Drainage Engg.2(1+1)

Lesson 12 Vertical Drainage and Biodrainage Systems

12.2 Vertical Drainage System

In conventional horizontal pipe subsurface drainage systems, the flow of water through the soil profile is a combination of horizontal and radial flow. For most part of the flow domain, from the mid-spacing up to the drain, the direction of flow remains essentially horizontal. In the radial flow zone, in the vicinity of the drain, there is a vertical component of the flow velocity. The drawdown in the case of horizontal pipe subsurface drainage system is limited to a maximum of depth of the drain from the soil surface, which seldom exceeds 2 m. Consequently, the vertical flow component is small (negligible). Once the excess water is collected in the drain, its outward flow takes place at a low gradient (normally not more than 0.2%) to limit the depth of installation and to permit gravity outfall (Bhattacharya and Michael, 2003). In contrast, a tubewell dewaters the soil profile from much greater depths. The outflow through a tubewell is directed vertically upwards, and if the drawdown is large and the cone of depression is evenly spread (as in uniform coarse soils), there is a substantial component of vertical flow even within the soil profile. Therefore, drainage by tubewells is known as vertical drainage. Another case of vertical drainage is when excess accumulated surface runoff water is to be disposed by recharging it into deeper aquifers through a tubewell (i.e., recharge well), wherein the flow through the recharge well is vertically downward.

12.2.1 Vertical Drainage using Tubewells

Tubewell drainage is a technique of controlling water table and salinity in agricultural areas. It consists of pumping an amount of groundwater equal to the drainable surplus using a series of wells (Fig. 12.1). Tubewell drainage is not new, but it has not been widely used. Early attempts to use a series of pumped wells for land drainage and salinity control were made in the U.S.A. and the former U.S.S.R. more than half a century ago (Boehmer and Boonstra, 1994).

Fig. 12.1. Schematic of tubewell drainage technique: Two wells tapping an unconfined aquifer in series. (Source: Michael et al., 2008)

A review of studies and experiences with tubewell drainage in various countries shows that this technique cannot simply be regarded as a substitute for the conventional technique of subsurface drainage (Boehmer and Boonstra, 1994). The success of tubewell drainage depends on many factors, including the hydrogeological conditions of the area, physical properties of the aquifer to be pumped, and the physical properties of the overlying fine-textured layers. Another important factor is that skilled personnel are needed to operate and maintain tubewells, and to monitor water tables and the quality of the pumped water.

Tubewells may be shallow or deep. A shallow tubewell is that which draws groundwater from the top unconfined aquifer. The depth of such an aquifer is highly variable from one region to another, but it usually ranges from a few metres to 30 or 60 m. Deep tubewells are those which draw water from a deep confined aquifer or from multiple aquifers. When the top unconfined aquifer is pumped, its effect on water table decline is clearly visible in the upper soil layer within a short period of pumping. If a deep tubewell is pumped, its effect on water table decline in the upper soil layer will be slow and will be visible if the pumped aquifers have hydraulic connection with the upper soil layers. If the pumped aquifer is fully confined, pumping from it may not have any effect on the water table in the overlying unconfined soil system. It is worth mentioning that in most cases, the planned tubewell constructions have been for the purposes other than drainage, i.e., to supply water for irrigation, domestic and industrial purposes.

Today, the groundwater scenario is that excessive groundwater pumping has resulted in an alarming decline of groundwater levels in several states of India due to the fact that annual groundwater withdrawal is much higher than the naturally occurring annual groundwater recharge (Bhattacharya and Michael, 2003). It is also a fact that groundwater withdrawal in the command areas of purely surface irrigation projects (e.g., Kosi and Gandak projects in Bihar) has benefitted the command area by lowering the water table and saving land from waterlogging and from possible chemical degradation (Prasad and Prasad, 1996). In these two projects, groundwater withdrawal, even for irrigation, was not a planned activity. However, plentiful availability of groundwater at low lift and a generally inefficient functioning of the irrigation projects led the enterprising and surface water deprived farmers to sink and operate a large number of shallow tubewells. This gave the desired drainage effect by lowering the water table, simultaneously ensuring availability of irrigation on demand by the farmers.

