インド・パンジャーブ平原における水文循環の変貌

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The Punjab is the area where the perennial irrigation canals of large-scale were first constructed in the Subcontinent of India. It is indeed true that the larger part of the Punjab has been benefited by the canal networks, while, the area was confronted with a rapid rising of the ground water table, with the result of waterlogging and salinity of fertile land. The serious rising of the water table came to be stabilized with the decrease of rainfall and a series of anti-waterlogging ameliorations, such as the excavation of drains and the boring of tubewells in the first half of the 1960s. With the development of the “green revolution”, tubewells were bored in increasing number and this has resulted in lowering of the water table in certain areas. This paper aims to discuss such a water problem in the Punjab in the light of behaviour of hydrologic cycle. The author points out that little understanding of hydrologic cycle of surface and ground water in the time of water use has caused the above water problem. The results obtained may be summarized as follows : 1) In the natural environment existed before the inception of perennial canal irrigation, the system of hydrologic cycle in the Punjab was characterized by a considerable deep level of ground water, most of which were recharged by the influent seepage from river to the doab. In such deep ground water areas of the doab, the water of heavy showers in August and early September were almost lost by the manners of surface run-off and evapotranspiration and rarely reached to ground water table through the soil and aeration zones. 2) The rising of water table caused by canal irrigations has greatly modified the linkages of hydrologic cycle. That is, vertical transfers of water between the surface and the ground water storages has become actively and resulted in the increase of evapotranspiration. And the lateral transfer of water from river to the doab has turned the outfluent from the doab to river. 3) Irrigation tracts in the Punjab can be grouped into two types in the light of mode of fluctuation of ground water table. In the first group (UBDC, SC-A, FC), the water table was rised by 0.10-0.13 meter/year from 1942 to 1955 and has been stabilized at 2.0-3.5 meters below the surface since 1956. In the second group (SC-B, WYC-B), the water table has been continuously rised by 1.55-0.65 meters/year since 1954 when the Bhakra Canal System was opened. Fluctuations of the water table of the first group can be divided into five stages. These modes and stages of fluctuation were discussed in connection with changed in rainfall and irrigation water managements. 4) Investigations were made into relationships between the fluctuation of water table and its controlling factors in UBDC. On the ground of the above investigations, calculation was made regarding the critical amount of rainfall and rainfall plus irrigation water, less than which the ground water table does not fluctuate. The critical amount of rainfall and rainfall plus irrigation water is 660 mm and 1, 216 mm in year, respectively. Based on these, irrigation water at the monsoon period should be controlled. 5) In the first group of irrigation tracts, where tubewells are intensively bored, the water table has turned the tendency of lowering in the latter half of the 1960s. Total decrease in amount of ground water storage calculated based on the above lowering on the above lowering are about 250 million cubic meter per year. Therefore, it seems that pumping of the ground water by tubewells has already exceeded to the safe yield. The author advocates that a detail calculation of the water balance on irrigation tracts be made in order to make a well-balanced management of water and to prevent cultivators from the disorder of water use which would be caused by the over-lowering of the water table.

インド・パンジャーブ平原における水文循環の変貌

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