HYDROLOGY OF BORNEO S PEAT SWAMPS Henk Ritzema and Henk Wösten Alterra The Netherlands 1
HYDROLOGY OF BORNEO S PEAT SWAMPS 1. Introduction...1 2. Climate...1 2.1.1 Evaporation...1 2.1.2 Rainfall...2 2.1.3 Dry spells...4 3 Topography...5 3.1 SURFACE TOPOGRAPHY...5 3.2 MINERAL SUBSOIL...6 3.3 CATCHMENTS...6 3.2.3 Drainage Base...7 3.2.4 Drainability...8 3.2.5 Water Management...10 4 The water balance in a peat swamp...12 4.1 WATER LEVELS...12 4.2 DISCHARGES / RUN-OFF...13 4.3 STORAGE CAPACITY...14 5 Gaps in knowledge...15 References...16 2
1. Introduction The lowland peat swamps of Borneo are purely rain-fed. They have their origins in the topographic conditions that lead to semi-permanent waterlogging. Under natural conditions, they are formed by the accumulation of vegetation, which is deposited on the waterlogged soils faster than it can decay. Hydrology is an important (if not the most important) factor in the formation and functioning of peat swamp ecosystems. The hydrology of a peat swamp depends on the climate, topographic conditions, natural subsoil, and drainage base. Any changes in the hydrology, especially those from the introduction of drainage, will have oftenirreversible effects on the functioning of these fragile ecosystems. A better understanding of the hydrology of peat swamps will make it possible to develop and manage them in a more sustainable way. 2. Climate The climate in Borneo is characterised by its uniform temperature, high humidity, and high rainfall intensity. The mean monthly temperature is stable and varies between 24 C and 27 C. According to the Köppen classification system, which is based on precipitation and temperature, the climate is a tropical rain climate without a dry season and a long-term mean precipitation in the driest month higher than 60 mm (Class Af). The climate is influenced by two monsoon winds, namely the Northeast Monsoon from November to February and the Southwest Monsoon from April to September. 2.1.1 Evaporation The average evaporation is fairly constant, varying between 3.5 mm/d and 4.8 mm/d with a total of around 1500mm per year (Table 1). Table 1 Mean monthly evaporation and rainfall (mm) in Central Sarawak and South Kalimantan Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year Mukah, Central Sarawak: Evaporation 116 119 140 136 136 133 131 137 128 129 109 115 1529 Rainfall 626 428 323 179 168 168 174 187 251 266 323 530 3623 Banjarmasin, South Kalimantan: Evaporation 109 111 125 133 125 117 126 136 143 139 115 114 1492 Rainfall 436 298 323 269 206 156 156 98 70 141 273 397 2823 1
2.1.2 Rainfall For the peat swamps, rainfall, in particular the rainfall that is in excess of evaporation, is the most important hydrology parameter. The annual rainfall is much higher than the annual evaporation (Table 1). The rainfall, however, is much more irregular, both in time and space. Along the coast of Sarawak, the annual rainfall varies from 2800 mm to 4700 mm (Table 2) with an overall average of around 3600 mm. In South Kalimantan, rainfall is significant lower, varying between 1900 and 3000 mm per year with an overall average of around 2800 mm. Differences is space (Figure 1) are linked closely to the average rainfall intensity and not so much to the number of days of rainfall. Table 2 Year Total annual rainfall at Mukah Airfield, Central Region of Sarawak Rainfall (mm) Year Rainfall (mm) Year Rainfall (mm) 1965 3429 1977 4159 1989 4099 1966 3291 1978 3387 1990 3272 1967 3892 1979 3066 1991 3283 1968 3401 1980 3486 1992 3319 1969 3717 1981 3739 1993 3454 1970 3877 1982 3796 1994 4031 1971 4516 1983 4631 1995 3809 1972 2878 1984 4453 1996 3813 1973 3823 1985 3560 1997 2748 1974 3478 1986 4116 1998 3198 1975 3742 1987 2783 1999 2985 1976 3147 1988 4698 2
Figure 1 Map of the mean annual rainfall isohytes in the Central Region of Sarawak. The rainfall is also not well distributed over the year (Figure 2). In the coastal region of Sarawak, during the Northeas t Monsoon, when rainfall may exceed 600 mm/month, the wettest months are December February. During these months rainfall can exceed 300 mm/d (Table 3). During the dry season (March November) the average rainfall is about 200 300mm/month, which still exceeds the rate of evaporation. This is not the case in South Kalimantan, where during the driest months (August and September) evaporation exceeds rainfall. Mukah, Sarawak (mm) 700 600 500 400 300 200 100 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec P Eto 3
