Summary of research project: |
Objective and technical fields:
Soil salinization control in rice systems 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 subsurface horizontal drainage and raising their efficiency.
Scientific and technical approach:
Study of the water-salt regime of soils of rice fields with horizontal drainage. Determination of subsurface drainage efficiency in comparison with open drainage and optimal limits of water-salt regime regulation on rice fields guaranteeing raising the soil efficiency.
Environment characteristics:
Climate is characterized by small precipitation (average annual precipitation does not exceed 210 mm), high thermal regime (sum of positive temperatures reaches 4800o) with considerable amplitudes of air temperature (for a day and for a year). Average annual temperature is about 12oC, relative air humidity is about 54% varying from 31-33% in summer months to 77-82% in winter months. Maximum evaporativity is in July (more than 280 mm), minimum evaporativity is in January and December (about 15 mm). Relief is uniform, flat. Inclination is 0,0002-0,0003. Topsoil is mostly made of loam. The depth of cover deposit is 2,1 m on the average for the plot, the coefficient of permeability is 1,45 m/day. Underlying rocks are fine-grained sands (their depth can be up to 60 m), the coefficient of permeability is 12 m/day. Depth of water table before development of lands was 4-5 m, salinity of ground waters was on the average 4,4 g/l. The type of salinity is chloride-sulphate.
Soils of the testing plot had high degree of salinity before the beginning of studies. Salt content in a soil layer 3 m deep was 0,313-2,094%, including chlorine ions - 0,045-0,182%; sulphates - 0,115-1,195%. Sodium (0,072-0,262%) and calcium - 0,021-0,346% predominated among cations. The type of soil salinity is chloride-sulphate. Before irrigation soils of the testing plot had the features of desertification. Volume mass of soils was 1,38-1,51 g/cm3, specific mass was 2,62-2,72 g/cm3, porosity was 44-47%, the least moisture capacity was 18,5-22,7%, the coefficient of permeability was 0,37 m/day.
Parameters of Pilot Projects and Technical Solutions:
Irrigated area is 80 ha. Open drain spacing is 400 m, subsurface drain spacing is 100 to 600 m. Average depth of subsurface horizontal drainage is 1,6 m. Subsurface horizontal drains are made of asbestos-cement pipes 200 mm in diameter with solid gravel wall treatment. Drainage modulus of surface drains with 400 m drain spacing was 0,36-0,40 l/sec. ha in the first year of rice growing, in the following years it was 0,20-0,22 l/sec. ha in the period of keeping a water layer in rice checks as a result of heaving and failure of slopes. Drainage modulus of subsurface horizontal drains with 400 m drain spacing (optimum variant) was 0,5-0,6 l/sec. ha. Irrigation network included group distributors and check ditches.
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 testing-production plot with horizontal drainage is located in «Komsomol» state farm. Development of the lands of this state farm started in 1971. Studies which were carried out on this plot for many years showed that subsurface horizontal drainage (SHD) is more efficient than open horizontal drainage (OHD). Efficient operation of SHD allowed:
- to increase drainability of irrigated lands. Annual volume of drainage outflow varied from 5,3 to 7,3 th. m3/ha with the duty of water equal to 19,7-22,9 th. m3/ha in comparison with 2,1-3,0 th. m3/ha on the OHD plot with the duty of water equal to 17,7-25,2 th. m3/ha;
- to desalinate soils and make them slightly saline even in one year of rice growing. In 2 years of rice growing salt content in 0-2 m soil layer decreased by 2 times on a solid residual (it was 154 t/ha) and decreased by 3 times on toxic compounds (it was 55 t/ha). In a soil layer 1 m deep salt content decreased down to 24 t/ha i.e. by 4,5 times.;
- to desalinate surface levels of ground waters; their salinity decreased by almost 2 times on the SHD plot (2,2 g/l) and by 1,5 times on the OHD plot (3,4 g/l);
- to discover that ground waters in the period of rice growing have the seasonal character of fluctuation: rise and joining with irrigation waters during flooding, maintenance of this level during a growing period and its reduction in the period of rice fields draining. After rice growing water table rose from 4-5 m to 2-2,5 m;
- to increase the rate of water table reduction by almost 1,5 times during draining of rice checks and to reduce it more rapidly to the depth which allows to carry out mechanized harvesting of rice in time;
- to increase crop yield by 10-20% due to creation of more favorable reclamation conditions both in growing and non-growing periods.
Analysis of water balances of the testing plots showed that drainage outflow by open drains varied from 2960 to 2070 m3/ha (9,1-11,6% of the total balance expenses) while drainage outflow by the SHD plot varied from 5330 to 7100 m3/ha (25,8-30,5%). Other balance expenses were: total evaporation - 12690-13630 m3/ha on the OHD plot and 11750-13880 m3/ha on the SHD plot; ground water outflow - 3520-9800 m3/ha on the OHD plot and 1340-7300 m3/ha on the SHD plot. Duty of water which was 89-95% of the balance income varied from 25170 m3/ha to 17710 m3/ha on the OHD plots and from 25070 m3/ha to 19000 m3/ha on the SHD plot.
Initial salt content of soils on the OHD plot was 379,2 t/ha, initial salt content in ground waters was 310,5 t/ha. In a year of rice growing salt content decreased down to 231,2 t/ha on the OHD plot and to 213 t/ha on the SHD plot; in 5 years of rice growing it decreased down to 145,8 t/ha on the OHD plot and to 124,8 t/ha on the SHD plot. Salt content of ground waters for the same period increased up to 388,6 t/ha on the OHD plot and up to 374,7 t/ha on the SHD plot. Salt inflow with irrigation water to the OHD and SHD plots was correspondingly 20,0-24,6 t/ha and 20,6-27,6 t/ha; salt removal with drainage waters - 10,4-18,4 t/ha and 24,7-36,1 t/ha; salt removal with ground water outflow - 17,3-49,6 t/ha and 6,2-39,2 t/ha. Rice yield on the SHD plot was 50-60 t/ha, rice yield on the OHD plot was 41-46 t/ha.
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