C. Relative air humidity is 70-80 % in January and 31-45 % in July-August. Humidity deficit is 22 mb and 27-23 mb respectively under average value 12 mb. Wind velocity varies from 2.5 to 3.5 m/sec. Evaporativity is 1540 mm. Annual precipitation is 120-140 mm.
Upper quaternary alluvial sediments are represented by loam, sandy loam and clay, which underlaid by sand. Quaternary depositions thickness is near 30 m. They are underlaid by neogen sediments (clay and sandstone). Cover loam is heavy with permeability coefficient 1.0 m/day and thickness of 1.8-2.2 m; sand thickness is 27-30 m, permeability coefficient is about 10 m/day.
Groundwater level is 1.6-2.0 m within the growing period and 2.5-3.0 m in autumn-winter time. Groundwater salinity is from 1.0 g/l (close to irrigation canals) to 3-10 g/l. Chemical composition – sulfate-chloride.
Soil density within the 1 m-layer is 1.58 g/cu.cm, solid rock density 2.69 g/cu.cm; porosity – 41 %, least moisture capacity is 21 % of dry rock weight, water yield – 7.8 %.
Soil salinity within the unsaturated zone in initial state – 0.32 % of dry rock weight including chlorine-ion – 0.03 %. Salinity type is sulfate-sodium-calcium.
Parameters of Pilot Projects and Technical Solutions:
Pilot plot area is 55 ha (including irrigated area 49.5 ha (net). Main crop is cotton. Irrigation network extent is 950 m. Open drainage-collector network is 1280 m, close drains – 990 m. Distance between drains is 500 m, drain depth is 3.06 m for experimental and 2.55 m for control drains. Drainage modulus for growing period: experimental drains – 0.25 l/sec, control drains – 0.06 l/sec.
Methodology:
For study of soil salinity changes, drainage and groundwater salinity, drainage outflow volume observation wells' transect was constructed. Drains' mouths were equipped by water-meters.
Drains' reclamation efficiency of the same construction but positioned into different soils (experimental drain - in sand with permeability coefficient 10 m/day, control drain - in loam with coefficient 1.0 m/day) was compared.
Results:
Close horizontal drainage in Turkmenistan was constructed over lot of schemes and positively influences on irrigated lands. But all close drains are located in loam that causes a small distance between them and as a consequence their large extent and high expenses.
Simple calculations show that if drains are located in sand, drainage outflow increases that allows to make distance between drains bigger and save money for their construction.
Within the pilot site drainage without slope with flooded mouths system has been established. Under these conditions deep open collectors are impossible to construct and maintain.
Experimental drains' drainage modulus within the first growing period after construction was 0.27 l/sec/ha that is 4.5 times higher than by drains, which are positioned in loam. Average annual drainage modulus is 0.2-0.05 l/sec/ha.
Groundwater salinity annually almost did not change and was 1.3-2 g/l in TDS and 0.2-0.5 in chlorine. In the middle between drains it was 2-3 times more. In long-term period chemical composition was stable except sulfate (SO4), which increased 2-3 times. Salinity decreased to 1.5-2.0 g/l in TDS and to 0.2-0.3 g/l in chlorine.
Drainage effluent salinity changes were 0.5-1.0 g/l in TDS and 0.15-0.3 in chlorine.
There is a big share of SO4 (to 1.0 g/l) in drainage effluent. Long-term drainage effluent salinity decreased to 1-2 g/l in TDS and 0.5 and 0.3 g/l in SO4 and Cl.
Groundwater regime regulation limit was much more on background of drains without slope and was 0.5-0.7. Average groundwater level decreasing rate was near 20 cm/day to compare with 5 cm/day under ordinary drains.
Groundwater decreasing rate raise allowed to apply irrigation leaching regime. Soil water overflow into groundwater was 1000-1500 cu.m/year and salt removal from loam 8-12 t/ha.
Average water duty to irrigated lands was 8000 cu.m/ha including 5000 cu.m/ha during growing period and 3000 cu.m/ha for leaching and moisture stock. Besides there were losses from irrigation canals in amount of 3500 cu.m/ha and precipitations in amount of 1200 cu.m/ha. Evapotranspiration value was 8000-9000 cu.m/ha. Drainage outflow equaled 4500 cu.m/ha.
Together with irrigation water 7.5 t/ha salts were allocated to irrigated lands and 9.0 t/ha were removed that is an approval of negative salt balance. Water and salt exchange active zone was more than 20 m.
Thus deep drainage without slope with flooded mouths positioned in sand allowed:
- to decrease groundwater level more uniformly and desalinizate soils and groundwater all over the drains;
- regulate groundwater level on optimal depth depending on phase of germination of agricultural crops;
- to accelerate groundwater level decrease after leaching and irrigation;
- to enlarge drainage outflow modulus 3-4 times to compare with drainage positioned in loam.
