Hydro-ecology: Linking Hydrology- and Aquatic Ecology (Proceedings of Workshop i ÎW2 held at Birmingham. UK, July 1999). IAHS Publ. no. 266. 2001. 101

Streamflow regime change and ecological response in the Lake in

LEKAN OYEBANDE Department of Geography, University of Lagos, PO Box 160, Akoka, Lagos, Nigeria e-mail: [email protected]

Abstract Three major factors account for the streamflow regime change in the Komadugu- system. The combined effects of the Sahelian drought and the partial conversion of flood flows to dry-season releases by reservoir has decreased the coirtribution from the Hadejia to the Komadugu- Yobe River since 1974. Blockages by weed growth and siltation in the Hadejia have also jointly contributed to the decline in river flow. The change in the flood regime has directly impacted certain components of the Yobe basin ecosystems including plant and fish species diversity and quality. A certain minimum flood extent is required in the Jama'are and Hadejia rivers to sustain their rich wetland ecosystems at reasonable level, particularly areas of high biodiversity such as the Dagona water fowl sanctuary, the Gorgoram forest reserve and the fish resources.

Key words Remote sensing and G1S; flooding; drought; ecology; wetland; birds; dams; Sahel; siltation; degradation

CLIMATE AND HYDROLOGY

The part of the Lake Chad basin within Nigeria is drained by five river systems. The largest of them, the Komadugu-Yobe river system is the focus of this paper. The Komadugu-Yobe river system drains an area of 147 840 km2 (85 470 km2 within Nigeria), before reaching Lake Chad. The Komadugu-Yobe River in turn comprises three main rivers: the Hadejia, Jama'are, and the Misau. The Hadejia is formed by the confluence of the Challawa and rivers about 20 km south of Kano (Fig. 1). For the first 48 km of its course the maintains a gradient of approximately 1 m km"1. As it descends from the crystalline basement complex and enters the ancient lacustrine basin of Lake Chad however, this gradient reduces abruptly and the channel becomes poorly defined and characterized by numerous small oxbow lakes. The Jama'are River, like the Kano, has its headwaters in the where it begins with a relatively high gradient, cutting through volcanic and metamorphic rock terrains before entering the lacustrine basin of Lake Chad northeast of Foggo. The Misau River rises north of Bauchi and flows northeast to join the Komadugu-Yobe River about 128 km from Lake Chad near the town of Damasak. Most of the headwaters have high sediment carrying capacities, hence the deposition of their load of silt and fine sand ftirther downstream, and the resulting aggraded valleys of poorly-defined channels with numerous small oxbow lakes. Annual rainfall varies from less than 400 mm in the north to over 1200 mm in the south, and the length of rainy season from 3'/2 to 6 months. Four rainfall zones have been identified (Northeast Arid Zone Development Programme, 1995). Zones (3) and Fig. 1 The Yobe basin. Streamflow regime change and ecological response in the Lake Chad basin in Nigeria 103

(4), the two northernmost zones do not experience overall water surplus anytime during the year. These two zones receive 400-600 mm and less than 400 mm of rainfall annually respectively whereas potential evaporation can range from 3000 to more than 4000 mm annually. The result is that high evaporation and low rainfall preclude surface streams except flashy flows generated during intense falls of rain. The northernmost zone (Zone 4) is the driest, real Sahelian zone typified by conditions in Damasak, Gashua, Geidam and Kukawa. The water balance is such that soil moisture deficits occur from September to July and soil moisture recharge only occurs during August and September. Surface streams are rare while those that flow from humid zones have their water depleted by evaporation and infiltration. The coefficient of runoff ranges from 13 to 22% and approximately 80%o of the runoff occurs from July to September with peaks occurring in late August. About 70%o of the total annual flow is lost downstream as far as Gashua, and another 10% between Gashua and Geidam. Water losses among the streams of the Yobe system are due very largely to évapotranspiration in poorly drained flood plains and also to a small but significant portion of surface runoff, which infiltrates to groundwater storage. Such infiltration, which serves to recharge the upper zone aquifer of Lake Chad (the Lake Chad basin has three regional aquifers: the upper, middle, and lower zone aquifers) is possible in the area west of Gashua (Table 1). Alkali & Carter (1996) estimated the recharge of the upper zone aquifer in three sections Gashua-Geidam-Damasak-Yau (a distance of 286 km) as 16.98 106 m3 year"1. Inflow from Lake Chad adds 0.73 106 m3 year"1. In analyses of water losses from the Komadugu-Yobe River between Gashua and Geidam, the possibility exists that floodwaters from the Komadugu-Yobe may cross over the flood plain to drain through the Misau River or vice versa (Diyam, 1996; Oyebande, 1997). Similar losses occur along the Misau River. The Hadejia splits into three channels between Hadejia and Likori (downstream of the Madachi Swamps and near the main road). The three channels are the old Hadejia