Banjarmasin, Kalimantan (mm) 700 600 500 400 300 200 100 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec P ETo Figure 2 Mean monthly rainfall and evaporation in a) Mukah, Central Sarawak and b) Banjarmasin, Central Kalimantan Table 3 Rainfall duration frequency in for two rainfall stations in the Central Region of Sarawak (DID) Duration (days) Rainfall (mm) for various return periods Sibu Airport (1953 1990) Mukah JKR (1934 1990) 5 years 10 years 5 years 10 years 1 156 178 264 307 2 181 202 326 377 3 210 233 381 444 5 253 278 480 563 7 294 326 535 628 14 427 471 702 808 30 624 681 1056 1213 2.1.3 Dry spells Despite the tropical rain climate, the peat swamps suffer from water shortage during prolonged dry period. In Sarawak, these dry periods that last for two weeks and have negligible rainfall (<10 mm/fortnight) occur at least once or twice every year. The average four-week minimum rainfall varies from 50 to 100 mm. This amount is often less than the evapotranspiration, which is around 3 mm/day (or 84 mm every four weeks). In South Kalimantan, the average dry season (monthly rainfall < 100 mm) can last for 3 to 4 months (Figure 3). During this period the rainfall deficit is around 100 mm. In extreme dry years (probability of exceedance 10%) this period can be extended to seven months. Without water conservation, evaporation can lead to slight but persistent moisture deficits and so to increased oxidation. In very dry years, the water table can fall below 1 m below soil surface. 4
3 Topography The peat swamps are bordered by the sea and by rivers, and have a dome-shaped surface (Figure 3). On the seaward side of the swamps, the borders consist of mudflats or sandy beach deposits. On the landward side, there are sometimes very narrow levees or no levees at all. Along the rivers, levees of mineral soils form the boundaries. These levees are prone to flooding. Figure 3 Topography of a peat dome (Melling 2000). 3.1 Surface topography The youngest peat swamps are found close to the coast. The ground surface of the young swamps rises gently from the edges to form a convex shape (dome) with slopes of 1 2 m/km. The highest point may be only 3 4 m above mean sea level. In the older, more developed swamps, the convexity at the edges is more pronounced. A rise of 6 m over the first 250 m has been recorded. The central bog planes are almost flat, with a rise of less than 0.5 m/km (Tie, 1991). 5
3.2 Mineral subsoil The basal mineral substrata of the peat swamps consist of sand or clay. The topography of the subsoil usually drops gently from the riverbanks or the coast to the centre of the swamps. This is what gives the peat deposits their characteristic lens-shaped cross-section. Where old riverbeds or levees are buried under the peat, there are small rises and drops in the mineral substrata. Levelling of transects across various peat domes has shown that the mineral substrata usually lie within 1 2 m above or below mean sea level (Figure 4). The mineral substratum under peat soils is often sulphidic. When this is the case, the peat layer acts as a protective wet sponge that keeps the underlying mineral subsoil in a wet, anaerobic condition. Once the peat has disappeared, however, the mineral subsoil will surface, available pyrite will oxidise, and acid sulphate soils with very low ph values will form. Acid sulphate soils are problem soils that can be farmed only under conditions of wellcontrolled water management. Box 2 presents the feasibility of farming on acid sulphate soils at different locations in the landscape. 5 4 South China Sea Level (m) 3 2 1 Drain Stream PEAT Streams PEAT Batang Balingian 0 MINERAL SUBSOIL 0 1000 2000 3000 4000 5000 6000 7000 8000-1 -2 Chainage (m) Figure 4. Cross-section through a peat dome (PS Konsultant 1998) 3.3 Catchments The dome-shaped surface of the peat swamps causes rainwater to drain off to different sides. This phenomenon divides a peat swamp into several catchments (Figure 5). A catchment is the area from which a stream collects water. Water divides form the boundaries of a catchment. Contrary to upland catchments, peat swamps have minimal topographic gradients, which makes it difficult to distinguish the catchment boundaries. To establish catchment boundaries, it is best to combine data from survey lines with information on land use, vegetation, and drainage patterns. Aerial photographs, maps, and satellite images (e.g. LANDSAT images) can provide valuable information. Because catchments in lowland swamps have a low relief and are often inter-linked, the catchment boundaries are not fixed. Under natural circumstances these boundaries can vary seasonally, due to extreme rainfall, drought, or tidal effects. 6
