After the 1970s' peak, the worldwide rate of irrigation development rapidly declined, and most financial assistance has been concentrated on operation and maintenance rather than new development of irrigated lands.
In spite of this effort, most irrigation departments in developing countries face heavy burdens of operation and maintenance costs. An analysis of this phenomenon ascertained that more emphasis had been placed on operational work than on maintenance work. It identified the maintenance and repair of canal systems as main drains on budgets. The most expensive work is repairing lined canals. Collapse of canal linings begins with seepage through the joints.
After displaying flaws in conventional jointing methods, the study disclosed a new method of making a construction joint, called the“two-stage compulsory filling method.”This technique is characterized by two-stage placement of mortar with different cement contents and zero slump. Heavy blows of a wooden plate can drive masses of mortar into the joint, and result in meeting a full set of requirements for the construction joint. This method can be applied to the expansion joint due to its flexibility.
It has been proven that, if existing techniques are translated into a method truly required in situ, we can improve canal performance.

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This paper describes the irrigation management on paddybased agriculture in the deltas of the Aral Sea basin. The Aral Sea is now desiccating, and has shrunk almost to 50% of the original surface area (in the 1950s) as a result of water withdrawals from Amu Dar'yra and Syr Dar'ya rivers mainly for irrgation. The authors and their co-workers studied the irrigation scheme in the deltas of the Ili river, Syr Dar'ya and Amu Dar'ya, which are mostly irrigated lands with a paddy-based crop rotation system. It became clear that massive amounts of water for irrigating paddy were consumed in the deltas.
In the Aral Sea basin, the area devoted to rice is 428, 000 ha occupying 6% of total irrigated area, and more than 3, 000mm of irrigation water is used for this crop. Rice is reported to consume more than 13% of total irrigation water (89km³). The irrigation requirement of 3, 500mm appear to be excessive if the annual evaporation of 2, 200mm in the region is taken into account. By introducing water-saving measure or eliminating paddy fields having extremely high percolation rates, average irrigation requirement may well be considerably reduced, say down to 2, 000mm.
During the course of our investigation in the rice area of the Aral Sea basin, where rice is grown in rotation with other crops such as wheat, corn, and alfalfa (e.g., 3 crops of rice in 7 or 8 years), irrigation amounted to 3, 500 to 5, 500 mm/crop (from May to September). For crops other than rice, average irrigation requirement is 800mm/crop. It is obvious that most paddy irrigation water is used to leach salt that accumulated in the soil during the non-rice crop cultivation period. Water requirements for paddy rice cultivation should be considered taking into account the leaching function of irrigation water. Analysis of water balance in the fields revealed that water applied to paddy raises groundwater level to a certain level, so that groundwater is available to irrigate crops other than rice such as wheat and alfalfa. The new conceptual model of paddy-based irrigation is essential to solve the problems of water saving in the Aral deltas. In actual practice however, there are many local variations in the irrigation management.
The major findings of this report are as follows;
1) Under the current irrigation practice, massive irrigation supply to rice crops appears to be necessary as:
i) accumulated salt must be leached away,
ii) water is supplied to non-rice crops from a raised groundwater table.
2) To improve water application efficiency the following measures are to be considered effective:
i) Proper surface irrigation practices for non-rice crops (e.g., furrow irrigation) to avoid reliance on water supply from the groundwater table which is raised by water supplied to rice crops. As farmers are not used to such irrigation techniques, it may take considerable time to modify their irrigation practices.
ii) alternatives to i), reclamation of non-cropped area (amounting to some 40% of entire irrigation command area within state farms and collective farms where groundwater table is raised high enough to promote evaporation loss. For this purpose, however, proper land leveling and drainage system must be implemented.

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The Bangkok clay is characterized by compacted subsoil which is one of the causes of poor drainage and restricted agricultural production in the Bangkok plain. Subsoil improvement practices (subsoiling, deep plowing, and a combination of the two) were tested to see whether they improve the soil physical properties and thereby plant growth. The soybean was selected as a reference crop. The results showed that the subsoil improvement practices altered soil structure, enhanced soil permeability, reduced soil hardness, induced subsurface drainage, and improved soil aeration. The soil physical condition was improved mainly due to the loosening of the compacted subsoil and break up of the plow pan. Thus, favorable soil physical conditions for plant growth were achieved. The combination treatment of subsoil improvement had the most significant effects on improving soil physical properties. The plant growth parameters of the soybean, including root length, dry matter production, and yield, were significantly higher in the combination treatment than in other treatments.

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For a double rectangular nozzle, the inside contracting angle, length-to-width ratio of the rectangular slot, and spacing between two rectangular slots are its main three design parameters. Water distribution profiles and droplet diameters for nozzles having different design parameters were tested. It was found that water distribution and droplet diameter are highly sensitive to both the spacing between two rectangular slots and the length-to-width ratio of the slot; however, the hydraulic performance of a double rectangular nozzle appears to be independent of the inside contracting angle. The effect of the diffuse extent of jets formed by double rectangular nozzles on the sprinkler rotation speed was also investigated, and it was found that the speed decreases as the diffuse extent of the jet increases, and either a large slot spacing or a small length-to-width ratio may lead to a low rotation speed.

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The effect of long-term irrigation and intensive cultivation on three soil series was investigated. These soils were from a calcareous belt in the G-Kirrigation project area in Bangladesh, which had experienced declining crop yields. In order to evaluate the effect, if any, of irrigation and intensive cultivation on soil properties, three soil series from irrigated and adjoining non-irrigated areas located in the same catena were studied. Soil properties such as organic carbon, CEC, electrical conductivity, potassium adsorption ratio, exchangeable sodium percentage, and ionic strength did not show any distinct changes between the investigated areas. The major changes identified between the irrigated and non-irrigated soils were pH, void ratio, maximum water holding capacity, water-soluble Na, ferrous iron, sodium adsorption ratio, available nitrogen, and free carbonate content. In this study, the correlations between the following soil properties were demonstrated in all the series: (i) exchangeable and water-soluble sodium, (ii) exchangeable and water-soluble potassium, (iii) void ratio and maximum water holding capacity. By applying t-test, the effects of cations on other cations, sodium adsorption ratio on free carbonate, and water-soluble sodium on water-soluble potassium were found to be insignificant. The present study indicated the necessity of further investigation of the relationship between the declining fertility and changes in soil carbonate.

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