Table 1 Annual river flow and groundwater recharge. (a) River flow balance in the Hadejia-Nguru Wetlands Year 1975 1977 1979 1981 1983 1984 1985 1987 1989 1991 1993 1995 Outflow(106 1139 907 1016 993 699 381 931 682 1208 1425 1004 <115? m3) Outflow as % of 37 35 34 39 52 48 53 44 49 52 52 <5? inflow (b) Groundwater recharge between Hadejia and Nguru, 1991-1995 along the Hadejia River Changes in groundwater level 1991 1992 1993 1994 1995 Observed rise in groundwater level (average, cm) - 198 200 255 166 Groundwater level rise due to rain (cm) 21 71 123 39 Groundwater level rise due to river bed recharge 129 132 127 (cm)

(c) Percentage of flow contributed by the Jama'are, Hadejia and Kafin Hausa rivers River 1991 1992 1993 1994 1995 1991- 1995 Jama'are 48 48 45 50 39 Averaj je = 46 Hadejia 47 48 51 45 53 Averaj ?e = 49 Kafin Hausa 5 4 4 5 8 Averaj ze = 5 Source: Hadejia-Nguru Wetlands Conservation Project (1996), Goes & Zabadum (1996). 104 Lekan Oyebande which leads to Gashua to form together with the Jama'are the Yobe River; the Marma channel which flows into the non-returning Nguru Lake; and the relatively small Burum Gana River (Fig. 1). An analysis of four years of pre-Tiga records (1964-1967) in the Yobe river system indicates that an average of more than 68% of measured runoff was lost from the Yobe system upstream of Gashua. Only 18% of total runoff reached Geidam (Oyebande & Nwa, 1980). Along the Misau River the average flow lost between Kari and Dapchi was 68% of the flow at Kari. Based on this analysis it is estimated that less than 10% of the total surface runoff from the Yobe river system reached Lake Chad even in the pre-Tiga era.

STREAMFLOW REGIME CHANGES AND THEIR CAUSES

Figure 2 shows the annual runoff of Hadejia at Hadejia and Komadugu-Yobe at Gashua between 1963 and 1996. The level of runoff dropped suddenly and significantly in

mean

(b) ! .J July - Nov [__"] Dry season Fig. 2 Annual runoff at Hadejia (1964-1995) and (b) at Gashua (1963-1996). Streamflow regime change and ecological response in the Lake Chad basin in Nigeria 105

1972/73 and remained low until 1975/76. In the case of the runoff at Hadejia, low flow or dry season flow increased at the expense of flood flows after 1976. These changes in streamflow regime coincided with the onset of the sahelian drought and the closure of Tiga Dam, which was filling during 1974-1976. (Oyebande & Nwa, 1980). A basin hydrological model that used monthly water balance data for the period 1964 to 1987 was used to simulate flood extent and groundwater storage in the Yobe Basin (Hollis & Thompson, 1993; Thompson & Hollis, 1995). The model indicated that the recharge of groundwater was lower than évapotranspiration from flooded soils, but higher than the discharge of Gashua (24% of river inflow). Groundwater storage was largely stable during 1964-1971 and 1975-1983, but fell in the early 1970s and especially in the late 1980s by an estimated aggregate of 5000 x 109 m3 as a result of drought and reduced flooding. The situation in the 1990s is indicated in Table 1, which shows that secondary recharge, that is, riverfed recharge, is consistently higher than direct or rainfed recharge.

