Summary of research project: |
Objective and technical fields:
Finding the possibilities for optimization of drainage parameters, elaboration of measures on depletion of water table and control of soil salinization on the basis of raised drainability of lands and leaching through construction of subsurface horizontal drainage. Determination of meliorative efficiency, capacity for operation of various drainage structures and elaboration of proposals for their improvement.
Scientific and technical approach:
Control of soil salinization, raising the soil productivity through increased drainability, leaching and leaching irrigation regime. Studies included scientific grounding of measures on improvement of water use and control of water-salt soil regimes.
Environment characteristics:
Environmental conditions of the experimental plot are typical for the most part of Yavan-Obikik valley. The plot is located between the Vakhsh and Kafirnigan rivers within the right-bank part of Obikik valley. Climate is continental and dry. Average annual temperature is 16,8oC. Sum of positive temperatures (above 10oC) is 5665-5467oC. Annual precipitation is 670 mm. Average annual relative air humidity is 49%. Evaporativity is 1816 mm. Coefficient of humidity is 0,37.
The valley is slightly corrugated sloping with inclination of 0,023 and the land surface height equal to 519 m in the east and 570 m in the west. Soils include alluvial loam soils of red-cinnamon color. Volume weight in natural condition is 1,8-1,82 t/m3. Plasticity is 11-16%, percentage of clay fraction is 50%. Soil density is 2,68 t/m3, volume weight is 1,8-1,82 t/m3. Coefficient of permeability is 0,59-1,23 m/day. Rate of water table rise does not exceed 4 cm/day, average rate is 2 cm/day. Maximum depth of water table is 1,0-1,5 m. Maximum ground water salinity (4-5 g/l) is in February, minimum ground water salinity (1,0-1,5 g/l) is in summer months. Type of salinity is invariable: sulphate calcium-sodium. Annual amplitude of salinity is 2,5 g/l. Maximum salt accumulation occurs within autumn months. Total salt percentage in 0-100 cm soil layer is 0,25% of soil weight. Relation between chlorine ions and sulphate is 1-2,5 (55%) and 1-0,2 (45%). Rate of drainage runoff on the experimental plot varies from 0,03-0,05 to 0,23 l/sec/ha. In new longitudinal and transversal drains rate of runoff does not exceed 0,09 l/sec/ha.
Average rate of runoff for a growing season is 0,17 l/sec/ha.
Parameters of Pilot Projects and Technical Solutions:
Net irrigated area of the experimental plot is 70 ha, gross area of the experimental plot is 75 ha. Designed extent of the surface drainage system is 760 m, actual extent is 715 m, specific extent is 84 m/ha. Drain spacing is 170-340 m. Depth of drains laying is 1,8-3,5 m. Pipes are polyvinylchlorine. Filters are sandy-gravel. The irrigation system consists of earthen canals of 35-36 m/ha, canal efficiency is 0,58-0,72 against 0,84 according to the project. Total efficiency of the irrigation system is 0,6-0,73. Canal filtration is 20%, infiltration on the fields is 17,6%, release is 5,4%.
Methodology:
Field natural studies on the dynamics of ground waters, flow of salts in soils of the aeration zone, registration of drainage waters, study of chemical composition and salinity of drainage and ground waters were made. Observations over ground water regime were carried out for 20 wells 2-10 m deep. Salt examination of soils was made on 6 salt stations where each quarter soil examination was made to 2,0 m deep within the following intervals: 0-25, 25-50, 50-75, 75-100, 100-150 and 150-200 cm. Twice a year geochemical profile was bored for determination of desalinization effect of drains. Salt examination was made for 5 drains. For solving the problems, except natural observations and studies data of other water and agricultural organizations were used.