A case study of planned groundwater withdrawal and its effect on water table behavior in the irrigation command (Hisar district in Haryana) of the Fatehabad branch of the Bhakra canal system has been reported by Dua and Puri (1996). Along the branch canal, 50 strainer type and gravel packed tubewells of depth varying between 27 and 38 m were commissioned at a spacing of 120 m. The tubewell discharge varied between 10-15 L/s. The pumped water was fed into the branch canal for augmenting water supply. Tubewell spacing, depth and the pumping rate were fixed keeping in mind the aquifer characteristics, presence of saline water at deeper depths and for avoiding upconing of the saline water zone into the fresh water zone during pumping, respectively. Initial and subsequent (after more than a year, from July 1995 to October 1996) water table observations were taken in a number of piezometers installed perpendicular to the branch canal at several locations. The quality of the pumped water was also monitored. These observations revealed a decline in the water table varying between 0.32 m and 3.09 m (average = 2.13 m, for 12 locations, covering a maximum distance of 600 m from the branch canal). The increase in the electrical conductivity (EC) of the pumped water after more than a year was small (0.1 dS/m).

The above-mentioned facts confirm a close linkage between groundwater pumping and water table decline both on a short-term and a long-term basis. Therefore, it follows that a planned groundwater pumping in shallow water table regions will provide water supply for certain purposes and at the same time, will ensure a favourable soil water regime in the root zone through water table management. Generally, the availability of good-quality groundwater is a prerequisite for the success of such an endeavor (Bhattacharya and Michael, 2003). However, if an irrigated area receives water from good-quality surface water sources, the poor-quality groundwater can be pumped and mixed with the good-quality surface water in a suitable proportion to meet the irrigation water demand. This is one feasible conjunctive use technique to control the rise of water table in the regions having poor-quality groundwater.

The discharge capacity of a tubewell is governed by the aquifer properties namely, transmissivity and storage coefficient or specific yield. The actual discharge determines the extent of drawdown. The discharge and the aquifer properties together, govern the shape and the spread of the cone of depression (i.e., the volume of soil being drained). The pumping schedule, non-pumping hours and the density of tubewell network determine the residuals drawdown after selected intervals of time and its cumulative effect over the years in a certain region. It is theoretically possible to predict the combined effect of the above factors on the water-table scenario of a region. However, the best method is to regularly monitor water table in a network of observation wells over a region and analyze the data to enable proper interpretation in terms of future possibilities of water-table rise or fall. In India, the Central Groundwater Board (CGWB) and the State agencies collect pre- and the post-monsoon groundwater-level data in different basins or sub-basins. These data should be analyzed along with the data of past developments in the region in terms of agriculture, urbanization and industrialization in order to obtain a clear picture of the possible future scenarios of water table behavior. Also, suitable theoretical analyses are necessary to estimate a proper tubewell density and a desirable rate of groundwater pumping to maintain a favourable groundwater balance. The interested readers should refer to the standard textbooks on groundwater hydrology for the detailed information about well hydraulics.

12.2.2 Vertical Drainage using Multiple Well-Point System

A multiple well-point system consists of a network of closely spaced shallow tubewells to dewater a waterlogged area where the water table is close to the groundwater or very close to the root zone and pumping by a single or a few scattered deeper tubewells are not adequate to lower the water table. It is also suitable when the deep well pumping or pumping at a high rate from a single tubewell may be hazardous due to the presence of poor-quality water at deeper depths (Bhattacharya and Michael, 2003). Thus, a multiple well-point system of groundwater pumping is essentially a drainage method. In this system, shallow tubewells are closely spaced to produce interference effect when they are simultaneously pumped. Khepar et al. (1971) reported that a battery of two tubewells, spaced at an interval of 3 m can achieve a drawdown of 1.74 m at the mid-point due to well interference between the two, whereas the drawdown effect on the outer region was the same as if the single well were being pumped. Therefore, to cause an effective drawdown over a larger waterlogged area, there should be several tubewells located within their radii of influence. Note that well interference is not desirable for the tubewells constructed for water supply purposes.

