1. Water-Logging, Land Salinity and Infertility
The introduction of surface irrigation from canals and tanks has resulted in the rise in the groundwater-table, continuous rise in water-logging and land salinization. Irrigation water unlike rainwater contains considerable amount of salt in the solution.
Salts like potassium sulphate or nitrates can be of direct benefit to plants, others such as calcium sulphate can contribute to improve the soil structure. On the other hand, salts such as sodium chloride, or compounds containing boron may have detrimental effects on the soil or the plant. Water-logging is caused when the salt of the soil comes up to the surface due to the rise of sub-soil water-level. Dams built for irrigation purposes bring water-logging in two ways.
Firstly, canals intersect drainage lines and thus cause rain or flood water to be held up and secondly, reservoirs and canals cause their own water to seep until water reaches the root zone level. If the sub-soil outflow is not enough to balance the inflow, the root zone level rises, and all the salt of the soil comes to the surface and makes the land unfit for the cultivation. In Surat and adjacent areas of Gujarat before the introduction of irrigation (Kakrapar Project in 1959 and Ukai Project in 1972) water table was between 6 to 9 metres from the ground level P x 2.
In 1987-88, the water table had come within 0 to 3 metres. Soil salinity has also increased; pH value has increased from 7.9 to 8.9. Kheda and surrounding area of Gujarat has been brought under canal irrigation, since 1959 through the pick-up weir at Kanakbori, supplemented in 1976 by construction of Kadna reservoir on Mahi River. The use of water under inadequate drainage has led to rapid rise in the water table.
In the beginning, many major dams were built for providing irrigation, which made possible considerable increase in crop production. But after two or three decades, this irrigation policy started showing its adverse effects. In Hoshangabad district of Madhya Pradesh due to the Tawa irrigation project, a large amount of good cultivated land has been affected by salinity and became infertile.
This area was rich in wheat production and now cannot even produce millets. Some conscious farmers of this area have started a “Mitti Bachao” (save the soil) movement. In several other places in India, particularly in Punjab and Haryana, the soil has become waterlogged and saline. Some of the lands become totally unfit for cultivation. In many places land has become so infertile that it cannot produce even the crop, which was being produced before the introduction of irrigation.
In Punjab in one year (1942-43), more than 280,000 hectares of land became useless as a result of salt accumulation and water logging. In Sonipat, Rohtak, Jind and Gurgaon districts of Haryana, Gram and Bajra are disappearing and cultivation of wheat, sugarcane and Jowar is becoming difficult because of water logging. Water logging may become serious where river irrigation is practised as a result of percolation of water from a dense network of rivers and canals.
In Uttar Pradesh and Punjab due to water seepage and in the case of Karz-kum canal in Russia, about 43 per cent water gets lost through seepage and converts large areas unfit for cultivation. According to a soil survey report in Egypt, about 50 per cent of irrigated land has deteriorated to the extent that they are low, medium or poor soils. Many of the developing countries are suffering from water logging and salinization of land.
In the name of development, more and more irrigation projects are coming up and making agricultural land infertile. Excessive irrigation does not leave land even that much productive as before irrigation. For a short period, irrigation increases production remarkably and then it makes land worse off. Generally, developing countries face this problem as water management is not proper.
2. Silt Deprivation (Siltation)
After construction of dams, silt, which is brought by the rivers, gathers in the reservoirs. This silt makes alluvial plains fertile along the rivers. Now, because of the construction of dams, this fertilizing effect has disappeared. On the contrary, siltation causes riverbed degradation and coastal erosion.
The dam holds up the silt and releases only clear water. Siltation reduces the life of reservoirs, and once reservoirs are filled up with sediment, they cannot be of much use further. The life span of reservoirs has frequently proven much shorter than planned, particularly when precautions are not taken to protect the catchment area. The cutting of trees and destruction of natural vegetation on mountain slopes increase the rate of erosion and result in siltation of rivers, reservoirs and irrigation canals.
The life expectancy of the Bhakara dam initially estimated to be 88 years is now expected to be 47 years and Hirakud dam from 110 years to 35 years, because of the high rate of siltation.
The life of Mangala dam in Pakistan was estimated to be 100 years. Its present expectancy is 50 years.
The Ksob dam in Algeria became first partially and now has been rendered completely useless within 10 years. In South Africa, reservoirs are filled up with sediments within 15 years. In Philippines, the expected life of Ambuklao dam has reduced from 60 years to only 32 years, due to the increased erosion, which is leading to massive silting of the reservoir.
The flood silt helps the delta shores to resist well against sea waves and currents. After construction of dams, silt, which forms a natural defence, does get eroded. Silt deposition at the mouth of the river provides food to the fish. When siltation does not take place, fish stop coming towards the seashore and the fish industry suffers. Thus, majority of experiences show that siltation causes many natural hazards.
The life expectancy of reservoirs also causes environmental imbalance such as delta, erosion, riverbed erosion and non-availability of food for fish for want of suitable vegetation in the offshore region.
3. Health Hazards
Large dams have serious health hazards to human beings. There is a steep increase in diseases like schistosomiasis (a debilitating intestinal and urinary disease caused by the larvae of a blood fluke), cerebral and spinal meningitis, pneumonia, measles, and malaria. These diseases emerge because of continuous standing water in the command area of the dam region.
In the case of the Aswan dam in Egypt, schistosomiasis is prevalent among a large population. According to a study conducted between the Aswan dam and Cairo city, schistosomiasis has increased from five per cent in 1930 to 35 per cent in 1972. The Kano river project in Nigeria was supposed to improve health through provision of clinics. But in reality, very limited clinics were provided and that too at inconvenient locations. On the contrary, there has been a dramatic increase in several waterborne diseases. While building dams, this aspect remains totally neglected and safety and precautions are not taken into account.