Irrigation and Drainage Systems

, Volume 22, Issue 3, pp 271–285

Sensitive variables controlling salinity and water table in a bio-drainage system

Authors

    • Department of Irrigation and DrainageUniversity of Shahid Chamran
  • Heydar Ali Kashkouli
    • Department of Irrigation and DrainageUniversity of Shahid Chamran
  • Ebrahim Pazira
    • Agricultural Engineering Research Institute
Article

DOI: 10.1007/s10795-008-9056-4

Cite this article as:
Akram, S., Kashkouli, H.A. & Pazira, E. Irrig Drainage Syst (2008) 22: 271. doi:10.1007/s10795-008-9056-4

Abstract

Bio-drainage can be considered as an important part of sustainable irrigation water management. Bio-drainage has potential for managing shallow water conditions in arid and semiarid areas especially when traditional subsurface drains are not available. Bio-drainage theory does not go back too far. The relationship between soil characteristics, water management regimes, and climatic conditions is not yet well defined. This study attempted to use a mathematical model (SAHYSMOD) to evaluate factors affecting design and operation of a bio-drainage system and study its sensitivity to different variables. The study showed that the major constraint of bio-drainage is salt accumulation in tree plantation strips in arid and semiarid regions. Maximum soil water salinity which can be controlled by bio-drainage is around 3 dS m−1 in rather medium run and sustainability may only be achieved where a salt removal mechanism is considered. The study also showed that the effectiveness of the system is higher where the neighboring strips are narrower. It also showed that bio-drainage is very sensitive to the amount of applied water. While the barrier depth does not have an important effect on water table draw down, it does have a great influence on lowering the salinization rate of tree plantation strips. The application of bio-drainage could be economically controversial since in humid areas water is sufficient for agricultural crops, allocating parts of the expensive land to mostly non-fruit trees may not be feasible, while in arid and semiarid regions there is usually enough cheap land to grow trees.

Keywords

Bio-drainageDrainageSalinitySAHYSMODWater table

Introduction

Drainage can be considered as an important part of sustainable and Integrated Water Resources Management. Drainage water in arid and semi arid regions is usually saline, and sometimes incorporated with nutrients, toxic materials left from residues of pesticides and herbicides, as well as trace elements. These substances are not suitable for the environment; their disposal to surface water such as rivers, lakes, and wetlands might be quite harmful. Nontraditional drainage methods could be utilized to maintain shallow water table and soil salinity at harmless levels, and prevent pollutant disposal to the surface aquatic bodies.

Among the main environmentally friendly drainage systems are bio-drainage, dry drainage, controlled drainage and agroforestry. Bio-drainage is a natural system, in which tree plantation strip absorbs deep percolation losses of irrigation water applied to the neighboring crop strip and dispose excess water through evapotranspiration. In other words the concept of bio-drainage is based on evapotranspiration from tree plantation strips located adjacent to the irrigated crop strips (Chhabra and Thakur 1998, Liaghat 2004). Hydraulically speaking, the crop strips and the tree plantation ones are “sources” and “sinks”, respectively. The method could be economically feasible in some parts of the world since it only needs a fixed capital investment at the beginning, while it produces fiber, wood, and animal fodder in successive years.

It is believed that bio-drainage technology is capable of maintaining water table at a desired level and to some extent relieves waterlogging and canal seepage problems, if designed properly (Smedema 1997). It is doubtful, however, if it can maintain soil salinity to an extent that crops could be grown economically (Heuperman et al. 2002). In saline environments, hybrid systems that combine bio-drainage and conventional engineering-based technology will be needed to achieve sustainability. Under this scenario the bio-drainage component improves overall system efficiency and minimizes the high-capital inputs associated with engineering-based drainage methods (Heuperman 2003).

According to Kapoor and Denecke (2001) bio-drainage could be used in various regions ranging from humid to semi arid areas, except when the ground water EC is greater than 12 dS m−1. Compared to traditional drainage systems i.e. horizontal and vertical, bio-drainage is the most cost effective and environmentally friendly option. Using bio-drainage, the requirements of conventional drainage systems such as field drains, conveyance systems, effluent disposal facilities, and pumping stations could be eliminated. In most cases bio-drainage could have the first priority compared with horizontal relief drains and/or vertical tube wells. The main constraint of bio-drainage is the need for salt removal and extra land for tree plantation. In semiarid areas this is not a real limitation since available irrigation water usually does not suffice the land available.