Causes of change in flow regime

Two major and primary factors account for the streamflow regime change. The first is the prolonged Sahelian drought, which began in the mid-sixties. The droughts of 1971/72 and especially that of 1983 and 1984 devastated many parts of northern Nigeria, in particular the eastern half of the Komadugu-Yobe basin. Annual rainfall data for Kano in 1983 and 1984 amounted to only 60%> and 57% of the long-term average for the respective years (1905-1982). The corresponding values for Nguru station are 44 and 65% of the 1942-1982 average (Oyebande, 1997). The Ngadda, Yedseram and Komadugu Gana rivers did not flow at all, and the Lake Chad level dropped to an all time low, below the minimum of 2.0 m recorded in 1907. The Misau River, which derives from the Komadugu Gana and usually carries flash floods during the months of July-September was completely dry at Kari. It was observed that all the tributaries of Jama'are and Komadugu Gana around the northern parts of also dried up. In the pre-Tiga period, there was a very high concentration of flow in the Hadejia during June-October, accounting for 98-99% of the annual flow. The post- dam period witnessed a reduction to 78-79%> concentration. Correspondingly, the extent of wetlands fed by floods progressively decreased from 2350 km" in 1969 to 700 km2 in 1987, 893 km2 in 1991 (see also Table 2). Analysis by Hollis et al. (1993) has indicated that drought lowered the streamflow at Gashua by up to 23%. 1984 is credited with the minimum peak flow of the Hadejia at Wudil (upstream of Hadejia) and Kafin Hausa, Jama'are at Bunga and Katagun, Yobe at Gashua, while 1973 recorded the minimum in Yobe at Geidam and Damasak. Water resource development projects (dams and their reservoirs and associated irrigation schemes) constitute the second principal driver of streamflow regime change

Table 2 Maximum flood extent between Hadejia and Gashua, 1991-1995 (in km").

Year 1991 1992 1993 1994 1995 Area 893 545 387 T728 967 Source: Goes & Zabadum (1996). 106 Lekan Oyebande in the Komadugu-Yobe basin. Three large structures and many small ones control the Hadejia river system. The first of the three large structures, the Tiga Dam, was completed in 1974 with an active storage capacity of 1400 106 m3. The height was lowered by 20% in 1992 to reduce the risk of instability; this change in height resulted in reduction of the storage capacity by 31%>, and of evaporation at full level by 8% of the inflow. The other two, the (capacity of 114 106 mJ), and the Hadejia barrage with a storage capacity of 12 106 m3, were both completed in 1992. The barrage supplies water to the Hadejia Valley Project. The dams control about 80%) of the total inflow of the Hadejia River. As may be expected, the operation of the reservoirs of the dams has significantly influenced the river flow regime of the Hadejia river system. Since the mid-1970s, the Tiga Dam began to provide dry-season releases that altered the river regime from zero flows during that season (Oyebande & Nwa, 1980) to a perennial regime. Unfortunately, the dry-season releases did not appear to be beneficial to areas and ecology downstream of Gashua (Fig. 2). The hydro-agricultural schemes that threaten the wetland ecosystem were planned in the 1960s and early 1970s using data for the relatively wet period up to 1972. Droughts lowered the flow at Gashua by 23% while the Tiga dam lowered it further by the same magnitude (Hollis et al, 1993; Hollis et al, 1993). The combined effect of Tiga and other planned dams and the Hadejia Pond has been a desiccation of the wetlands and lowering of the water table downstream of Hadejia, in particular. By simple water balance calculations, it can be shown that a decrease of flow upstream of Hadejia due to evaporation from the Tiga reservoir with a rate of 425 106m3 annually causes a reduction of flow in Gashua of 56 x 106 m3 annually. When water use for urban and irrigation activities, e.g. the Kano River Irrigation Project (KRIP), in the middle Hadejia basin (about 22 m3 s"1, ranging from 18 to 25 m3 s"1) is added, the result is a reduction of flow at Gashua of 60 x 106 m3 annually. With all the dams (excluding the uncompleted , which alone could generate an extra evaporation of 425 1 06 m3 or a reduction of 162 1 06mJ in the low at Gashua) operating as designed the total reduction is expected to reach 76 * 106m3 per annum. Prior to the construction of Tiga dam, more than 90% of the river flows were lost before they

Table 3 Result of 1996 dry season test releases from Tiga and Challawa Gorge Dams.