K u a l a M a t u Jemoreng c atchment 5 m B a t a n g M atu Matu 2 m 3 m 4 m 5 m 6m 7 m N S e k aan c a t c h m e n t 0 5 K i l o m e t r e s Figure 5 The catchment of a peat swamp (adapted from SWRC,1997). The dotted lines are the catchment boundaries. Also visible are contour lines of the Jemoreng catchment. Certain types of land use (e.g. intensive farming and logging of forests) influence the groundwater table and the boundaries of the catchments. The groundwater table in a drained area influences the water table in an adjacent non-drained area. Activities that have long-term impacts are: S Digging of transportation canals that connect streams or rivers S Construction of drains S Pumping of groundwater S Building of roads through swamp lands S Drainage for agriculture. As drained areas influence water tables in contiguous undrained areas, so, too, do catchment areas influence each other. This influence can cause the hydrology of lowland peat swamps to behave in uncontrolled or unexpected manner. Activities or projects that take place in one catchment can influence activities in another and lead to conflicts of interest. It is easy to imagine the conflicting interests that would occur if, for instance, an agricultural project were adjacent to a water-supply area. The drainage in the agricultural project would lower the water table in the water-supply area, reducing the volume of water available for domestic and industrial use. 3.2.3 Drainage Base Tidal ranges along the Borneo coast may vary from about 2 m to almost 6 m. The influence of the tide is not restricted to the coastal area. It can move up rivers, reaching as far as 200 km inland. Most of the peat lands are located in areas under tidal influence. 7
3.2.4 Drainability Drainability refers to drainage by gravity, thus drainage without the aid of mechanical devices such as pumps. Different classifications are used in Indonesia and Malaysia. In Sarawak, the long-term drainability is assessed on the basis of the mineral subsoil level, rather than the present (peat) ground surface (Box 1). The drainage base is defined as the water level in the adjacent river or stream, below which natural drainage by gravity cannot be achieved (conveyance losses add an additional hydraulic head of at least 20cm per kilometre). In Indonesia, four land categories are commonly distinguished in tidal swamp areas. The classification is mainly based on the potential for irrigation, but they have be adjusted to incorporate the drainage potential (Box 2). Gravity drainage Gravity drainage of an agricultural scheme in peat lands is possible when the water level inside the scheme is higher than the (outside) water level of the river. In the lower-lying areas, drainage may be possible only during low tides. In such a case, control structures and bunds are needed to prevent water entering the scheme area during high tide. Adequate storage should be available inside the scheme, so that it is possible to keep the excess water until it can be discharged during low tide. Box 1 Drainability classes (Agrosol, 1997) Figure 6 The drainability concept. Drainability refers to drainage by gravity, thus drainage without the aid of mechanical devices such as pumps. Long-term drainability is assessed on the basis of the mineral subsoil level, rather than the present (peat) ground surface. The drainage base is defined as the water level in the adjacent river or stream, below which natural drainage by gravity cannot be achieved (conveyance losses add an additional hydraulic head of at least 20cm per kilometer). To assess drainability, the following classification is used: 8
Very good: Good: Moderate: Poor: Mineral subsoil surface is above the drainage base established at High Water Level (HWL); therefore natural drainage can be achieved at all tide levels, including high tides. Mineral subsoil surface is between drainage base established at HWL and Mean Water Level (MWL). Mineral subsoil surface is between drainage base established at MWL and Low Water Level (LWL). Mineral subsoil surface is below drainage base established at LWL; therefore natural drainage cannot be achieved at any tides, even low tides. Note: This classification differs slightly between the various studies (see e.g. Alan Tan and Lim Hiok Hwa, 1999). Box 2 Tidal land Classification in Indonesia (AARD & LAWOO 1992) elevation of the water level class A class B class C class D land average river water level high tide mean sea level low tide river mouth Figure 7 distance from river mouth Tidal land classification is based on the water levels in the main rivers. Type A Type B TypeC TypeD Areas between mean low tide and mean neap tide. Daily flooding and drainage. These areas appear close to the sea. The potential for irrigation and drainage is good