WATER UTILIZATION AND SALTS ACCUMULATION IN ARID LAND REGIONS

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  • 乾燥地における水利用と塩類集積
  • カンソウチ ニ オケル ミズ リヨウ ト エンルイ シュウセキ

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Abstract

More than 30% of the land surface of the earth consists of arid and semi-arid soils, which are generally too dry to produce a good yield. If enough fresh water is avairable and the soil condition is suitable, these soils can be irrigated and used for agricultural land. These regions therefore are expected to be the new promissing land for food production in future. From this background, recently much attention to arid and semi-arid soils has been paid, and more agricultural development projects are being planned and carried out. However, many projects fail in the past and even now, because some years after irrigation the salinity or alkali hazard increases. In this paper, the soil profile investigations of salts accumulation related to the depth of ground water level, some field experiments using lysimeter and the model experiments to clearfy the mechanisms of salt accumulation are illustrated.<br> From the field investigations and lysimeter experiments of irrigated lands in Khuzistan State, Iran, the following cases have been pointed out as the essential processes of soil salinization.<br> Case 1; Shallow groundwater formation due to seepage from earth canal and intensive irrigation such as basin irrigation.<br> Case 2; Logging of irrigation water at the extremely hard pan under plough layer caused by the compaction of tractor loading.<br> Case 3; Salts addition to the soil irrigated by water having high electric conductivity.<br> Case 4; Low leaching resulted from the extremely high evapotranspiration under the cultivation during summer dry season.<br> Case 1 is most dominant process to develop the soil salinization in this regions. Salt contents at surface and subsoil of the field with shallow groundwater table are 131.4 and 20.64mS/cm in EC, respectively. These salts are mixtures of sodium and magnesium salts which are mobile cations through soil profile. Because the critical value of EC for plant growth is 4mS/cm, these findings show that amelioration for these soils may be practically impossible.<br> Case 2 is repeatedly found in desalinized fields with clayey texture. The continual cultivation in desalinized field causes an extremely hard pan to form under plough layer due to the compaction of tractor loading. This hard pan is apt to become a impermeable layer logged with irrigation water, and provides the “secondary site” which functions to supply sodium salts to surface horizon. In the cultivated land with drainage system, salts exclusion effects by irrigation water is surely promoted, however, one should pay the attention that there occurs unfavorable changes in soil chemical and physical conditions by the participation of artificial effects.<br> Case 3 comes from quality of irrigation water. Chemical analyses of the irrigation water applied show that about 1.8 kg of total dissolved salts are contained in 1m3 of irrigation water, and that pHc of the irrigation water is far less than 8.4. Low value of pHc means that calcium carbonate dissolved in the irrigation water has a tendency to precipitate in soil when it contacts with soil. Lysimeter experiments reveal the fact that about 65% of the dissolved salts supplied by the irrigation water are drained away from the soil and 35% remain. Considerable differences in the ease of leaching tendency among ions are found. Bicarbonate, chlorine, calcium and magnesium remain in the soil column with comparative ease.<br> Case 4 occurs in the cultivation under a limited water supply. Leaching fraction is the ratio between the amount of water drained below the root zone and the amount applied in irrigation. Soil salinization has progressed quickly during about two months under the condition that leaching fraction has controlled to only 1% even in applying 760mm of the irrigation water. The yield decrease of alfalfa resulting from the soil salinization is 25% of the yield expected from the case without salinity hazard.

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