Site Period Average flow % Flow arriving % Volume (nrV) arriving Tiga + Challawa 12 February 22.5 100.0 - Hadejia 12-25 February 4.7 20.9 - Likori 12-29 February 1.4 6.2 - Tiga + Challawa 15 February-3 March 62.5 100.0 100.0 Kafin Hausa 21 March 3.9 6.2 - Hadejia* 12-24 March 39.5 63.2 50.6 Likori 17-27 March 22.1 35.4 24.8 Kasaga 5-15 April 4.0 6.4 7.3 Gaborua** 3-6 April 0.2 0.3 0.1 Tiga + Challawa 14 March-6 June 32.5 100.0 - Hadejia 3 April-15 May 11.0 33.8 - Likori 5 April-15 May 6.6 20.3 - Source: Hadejia-Nguru Wetlands Conservation Project (1996). Streamflow regime change and ecological response in the Lake Chad basin in Nigeria 107 entered Lake Chad. But the post-dam pattern of losses seems to have worsened both between Hadejia and Gashua and from Gashua to Geidam. Gashua was selected because it is an important station located just upstream of where the Hadejia and Jama'are river systems form Komadugu Yobe. Geidam is the next most reliable station to assess the changes along the Yobe. Developments in the Jama'are basin are also planned. Construction work on the Kafin-Zaki dam which has a capacity greater than Tiga dam has been suspended. If the development of the Jama'are and Hadejia river basins goes as planned, then an irrigation water requirement of 1000 x 106 mJ annually will be raised. The surface area of the reservoirs to provide this will be about 800 km2 causing an extra evaporation of 2040 x 106 mJ per annum, and a reduction in flow at Gashua of at least 1275 * 106 m3 per annum. This is equal to the total annual flow at Gashua during an average year (Diyam, 1996). At the Hadejia-Nguru Wetlands, the flood extent has been reduced by the Tiga dam, but the Challawa dam has added to the problem. Thus in the absence of river training activities and big floods to clear the river channels, weeds and silt blockages divert flows on to the flood plains. For instance, these impediments have stopped the flow in the Old Hadejia River (Fig. 1), so that Marma Channel has received more water since the 1970s, and its long-term flow is now 294 1 06 m3 with a coefficient of variation (s/p:) of 50%. According to Diyam (1996), the channel starts to spill into the flood plain when the discharge at Likori (downstream of Madachi) exceeds approximately 17 m3 s"1 (on average from 24 July to 12 October). A flood volume of less than 200 106 mJ results in a very limited flood event (<160 km2 inundated area), but a flood volume of 350 106 m3 would inundate an area greater than 250 km2 (Goes & Zabadum, 1996). It affects the whole area of the Hadejia River from Hadejia through Marma Channel to Nguru Lake (see Table 4). The Burum Gana carries some 16%> of the flow in the Marma Channel. The Kafin Hausa, a distributary of the Hadejia does not start flowing until the discharge of the Hadejia at Hadejia exceeds 450 106 mJ. This level of flow with the corresponding river stage is required to initiate flow in the Kafin Hausa because the river suffers severely from sedimentation which has reduced its peak discharge to a third (~25 m3 s"1). The reach of the Hadejia at Gashua has extensive fadamas (wetlands). The river starts spilling into the flood plain when the discharge at Hadejia exceeds about 9 m3 s"1

Table 4 Number of water-related birds and flood extent in the Hadejia-Nguru Wetlands.