because there is sufficient tidal fluctuation to allow a daily flooding and drainage of the soil surface Areas between mean neap tide and mean spring tide. Springtide flooding and daily drainage. Irrigation is only possible during springtide. Absence of daily flooding requires water conservation measures. Daily drainage is always possible. Area above spring tide. No tidal flooding, permanent drainage. In these areas drainage of excess rainfall is always possible due to absence of high water levels in the canals. Area outside the influence of daily tide. No tidal flooding, limited drainage. Due to the absence of a drainage infrastructure, this area can not be drained. The water table drops during the dry season when evaporation exceeds rainfall. 9
The assumption is that a peat swamp can be economically drained on a long-term basis only if the mineral subsoil level is above the mean water level in a nearby stream, river, or sea, into which the drainage water will be discharged. At present, also areas are being developed where the mineral subsoil is below mean sea level. Gravity drainage is possible in these peat swamps because of their dome-shaped morphology. Uncontrolled drainage and the subsequent excessive subsidence could put an end to this quickly: it may take 50 to 750 years before the overlaying peat soil has disappeared. Pumped drainage Currently, pumped drainage of peat swamps is not an option, and the drainability of areas that lie below mean water level is classified as moderate or poor. In Mukah, a design rainstorm with a return period of five years may produce 480 mm of rain in 5 days. If we neglect the storage within the area, 11 l/sec/ha will have to be evacuated in this five-day period. Pumped drainage might not be an option at present, but it might be one in future. Then, and on a limited scale, it might be more economical for crops with a very high rate of return (e.g. horticultural crops). The alternative is to give the land back to nature. 3.2.5 Water Management Peat under natural conditions is waterlogged for most time of the year. Using peat land for agriculture requires a water management system that will lower the water table and guarantee a timely removal of excess rainfall. Peat, however, is a precious resource that should be handled with care to prolong its life. To avoid excessive subsidence, and to reduce water stress in dry periods, the level of the water table has to be controlled (Figure 68. Figure 8 Peat land requires a water management system that controls the water table. The functions of the water management system are somewhat conflicting: on one hand there is the removal of excess water, which requires unrestricted outflow conditions, and on the other hand is the control of the water table and water conservation, which can be achieved only by restricting the outflow. To fulfil these requirements, two conditions have to be considered: the normal-water-level criterion for normal (= average) conditions and the highwater-level criterion for short periods of extreme rainfall: 10
S S The normal-water-level criterion refers to maintaining the water level at a level that is deep enough to enhance the agricultural use of the land but, at the same time, shallow enough to sustain the peat; The high-water-level criterion refers to removing excess rainfall during extreme events. The design of the water management system should also be based on the specific soil hydraulic characteristics of peat (i.e. on the very high infiltration rate, storage capacity and permeability). Because of these unique characteristics, excess rainfall will not be removed as surface runoff but mainly as groundwater runoff. For conditions in Borneo, with its humid climate and prolonged periods of rather uniform rainfall, the steady-state approach (e.g. the Hooghoudt Equation) can be used to calculate drain spacing. The simplicity and the limited requirement of input data make it very suitable. The spacing of the drains should be based on the drainage requirements during normal-water-level conditions and the dimensions of the drains on the high-water-level conditions. The design water level in the water management system will depend on the seasons. During the monsoon season a lower level will have to be maintained to increase the discharge capacity and during the dry season a higher level will have to be maintained to conserve water. Consequently, structures are needed to control the water levels in the system. Because peat is so highly permeable, we recommend a cascade of closely spaced structures with small differences in head. The dynamic storage capacity in the drainage system is small compared