Definition

Although the concept of using vegetation to remove excess water and dry up soil profiles is not new, the term bio-drainage that covers this process was introduced only in the late 1990s (Heuperman 2003 and Denecke 2001).“Bio-drainage” is defined as the use of vegetation to manage water fluxes in the landscape through evapotranspiration (Australian Institute of Sustainable Irrigated Agriculture; http://www.wca-infonet.org/id/1295). Traditionally management strategies to address this problem have often focused on engineering approaches such as deep open ditches, vertical drainage or horizontal subsurface drainage which all require expensive capital investment and operation and maintenance. Bio-drainage is an alternative technique that has recently attracted interest in drainage and environmental management circles. Bio-drainage can be either remedial (lowering water tables after they have risen), or preventative (intercepting soil water before it reaches the water table). Bio-drainage is a natural system in which plants highly resistant to soil salinity are usually grown in strips adjacent to irrigated crop strips. Plants due to their evapotranspiration (mostly transpiration) create a medium of lower water potential, thus, subsurface drainage water of crop strip moves towards the plantation strip (Fig. 1), where water moves to the atmosphere and salts remain in the soil.
https://static-content.springer.com/image/art%3A10.1007%2Fs10795-008-9056-4/MediaObjects/10795_2008_9056_Fig1_HTML.gif
Fig. 1

Schematic representation of a bio-drainage system

In bio-drainage studies the designer must be concerned about a broad range of variables affecting water balance, salt balance, area to be allocated to plantation, plant species, consumptive use of plants, groundwater quality, soil characteristics, and more importantly the ratio of the plantation width to the width of the crop strip (Akram 2006). Plant species should be selected from among high evapotranspirative, salt resistant deep rooted ones, such as Tamarix troupii, Acacia tortilis, Acacia nilotica, and different eucalyptus species, particularly Eucalyptus camaldulensis.

Heuperman et al. (2002) mentioned two major conclusions: Bio-drainage can effectively contribute to strongly reducing the problems as experienced from waterlogging in irrigated agriculture and non-irrigated agriculture and the problems associated with a rise in salinity in the root zone can be effectively delayed using bio-drainage systems in semiarid and arid areas.

Materials and methods

Needs to use a mathematical model

Numerous variables affect performance of bio-drainage (Akram 2006, Heuperman et al. 2002, Singh et al. 2002). Field experiments need quite different types of soils (both from physical and chemical points of view), climates, water quality, crop and tree species having different depths of irrigation and evapotranspiration, respectively, and above all several years time to monitor the results for different widths of crop and plantation strips. In other words it is almost impossible to do the research in the field to evaluate the effects of so many parameters on the performance of bio-drainage. So, it seems that a mathematical model is the most suitable way to simulate water table, and salt accumulation in the soil simultaneously, both for crop and plantation strips in different soil and climate conditions. No doubt the most promising results should be tested in the field.

Figure 1 shows that in bio-drainage there is a vertical flow i.e. irrigation water applied to crop strips and evapotranspiration from adjacent tree planted strips, and a horizontal flow between the neighboring strips. So, at least a 2D model is needed. SAHYSMOD simulation model was found to be an appropriate one. It combines the agro-hydro-salinity model, SALTMOD (Oosterbaan 2002), and the nodal ground water model (Standard Groundwater Model Package (SGMP), developed by Boonstra and de Ridder 1990) which was made by K.V.G.K. Rao. The calculation programmes were elaborated in FORTRAN and the user shell in Turbo Pascal (SAHYSMOD working group of ILRI 2003).

SAHYSMOD is a computer program for the prediction of the salinity of soil moisture, ground and drainage water, the depth of the water table, and the drain discharge in irrigated agricultural lands, using different geohydrologic conditions, varying water management options, including the use of ground water for irrigation, and several cropping rotation schedules, whereby the spatial variations are accounted for through a network of polygons. SAHYSMOD uses Block Centered Finite Difference method to solve the well known Bousinesque equation.

