Abstract. The use of peatlands in the humid tropics requires drainage to remove excess rainfall. The design principles for the drainage systems currently being ...
Irrigation and Drainage Systems 12: 123–139, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands.
A new approach to water management of tropical peatlands: a case study from Malaysia H.P. RITZEMA1, ABD. MUTALIB MAT HASSAN2 & R.P. MOENS3
1 International Institute for Land Reclamation and Improvement/ILRI, P.O. Box 45, 6700 AA Wageningen, The Netherlands; 2 Department of Irrigation and Drainage, Kuala Lumpur, Malaysia; 3 Grontmij Consulting Engineers, De Bilt, The Netherlands
Accepted 17 December 1997
Abstract. The use of peatlands in the humid tropics requires drainage to remove excess rainfall. The design principles for the drainage systems currently being implemented on peatlands are the same as for mineral soils. The objective of such systems is the timely removal of excess rainfall by surface runoff. For peatlands, with their different soil-hydraulic characteristics, these systems have resulted in poor watertable control and high rates of irreversible subsidence. Concerns about this rate of subsidence and the level of sustainability of the present land use have prompted a study to develop a new water management system. This new system includes a shift from a drainage system that focuses on discharge of excess water towards a system that combines drainage and water conservation. In the new two-step design, the drain spacing and corresponding drain discharges are obtained with a steady-state approach. These outputs are used to calculate the capacity of the drains, including control structures, using an unsteady-state approach. The new system results in a shallower but more narrowly spaced drainage system and maintains a more constant but relatively high watertable and reduces subsidence. It should be remembered however, that even with the improved water management, subsidence cannot be arrested; it is the price one has to pay for the use of tropical peatlands. Key words: drainage system design, humid tropics, peatlands, water management
Introduction Peatlands in the humid tropics cover some 36 million ha, 20 million of which are found in South-East Asia (Andriesse 1988). Water management is the key to sustainable use of these peatlands for agriculture, forestry, and to conserve the natural ecosystem. Drainage is needed to remove excess rainfall and to control the watertable. In the humid tropics, the emphasis is on drainage and not on water conservation (Smedema 1987). In Malaysia, the currently installed systems are aiming primarily to create flood-free conditions and to eliminate waterlogging (Welch & Nor 1989). As soon as the peat soils are drained, however, the process of irreversible subsidence commences, which is a well-known and hard-to-overcome constraint to their development and
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124 use (Aminuddin 1994; Andriesse 1994). Subsidence can be reduced by maintaining a high watertable (Schothorst 1982). The high permeability of peat, however, makes watertable control difficult (Salmah 1992). This paper presents the findings of a study on the development of an improved water management system for peatlands. The study was part of an inter-disciplinary study on peat soil management for the Western Johore Integrated Agricultural Development Project/IADP. The IADP started in 1974, its main objective being to raise the income and standard of living of the rural community in the western part of the State of Johore, in the South of Peninsular Malaysia (Lim 1994). It aims to achieve integrated agricultural development by providing a basic engineering infrastructure, agricultural land development, extension services, credit, and improved marketing facilities. The drainage system installed in the peatlands is the same as that in the mineral-soil areas. Both systems are based on the assumption that all excess rainfall will be evacuated by surface runoff. In the peat areas, this has resulted in excessive rates of subsidence, ranging from 2 to 5 cm/day (Wösten et al. 1997), and watertable levels which are often outside the range for optimum crop production. To counterbalance these negative effects, the drainage of peat areas must be approached from a total water management perspective whereby not only the removal of excess rainfall is considered but also the crop water requirements (Salmah et al. 1994). The study approach to develop such an improved water management system was as follows: Field data were used to analyse the present water management system. Next, a steady-state drainage model was used to predict the effect of alternative water management options on the depth of the watertable and the drain discharge, and an unsteady-state drainage model was used to simulate the fluctuation in the water levels in the drains and to predict the effect of control structures on these levels. This paper presents the set-up of the study, the main results, and the conclusions.
