Drainage flow and drainage module

Drainage flow and drainage module

Drainage flow through collector and drainage network is measured in terms of the water volume (m³) diverted from a drained land for a certain period of time.

Drainage flow is the volume (or discharge) of water diverted by collector and drainage network (or by its separate components) from a reclaimed area for a certain period of time; it is measured in l/s, m³/s, m³/ha (for month, year, etc.). Drainage flow depends on soil permeability, degree of drainage of area, depth of laying of collector and drainage network, groundwater recharging conditions, specific rate of drainage flow, and other factors.

For cotton zone, the value of drainage flow with which favourable ameliorative condition of lands are provided comes to 15-35% of water supply or 1.5-10.0 ths m³/ha per year.

Drainage flow module (specific rate of drainage flow) is a quantitative characteristic of the groundwater flow from a unit of a drained land (ha). Drainage flow modulus (qd) is measured in l/s per hectare.

Drainage flow modulus is used to determine the design discharge of drains and collectors provided that optimal meliorative regime is maintained.

Source: Cotton Growing Encyclopedia (in two volumes). Volume 1,
A– M. Editorial Board: Mirakhmedov, S.M. (Editor-in-Chief) et al. Tashkent,
Office of Editor-in-Chief of Uzbek Soviet Encyclopedia, 1985, pp. 544

To compute the space between drains not only lithological texture and soil permeability are to be taken into consideration, but also drainage flow modulus determined based on water and salt balance as the groundwater volume (load on drainage) to be diverted from a drained land to maintain groundwater table at a required depth. Drainage flow modulus implies the discharge (diversion) of drainage water per time unit from one hectare (l/sec/ha). On irrigated lands subject to salinization, average annual drainage flow modulus is usually 0.15 l/sec/ha for heavy, 0.20 l/sec/ha for medium, and 0.25 l/sec/ha for light soils.

Source: Yusupov, A., Rakhimov, N., Umarov, P. Irrigated land drainage manual. Tashkent, 2012

The cross-sectional sizes of permanent horizontal open drains are estimated by hydraulic calculations. Design discharges are calculated according to the drainage flow modulus during the most stressed period of the target year. Drainage flow modulus means the discharge of the water diverted from a unit area, l/s per ha.

Q_dr = ΔW_gw^design / 86,4 * t_design

where:

ΔW_gw^design is the groundwater volume to be diverted within a stressed period of the target year, m³/ha;

t_designis the duration of a base period, days.

Source: Land reclamation. Golovanov, A.I., Aydarov, I.P., Grigiriv, M.S., et al.
Edited by Golovanov, A.I.. Moscow, “Kolos” Publishing House, 2011, p. 824

Drainage flow determination methods

Drainage flow is determined by several methods for designing of drainage. Average annual and average monthly drainage flow is determined by computing water through by the following formulas:

D = (I – O) + (I₁ – O₁) + (P + S_f) + (S – W) – E_S – (E + T_r) + R

or D = (I - O₁) + αF_C + q + R

where:

D is drainage flow, m³/ha;

I and O is inflow and outflow of surface water respectively, m³/ha;

I₁ and O₁ is inflow and outflow of groundwater respectively, m³/ha;

P is atmospheric precipitation, m³/ha;

S_f is surface flow, m³/ha;

S is supply of irrigation water, m³/ha;

W is water escape from field surfaces, m³/ha;

E_s is evaporation of surface water, m³/ha;

E + T_r is evaporation and transpiration, m³/ha;

+ R is recharge of groundwater by confined groundwater (+) or groundwater overflow downward (-), m³/ha;

F_c is seepage losses from canals, m³/ha;

α is the coefficient indicating the share of seepage from canals going to recharge of groundwater;

+ q is the vertical moisture exchange between soil water and ground water, m³/ha.

In operating systems of collector and drainage network, drainage flow is determined by measuring the flow on gauging stations or by studying the operation of its separate elements to determine value of drainage flow modulus.

Drainage flow modulus is a quantitative characteristic of groundwater flow from unit drainage area (ha). Drainage flow modulus is used to determine the design discharges of drains and collectors provided optimum ameliorative regime is maintained. For cotton-growing areas, drainage flow modulus comes to 0.15-0.40 l/s from one hectare; the values of modulus are determined by calculated (analytical) and experimental methods.

Drainage flow modulus value varies with time qd = f(t) depending on acting pressure. Maximum drainage flow modulus values occur during the period of leaching irrigation; minimum one, during the period of reduction or absence of recharge of groundwater and lowering of its level. Having compared the design and actual drainage flow modulus, one can infer about the necessity to intensify drainage. Design discharges of drains and collectors are estimated by the following formulas:

Q_d = q_d * Ω_d

Q_c = q_d * Ω_c

where:

Ω_d is the area drained by the drain;

Ω_c is the area drained by the collector;

q_d is the design value of drainage flow module.

At the stage of designing the collector and drainage network, two stages of system operation can be distinguished: meliorative (m) and operational (o). The design values of drainage flow modulus at these stages are different. At that, q_dm > q_do. Reinforcement of the collector and drainage network at the meliorative stage is carried out by making temporary drains. The issue of economically sound combination of permanent and temporary elements of collector and drainage network to set required intensity of soil water and ground water diversion during the periods of their maximum inflow to drainage structures is resolved based on the drainage flow modulus value.

