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Objective and technical fields:
Soil salinization prevention and irrigated lands productivity increase on base of land drainability improvement by means of close horizontal drainage. Objectives: drainage optimal parameters definition on salinizated deserted and medow-deserted lands within alluvial delta of Zerafshan river. Reclamation efficiency and close horizontal drainage parameters determination. Soil water -salt regime management on background of such drainage.
Scientific and technical approach:
Irrigated lands drainability strengthening for salinization prevention is achieved by means of close horizontal drainage construction and creation of downward flow under leaching.
Environment characteristics:
Climate is characterized by hot summer, relatively cold winter, low precipitation and lot of sunny days. Average air temperature is 14.8 oC with maximum 48 oC in summer and minimum -28 oC in winter. Duration of the non- freezing period is 213 - 214 days. Average air temperature during growing season is 22.8 -24.4 oC. Sum of positive temperature is 4680.7 -4794 oC. Annual precipitation is 114 125 mm. Mainly rainfall occurres during non -growing period (70 -75 %). Evaporativity is 1350 mm. Wind is very active, only 16 -17 days a year it is absent. Landscape is flat with slope 0.002 -0.0003. Soils are medow -deserted: heavy loam -10 %, middle loam - 20 %, light loam -41 %, sandy loam -29 %. Lithology: multilayered thickness constituted by sandy loam, loam, clay with permeability K = 0.1 -0.4 m/day. There are two sites with typical conditions:
1. Heavy and middle loam soils prevail with permeability Kp=0.12 -0.3 m/day, soils are middle and strongly salinizated, groundwater salinity is 4 -60 g/l.
2. Light loam and sandy loam with Kp =0.5 m/day, soils slightly and non -salinizated, groundwater salinity is 2 -15 g/l.
Main crop until 1993 -cotton (80 %), since 1993 wheat (90 %), corn for silage (5 -6 %), cotton (3 -4 %), vegetables -melons (3 -4 %). Irrigation depth for cotton 2.8 -2.0 th.cu.m/ha with 3 -4 irrigations; for corn 1.2 -1.6 th.cu.m/ha, 5 -6 irrigations; for wheat 1.8 -2.2 th.cu.m/ha, 4 -5 irrigations.
Parameters of Pilot Projects and Technical Solutions:
Irrigated area is 250 ha (gross). Close horizontal drainage parameters are as follows: eight close drains are constructed within the 1-st site (depth is 2.4 -2.6 m, distance between drains is 80 -150 m, length is 400 -650 m). Three close drains are constructed within the 2-nd site (depth is 2.2 -2.4 m, distance between drains is 300 -400 m, length is 350 -400 m). Tubes are plastic with screen cover and sand -gravel strewed. Irrigation network is represented by concrete flumes (90 %) and earthen ones (10 %). South border is Amubukhara canal, north and west borders are collectors.
Methodology:
Field investigations of water, salt movement and water -salt balance elements measurement. Site was equipped by all means of accounting and measurement. Multicriterial analysis was used for data processing and summarizing.
Results:
Four years observations of water -salt regime regulation on background of drainage showed:
- drainage parameters and leaching regime of irrigation provided soil desalinization within unsaturated zone and groundwater upper layer;
- areas of strongly and middle salinizated soils decreased by 3.5 times; salt removal within 4 years was 274 t/ha. Soil desalinization dynamics was intensive and within 3 years strongly salinizated soils became slightly or non -salinizated ones;
- drainage effluent salinity depends on soil salinization level and was for strongly salinizated soils 20 -52 g/l; for less salinizated soils -9.4 -8.5 g/l with decrease almost 2 times to the end of 1994;
- groundwater average level during growing season was 1.67 -2.08 m. Depletion rate after irrigation was 0.08 -1.13 m/day;
- groundwater salinity to 5 g/l area increased by 3 times from 23 to 76 ha, more than 20 g/l -decreased 2.7 times from 127 to 48 ha;
- water duty was 102000 -8000 cu.m/ha in spite of low water supply. Measures undertaken allowed to create leaching regime and change structure of water -salt balance. Balance was supported negative with salt removal 18.47 -33.18 t/ha. Drainage outflow was 2437 -1623 cu.m/ha. Drainage modulus was 0.05 -0.08 l/sec. Leaching regime ratio was for structure with prevailing cotton -1.16; with prevailing wheat -1.2; drainage outflow was constituted for about 25 % by water supply. Total evaporation decreased that provided high rate of soil and groundwater desalinization;
- drains with relatively shallow depth required less expenses for their construction and maintenance;
- drains hydraulic and hydrotechnical parameters are quite high, discharge achieves 5.17 l/sec. Zone of active water and salt exchange was 4 -5 m.
Analysis of results permitted to recommend for deserted and medow -deserted soil on delta sediments the following drainage parameters:
- optimal depth for light mechanical composition of soil is 1.9 -2.3 m under groundwater depth 1.6 -1.75 m and distance between drains 80 -150 m;
- optimal depth for heavy mechanical composition of soil is 2.00 -2.40 m under groundwater depth 1.65 -1.95 m and distance between drains 80 -150 m.
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