If one wishes to determine the effect of variations of a certain parameter on the value of other parameters, the program must be run repeatedly according to a user-designed scenario. This procedure is used here in this research. It must keep in mind that although ecohydrological models offer predictions for the future, they may become inaccurate due to over- or under parameterization (Singh 2005).

SALTMOD, SGMP, and SAHYSMOD have all been evaluated in the past and have proven their credibility (Shirahatti et al. 2001).

Model characteristics

  • Nodal Network

The study area was divided into a nodal network. The trial and error showed that 40 rectangle long (1,000 m) narrow (12.5 m) polygons can serve the purpose. As an example if one allocates a 100 m strip to crop and 100, 75, 50, 25, or 12.5 m to tree plantation strips, he would have a crop to plantation width ratio (Lc/Lp) equal to 1, 1.33, 2, 4, and 8, respectively. Then he could select the most suitable answer which is the highest ratio of Lc/Lp that could maintain water table and salinity at a desired level. In this example eight adjacent polygons, each with 12.5 m width, are allocated to crop strips, while at the same time eight, six, four, two, and one neighboring strips are allocated to tree plantations, respectively.
  • Model components

The model consists of three parts:
  1. (a)

    An agronomic water balance model, which calculates for each polygon the downward and upward water fluxes in the soil profile (SALTMOD);

     
  2. (b)

    A groundwater model of the aquifer, which calculates the groundwater flows into and from each polygon and the groundwater levels per polygon (SGMP); and

     
  3. (c)

    A salt balance model which runs parallel to the water balance model and determines average EC in the soil profiles of both crop strips and tree planted strips which are carried by water from crop strips, and then evapotranspirated from tree plantation strips. This study is confined only to the transfer of Total Dissolved Solids (TDS) represented by EC, and not to any specific salt.

     
  • Seasonal approach based on daily calculations

The model is based on seasonal input data and returns seasonal outputs. It is especially developed to predict long term trends, which could be more reliable when it is on a seasonal rather than on a daily basis. When the trend is sufficiently clear, one can predict the future of the project (SAHYSMOD working group in ILRI 2005). The computer program, however, performs calculations on daily basis. For this purpose, the seasonal water balance factors given with the input are changed automatically to daily values and the results are obtained by summations of the daily calculated values. Groundwater levels and soil salinity at the end of the season are found by accumulating the daily changes.
  • Cropping patterns/rotations

The input data on irrigation, evaporation, and surface runoff could be specified seasonally for two cropping patterns as well as one for non-irrigated land. In this study, however, neither a specific crop nor a particular tree is directly considered. Instead, net daily water moved below the root zone in crop strips, and evapotranspiration rate from plantation strips were used.
  • Soil strata

Four different reservoirs on and in the soil profile are considered (Fig. 2):
  • – A surface reservoir(s);

  • – An upper soil reservoir in the root zone (r) which can be saturated, unsaturated, or partly saturated, depending on the water balance. All water movements in this zone are vertical, either upward or downward, depending on the water balance. SALTMOD takes care of the vertical flow;

  • – A transitional soil reservoir zone (x) which can also be saturated, unsaturated or partly saturated. All flows in this zone are horizontal, except the flow to traditional subsurface drains, which is radial. SGMP takes care of the horizontal flow between and amongst the neighboring polygons; and

  • – A deep reservoir or main aquifer.

https://static-content.springer.com/image/art%3A10.1007%2Fs10795-008-9056-4/MediaObjects/10795_2008_9056_Fig2_HTML.gif
Fig. 2

Concept of four soil reservoirs with hydrological inflow and outflow components

Where:
Gu

Subsurface drainage water used for irrigation (cubic meters per square meter total polygonal area);

So

Outgoing surface runoff or surface drain water from irrigated land (cubic meters per square meter irrigated area);

Ea

Total actual evapotranspiration (cubic meters per square meter total area);

