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Objective and technical fields:
Prevention of the soil salinization on the base of regulated drainage of a new generation-vacuum vertical drainage system. Objectives: Management by water, air, salt and nutrient regime of irrigated lands, soil fertility increase, drainage outflow cutdown, water resources rational use.
Scientific and technical approach:
Creation of optimal for plants soil’s water, salt, air and nutrient regime based on leaching and minimum irrigation by fresh groundwater. Investigation’s importance: soils and groundwater desalinization, groundwater level support at optimal depth for plants depending on development phase, water saving, soil fertility increase.
Environment characteristics:
Climate is continental. Sum of effective temperatures (more than 10oC ) is 4900-5000oC. Air relative humidity in winter is 70-80 %, in summer - 30-45 %. Wind velocity is 2.5-3.5 m/sec. Evaporation from water surface is 1510 mm/year. Annual precipitation is 140 mm. Relief slope is 0.00023 and directed to the north-west. Cover layer is constituted by loam and sand loam and has thickness of 1.5-2.5 m. Its permeability coefficient is 1.0 m/day. It is underlaid by coarse and middle-grained sand with permeability 15-20 m/day. Its thickness is 30-50 m. Beneath there are neogen sediments which constitute impermeable layer. Groundwater level was 1.2-1.5 m in spring and 1.8-2.5 m in autumn-winter period. Groundwater salinity was 5 g/l, soil salinization - about 1 % on solid residue and 0.03 5 on chlorine-ion. Salinization type is chlorure-sulfate.
Parameters of Pilot Projects and Technical Solutions:
Irrigated area is 1500 ha (net). Main agricultural crop is cotton. Irrigation network is represented by earthly canals with extent of 40 km. There are vacuum drainage system within the pilot site for small drainage outflow volume by pumping. Distance between collectors is 500-800m, depth is 2-2.5 m. Vertical drains connected to collectors have depth of 6m. All system is made from polyethylene tubes. Vertical drains’ screens are positioned in sand and collectors - in loam.
Methodology:
Observations on soil salinity dynamics, groundwater level and salinity, irrigation depth were performed. Pilot plots were equipped by weirs for irrigation depth measurement and by wells for groundwater measurement. Drainage outflow volume depended on cotton development phases and groundwater salinity.
Results:
Pilot site lands are middle salinizated and need annual leaching. Cotton yield was 2.2 t/ha. Open horizontal drainage and irrigation leaching regime did not give expected results - seasonal salinization was progressing, soil fertility remained low. Vacuum drainage systems differ from other types of drainage because they remove water by gravity as well as by pumping. Regulated drainage operation allowed: to create groundwater flexible regime for soil water and salt regime optimization. System’s drainage modulus varies within broad range 0.04-0.1 L/sec/ha under gravity and 0.5-0.6 L/sec/ha under irrigation and leaching. Long-term common drainage modulus is 0.1 L/sec/ha and by pumping - 0.5-0.6 L/sec/ha. Groundwater level depending on plant’s phase of germination was kept on the depth from 1.2-1.3 m to 3.0 -3.5 m; to accelerate groundwater level lowering from 1-3cm/day to 12-20 cm/day; to decrease water volume for 1 ton salt removed from the soil from 330 to 230 cu. m.; to accelerate soil desalinization. Initial soil salinization was 0.67% for unsaturated zone and 0.96 for loam on solid residue and 0.02-0.03 % on chlorine. After 3 years unsaturated zone salinization was decreased to 0.43 % and 0.01 % on chlorine. Salt current in loam decreased on 0.37 %, chlorine - on 0.02 %. After 6 years loam and soil unsaturated zone salinity became equal and were 0.20 % on dry residue and 0.01 % on chlorine i.e. soil passed to slightly salinizated; to create negative salt balance of unsaturated zone and decrease significantly groundwater salinity. Within 10 years of drainage operation 53.2 t/ha salt came and 137.2 t/ha were removed by drainage system; to decrease groundwater salinity within soil 10 m - layer and create «fresh pillow», which prevents soil secondary salinization. Initial groundwater salinity at depth of 2 m was 3.5 g/l, at depth of 3,4,6 and 10 m- 3,7;5,8;5,5 and 3.4 g/l respectively. After leaching with depth 5150 cu. m/ha in the first year of operation groundwater salinity increased in soil 4 m- layer on 3.5-4 g/l and became 7.4; 8.1 and 6.2 at depth 2, 3 and 4 m. After 3 years due to irrigation leaching regime groundwater salinity decreased to 1.8 - 2.0 g/l at depth of 6 m and after next 6 years more than 10 m. Water and salt exchange active zone encompassed 5-10 m depth under gravity and more than 30 m under pumping. Drainage water salinity at the initial stage of operation was 8 g/l and was not dependent on method of performance. Along with groundwater upper layer freshening drainage effluent salinity was decreasing under gravity and remained the same under pumping. Annually drainage effluent salinity was as follow: at the first year of performance: from 7.8 to 7.3 g/l under gravity and 8.9 -9.2 g/l under pumping; at the second year: 5.5-3.9 g/l (gravity) and 8.9-6.2 g/l (pumping); at the third year: 3.3-4.6 g/l (gravity) and 6.6-7.5 g/l (pumping). In further years with increase of freshening depth drainage effluent salinity was 1.7-1.8 g/l. Due to soil and groundwater salinity decrease land fertility increased that resulted in cotton yield which was 3.1-3.5 t/ha.
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