The result of operating a multiple well-point system will be that all the individual cones of depression will superimpose on one another, giving a larger overall water table decline during pumping over a larger area. Pumping from such a network can be accomplished using a single pump installed on the main line after joining all the well points by pipes having air-tight joints (Bhattacharya and Michael, 2003). Alternatively, individual rows of the well points can be joined by a pipe which delivers water in a common sump well. By using bends, the delivery points of these pipes are kept sufficiently deep in the sump well. A single pump draws water from the sump well causing a drawdown in it, adequately below the surrounding water table. This enables the pipelines connected to the well points draw groundwater and deliver it into the sump well. A specific situation where such a system is useful is when the water table is shallow and the soil is fine textured and non-cohesive (Bhattacharya and Michael, 2003). Under these conditions, making stable open drains or trenches for installation of a pipe subsurface drainage system become a difficult task due to immediate collapse of the drains or the trenches. Further, to take the full advantage of the system, the groundwater quality should be such that it can be used for irrigation or other purposes.

In brackish (saline) groundwater regions, the extraction of groundwater is low as the water cannot be used for irrigation. Use of only surface water for irrigation causes a gradual rise of the poor-quality groundwater. Conjunctive use of surface water and groundwater can be adopted in such places for controlling water table as well as for increasing irrigation water availability. Gupta et al. (1987) studied this aspect in the Faridkot region of south-west Punjab where the EC of the groundwater was about 8 dS/m and the land had turned waterlogged and saline due to prolonged irrigation with surface water only. This study revealed that of the total water applied in irrigation, the groundwater and the surface water mixed in the proportion of 40:60 resulted in a maximum return from irrigation. Obviously, the water table would decline when 40% of the irrigation water is withdrawn from the aquifer rather than irrigating with only surface water.

Shakya et al. (1995a,b) conducted an experiment with a multiple well-point system in a 16 ha area near Golewala village in south-west Punjab. The discharge from each of the well points was approximately 3 L/s. The system was operated intermittently with 8 hours of pumping followed by 16 hours of recovery in 24 hours and continued for at least a week at a time. The residual drawdown at the end of pumping was 24 cm near a well point, but only a few centimetres at the farther point, i.e., at the intersection of the diagonals of a rectangle formed by joining the locations of four adjacent well points. The longer was the total duration of pumping, the more was the residual drawdown. This study was carried out in a small area (16 ha) within a vast expanse of a shallow water-table region. Therefore, the temporary drawdown during and immediately after pumping was recovered soon due to the inflow of groundwater from the surrounding area. The major conclusion of their study was that the multiple well-point system could be used for the control of water table, and hence for the drainage of waterlogged areas. However, for effective and long lasting drainage effects, such a system is to be adopted over a relatively larger area. The functioning and the results of this study were demonstrated to the local villagers. A large number of the villagers having their own tubewells but of low discharge to avoid upconing of saline groundwater from deeper depths adopted the system with a minimum of two well points to a maximum of 12 well points, depending on the size of their farm holdings.

12.2.3 Vertical Drainage using Dug Well

Dug wells tap shallow unconfined aquifers. Therefore, pumping from dug wells has an immediate effect on the shallow water table. However, since the dug wells are shallow, have small discharge capacity, and have a small zone of influence, the effect of water table decline is limited to a small area surrounding the wells. The constraints of low discharge can be partially avoided by withdrawing water at a high rate to cause adequate drawdown within a short period and stop pumping thereafter to allow recovery (Bhattacharya and Michael, 2003). If the rate of recovery is very slow, as in the situations where the surrounding soil/porous medium has a low saturated hydraulic conductivity, the drawdown effect in the close vicinity of the well lasts longer. This period can be farther lengthened by pumping a number of times in a day. The water so pumped can be used for irrigation in lieu of using the canal water, if the area is canal irrigated.