Source: Cotton Growing Encyclopedia (in two volumes). Volume 1, A– M.
Editorial Board: Mirakhmedov, S.M. (Editor-in-Chief) et al. Tashkent,
Office of Editor-in-Chief of Uzbek Soviet Encyclopedia, 1985, pp. 544

Drainage flow utilization

The possibility to use collector and drainage waters for irrigation purpose depends on its quality. If there are no toxic salts and agricultural chemicals in drainage water, it can be supplied to irrigation network. This possibility most often occurs when pumping out groundwater by vertical drainage. In addition, collector and drainage waters may contain elements of inorganic nutrition of plants and good micronutrient elements in quantity exceeding their content in river water, which came to groundwater under improper storage of fertilizers and application of their excess dozes. Such water has fertilizer value. However, in many cases toxic substances represented by the pesticides washed out from soil by seepage water are often found in such water.

Horizontal drainage working on saline lands diverts highly saline water that can be added only to irrigation water by determining required correlation of saline and fresh water in terms of ion content most exceeding maximum permissible concentration. Depending on grain-size distribution of soil, collector & drainage water with salinity less than 5…6 g/l is used directly for irrigation or diluted with river water. With higher salinity, this water is used for leaching of saline lands as well as for irrigation of salinity-resistant plants- halophytes cultivated for fixation of shifting sands and applied as additives for roots.

Widespread alternative of drainage water utilization consists in its evaporation. For this purpose, they use special structures, viz. man-made evaporating ponds or natural depressions for accumulation of flow. The area of an evaporating pond is calculated through the volume of drainage flow, sheet of water losses to evaporation and seepage. Drawback of this method consists in allocation of considerable areas for evaporating ponds.

One of the methods of utilization of waste and collector & drainage water of irrigation systems involves natural freezing. Desalination of drainage water by this method is based on the effect of division of freezing saline water into free-saline crystals and brine; the latter separates (filters out) from ice massif at the first stage of spring thaw. Fresh water forms in the result of melting of the massif after diversion of residual brines. To perform freezing out, drainage flow is accumulated in a basin during the warm (frost-free) season. When winter comes, the drainage flow goes by gravity to desalination grounds. Desalination grounds represent sites (check plots) where ice builds up. When ice layer thickness becomes 0.5-0.6 m, water under the ice is discharged into an evaporating pond. The first portions of the water formed in the result of spring ice melting are supplied to the evaporating pond too. Desalinated water of good quality is accumulated.

To reduce seepage losses from the surface of desalination grounds, it is advised to create there an impermeable membrane from soil that is preliminarily moistened and frozen to a depth of 5-10 cm. In summer, check plots are used in agriculture.

Source: skyrage.ru

Use of drainage water for various purposes

Collector and drainage water being an accompanying by-product of irrigation agriculture can serve also as additional reserve for usage as the source of irrigation of salinity-resistant crops, as a water body for maintaining biodiversity, fishery and hunting field, for recreation of people, and other economical purposes.

However, its pollution by the residuals of mineral fertilizers and pesticides used in agriculture considerably limits the possibility of reuse of collector and drainage waters for economic use. Accumulation, transport and diversion of drainage water are associated with large material inputs.

Formation of large quantity of collector & drainage flow under present-day conditions is generally attributed to low efficiency of used methods and practices of water application, techniques of crop irrigation. According to scientific recommendations, with high-technology based management of irrigated agriculture the volume of formed drainage flow should not excess 10% of the volume of the water supplied to the irrigated fields. However, because of huge unproductive losses of irrigation water the share of collector and drainage waters in the volume of the water supplied for irrigation comes to 20-55%. If taking into consideration that not the entire irrigated area in the region is provided by collector and drainage network and especially in mountain areas where the so-called “dry drainage” is mainly used, it is obvious that enormous quantity of water was consumed per each irrigated hectare and specific drainage flow comes to considerable values. This is merely an unproductive use of river water and its conversion from good-quality water into man-caused polluted water.

That is why when developing measures for sustainable management of drainage flow efforts first must be focused on water saving and widespread reduction of unproductive losses of irrigation water and accordingly specified drainage flow per irrigated hectare and in whole reduction of collector and drainage water volume.

During the initial periods of intense expansion of irrigated areas and mass construction of collector and drainage systems, diversion of drainage water to stem streams became widely used for the purpose of its reuse. Such “reuse and cyclic use” of water resources provided for in old “schemes” and drafts of Integrated Water Resources Use plans was substantiated by the need for enhancement of the irrigation capacity of river runoffs (available water resources). The schemes of integrated Aral sea basin water resources use drawn up by design institutes forecasted the possibility to enhance the irrigation capacity of river runoffs by up to 15-20 % owing to reuse of return water.

However, trend of irrigated agriculture development in Central Asia for past decades shows that “reuse and cyclic use” of available water resources from stem streams is admissible till a certain limit of return of collector & drainage waters, after which it will cause great damage to not only drinking water supply, but to other sectors of the national economy and especially to the development of agricultural sector, leading thus to degradation of river water quality.

At present, in addition to implementation of organizational and technical measures for improvement of water use regime in irrigated agriculture so that to reduce the volumes of generated return waste and drainage waters as much as possible, the following options can be proposed for management and placement of drainage and waste water formed in the Aral sea basin:

Use of collector and drainage waters at the place of their formation for irrigation of saline-resistant crops, including by mixing that water with river water;
Use of collector and drainage waters in the zones of main off-take canals for irrigation of saline-resistant crops on light desert and sandy and other soils;
Properly organized diversion of drainage and waste waters from a cultured zone to man-made water reservoirs and use of these reservoirs for fish breeding, recreation and sports purposes; this may also involve diversion of drainage and waste waters to the lakes of the Amu-Darya and Syr-Darya river deltas to create watered zones and save biological productivity and ecological balance;
In future, an option of drainage and waste water use by its desalination can be considered for meeting different needs, including for irrigation of crops.

Source: Akmammedov, M. Sustainable drainage water management at the transboundary level