Gd

Total amount of subsurface drainage water (cubic meters per square meter total area);

Gi

Horizontally incoming ground water flow into a polygon through the aquifer (cubic meters per day); and

Go

Horizontally outgoing ground water flow from a polygon through the aquifer (cubic meters per day);

Gw

Ground water pumped from wells in the aquifer (cubic meters per square meter total area);

Ig

Gross amount of field irrigation water (cubic meters per day per square meter total area);

Io

Gross amount of outgoing field irrigation water (cubic meters per day per square meter total area);

Lc

Percolation from the irrigation canal system (cubic meters per square meter total area);

Lr

Percolation from the root zone (cubic meters per square meter total area);

λI

Infiltration through the soil surface into the root zone (cubic meters per square meter non-irrigated area);

Pp

Rainfall/precipitation (cubic meters per square meter total area);

VL

Vertical downward drainage into the aquifer (cubic meters per square meter total area); and

VR

Velocity of vertical upward seepage from the aquifer (meters per day).

  • Agricultural water balances

The agricultural water balances can be calculated for each soil reservoir separately. The excess water leaving one reservoir will be the incoming water for the next one. Under certain conditions, the height of the water table influences the water balance components. For example a rise of the water table towards the soil surface may lead to an increase of capillary rise, actual evaporation, and subsurface drainage, or a change in percolation losses. This, in turn, leads to a change of the water balance, which again influences the height of the water table, etc. This chain of reactions is one of the reasons why a computer model must be used.
  • Salt balances

The salt balances are calculated for each soil reservoir separately. They are based on their water balances, using the salt concentrations of the incoming and outgoing water. Some concentrations must be given as input data, like the initial salt concentrations of the irrigation water. A salt concentration of outgoing water by subsurface drainage was assumed to be zero because no drainage system was included in bio-drainage.
  • Ground water flow

The model calculates the groundwater levels and the incoming and outgoing ground water flows between the polygons by a numerical solution of the well known Boussinesque equation. The levels and flows influence each other mutually. The groundwater situation is further determined by the vertical recharge that is calculated from the agricultural water balances. These depend again on the levels of the ground water. If one wishes to impose a zero flow condition at the external nodes, the conductivity can be set at zero.
  • Drains

The subsurface drainage effects can be simulated through drains or pumped wells. The subsurface drains, if any, are characterized by drain depth and drainage capacity. By installing a drainage system with zero capacity one obtains the opportunity to run the model for bio-drainage. Incorporation of subsurface drains with bio-drainage could be the subject of another study with the hope of reaching acceptable sustainability.
  • Annual input changes

The program can be run either with fixed input data for the number of years determined by the user or with variable input data. The first option was used to predict future developments based on long-term average input values, as it will be difficult to assess the future values of the input data year by year.

Inputs

Input data for the SAHYSMOD model included climatic data, soil properties, crop parameters, and irrigation and drainage system layout. In order to study the effect of different parameters, the following variables were considered:
  • Soil and water:
    • four values for hydraulic conductivity, i.e. K = 0.25, 0.5, 1.0, and 2.0 m day−1;

    • five values for depth to the barrier, i.e. d = 2, 4, 6, 8, and 10 m;

    • two values for initial water table depth in crop strips, i.e. 0.2, and 0.4 m;

    • four values for initial irrigation water salinity, i.e. C1 = 1, 2, 5, and 10 dS m−1; and

    • four values for initial average soil salinity, i.e. C0 = 1.5, 3, 7.5, and 15 dS m−1.

  • Recharge and discharge:

    The study is done without considering any particular crop in crop strips, using different drainage rates instead. The same is true for plantation strips in which different values for evapotranspiration were considered.
    • Three values for drainage rate, i.e. q = 1, 2, and 3 mm day−1;

    • One value for annual precipitation, i.e. 250 mm/year equally distributed; and

    • Two values for evapotranspiration rate from plantation strips i.e. 3,000, and 4,000 mm/season. Although 4,000 mm/season seems to be quite a high value, it was included to verify if bio-drainage can still work under this extreme condition without crucial impairment of the trees due to salt accumulation.