Materials and methods Description of the project area The project area is fairly flat and relatively low-lying (Lim 1994). It measures 359 000 ha, of which about 95 000 ha are peat soils. Rainfall and evaporation are fairly evenly distributed over the year (Figure 1). Over the period 1950– 1992, the long-term average monthly rainfall varied from 165 mm in the driest month (February) to 275 mm in the wettest month (November). Fluctuations from year to year, however, are high (e.g. in 1994, monthly rainfall varied from 28 mm in July to 544 mm in November). Evaporation is even more con-
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Figure 1. Monthly rainfall and evaporation in 1994 and in the period 1950–1992.
stant, varying between 4 and 5 mm/day. Although there is an overall excess of rainfall, periods with rainfall deficit can occur, as happened in the period July–September 1994. The peat, which varies in depth between 1 m and 10 m, has a high infiltration, water-holding, and watertransmitting capacity (Ong & Yogeswaran 1994). In the study area, the hydraulic conductivity varies between 4 and 60 m/d and the drainable pore space, which was calculated from the instantaneous rise of the ground water level after heavy rainfall, is about 0.55. The main crops are oil palm, rubber, and pineapple. Although information on crop yields on peat is scarce, they can be as high as on mineral soils (Singh 1993). For optimum crop yields, the watertable may fluctuate between 0.20–0.40 m for shallow-rooting crops (e.g. cassava, soybeans, vegetables, pineapples, and sago) and 0.60–0.90 m for deep-rooting crops (e.g. oil palm, rubber, pineapples, maize, and groundnuts). Outside this range, yield reductions are as high as 50 per cent per 0.3 m change in the watertable (Zahari et al. 1989). Description of the present drainage system The drainage system was designed to create flood-free conditions and to eliminate waterlogging, with emphasis on drainage and not on water conservation (Lim 1994). Drainage is provided through an open canal system with tertiary (field) drains spaced at intervals of 200 m and secondary drains spaced at intervals of 800 m. To minimize costs, the secondary drains are constructed perpendicular to the contour lines. In the study area, the longitudinal slope of the secondary drains is about 2 m/km. Drainage is by gravity. Structures at the downstream end of the secondary drains prevent backflow during high
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Figure 2. Layout of the drainage system in the test plot.
river stages. The design drain discharge, which is based on a maximum duration of flooding resulting from a 1-in-5-year rainstorm, varies from 6 l/s/ha (52 mm/d) to 10 l/s/ha (86 mm/d). It is assumed that all excess rainfall is evacuated by surface runoff. The secondary drains were installed in 1973, and the construction of the tertiary drains was left to the farmers. The installation of the secondary drains appeared to be sufficient to control the flooding, probably because of the high water-holding and water-transmitting capacity of the peat. Consequently, the farmers did not construct the tertiary drains. To monitor the effect of the tertiary drains, the project constructed a test plot with tertiary drains and control structures in the secondary drains (Figure 2). Rainfall, subsidence rates, and ground and surface water levels were collected in the test plot and in a nearby control plot. Simulation of the water management The flow of drainage water can be divided into groundwater flow through the soil towards the drains and open-water flow in the drainage system. Groundwater flow is influenced by the recharge to the watertable (rainfall minus crop evapotranspiration), the soil hydraulic parameters (drainable pore space, hydraulic conductivity, and the thickness of the peat layer), and the characteristics of the drainage system (depth and spacing of the drains). The open-water
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127 flow depends on the characteristics of the drainage system (cross-sectional area, longitudinal slope, and control structures), the inflow of groundwater and surface water, and the outlet conditions. Because of the large waterholding capacity of peat soil, fluctuations in the watertable are relatively small, even during periods of high rainfall. Thus a steady-state approach can be used to simulate the depth of the watertable and the groundwater flow towards the drains (Oosterbaan 1994). Storage capacity in the drains, however, is small compared to the total flow. Thus to simulate the flow of water in the open drainage system an unsteady-state approach is used. Simulation of the watertable and corresponding drain discharges A spreadsheet computer model based on the Hooghoudt Equation was developed to simulate the depth of the watertable and the discharge to the drainage system for different depths and spacings of the drains. Although the Hooghoudt equation is based on a steady-state approach, it can also be used to simulate the fluctuation of the watertable over a period of non-uniform distribution of recharge, if this period is divided into time intervals during which the recharge to the groundwater is assumed to be constant (Ritzema 1994). In the model, the depth of the watertable and the corresponding drain discharges are calculated with the following equations: ht = ht −1t + qt =
(rt − qt −1t ) 1t µ
8 K d ht −1t + 4 k h2t −1t , L2
(1)
(2)
where ht depth of the watertable with respect to the water level in the drain at time t (m) qt drain discharge at time t (m/d) K hydraulic conductivity (m/d) rt recharge at time t (m/d) d equivalent depth of the soil layer below drain level (m) µ drainable pore space (–) L drain spacing (m) 1t time step (d)