to the recharge by excess rainfall and the corresponding discharge. Therefore it is possible to use the steady-state approach for the design. The above considerations result in a water management system with narrowly spaced drains in combination with an intensive network of control structures. As a consequence of maintaining high water levels the percentage of the area occupied by the water management system will be high: between 15 and 20% compared to less than 5% in mineral soil areas. The layout of the water management system should make use of the dome-shaped topography of the peat lands. Field drains should be located parallel to the contour lines and collector drains perpendicular to these. Water storage is needed to replenish the groundwater during prolonged dry periods. The best place to store water is in the centre of the peat dome. To minimise the effects of rapid initial subsidence in the first few years after reclamation, we recommend a two-phase approach to implementing the water management system. In the first phase, the area is opened and the main drainage system is installed. In the second phase, the field drainage system is installed. A time delay of at least 1 to 2 years is recommended between the two phases. We must remember, however, that the continuous subsidence of the peat will cause the land s surface to fall and make it necessary to upgrade the system at regular intervals, probably every 5 to 10 years. Continuous subsidence restricts the future drainability. In most peat swamps, the subsidence will ultimately lead to a shift from gravity drainage to pumped drainage or, if this is not feasible, to a return to nature. 11
4 The water balance in a peat swamp The lowland peat swamps of Borneo are completely rain-fed. No flow from upland areas enters these swamps. The rainfall either evaporates or is transported from the swamps as near surface run-off, inter-flow, or groundwater flow. The general water balance of a peat swamp can be written as follows: Where: P E Q S P = E + Q + S total rainfall (m) total evapotranspiration (m) total discharge (m) change in storage (m) Under natural conditions, the groundwater table will rise due to rainfall and fall due to evapotranspiration and the outflow of excess rainfall by surface flow, groundwater flow, or interflow. The resulting change in storage can be considerable over short periods (i.e. days or weeks). Over the years, however, this change in storage will be negligible compared to the total in- and outflow. 4.1 Water levels The fluctuation of the water level in a peat swamp depends mainly on rainfall because evaporation and (groundwater) outflow are fairly constant. During the wet season (November February), the rainfall always exceeds the combination of evaporation (Figure 2) and groundwater run-off. Thus, in this period, the water level is always above the soil s surface (Figure 9). These wet conditions are favourable for peat accumulation. During the drier months of the year, when dry spells can last for weeks, the water level in the swamp can drop below the soil surface. Observations in different swamps have shown that the drop of the water table is not the same throughout the whole swamp. Between the dry and wet seasons, the water table in a peat swamp can fluctuate up to 0.58 m near the edge of a peat dome (Tie, 1991). In the centre, the seasonal fluctuation is slightly smaller (0.45 m). The relatively steep periphery has a deeper water table than the flat centre. Under natural conditions, fluctuation of the water table will be as follows (Ong and Yogeswaran, 1991): On hot and non-rainy days, surface water may drop 10 15 mm daily. When initial water levels are below soil surface, the drop is 5 10 mm. Water levels drop fastest between 0900 hrs and 1700 hrs The maximum drop of the water table depends on the length of the dry spell. The maximum depth of the water table varies for different swamps from 0.3 1.0 m below soil surface. 12
Figure 9 Example of the fluctuation of the water level in a peat swamp in Penibong, Central Sarawak in 1991 (Ong and Yogeswaran, 1991). 4.2 Discharges / Run-off Under natural, undrained conditions, there are three types of outflow from the peat body of a swamp: Surface run-off or depression flow; Sub-surface flow or interflow, and; Deep groundwater flow. Because of the predominantly high water levels in a peat swamp, surface flow will account for most of the natural outflow (Table 4). In the study, conducted in the Kut Catchment in 13