  • Land layout:
    • Parallel strips, with high ratio of their lengths to their widths in order to minimize the effect of the boundaries;

    • 19 values for strip widths as shown in Table 1.

Table 1

Different combinations of plant and crop widths

Crop strip width (m)

Plant strip width (m)

Ratio

(Lc)

(Lp)

(Lp/Lc)

12.5

50

4

25

100

4

25

75

3

25

50

2

50

100

2

50

75

1.5

75

100

1.33

25

25

1

50

50

1

75

75

1

100

100

1

100

75

0.75

75

50

0.66

25

12.5

0.5

50

25

0.5

100

50

0.5

75

25

0.33

50

12.5

0.25

100

25

0.25

Accepting criteria

The model was run 18,240 times for the combination of different variables. Only 69 options satisfied the following criteria:
  1. 1.

    The average soil salinity of crop strips evenly distributed in the soil profile does not exceed 8 dS m−1 after 20 years, in which still some annual crops such as alfalfa, barley and sugar beets could be grown;

     
  2. 2.

    The average soil salinity of plantation strips uniformly distributed in the soil profile does not exceed 32 dS m−1 after 20 years, which a few trees and bushes such as species of eucalyptus, acacia, atriplex, and tamarix could still tolerate;

     
  3. 3.

    The water table in crop strips is not shallower than 0.5 m, suitable for some annual shallow rooted crops;

     
  4. 4.

    The water table in the midpoint of plantation strips is not shallower than 1.2 m;

     
  5. 5.

    In case where several plantation widths (Lp) satisfy the above criteria for a given crop width (Lc), the one with minimum Lp/Lc ratio i.e. the more economical one would be accepted.

     

Results and discussion

The effect of hydraulic conductivity on water table

The greater the Lp/Lc, the higher the water table draw down in both crop strip and plantation strip. In other words, the highest curve in Fig. 3a is for Lp/Lc = 4, while the lowest one is for Lp/Lc = 0.25. Note that the left value in all legends corresponds to Lp and the right one corresponds to Lc, both in meters. Increasing hydraulic conductivity does not have a significant role in lowering water table in the plantation strip (Fig. 3a). This is probably because the water movement occurs due to a potential difference created by evapotranspiration from plantation strip. As long as the potential difference is at such an extent that soil is able to transmit water, hydraulic conductivity does not have so much effect on water table draw down (Akram 2006). More research is needed for q > 3 mm day−1.
https://static-content.springer.com/image/art%3A10.1007%2Fs10795-008-9056-4/MediaObjects/10795_2008_9056_Fig3_HTML.gif
Fig. 3

The effect of the hydraulic conductivity on the depth of water table in crop strips (a) and plantation strips (b)

Figure 3a also shows that very low values of hydraulic conductivity may not be able to draw water table down to the desired level in crop strips. This result is quite the same as what one can expect from conventional drainage systems. However, the problem may not be important when Lp/Lc has higher values. In almost all conditions when the hydraulic conductivity exceeds 1 m day−1, the water table does not fall any more in crop strips. So, bio-drainage is less effective in heavier soils, hence, more attention should be paid in its design. Higher values of Lp/Lc, however, will solve the problem, but of course with the expense of allocating more land to plantation area.

The effect of hydraulic conductivity on soil salinity

Increasing hydraulic conductivity does not have a significant role in increasing average salinity either in crop strip or in plantation strip since the water movement occurs due to potential difference created by evapotranspiration from tree plantation strip. This is true at least for q < 3 mm day−1. The lower the Lp/Lc ratio, the higher the salt concentration in plantation strips. This is due to salt balance, in which the salt mass transferred to the plantation strip remains constant while the plantation strip width decreases, and consequently the salt concentration increases.