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128 Simulation of the water levels in the open drains An unsteady-state computer model, DUFLOW, was used to simulate the fluctuation in the water levels in the drains, to simulate the effect of control structures on these water levels, and to calculate the capacity of these structures in relation to the drainage requirements. DUFLOW is a micro-computer package for the simulation of flow and water quality in open channel systems based on the one-dimensional partial differential equations (IHE 1995). These equations are the mathematical translation of the laws of conservation of mass and of momentum (energy), i.e. B
δQ δH + =0 δt δx
(3)
δH δ(αQv) g|Q|Q δQ + gA + + 2 = bγ w 2 cos(8 − φ), δt δx δx C AR where t x H(x,t) v(x,t) Q(x,t) R(x,H) A(x,H) b(x,H) B(x,H) g C(x,h) w(t) 8(t) φ(t) γ (x) α
(4)
time (s) distance along channel axis (m) water level with respect to reference level (m) mean velocity (averaged over the cross-sectional area) (m/s) diacharge at location x and time t (m 3 /s) hydraulic radius of cross-section (m) cross-sectional flow area (m2 ) cross-sectional flow width (m) cross-sectional stoarge width (m) acceleration due to gravity (m/s2 ) coefficient of De Ch´ezy (m2 /s) wind velocity (m/s) wind direction (degrees) channel direction (degrees) wind conversion coefficient (–) correction factor for non-uniformity of velocity distribution (–)
In DUFLOW these two equations are discretized in space and time. To find a solution for this set of discretized equations, additional conditions for the
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129 physical boundaries of the network and the initial conditions have to be specified. The schematic representation of the canal system is built up out of three types of elements: open channel sections, control structures (weirs, culverts, siphons, and pumps) and boundary conditions (water levels and/or discharges). Very detailed schematization is not necessary because of the nature of the equations used in the programme, i.e. for one-dimensional flow only. New design approach The new design approach has to fulfil two objectives: to provide adequate drainage for agricultural use and to reduce subsidence of the peat by maintaining a high watertable. Note that these two objectives are sometimes conflicting. On the one hand a drainage system with a high capacity is required to provide adequate drainage during periods of high rainfall intensities. On the other hand, a drainage system with a reduced drainage capacity is required to maintain high watertables during periods with no rainfall. Thus a flexible system is needed, in which the capacity can be adjusted to the season. Compared with mineral soils, peat has a very high capacity for infiltration and water transmission. Consequently, almost all excess rainfall infiltrates into the soil and moves towards the drainage system as groundwater flow. Hence the drainage system should be designed for watertable control and not for surface runoff, as in mineral soils (Figure 3). The agricultural use defines the range in which the watertable may fluctuate. For shallow-rooting crops this range is 0.20–0.40 m and for deep-rooting crops 0.60–0.90 m. To fulfil the second objective, the watertable should be kept as high as possible the whole year around. Thus the watertable midway between the drains should be kept at the upper limit of the range for optimum crop production and the water level in the drain should be kept at the lower limit. The difference between the two levels (the watertable head) defines the drain spacing. In its turn, this drain spacing determines the rise of the watertable after heavy rain and its fall during prolonged dry spells. To control the water level in the drains, control structures are required. If, during heavy rain, the watertable rises to the soil surface and the period of waterlogging exceeds the design criteria, the water level in the drain should be lowered temporarily so as to create extra head. If, on the other hand, the watertable falls too low during a prolonged period of dry weather, the water level in the drain should be raised. Based on the above considerations, a two-step design approach is proposed:
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Figure 3. The existing drainage criteria are based on surface runoff. The new criteria are based on groundwater runoff.
Step 1. Drain spacings are calculated based on average (steady-state) conditions, i.e. a year with average rainfall. The upper limit of the desired watertable for crop production defines the watertable in the soil midway between the drains, and the lower limit defines the water level in the drain. The Hooghoudt spreadsheet model can be used to calculate the drain spacing and the corresponding drain discharges.
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131 Step 2. Control structures are designed to control the water level in the drains under extreme wet and dry conditions, i.e. with a frequency of occurrence of 1×5-year. If the watertable rises to the soil surface and the period of flooding exceeds the design norm, the water level in the drains should be lowered. On the other hand, if the watertable falls below the lower limit, the water level in the drains should be raised. The Duflow model can be used to calculate the capacity (dimensions) of the drainage system, including the position and height of the control structures. The output of the Hooghoudt model, i.e. the discharge to the drainage system, is used as input in the DUFLOW model.
