Summary of research project: |
Objective and technical fields:
Control of soil salinity on the basis of soil and ground waters desalinization by various types of leaching. Objective is to find profitable, water saving, technologically and ecologically acceptable types of leaching, to assess the role of the vertical drainage.
Scientific and technical approach:
Control of soil salinity, creation and restoration of desalinized soils fertility with strengthened drainability of the territory by vertical drainage and waters pumped by the vertical drainage system, and for rice irrigated by fresh (irrigation) water.
Environment characteristics:
Environmental conditions of the plot are typical for Vakhsh valley. Climate is subtropically continental, sum of positive temperatures is 5500-6000oC, average annual temperature is 15,5oC, duration of the frost-free period is 234 days, precipitation is 290 mm per year, evaporativity is more than 1500-1600 mm per year. The plot is situated in the zone of intensive salt accumulation, on the third alluvial terrace of the Vakhsh river. Relief is cup-shaped. The plot is divided into two parts: north-eastern part with inclination of 0,01 and western gentle part with inclination of 0,0017. Upper soil level is melkozem, its sickness is up to 6-11 m, deeper there is a sick shingle layer more than 100 m sick with sand fill, its coefficient of permeability is up to 10-20 m/day. Melkozem is alternative loam and sandy loam layers with clay lenses. Its coefficient of permeability is 1,0-0,025 m/day.
Predominating initial depth of water table was 0,5-3,0 m. Salinity of ground waters was very high in the south-western part of the plot reaching 50 g/l and more, near canals it was 3-5 g/l and less. Zones with salinity of 10-24 g/l predominated. In the shingle roof salinity of ground waters was up to 10-15 g/l.
Type of the ground water salinity was chloride-sulphate-sodium-calcium. Ground water head was 0,2-0,6 m.
Soils of the plot were saline soils and saline gray soils and their meadow subtypes. Before development 72% of the lands were saline, 15% of the lands were heavily salinized and only 13% of the lands had slight and no salinity. Salt content in 0-2 m soil layer was 80-150 t/ha. Type of salinity was chloride-sulphate and sulphate-chloride. 40% of the territory had high and middle gypsum content. Soils texture was light near canals and heavy clay in the central part of the cup. Soils often contained clay pans of bluish and brown color. With increase of gypsum content absorption capacity decreased from 10-12 to 7-9 mg. eqv. per 100 g of soil. Volume soil weight was 1,28-1,72 g/cm3, specific soil weight was 2,48-2,81 g/cm3, porosity is 37-58%. In gypsum-bearing layers with increase of gypsum content specific weight decreased down to 2,4 g/cm3, volume weight decreased down to 1,3 g/cm3. Coefficient of permeability of gypsum-bearing light, middle and heavy loam was 6-12 and 1-2 m/day. Coefficient of permeability of clay soil with low gypsum content was 0,025 m/day, for clay soil with middle and high gypsum content it was 0,5-1,0 m/day.
Parameters of Pilot Projects and Technical Solutions:
On the territory of the experimental plot with the area of 400 ha near one of three vertical drainage wells in zones of heavy salinized and saline soils on the territory of 10-20 ha leaching of various types (with different technologies and used water) was made. Salinized (4-10 g/l) ground waters pumped by vertical drainage wells or irrigation fresh waters with salinity less than 0,5 g/l were used. Area of leached checks was 0,01-1 ha, height of their borders varied from 40-50 to 1 m and more. Time of each type of leaching was different: in winter-spring period and in summer-growing season with the duration from several months to 2 years, continuous and with intervals, with and without release of leaching water.
Methodology:
Leaching was studied for estimation of its effect on soil-reclamation and ecological-economic parameters:
- use of salinized ground waters pumped by vertical drainage wells in the zones of its formation;
- creation of checks with different size and height of their borders;
- winter-spring and summer dates of leaching;
- continuous and interrupted leaching;
- with intensive or interrupted operation of vertical drainage wells;
- with or without release of water from checks;
- with one-year or two-year rice growing.
Monitoring of desalinization was made by creation of hydroposts on experimental leaching plots for accounting of leaching and released waters and their salinity; by determination of constant areas for selection of soil and ground water samples for chemical analysis: before 1 m depth - each 20 cm, deeper - each 50 cm, to the depth of the shingle roof.
Systems analysis of data of field observations was used.
Results:
a) Leaching of saline soils (20 ha) by salinized ground water pumped by vertical drainage well No 2 to checks with the area of 0,4-0,6 ha. Borders were cut horizontally with the difference of heights equal to 10 cm. Time of leaching was December - July without release.
Gross leaching norm was 19 000 m3/ha, net leaching norm was 15 200 m3/ha. Chlorine-ion content in 0-40 cm layer decreased from 46 to 4,5 t/ha or more than by 90%, in 1 m layer it decreased from 76 to 11 t/ha or by 85%. Desalinization took place in deeper layers, too. In 500-700 cm soil layer chorine-ion content decreased from 42 to 28 t/ha (66%). Soil layer 1 m deep remained slightly salinized (0,078% on chlorine). For achieving desired condition (less than 0,03% on chlorine) additionally 4 000 m3/ha of irrigation water was given.
b) Leaching of saline lands by water flooding to big checks (1-1,2 ha) and borders 1 m high, with 15 cm difference of heights with continuous and interrupted operation of vertical drainage wells. With continuous operation of vertical drainage wells leaching norm of 10 000 m3/ha was given during 8 days. Soil salinity in 1,5 m soil layer decreased from 520 to 197 t/ha (38%), on chlorine-ion it decreased from 50 to 12 t/ha (24%). However, remained soil salinity (0,05% on chlorine-ion) was higher than the desired content. Artificial prolongation of the leaching time by 4-5 times by creation of 3 intervals of water supply through interruption of vertical drainage wells operation (10-20 days) allowed to achieve required effect with the same leaching norm. As a result, for the whole melkozem layer chlorine content decreased down to 0,015-0,017%, i.e. it was 2 times less than during fast leaching.
c) Leaching of heavily salinized soils under rice (vertical drainage well No1) on the area of 23 ha under the conditions of good operation of the well and desalinization of the whole melkozem soil layer. Remained salinity in 0-5 m soil layer did not exceed 0,015% on chlorine-ion. Cotton near rice field grew normally. Rice yield for the years of leaching was 2,2 t/ha. The yield of cotton sown on the rice field in the next year was 1,8 t/ha, secondary rice sowing gave the yield of 4 t/ha.
Summarized analysis of results of studied leaching with vertical drainage wells operation gives the following conclusion:
- for leaching of saline lands and heavily salinized soils it is possible to use salinized (up to 4-6 g/l) ground waters;
- large-size leaching for 20 ha and more with vertical drainage wells operation does not cause negative impact on environment outside the borders of a leached field;
- Under delayed interrupted water supply to checks with intervals of 10-20 days with interruption of vertical drainage wells operation effect of salt removal is higher than effect of accelerated leaching with the same norm.
- With the increase of check borders size the area of not leached soil increases, this creates additional labor expenses and less uniform remained salinity.
Cost of leaching of saline lands and heavily salinized soils in the Republic was estimated as 300 ruble/ha. Crop yield (1,5-1,8 t/ha of cotton) even in the second year of development covered the costs for leaching.
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