The effect of barrier depth on water table

Increased barrier depth results in more draw down of the water table in crop strips. This result is quite the same as what one can expect from conventional drainage systems. In case the barrier is too shallow (2 m in the example shown in Fig. 4); the water table in crop strip cannot sufficiently fall down. Increased barrier depth results in sharp decline of the water table. The rate, however, does not remain constant and gradually decreases.
https://static-content.springer.com/image/art%3A10.1007%2Fs10795-008-9056-4/MediaObjects/10795_2008_9056_Fig4_HTML.gif
Fig. 4

The effect of the barrier depth on water table in crop strips

The water table depth in plantation strip is not a function of the location of the barrier. It, however, depends on the evapotranspiration rate of trees. The greater the Lp/Lc, the higher the water table draw down in plantation strip, the same as it was in the crop strip. The greater ratio, however, is not feasible since the land value of crop strip is more than the plantation strip. Bio-drainage could last longer where the impervious layer is too deep (Fig. 5).
https://static-content.springer.com/image/art%3A10.1007%2Fs10795-008-9056-4/MediaObjects/10795_2008_9056_Fig5_HTML.gif
Fig. 5

The effect of the barrier depth on salinity in plantation strips

The effect of barrier depth on salinity of the plantation strip

Increasing the depth to the barrier causes less salt concentration in plantation strips. This is due to salt balance, in which the salt mass transferred to the plantation strip remains constant. While the volume of the soil increases, the salt concentration must decrease. This means that bio-drainage is more feasible and can last longer in areas where the impervious layer is deeper.

The higher the Lp/Lc ratio, the lower the salt concentration in plantation strips. This means that with allocation of more land to tree plantation, the efficiency and the life of the bio-drainage system increases.

Sustainability of bio-drainage

Sustainable projects are those which save the physical, ecological, social, and economic environment in the long run. It is expected, however, that so called environmentally friendly drainage methods be really sustainable without any harm to the environment. Most authors believe that bio-drainage is able to maintain water table in a position not to be hazardous to the crop. However, some are doubtful about its sustainability from the point of view of soil salinity. In fact, according to the salt balance principle, salt accumulation in strips having upward flow, i.e. plantation strips, is inevitable unless some type of salt removal mechanism such as foliage harvesting, salt scraping from the soil surface, and/or leaching facility exists. Heuperman et al. (2002) and Heuperman (2003) suggest a combination of bio-drainage and a conventional one.

Bio-drainage and salinity in crop strips

Figure 6 shows the soil salinity of crop strips with the passage of time, in which it increases during the first 2 years and remains almost constant afterwards. The soil salinity is proportional to the irrigation water salinity. The figure also shows that because of low leaching fraction the crop strip salinity is mostly beyond the tolerance of most crops, unless the irrigation water quality is quite suitable.
https://static-content.springer.com/image/art%3A10.1007%2Fs10795-008-9056-4/MediaObjects/10795_2008_9056_Fig6_HTML.gif
Fig. 6

Relationship between average soil salinity with time in crop strips (C0 = initial average soil salinity)

Bio-drainage and salinity in plantation strips

Irrigation water quality does have a great influence on the salinity of plantation strips as shown in Fig. 7. The salinity of plantation strips increases exponentially with time i.e. salinity in the plantation strip (dS m−1) = 2.24 e0.11t (years); R2 = .97 for Lc = 25 m and Lp = 50 m, and initial soil water salinity of 1.5 dS m−1 as shown in Fig. 7. This, however, clearly shows that in case of irrigation with highly saline water, the soil salinity in plantation strips increases so fast that not only in the long run but even after 3 to 4 years plants die, and bio-drainage system fails to function.
https://static-content.springer.com/image/art%3A10.1007%2Fs10795-008-9056-4/MediaObjects/10795_2008_9056_Fig7_HTML.gif
Fig. 7

Relationship between salinity of irrigation water and soil salinity with time in plantation strips

Bio-drainage effectiveness and the widths of the strips

The results show that with a constant Lp/Lc ratio, the effectiveness of the system is higher when both Lp and Lc have smaller values. For example, when Lp/Lc = 1, the water draw down is higher and salt accumulation is lower when Lp = Lc = 25 m compared to when Lp = Lc = 75 m. The designer should therefore take the narrower strips into consideration to such an extent that it does not impair the work of agricultural machinery and does not make any other limitations.