Sharma (1999) studied the effect in terms of vertical drainage and improvement in crop yield by pumping from a dug well in a shallow water table farm holding in the Barna Command area of Madhya Pradesh. The crops were soybean in kharif and wheat in rabi. It was found that the temporary water table decline due to pumping from dug well and using the water for irrigation, instead of the available canal water, could give a 14.6% and 27.3% increase in yield, respectively for the kharif soybean and rabi wheat as compared to the area where the irrigation was only by canal water. In the rabi season, the unpumped dug wells in the canal irrigated region recorded a rise in water table by 2 cm, whereas the pumped dug wells recorded a decline in water table by 15 cm from the start to the closure of the canal during the rabi season. For the pumped dug well, the well water was used for irrigation, not the canal water. Water table observations made in a number of auger holes, in the areas under canal irrigation and under well irrigation, revealed a reduction in the SEW₁₀₀ (number of days when the water table were shallower than 100 cm) values in the rabi season from 76 in the canal irrigated region to 24 in the dug well irrigated region.

Based on the above information and discussion, it can be concluded that vertical drainage has many advantages. It is a means of controlling water table in the unconfined aquifer and leaky aquifer regions as well as it is a vital component of conjunctive use of surface water and groundwater which is considered an appropriate mechanism of land and water management against degradation, especially in canal command areas (Bhattacharya and Michael, 2003). Thus, it augments surface water supply and the additional water irrigates more area or makes more water available for irrigation in a given area. Also, with a proper mixing of surface water and groundwater, it is useful in irrigation even in the poor-quality groundwater regions. A constraint, perhaps in several countries, in adopting vertical drainage at all the places where it is needed is that it requires an assured supply of additional energy to operate a pumping system. In fact, the availability and cost of energy and the timely replacement of pumps and engines after their economic lifetime are the key determinants in the selection of vertical drainage.  

12.3 Biodrainage System

12.3.1 Concept of Biodrainage

All living plants transpire water. The source of the water is either irrigation water or groundwater. The transpiration capacity of a plant depends on its species root depth and spread, canopy area, leaf area and leaf structure. When the transpiration is met primarily by withdrawing groundwater, the process is known as biodrainage in the field of drainage engineering. The total water transpired from the groundwater reservoir in a region, and hence its drainage effect (i.e., effect on water table decline) in a region is a function of plant density and other plant factors. Rice plants transpire quite heavily but the process is not called biodrainage because the rice root system are shallow (30-40 cm deep) (Bhattacharya and Michael, 2003).

Medium to deep rooted plants in a shallow water table region may act as small capacity tubewells, constantly pumping groundwater to maintain their transpiration rates (FAO, 2002; Bhattacharya and Michael, 2003). The difference between the two is that in case of tubewell drainage, the area encompassed by the tubewell network is available for normal crop production but in the case of biodrainage by a cluster of plants, the area within the cluster cannot be used for normal crop production.

Use of plants to supplement the drainage effect of conventional drainage systems in reclaiming polders in the Netherlands has been reported by Raadsma (1974). Weed of a certain species were aerially sown over the area to be reclaimed from waterlogging, besides providing shallow trenches. Figures 12.2(a,b) illustrate the application of biodrainage systems in controlling waterlogging in the canal command of Indira Gandhi Nahar Project (IGNP), Rajasthan, India. Besides canal commands, the prospective sites for tree plantations for the purpose of biodrainage are government lands and fallow lands with low productivity (Bhattacharya and Michael, 2003).

