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Nadide Demir General Directorate of State Hydraulic Works, Ankara, Turkey Abstract An essential requirement for successfully irrigated agriculture is the development and maintenance of a soil zone in which the moisture-oxygen-salt balance is favorable for plant growth. Plants require both moisture and oxygen to live. If a saline water table rises and remains in the root zone, it results high saline moisture condition. Agricultural production is negatively affected by this condition. For sustainable irrigated agriculture, drainage is equally important. Water table observations and evaluations are critical to check whether drainage systems are functional or not. And water table level indicates that excessive water is harmful for plant root zone. For that purposes, continuous monitoring of water table level and it' s quality and controlling of expected water table depth are required. These practices have still been carried on 1 194 353 ha areas, by DSI managed schemes that covers 2 251 625 ha areas. Introduction A prime requirement for successfully irrigated agriculture is the development and maintenance of a soil zone in which the moisture-oxygen-salt balance is favorable for plant growth. Plants require both moisture and oxygen to live. When a saline water table rises and remains in the root zone than about 48 hours, resulting in an abnormally high saline moisture condition, agricultural production is usually seriously affected. The presence of oxygen in the interstices of the soil in the root zone is as necessary as water for both seed germination and plant growth. The oxygen content of soil is governed by the rate of diffusion of oxygen through the soil pores. Also, the oxygen content is markedly affected by the moisture content of a soil. Soils with initially low moisture content normally can be expected to have relatively open pore structures between soil particles, allowing oxygen to freely permeate through the interstices. As the moisture content increases, air in the pores is displaced by water, thus forcing the air upward and subsequent expulsion to the atmosphere. However, once the oxygen has been expelled, the oxygen content recovery rate is extremely slow in a soil that is in transition from a moist or wet state to a dries state. This is because of the inherently slow rate of diffusion of gases through such soils and the phenomenon of capillary stresses which develop in soils when the water content does not completely fill the voids. The proper balance between soil moisture and oxygen is maintained to a considerable extent by adequate drainage. Studies of the water table produce information necessary for the solution of a drainage problem. Areas where a high water table has developed or is anticipated must be mapped. Information concerning depths, trends and movements is essential for an understanding of the problem. The water table investigation provides data on the position, extent, and fluctuations of the water table; the quantity and direction of movement of the groundwater, and an indication of water sources and areas of discharge. The investigation is made using observation holes and piozemeters, and analyses of periodic measurements. Materials and Methods General Directorate of State Hydraulic Works (DSI) has 2 251 625 ha irrigation networks are opened to operation. 360 775 ha irrigation networks are managed by ground water irrigation cooperatives. Water table monitoring of these networks are not being conducted by DSI.They are small networks and irrigation is done with groundwater. In other schemes water table is controlled by DSI. Most of these schemes have significant drainage problems as they are large irrigation schemes (Çukurova, Gediz, Konya basins with GAP project areas). Some irrigation areas which have suitable topographic conditions and do not require observation wells. Operation and Maintenance Department of DSI is responsible for water table management. Water table monitoring has been done in 128 irrigation schemes covering 1 194 353 ha. These schemes were placed 10 812 observation wells. Observation network is designed with observation wells which performance per 100 ha has to have one . Selection of the location for wells should be made in the field, where conditions that might affect water table can be readily observed. Wells should be located to eliminate the effect of ponds, lakes , road, border ditches, canals, laterals, rivers and similar water holding reservoirs on water table . Each well perform per 100 ha, and measurements are done once in a month. Water table level is measured with a water level indicator. The objective of the measurements is to establish a record of the water table fluctuations over a period of time that will reflect all factors affecting the water table. After recording the measurements, lines of equal water table maps are drawn. These maps;
2) Depth to lowest water table map: one year's recording result and lowest water table values are used relating to each well. If water table is between 0-1m, this location requires farm drainage 3) Lines of equal the most intensive irrigation map; this map is drawn, measurements are done when irrigation is the most intensive in a month. And it shows how irrigation influences water table. 4) Water table quality map; salt concentration (ECx10-6 250C values) is measured with an electrical conductivity meter at each well. By plotting all the EC x10-6 250C values on a map, lines of equal electrical conductivity (equal salinity) can be drawn. Usually critical watertable salinity is more than 5000 micromhos/cm. 5) Locations which show with high water table level and salinity; on the lines of equal the most intensive irrigation map, locations where water table is 0-1m and on the water table quality map, locations where water table has more than 5000 micromhos/cm salinity are determined. These locations are drawn one map and show with high water table and salinity. 6) Water table contour map; a water table contour map is a map of the phreatic surface;it can be prepared for a specific date, but preferably as a mean for a longer period. Conclusions According to 2000 year' s evaluation results; locations which show with high water table level and salinity has a little ratio (% 0,43). Water table observation areas have 94 504 ha high water table (% 8). This high water table is, at the most intensive irrigated month. 47 261 ha areas has water table salinity problem which they are> 5000 micromhos/cm.  Explanation: Irrigation networks are opened to management by DSI in 2000 year's : 1 875 104 ha Water table monitoring areas in 2000 year' s : 1 194 353 ha The ratio of water table monitoring areas to irrigation networks are opened to management : %64 Numbers of observations wells : 10 812 Some irrigation areas which have suitable topographic conditions (gradient> %3-5) and do not require watere table observation. For that reason there is a difference between irrigation networks are opened to management by DSI 1 875 104 ha and water table monitoring areas 1 194 353 ha. 2000 year' s evaluation results compared with last year' s, reveal that the area of problematic soils have not been changed. Drought seasons and clearing of drainage canals are main factors of this no change. In 1986, drainage rehabilitation works were initiated in pilot irrigation projects under World Bank Loan. Rehabilitation works positively effected drainage performance of these schemes. Water table level management is critical for soil conservation and enables desired productivity. Continued monitoring of water table levels is therefore inevitable.  |