High sensitivity of bio-drainage to salinity in arid and semi arid regions

The maximum initial soil water salinity that the bio-drainage system is able to control for only 5 years is about 5 dS m−1. Assuming soil water salinity is almost 1.5 times the salinity of irrigation water, one can conclude that the bio-drainage system cannot function properly when irrigation water salinity exceeds about 3.3 dS m−1. This is of course true where the annual rainfall is 250 mm, as was the case in this study. Since rain dilutes the soil water salinity, one can expect that salinity hazard will be less in humid areas (Fig. 8).
https://static-content.springer.com/image/art%3A10.1007%2Fs10795-008-9056-4/MediaObjects/10795_2008_9056_Fig8_HTML.gif
Fig. 8

a The trend of water table status with time, b the trend of salinity with time

Suitable regions for bio-drainage

Bio-drainage does have a high sensitivity to salinity in regions with arid and semi arid climates. If one wishes to find a solution to this problem, bio-drainage cannot be a good alternative to conventional drainage systems.

Bio-drainage needs a relatively high percentage of land out of the farming system. So, bio-drainage is more suitable in regions where the land is in abundance and cheap. In bio-drainage a considerable percentage of land is allocated to tree plantation which does not need to be irrigated. The system is therefore more suitable in regions where water is scarce, and/or expensive.

Lp (m)

Lc (m)

k (m day−1)

d (m)

q (mm day−1)

C0 (dS m−1)

C1 (dS m−1)

25

25

1.0

10.0

1.0

1.5

1.0

Conclusions

The followings can be concluded:

The major constraint of bio-drainage in arid and semi arid regions is salt balance and accumulation of salt in tree plantation strips. The sustainability of the system, however, is questionable except where the irrigation water is quite suitable and/or in humid regions with high annual precipitation. In saline environments, hybrid systems that combine bio-drainage and conventional engineering-based technology will be needed to achieve sustainability.
  • The effectiveness of the system is higher where the neighboring strips are narrower; i.e. 25 and 50 m instead of 50 and 100 m for crop strips and plantation strips, respectively. It could therefore be recommended that the designers take the narrowest possible strips into their consideration to such an extent that it does not impair the work of agricultural machinery and does not cause any other limitations.

  • Maximum initial soil water salinity which can be controlled by bio-drainage is about 5 dS m−1 in a 5 to 10 year time span. In many cases even an initial soil salinity of 3 dS m−1 cannot be tolerated in the rather longer run (for example 20 years).

  • In most cases salinity of crop strips in bio-drainage is independent from the hydraulic conductivity of the soil. The crop strips could therefore be reclaimed even in heavy soils with low hydraulic conductivity provided that the transmissivity of the soil can satisfy the evapotranspiration of the trees.

  • Barrier depth does not have an important effect on lowering water table in tree plantation strips. The higher barrier depth, however, has a great influence on lowering the salinization rate of tree plantation strips. A bio-drainage system could not be expected to be considered successful in areas where the barrier depth is too shallow, say less than 4 m in arid and semi arid regions.

  • Bio-drainage is very sensitive to the amount of applied water. The higher the water applied, the lower is its effectiveness in both water table and salinity control in plantation strips. Choosing a cropping pattern with lower consumptive use and/or increasing water application efficiency may therefore increase the life of the system.

  • In arid and semi arid areas, sustainability can only be achieved where some sort of salt removal mechanism is included in the system. These could be foliage harvest and scraping salts accumulated on the soil surface in plantation strips. It seems that in some cases bio-drainage can be complemented by traditional drainage systems as Heuperman 2000 and Heuperman et al. 2002 suggests. This is, of course, subject to further research.

In addition to direct conclusions from this research one may make the following indirect conclusion as well:
  • The application of this system is more feasible in regions with cheap land and high water price. The applicability of bio-drainage could be higher in areas where the water quality is relatively better and its price is lower while, at the same time, the land is relatively expensive. There is therefore usually a paradox between technical and economic points of view.

Acknowledgement

The authors would like to sincerely express their gratitude to R. J. Oosterbaan, from International Land Reclamation Institute (ILRI), Wageningen, The Netherlands for his generous assistance in modeling.

Copyright information

© Springer Science+Business Media B.V. 2008