Results and discussion Current conditions In the traditional system, there is only limited control in water management, consequently the level of the watertable fluctuates in time and in space. The fluctuation in time depends mainly on the recharge from rainfall. The variation is considerable, ranging from about 0.8 m midway between the drains to almost 1.0 m along the drains (Figure 4). Owing to the soil’s high drainable pore space, the watertable never reaches the soil surface. Even rainfall of more than 100 mm/d, such as occurred in October 1994, did not result in water ponding on the soil surface. Consequently, hardly any surface runoff occurs and peak flows in the drains are far below the design discharge, resulting in low water levels in the drains, on average between 1.5 and 2.5 m below ground level. The fluctuation of the watertable in space is also considerable: the average watertable drops from around 0.5 m midway between the drains to more than 1.0 m near the drains. The layout of the road system aggravates the situation. The farm roads, which are situated alongside the secondary drains (Figure 2), are built on a layer of compacted fill, consisting of mineral soil brought in from outside the project area. Consequently, the road, due to its high bulk density, subsides much faster than the surrounding peatland. To maintain a sufficient freeboard and to reduce the cost of road maintenance, the secondary drains are de-silted every 2–3 years, consequently lowering the watertable. A direct relation between watertable depth and soil subsidence was found: the rate of subsidence increases 0.4 cm per year for each 0.10 m lowering of the watertable (Wösten et al. 1997). Thus, due to the difference in the depth of the watertable, the subsidence near the drains is higher than midway between the drains, causing the ground surface to take on a parabolic shape (Figure 5). It is estimated that approximately 30 per cent of the total subsidence can be attributed to the unevenly distributed watertable.
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Figure 4. Fluctuation of the watertable midway and next to the secondary drains in the test plot in 1994.
Figure 5. Subsidence near the drains is higher than midway between the drains, causing a parabolic shape of the ground surface.
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Figure 6. Measured and simulated hydraulic head (= depth of the watertable midway between the drains minus the water level in the drains).
Summarizing the exisiting situation we can conclude that, although no flooding occurred (the reason why farmers did not construct tertairy drains!!), the watertable is often outside the range for optmium crop production. Midway between the drains, the average watertable level is often too shallow, and near the drains it is often too deep. These deep watertables have resulted in unacceptably high levels of subsidence. The difference in watertable depth also created different subsidence rates, which in their turn, resulted in a parabolic shape of the groundsurface. This makes water conservation complicated. Simulated watertables and drain discharges The Hooghoudt spreadsheet model was calibrated by comparing the measured and simulated hydraulic heads (= depth of the watertable midway between the drains minus the water level in the drains, Figure 6). After calibration, the model was used to calculate the drain spacings for steady-state (long-term average) rainfall conditions for shallow-rooting and deep-rooting crops and three classes of peat depth (Table 1). The calculated drain spacings vary from 120 m for the shallow-rooting crops on the shallow peat (peat depth < 1.5 m) to 320 m for the deep-rooting crops on the deep peat (peat depth > 3.0 m). The difference in drain spacing for the three peat classes can be explained by the difference in transmissivity (=the product of the hydraulic conductivity and the thickness of the peat layer
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134 Table 1. Calculated drain spacings for the three peat classes and the two crop types. Peat depth (m)
< 1.5 m 1.5–3.0 m > 3.0 m
Drain spacing (m) Shallow-rooting crops
Deep-rooting crops
120 200 270
140 230 320
Table 2. Simulated watertables and discharge rates. Shallow-rooting crops
Deep-rooting crops
Simulated watertable depth (m below ground level): – Average during normal year – Range during normal year – Maximum (1 × 5 year rainfall)
0.25 0.12–0.33 0.01
0.68 0.52–0.77 0.38
Simulated discharge (mm/d): – Average during normal year – Range during normal year – Maximum (1 × 5 year rainfall)
2.5 1.0–4.8 6.9
2.5 1.4–4.5 6.3
below the watertable). Next, these drain spacings were used to calculate the fluctuation in the watertable and the corresponding discharge for the rainfall pattern in a normal (long-term average) year and during extreme wet and dry periods (1×5-year return period). As the difference between the three peat classes is already negotiated in the spacing, the results for the three peat classes are almost the same, so they are grouped together (Table 2). Under extremely wet conditions, the watertable almost reaches the soil surface, but only for the shallow-rooting crops. During dry periods, it takes the watertable 20 days to fall to drain level for the shallow-rooting crops and 30 days for the deep-rooting crops. This corresponds to return periods of no-rain days of about 7 years for shallow-rooting crops and more than 10 years for deeprooting crops.
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Figure 7. Simulated water levels in the secondary drains.
