Biodrainage
Biodrainage implies lowering of groundwater table by planting trees.
Biological drainage (biodrainage) is a natural vegetable cover, wood species and agricultural crops with high transpiration capacity. During their life activity, they absorb large quantity of soil moisture, and as a result groundwater table lowers.
Alfalfa, willow, poplar and other crops and wood species serve as biodrainage. For example, during the growing season alfalfa consumes 4-20 ths m3/ha of water (depending on age, depth of groundwater level, grain-size distribution, thickness of sowing, and other conditions). At that, the share of groundwater in the total flow varies from 0 to 78%. In summer, during the period of intensive absorption of moisture by plants groundwater table under alfalfa fields is as a rule lower by 50-70 cm than that under cotton fields. Alfalfa has also high salinity resistance: it consumes saline groundwater with total salt content up to 5 g/l (depending on salinization type). These useful properties of alfalfa are used to maintain good amelioration condition of lands. It is the main predecessor of cotton in the crop rotation.
Willow and poplar consume 20-100 m3 of water (per tree) during the growing season. They are usually used for the plantations along inter-farm and on-farm canals. Capturing seepage water from canals they maintain good ameliorative condition of the area to a considerable degree.
Source: Cotton Growing Encyclopedia (two volumes). Volume 1, A-M.
Editorial Board: Mirakhmedov, S.M. (Editor-in-Chief) et al. Tashkent,
Chief Editorial Board of the Uzbek Soviet Encyclopaedia, 1985, p. 544
Biological drainage is represented by forest plantations and permanent grasses (alfalfa) with deep root system (3-4 m). Forest plantations along irrigation canals capture seepage and ground waters by their roots, and these waters are lost to transpiration. As a result, groundwater level lowers considerably. This process substantially reduces accumulation of salts in the active layer of soil.
Professor H.A. Akhmedov gives the following interesting information in his book “Drainage Reclamation” (1974): according to data by V.S. Malygin, a good drain intakes and diverts 54-62 m3 of groundwater a year per meter of its length, and one tree evaporates 50-90 m3 within this period. Consequently, forest shelter belt 5-10 m wide of 5-10 trees can remove groundwater from soil more than drains can.
Forest shelter belts along canals have the same depression curve as drains have. According to data by S.P. Suchkov, in the farm “Pakhtaaral” (the Golodnaya Steppe) sphere of influence of two-row forest belt of willows extended for a distance of 150-170 m. the difference of water horizons is 1-0.7 m.
Local species which can sustain high temperature, air dryness, winds and other adverse conditions are selected for forest belts. These requirements are met by mulberry, poplar, Chinese elm, oleaster, ailanthus, maple, willow, ash. Out of taller and more longevous plants are walnut and plane tree; out of horticultural crops are apricot tree, cherry tree, gean, etc.
Forest shelter belts are arranged along main and on-farm canals, group and delivery ditches perpendicularly to the direction of the winds prevailing in a particular area. They are not to obstruct the movement of agricultural machines and machinery during cleaning of the bottom and slopes of irrigation and drainage network.
Forest shelter belts are arranged in two directions bisecting one another at right angles with a glance of the efficiency of wind-weakening effect of plantations at the distance equal to 20-30-fold height of trees. Longitudinal forest belts are arranged at the interval of 600-800 m, the space between the cross belts is 1000-1500 m.
Efficiency of biological drainage
Under the influence of plantations of trees the value of overall groundwater depression fluctuates within the range of 150-200 m; in the case of plural-row plantations, it is even more. When carrying out computation, the depression length is taken as 200 m; in the case of double-sided plantation, 2 x 200 = 400 m. If the canal length is 500 m, the area of the plot where groundwater drawdown is carried out by forest plantations will be equal to 400 x 500 = 20 ha; 1 ha of plantation of trees can transpire 10-20 ths m3 of groundwater. Consequently, water balance irretrievably looses to the atmosphere: 15000 x 0.5 = 1500 m3.
In the middle of irrigated plots (in view of preservation of their area of no less than 10 ha) additional belt of plantation of trees is planted in lowlands to still bring down the groundwater level with breaking the depression curve on its crest. In this case, it is necessary to decide what is more advantageous: biological drainage or labor-consuming and expensive artificial drainage.
Forest ranges as compared with tile drainage and seepage-control measures require little capital investment. They make the microclimate of irrigated plots mild protecting the sowings of cotton and other crops from harmful impact of hot dry winds (garmsil). By shadowing canals, they reduce evaporation from water surface, lower wind speed above soil surface.
Orchard crops growing along irrigation networks bring in return every year, and wood species serve as the source of construction timber and casewood.
However, in spite of positive role of biological drainage in control of soil water balance, the salt balance hardly changes. Considerable volume of groundwater consumed by plants for transpiration does not affect the salts dissolved in the groundwater. These salts remain in soils and groundwater. Biological drainage gives good results not in first years after planting, but in 5-10 years.
Source: Baraev, F.A., Sherov, A.G., Baraev, A.A. Land reclamation and biotechnical drainage. Tashkent Institute of Irrigation and Melioration, Tashkent, 2005 The role of biological drainage can be performed not only by tree and grass plantations, but also by a number of other methods; in particular, arrangement of major crops cultivated on irrigated plots. These are combined sowings of winter wheat and alfalfa (clover) as well as cotton in case of special soil treatment.
Source: Baraev, F.A., Sherov, A.G., Baraev, A.A. Land reclamation and biotechnical drainage.
Tashkent Institute of Irrigation and Melioration, Tashkent, 2005
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