C. Relief is represented by a plain with sand cover thickness of 160 cm, underlaid by light and heavy loam. Soil volumetric mass varies with depth from 1.41 to 1.61 g/cu.cm, solid phase density is 2.70-2.73 g/cu.cm. The least soil moisture capacity within 1 m-layer is 8-10 % and beneath is 12-14 %.
Soil permeability is high (about 6.5 m/day). Salt content within soil 1 m-layer was 0.17 % for TDS and 0.008 % for chlorine, i.e. soils are practically non-salinizated.
Groundwater level varied within the limits of 2.0-2.5 m, water salinity was 2.8-5.0 g/l for TDS and 0.18-0.29 g/l for chlorine.
Parameters of Pilot Projects and Technical Solutions:
Field investigations were carried out for definition of salt and water migration under cotton irrigation by drainage brackish water in furrows. Irrigation regimes dependence on drainage water salinity were studied. Experiments were repeated 4 times. Control sites were irrigated by ordinary irrigation water from Karakum canal with salinity 0.5-0.6 g/l.
Experimental and control fields were equipped by observation wells for groundwater level and salinity measurement and by watermeters for irrigation water accounting.
Methodology:
Investigations were carried out within the field with area of 125 sq. m. Total area is 2000 sq. m. Agricultural crop is cotton. Number of irrigations – 12. Irrigation norm 600-800 cu.m/ha under drainage water irrigation and 500-700 cu.m/ha under fresh water irrigation. Irrigation norms were 8750 cu.m/ha and 7700 cu.m/ha respectively.
Results:
Drainage outflow is formatted as a result of water filtration from irrigated fields, Karakum canal and other canals, groundwater inflow from the Kapetdag mountain. Drainage water salinity during the growing season was 2.1-2.8 g/l. Salinity type – sulfate-sodium-magnesium. Cotton irrigation was performed by norm of 600-800 cu.m/ha under drainage water irrigation and 500-700 cu.m/ha inder irrigation by fresh water (control). Irrigation norms were 8750 and 7700 cu.m/ha respectively. Number of irrigations – 12.
Early spring additional fresh water was brought to experimental and control fields (for soil water stock) – 1000 cu.m/ha. Vegetation irrigations were appointed under soil moisture of 0.70 normal moisture.
To support soil optimal moisture during germination phases of the plants irrigation norms were differenciated: before blooming 600-700 cu.m/ha, within blooming and fruit-bearing period 700-800 cu.m/ha and during ripening 600-700 cu.m/ha. Under fresh water irrigation its norms were cut down on 100 cu.m/ha.
Groundwater salinity during last 2 years decreased from 4.8 to 2.8 g/l, its level varied within 2.0-2.5 m.
Insignificant salt accumulation in unsaturated zone of sand-desert soil is observed mainly at expense of SO4-anion and Ca-anion increase which are not toxic salts for plants.
Annually before sowing each cotton field was provided by organic fertilisers in amount of 15-20 t/ha. During growing season cotton was fed by ammonia saltpetre (200 kg/ha); phosphorus (200 kg/ha) and potassium (50 kg/ha).
Soil water-physical properties under irrigation by drainage brackish water or by fresh water during all period of study did not change.
Average cotton yield on sandy soils under irrigation by drainage brackish water was 3.54-4.39 t/ha and on the control fields 3.73-4.06 t/ha.
