Summary of research project: |
Objective and technical fields:
Soil salinization control in rice irrigation schemes through strengthening the drainability of a territory and leaching irrigation regime (in rice growing). Control over the water-salt regime of soils of rice fields with vertical drainage and raising their efficiency.
Scientific and technical approach:
Soil salinization control and raising their efficiency in rice growing are based on strengthening and regulation of the drainability of a territory through elaboration of the drainage regime guaranteeing optimal control over the water-salt regime of soils of rice fields in growing and non-growing seasons.
Environment characteristics:
The region is attributed to Kzylkum sub-zone of a dry hot zone of deserts and semi-deserts. Climate is characterized by small precipitation: average annual precipitation is 165-210 mm, the highest precipitation falls in spring and autumn. Total precipitation for a warm period (t >10oÑ) is 60-70 mm. Sum of positive temperatures (t>10oC) is 4600-4800o. Hydrothermal coefficient is 0,13-0,15. Thermal regime of a warm period is characterized by considerable amplitudes of temperature reaching 45-55oC. Humidity deficit varies from 8-9 mb in April and October to 32 mb in July. Evaporativity is 1240-1480 mm, natural moisture deficit is 1180 mm, average annual temperature is 11,7-12,0oC, average annual relative air humidity is 54-57%
Relief is uniform, flat. Inclination is 0,0002-0,0003. Topsoil is mostly made of loam. Soils consist of two-layer quaternary deposit: cover deposit is loam, its depth is up to 3 m, the coefficient of permeability is 1,35-1,45 m/day; underlying rocks are fine-grained sands, their depth is up to 60 m, the coefficient of permeability is 12 m/day and more. Depth of water table before development of lands was 4-5 m, salinity of ground waters was on the average 1,2-2,2 g/l near the surface and 0,8-1,5 g/l at the depth of 10-50 m. The type of salinity is chloride-sulphate, more seldom it is sulphate. In relation to cation composition ground waters are sodium-magnesium with high content of magnesium.
On the testing plot there are mostly meadow-gray soils (loams with clay and sandy loam interlayers). Its depth can be up to 3 m, coefficient of permeability is 1,45 m/day. Cover loams have the following water-physical properties: volume mass is 1,41-1,54 g/cm3, specific mass is 2,63-2,72 g/cm3, total porosity is 43-47%. As a result of high heterogeneity of soils the least moisture capacity varied from 18,8 to 25% to the weight of absolutely dry soil. Coefficient of permeability is 0,37 m/day. The most part of salt content was in the topsoil 1,5 m deep. Salt content was 0,34-0,40% on a solid residual, including chlorine ions (0,065-0,075%); sulphates (0,104-1,187%); sodium (0,069-0,083%). Total influence of toxic ions in 1 m layer was 2,05. Type of soil salinity is chloride-sulphate.
Parameters of Pilot Projects and Technical Solutions:
Irrigated area was 487,8 ha. There were 6 vertical drainage wells. They had the following parameters: depth - 50 m, diameter - 1270 mm, total length of split filters - 20 m. Vertical drainage wells discharge varied from 40 to 60 l/sec depending on their working regime. Specific discharge was 3-5 l/sec/m. Irrigation network included earthen canals of various levels: group distributors and check ditches with specific length of 25-30 m/ha. Water was supplied to the plot by 6P-2 canal and its distributors.
Methodology:
Field studies on determination of the components of water-salt balances and observations over water and salt moving in soils. Investigations included joint study of water and salt balances, reclamation state of the irrigated lands and crop yield. For these studies the testing plot was provided with necessary metering and accounting equipment.
Results:
The development of lands of the testing-production plot with vertical drainage which is located in «Kzylkum» state farm started in 1975. Before development soils were middle-salinized. The most part of salt content was in the topsoil 1 m deep: more than 50 y/ha, including 40 t/ha of toxic salts. In the soil layer 2 m deep salt content was app. 80 t/ha, including more than 60 t/ha of toxic salts. Soils included 8 chemical compounds: hydrocarbonates of sodium, calcium and magnesium; sulphates of magnesium, calcium and sodium; chlorides of sodium and magnesium. Sodium and than calcium compounds dominated. Gypsum is found only in the top layer 1,5 m deep.
As a result of land development quantitative and qualitative salt content at the different distance from a vertical drainage well (VDW) changed differently and depended mostly on the degree of drainability of plots and initial soil salinity. Irrespective on initial salinity chlorides were washed away from soil. On the plots where washed soil was of middle-salinized content of sulphate calcium decreased, too. On the whole in a year of rice growing soils of the testing plot became almost non-salinized: the sum of toxic salts in 2 m soil layer decreased down to 0,085% at the distance of 250 m (1st, 2nd zones), to 0,118% at the distance of 250-500 m (3rd zone) and to 0,135% at the distance more than 500 m (4th zone) from VDW. The biggest salt content reduction was in 0-1 m layer.
During a rice-lucerne crop rotation period salt content decreased during rice growing and increased during lucerne growing. In the zone of active influence of chimney drains intensity of salinity restoration was insignificant. Considerable increase of salinity was out of the active zone at the distance of more than 500 m from wells where salt content of soils reached initial salt content and sometimes even exceeded it. Second rice growing led to reduction of salt content both in the first and in the second years. Approximately 70% of the total desalinization (for 2 years) was in the first year. Cyclic change of salt content during crop rotation with its gradual quantitative reduction took place. After a year of rice growing, 2 years of lucerne growing and 2 years of second rice growing salt content in 0-2 m soil layer decreased down to 30 t/ha in the first zone, to 37 t/ha to the second zone, to 43 t/ha in the third zone and to 56 t/ha in the forth zone.
During operation of VDW conditions for creation of descending streams of irrigation water were made, which favored the inflow of fresh water due to which desalinization of the upper layers of ground waters took place. In 2 years of rice growing salinity of the ground waters at the depth of 5 m decreased down to 1,2-1,4 g/l on a solid residual and to 0,18-0,21 g/l on a chloride-ion. Type of salinity was chloride-sulphate.
Analysis of water balances of the testing plots showed that the main income item was the supply of water which was more than 80% of the sum of other items. In the years of studies It decreased from 28550 m3/ha down to 22830 m3/ha. Precipitation was 1740-2560 m3/ha, ground water inflow was 2030-3470 m3/ha. The main balance expenses were: total evaporation - 13340-13780 m3/ha and drainage outflow - 9930-11710 m3/ha which were correspondingly 50% and 38% of the sum of balance expenses. Ground water outflow was 2970-4880 m3/ha.
Analysis of salt balances shows desalinization of soils in all the years of rice growing. In 1975 soil salt content decreased from 91,5 t/ha down to 54,6 t/ha, ground water salinity changed from 263,4 t/ha to 297,2 t/ha; in 1979 soil salt content decreased from 47,2 down to 39,0 t/ha, ground water salinity changed from 328,0 t/ha to 334,5 t/ha. Salt was taken in with irrigation water (28,4-37,1 t/ha) and ground water inflow (3,2-5,2 t/ha), salt was removed with drainage waters (9,6-14,1 t/ha) and ground water outflow (4,2-7,9 t/ha).
Rice yield on the testing plot with vertical drainage was different in different zones. The best yield was in the second zone (6,8 t/ha) at the distance of 100-250 m from VDW, the poorest yield was in the forth zone (5 t/ha) at the distance of more than 500 m from wells.
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