponds are rare, but up to about 1600 mm of annual precipitation, farm ponds are a rapidly increasing fraction of the agricultural landscape. In moist climates, farm ponds make up 3-4 of agricultural land. Downing and coworkers used this relationship (Figure 8) with data on area under farming practice, pond size, and estimates of annual average precipitation to estimate the global area covered by farm pond impoundments. They found that 76 830 km2, worldwide, is covered with farm...
Ozone reverts to oxygen when it has been added and reacted, thus increasing somewhat the dissolved oxygen level of the effluent to be discharged, which is beneficial to the receiving water stream. Contact tanks are usually closed to recirculate the oxygen-enriched air to the ozonation unit. Advantages of ozonation over chlorination are that it does not produce dissolved solids and is affected neither by ammonia compounds present nor by the pH value of the effluent. On the other hand, ozonation has been used to oxidize ammonia and nitrites presented in fish culture facilities 31 .
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