Fig. 12.2. (a) Inundated area caused by leakage alongside IGNP main irrigation canal; (b) Trees in background are the biodrainage system that dried-up the inundated areas along the main canal. (Source: FAO, 2002)

12.3.2 Advantages and Disadvantages of Biodrainage

Kapoor (1998) and FAO (2002) have reported several advantages of biodrainage in comparison to the conventional drainage methods. The major advantages are that biodrainage is a low-cost measure, does not require gravity outlet and hence no physiographic constraint, operation and maintenance are required only at the plant establishment stage, no requirement of energy for operation and supply of fuel and fodder materials. Kapoor (1998) compared a row of trees, separated from another row at a distance, to a system of parallel subsurface drainage and suggested the use of one of the steady-state drainage design approaches for the determination of a suitable spacing between the tree rows when the water withdrawal rate from the tree row is known or can be estimated and is substituted for drainage coefficient in the design equation.

The disadvantages of biodrainage are that some area is required for growing the plants, which cannot be available for crop cultivation and that good-quality water should be available for plant establishment. According to Kapoor (1998), despite the advantages outweighing the disadvantages, it is not yet a recognized drainage method and it is necessary to conduct field studies on the evapotranspiration from afforested areas, chemical properties changes in the soil under the plants established for biodrainage, and the drainage effect within and outside the plantation area.

Other than providing drainage effect, which remains yet to be properly quantified in terms of the magnitude of water table decline under a specified tree density, certain tree species have been shown to improve soil chemical condition in the top 0-30 cm soil depth. Mishra et al. (2000) have reported changes in the chemical properties of the 0-30 cm soil under plantation densities of 5,000 to 20,000 trees per hectare after 10 years of plant establishment and have compared those with the corresponding chemical parameters in the unplanted area. The study was conducted at Dhaulakuan in Himachal Pradesh, India where the soils texture varies from sandy loam to loamy sand and the annual rainfall is 1600 mm. On average, in the top 15 cm soil of the planted area, the pH and EC registered declines varying from 4 to 9% (in the existing acidic range) and from 12 to 28%, respectively as compared to those of the unplanted area. For the 15-30 cm soil, the decrease in EC and pH varied from 2 to 39%. Other properties such as organic carbon, available N, P and K and exchangeable calcium and magnesium were reported to increase under the planted area varying from 14 to 60% in the top 15 cm of the soil and from 12 to 75% in the 15-30 cm soil.

12.3.3 Concluding Remarks

Research conducted in different parts of the world indicated that biodrainage is a viable non-conventional drainage technique (FAO, 2002). Much research on biodrainage has been completed, but more is required. Not all questions have been answered concerning the precise design of biodrainage systems, even in those areas where biodrainage systems have been found to be adequate in integrated water management of irrigation and drainage systems. Examples from several countries are available where vegetation, particularly trees and salt-tolerant plants, has been used to attain environmentally safe, and effective drainage and disposal systems (FAO, 2002).

More research on biodrainage is necessary to quantify the groundwater withdrawal rates by different tree species, their effect on water-table decline, their tolerance to waterlogging, irrigation water requirement at the time of their establishment and afterward, salt tolerance of the highly transpiring deep-rooted trees, salt balance in the soil under tree plantation, impact of the tees on the local environment (it has been found that the vegetation growth is usually poor under a eucalyptus tree), and the economics of biodrainage systems (FAO, 2002; Bhattacharya and Michael, 2003). Considering the capability of certain plants to grow satisfactorily under adverse soil and water environment, tree plantation may be a feasible alternative to yield remunerative products, especially in the chemically degraded and waterlogged soils where normal crop production is not possible. According to FAO (2002): “Drainage engineers should no longer ignore the opportunities that biodrainage systems can offer. When planning for projects, the agricultural sector increasingly feels pressure from other users of the environment. For example, it is becoming increasingly unacceptable to set aside land exclusively for routinely designed irrigation and drainage projects. This illustrates the possible advantages of biodrainage systems”.

References

Bhattacharya, A.K. and Michael, A.M. (2003). Land Drainage: Principles, Methods and Applications. Konark Publishers Pvt. Ltd., New Delhi.

Boehmer, W.K. and Boonstra, J. (1994). Tubewell Drainage Systems. In: H.P. Ritzema (Editor-in-Chief), Drainage Principles and Applications, International Institute for Land Reclamation and Improvement (ILRI), ILRI Publication 16, Wageningen, The Netherlands, pp. 931-964.