Central Sarawak, groundwater flow formed only a small component of the water balance, but it was still about 170 mm/year. Interflow, which is defined as the flow that takes place through an upper soil layer of higher permeability, was about 340 mm/year. Because the responses of the swamp were measured in an artificial drain, the results of the study may not be fully representative of a natural situation. Table 4 Water balance of Kpg. Kut catchment, Pulau Bruit (June 1988 June 1989) (SWRS, 1997) P (mm/y) E (mm/y) Q (mm/y) 2789 1248 1541 Q surface Q inter Q ground As a percentage of Q: 67% 22% 11% As a percentage of P: 37% 12% 6% 4.3 Storage Capacity In peat, rainfall can easily infiltrate into the soil. The storage capacity of peat works as a buffer during times of heavy rainfall; the deeper the water table, the larger the storage capacity. Although tropical peat has a high drainage pore space (in Sarawak it varies between 0.3 and 0.85), the water storage capacity in the peat is relatively small, because under natural, undrained, conditions, the water table fluctuates around the soil surfaces. Even in dry periods, when the water table can drop to 0.6 m below soil surface, the storage capacity in the soil will only be in the same order of magnitude as rainstorm with a 1-year return period. 14
5 Gaps in knowledge 5.1 Climate changes: 5.1.1 Rainfall: total rainfall and distribution 5.1.2 Dry periods: frequency and duration 5.2 Hydrology 5.2.1 Run-off under natural conditions: percentage Surface run-off or depression flow, sub-surface flow or interflow, and deep groundwater flow. 5.2.1 Water storage capacity of reclaimed peat soils 5.2.2 Relation between water table and peat accumulation or oxidation 5.2.3 Soil hydrologic characteristics of the deep peat: hydraulic conductivity, saturated thickness, transmissivity, drainable pore space and storativity. 5.2.4 Catchment changes after reclamation 5.2.5 Effect of buffer zoning 5.3 Water Management 5.3.1 Water level control: maintaining a high water table. 5.3.2 Water storage in the canal system/ removal of excess rainfall 5.3.3 Effect of subsidence on the canal system 5.3.4 Operation and maintenance of the water management system 15
References Agrosol. 1997. Soil/Peat Drainability and Oil Palm Feasibility Studies of Balingian Rural Growth Centre. Alan Tan and Lim Hiok Hwa 1999. 1999. Peat hydrology and water management. In: Proceedings Workshop on Working Towards Integrated Peatland Management for Sustainable Development, 17-18 August 99, Kuching, 11 pp. Department of Irrigation and Drainage & LAWOO, 1996. Western Johore Integrated Agricultural Development Project, peat soil management study. Kuala Lumpur, Malaysia and Wageningen, The Netherlands. Department of Irrigation and Drainage. 1962/97. Hydrological Yearbooks: edition 1962-1997. Department of Irrigation and Drainage, Sarwak. Department of Irrigation and Drainage. 2001. Water Management Guidelines for Agricultural Development in Lowland Peat Swamps of Sarawak. Department of Irrigation and Drainage, Sarawak. Kselik, R.A.L., K.W. Smilde, H.P. Ritzema, Kasdi Subagyono, S. Saragih, Mauliana Damanik and H. Suwardjo. 1993. Integrated research on water management, soil fertility and cropping systems on acid sulphate soils in South Kalimantan, Indonesia. In: D.L. Dent and M.E.F. van Mensvoort (eds.), Selected papers of the Ho Chi Minh City Symposium on acid sulphate soils. ILRI Publication 53: pp. 177-194. Melling, L. 2000. Dalat and Mukah sago plantation peat soil study. Final Report. Soil Branch, Department of Agriculture Sarawak. Ong,B.Y. and Yogeswaran,M., 1991. Peatland as a resource for water supply in Sarawak. Proceedings of the International Symposium on Tropical Peatland. Kuching, Sarawak. PS Konsultant 1998. Detailed Design and Construction Supervision of Flood Protection and Drainage Facilities for Balingian RGC Agricultural Development Project, Sibu Division, Sarawak (Inception Report), Department of Irrigation and Drainage, Kuching. pp.24. Ritzema, H.P., Mutalib Mat Hassan, A. and Moens, R.P. 1998. A New Approach to Water management of Tropical Peatlands: A Case Study from Malaysia. Irrigation and Drainage Systems 12 (1998) 2, p.123-139. Ritzema, H.P. and T.P. Tuong. 1994. Water-management strategies as a tool for the sustainable use of acid sulphate soils. Regional Workshop on the sustainable Use of Coastal Land in South-east Asia, 18-22 April 1994, Asian Institute of Technology, Bangkok, 34 p. 16
Sarawak Water Resources Council,1997. Sarawak Water Resources Study Projects - Final Report submitted by PS Konsultant, Montgomery Watson, Australia, The Centre for Water Reseach (CWR). Tie, Y.L. and Esterle, J.S., 1991. Formation of lowland peat domes in Sarawak, Malaysia. Proceedings of the International Symposium on Tropical Peatland. Kuching, Sarawak. 17