Simulated water levels in the open drains The DUFLOW model was calibrated for the design situation in 1973. Simulations were made with a Manning’s roughness coefficient of 0.04 (representing poor maintenance conditions, Bos 1994) and a constant downstream water level of 0.55 m+MSL (the average water level in the Benut River). After the model was calibrated, the water levels were simulated for the 1993 situation, with the control structures in the test plot (2600 m upstream of the outlet) and the actual bed levels in 1993 (Figure 7). Water levels were simulated for the design discharge (6 and 10 l/s/ha), and the average (0.3 l/s/ha) and maximum (1.1 l/s/ha) discharge found as output from the Hooghoudt model. Simulated water levels in the downstream
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Figure 8. A cascade of closely spaced control structures to maintain high water levels in the open drains.
section for the maximum discharge are slightly underestimated because river water levels during floods can be up to 2.0 m + M.S.L. These high river levels, however, do not influence the water level in the sections near the test plot. Next, the model was used to calculate the location, the height, and the capacity of the control structures (Figure 8). Fortunately, structures with fixed crest levels can be used because of the low peak discharges. These control structures are needed to maintain the required water level in the drains. As the permeability of the peat is high, water conservation is difficult, and the water can be retained only by a cascade of closely-spaced weirs with small drops. Further research is needed to check if these constant water levels can be maintained in spite of the loss of water by seepage flow through the peat layers underneath and along the structures. We can conclude that in Western Johore with its fairly evenly distributed rainfall pattern, good drainage conditions can be created if it is possible to maintain a constant water level in the drainage system throughout the year: 0.4 m for the shallow-rooting crops and 0.9 m for the deep-rooting crops. The average discharge is 2.5 mm/d (≈ 0.3 l/s/ha) and the maximum discharge is about 7 mm/d (≈ 1.1 l/s/ha). This maximum discharge corresponds to a 1×5-year return period. As the design discharge for the same return period was 10 l/s/ha, we can conclude that the existing drainage system is overdesigned. For areas with a more pronounced difference between the rainy season and the dry season, it will not be possible to maintain a constant water level throughout the year. In the rainy season a lower water level will have to be maintained to avoid water ponding, and in the dry season a higher water level will be needed to reduce drought stress and subsidence. Under such conditions, the control structures in the drains will have to be equipped with adjustable weirs or stop boards.
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137 Road design We have seen that in the current situation, the roads subside at a higher rate than the adjacent land surface. This trend can be stopped only if the design of the roads is changed. A prerequisite for a new design is that the roads should have sufficient bearing capacity without needing low watertables beneath the roadbed. Alternative foundation materials that have a bulk density similar to that of the removed peat soil (e.g. foamed concrete, rigid expanded polystyrene, and bales of dried peat) should be used. If such an alternative road design can be implemented, water levels in the drains can be maintained at a higher level, and the subsidence rates will decrease even further.
Conclusions In peatlands, water management is required to obtain optimum crop yields and, at the same time, to minimize subsidence. Under poor water management, crop yields may decrease by as much as 50 per cent, and subsidence can be excessive. The current design principles, which aim to remove excess rainfall, are the same as for mineral soils. They have resulted in sub-optimum growing conditions for the major crops and excessive subsidence, especially alongside the drains. We propose a new, two-step design approach that aims to optimize crop production while conserving the peat with a drainage system that: − Maintains a constant high watertable throughout the year within the range for optimum crop growth. This water conservation criterion is based on average conditions. − Avoids flooding and drought stress during extremely wet or dry periods. This drainage criterion is based on a 1×5-year return period. A spreadsheet model based on a steady-state approach, i.e. the Hooghoudt equation, was used to calculate optimum drain spacings and to simulate the fluctuation of the watertable and the groundwater flow towards the drains. The steady-state approach can be used because fluctuations in the watertable are relatively small, even during periods of high rainfall, because of the large water-holding capacity of peat soil. Storage capacity in the open drainage system, however, is small, thus an unsteady-state model, i.e. DUFLOW, was required to simulate the flow of water in the open drainage system and to predict the effect of control structures. The output of the Hooghoudt model, i.e. the groundwater flow to the drains, was used as input for the DUFLOW model. Both models proved to be a useful tool for analysing the existing situation and for developing an improved drainage system. The new design
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138 approach results in a more narrowly spaced drainage system (to obtain a more evenly distributed watertable) in combination with water level control by structures in the drainage system (to conserve water). Because the more constant and evenly distributed watertable is relatively high, it will improve crop yield and reduce subsidence. It should be remembered, however, that, even with the improved water management, subsidence cannot be arrested; it is the price one has to pay for the use of tropical peatlands.
Acknowledgement This study could not have been conducted without the data and support provided by the Department of Irrigation and Drainage of the Malaysian Ministry of Agriculture.
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