Dua, S.K. and Puri, T.S. (1996). Vertical drainage for removing drainage congestion in selected command area of Fatehabad branch: A case study. In: Technical Papers, National Workshop on Reclamation of Waterlogged, Saline and Alkaline Lands and Prevention thereof, Ministry of Water Resources, New Delhi, India.

FAO (2002). Biodrainage: Principles, Experiences and Applications. IPTRID, Food and Agriculture Organization (FAO), Knowledge Synthesis Report, No. 6, Rome, Italy.

Gupta, P.K., Khepar, S.D. and Kaushal, M.P. (1987). Conjunctive use approach for management of irrigated agriculture. Journal of Agricultural Engineering, XXIV(3): 307-316.

Kapoor, A.S. (1998). Biodrainage to overcome waterlogging and salinity problems in irrigated lands in dry arid regions. In: Technical Papers, National Seminar on Strategy for Prevention and Reclamation of Waterlogged Areas in Irrigation Commands, Ministry of Water Resources, New Delhi, India.

Khepar, S.D., Sondhi, S.K. and Arora, S.K. (1971). Hydraulic performance and economics of a battery of shallow tube wells in aquifers of limited thickness. Journal of Agricultural Engineering, 8(3): 17-23.

Michael, A.M., Khepar, S.D. and Sondhi, S.K. (2008). Water Well and Pump Engineering. Second Edition, Tata McGraw Hill Education Pvt. Ltd., New Delhi.

Mishra, V.K., Raina, J.N. and Nayak, B.K. (2000). Multipurpose trees enrich degraded watershed: An approach for rural rehabilitation. Journal of Soil and Water Conservation, 44(1&2): 60-71.

Prasad, T. and Prasad, R.S. (1996). Conjunctive irrigation as a strategy for prevention/remedy of waterlogging in humid alluvial plains for sustainably productive agriculture. In: Technical Papers - Supplementary, National Workshop on Reclamation of Waterlogged, Saline and Alkaline Lands and Prevention thereof, Ministry of Water Resources, New Delhi, India.

Raadsma, S. (1974). Current Drainage Practices in Flat Areas of Humid Regions in Europe. In: Jan Van Schilfgaarde (Editor), Drainage for Agriculture, Monograph No. 17, American Society of Agronomy, Madison, Wisconsin.

Shakya, S.K., Gupta, P.K. and Kumar, D. (1995a). Innovative Drainage Techniques for Waterlogged Sodic Soils. Bulletin No. 3, AICRP on Agricultural Drainage, Department of Soil and Water Engineering, PAU, Ludhiana, Punjab.

Shakya, S.K., Gupta, P.K. and Singh, S.R. (1995b). Multiple Well Point System for Irrigation/Drainage. Bulletin No. PAU/1995/T/584E, AICRP on Agricultural Drainage, Department of Soil and Water Engineering, PAU, Ludhiana, Punjab.

Sharma, R.K. (1999). Preventive measures under high water table condition of command area. Journal of Soil and Water Conservation, 43(1&2): 96-105.

Suggested Readings

Bhattacharya, A.K. and Michael, A.M. (2003). Land Drainage: Principles, Methods and Applications. Konark Publishers Pvt. Ltd., New Delhi, India.

Murty, V.V.N. and Jha, M.K. (2011). Land and Water Management Engineering. Sixth Edition, Kalyani Publishers, Ludhiana, India.

Ritzema (Editor-in-Chief) (1994). Drainage Principles and Applications. International Institute for Land Reclamation and Improvement (ILRI), ILRI Publication 16, Wageningen, The Netherlands.

Schwab, G.O., Fangmeier, D.D., Elliot, W.J. and Frevert, R.K. (2005). Soil and Water Conservation Engineering. Fourth Edition, John Wiley and Sons (Asia) Pte. Ltd., Singapore.

Smedema, L.K. and Rycroft, D.W. (1983). Land Drainage. Batsford Academic and Education Ltd., London.