Results:
The experimental plot was situated among irrigated lands of the collective farm and was surrounded by permanent canals which created large outside inflow and drainage load. Location of old drains parallel to distributory irrigation pipelines in the direction of inclination and inclination of ground water flow was not very effective. Therefore to raise the land use efficiency of irrigated lands and to reduce operational costs it was more preferable to use subsurface horizontal drainage systems transversally to laid drains (i.e. transversal to the direction of ground water flow). Water inflow to drains of transversal location increased from upper to lower drains. In ΞΑ-3-8-4 drain specific water discharge was 0,47-0,68 l/sec per 1 km of the drain. In ΞΑ-3-8-1 drain water discharge was 0,07-0,13 l/sec, i.e.0,5-0,93 l/sec per 1 km of the drain. In the lower drain (ΞΑ-3-8-3) specific water inflow was 0,67-1,18 l/sec/km. In longitudinal drains water discharge and water inflow sharply increased - by 2-2,5 times. Specific water inflow to the new subsurface drain ΞΑ-4-1-5 was on the average 0,28 l/sec/km of the drain in 1989 and 0,59 l/sec/km for 10 months of 1990. Comparisons of measurements of discharge in this drain and longitudinal drains showed than specific water inflow to longitudinal drains was 1,1-2,0 times higher in 87% of cases. Rate of runoff varied from 0,03-0,05 to 0,23 l/sec/ha. Type of salinity was between sulphate-chloride and chloride-sulphate. Relatively high percentage of chlorine-ion (0,027-0,071%) predetermined low degree of salinity. Average percentage of chlorine-ion was 0,035%. The most part of geochemical examinations in 0-100 cm soil layer (65%) showed reduction of salt content by 3-5% in the zone of transversal drains with slight tendency of chlorine-ion percentage increase by 1-2%. Average salt content reduced from 0,236 to 0,141%, chlorine-ion percentage reduced from 0,04% to 0,035%. Maximum water salinization (up to 5 g/l) was typical for February. Maximum dilution of water (up to 1,5-2,5 g/l) was in April-May due to precipitation and first irrigation waters. In summer months water dilution at the end of water application and salt accumulation in irrigation intervals alternated with maximum amplitude of variation up to 1 g/l. Type of salinity was invariable: sulphate-calcium-sodium. Annual amplitude of salinity variation reached 2,5 g/l.
These facts were proved by studies carried out on the experimental plot which can be described as follows:
a) water inflow of transversal drains was 1,5-2,5 times higher than water inflow of longitudinal drains with the same depth of pipe laying;
b) drainage of drain spacing 175-200 m wide by transversal drains on the average for a growing season was 1,7-2,1 m against 2,2-2,5 m between longitudinal drains at the distance of 80 m from each other and 1,43-1,5 m with drain spacing of 120 m;
c) rate of ground water depletion after water application was almost the same for all variants and parameters (p. b)
d) Desalinization tendencies begun between transversal drains and in the drain spacing 80 m wide of longitudinal drains. The most part of geochemical examinations in 0-100 m soil layer (65%) showed reduction of salt content by 3-5% with slight increase of chlorine-ion percentage by 1-2% in 55% of cases. Putting into operation the experimental plot with predominating growing of cotton, lucerne and other perennial plants gave the possibility:
- to rise land use efficiency, which was 0,85;
- to create high drainability of irrigated lands. Specific water inflow was 0,56-0,74 l/sec/km. Annual drainage outflow varied from 13,8 to 15,2 th. m3/ha;
- to provide high rate of ground water depletion after leaching and water application. Rate of ground water depletion was 6-10 cm/day while it was 1-2 cm/day on other plots;
- to maintain the optimal depth of water table. Depth of water table on the plot for a growing season was 1,0-1,5 m, it was 0,3-0,5 m deeper than on the surrounding territory;
- to create high rate of soil desalinization during leaching and under leaching regime of irrigation. Rate of desalinization on chlorine ion was very high and permissible limit of salt content was achieved;
- to make the water-salt balance on the plot negative. Geochemical examinations for 4 years in 0-100 cm soil layer showed reduction of total salt content by 3-5% in the zone of transversal additional drains. In the zone with longitudinal drains the tendency for desalinization was almost the same. Average salt content decreased from 0,236 to 0,141%, chlorine ion content decreased from 0,04 to 0,035%;
- to reduce ground water salinity to 2,0-2,5 g/l;
- to raise the productivity of cotton on cultivated plots from 2,1 t/ha up to 3,5-4,0 t/ha.
Thus, field observations on operation of the system of drains laid perpendicularly or at a obtuse angle to the direction of ground water flow showed its higher efficiency comparing with the longitudinal drains of the same construction with large drain spacing. Transversal scheme of drain laying is the most prospective on the lands with significant (more than 0,01) inclination. The extension of new drains reduces by 25% due to drain spacing equal to 170-200 m.
|