Hydrological unit 1994 1995 1996 1997 Hadejia River n.a. n.a. n.a. n.a. Marma Channel and 45 715 (61%) 120 709 (47%) 61 853 (32%) 202 440 (64%) Nguru Lake Flood (km2) 106 349 334 335 Old Hadejia and n.a. n.a. n.a. n.a. Burum Gana rivers Kafin Hausa 9378 (13%) 49 452(19%) 113 754 (60%) 67 995 (21%) Flood (km2) 17 78 74 135 Total (Birds) 55 093 (100%) 170 161 (100%) 175 607(100%) 270 435 (100%) Flood (km2) 123 427 408 470 Source: Poletera/. (1997). 108 Lekan Oyebande between June and mid-October. The flow in the Hadejia is important. During the period 1991-1995 the Hadejia contributed 49% of the total flow while the Jama'are is credited with 46%. Diyam (1996) found that when the discharge at Hadejia Bridge exceeds 40-50 m3 s"1, the discharge at Likori stops increasing, since most of the water above this level flows into the fadamas. It takes water 4-7 days to travel to Likori (downstream of Madachi) from Hadejia. The weed blockages of the Old Hadejia river in the Hadejia-Nguru Wetlands are severe, and as a result the Hadejia and Burum Gana rivers contribute little to the flow of the Komadugu-Yobe River. Disputes erupted between the downstream riparian states (Borno and Yobe) and six of their most severely affected local government areas. They blamed their plight (lack of adequate water for their needs on their upstream neighbours whom they accused of storing virtually all the water from the Tiga and Challawa dams, and releasing too little for downstream users and uses. A Committee of the main stakeholders was eventually set up in 1992 in search of a consensus approach. A scientific investigation of the issues included the following questions: - Are excessive utilization of water upstream and inadequate release of water by the reservoir operators actually the cause of the reduction in the extent of the wetlands? - If more water were released, will the downstream riparian users get more water? Test releases were conducted in the dry season of 1996 when cross-flows between the main rivers would be absent. The tests were planned jointly by the Hadejia-Nguru Wetlands Conseivation Project and the two basin development authorities in the area (Hadejia Jama'are and Lake Chad). Water releases were from Tiga and Challawa Dams in the scenarios shown in Table 3. The water release flooded most of the flood plains along the Hadejia river system, thus simulating almost perfectly the wet season condition. It also proved that dam outlets and the Hadejia valley project pond are adequate to generate artificial flooding in most of the wetlands. The third important observation from the tests is the steady percentage increase in flow arriving at Hadejia and Likori as the discharge released from the dams increases up to 62.5 mJ s"1. Water loss at Hadejia, at all three release rates, varied slightly between 17.8 and 23 mJ s"1, a quantity that represents water use in Kano City, Kano River Irrigation and other irrigation projects, groundwater recharge and evaporation. Finally, the tests also show that virtually no water from the Hadejia system leaves the Hadejia-Nguru Wetlands due to weed blockages (Typha reed beds) and siltation of the riverbed in the zone of the wetlands. The implication of this for the ecology downstream has been grave and has led to disputes between upstream and downstream communities.

ECOLOGICAL RESPONSE TO STREAMFLOW REGIME CHANGE

Land use and cover

A land-use map has been prepared to depict the distribution of the principal land uses along the Hadejia and Jama'are rivers in the Hadejia-Nguru Wetlands (Yelwa, 1996). Of the total area of 5107 km2, 47.7% is in the Hadejia and the rest in the Jama'are river section. Six land uses were identified. They are upland farms, locally known as Tudu, Streamflow regime change and ecological response in the Lake Chad basin in Nigeria 109 rainfed farmland planted with millet and sorghum. The second is flooded rice farms (fadamas), planted mainly with rice during the wet phase and with cassava and especially leguminous crops like groundnuts, cowpea, okra, potato, tomato and roselle during the dry phase. This is known as the residual (soil) moisture farming. The third consists of irrigated farms, planted mainly with wheat and vegetables. Next is the tree and shrub vegetation, the climax vegetation of the Sudano-sahel zone whose dominant species are Acacia seyal, A. arabica, A. albida, Ziziphus jujuba, Tamarindus indica, Adinsonia digitata and doum palm. The other two types are open water, comprising permanent surface water in river channels, ox-bow lakes and lakes; and rangelands dominated by grasses such as Andropogon gayanus and Vetiveria nigritana, predominantly used for grazing cattle. The proportions of the total area attributable to each of the six land uses are 35, 14, 2, 25, 2.5 and 21.5% respectively. Remote sensing and GIS based studies on land-use change between 1965 and 1995 conducted by the University of Lagos in northwestern Nigeria—the Sahel zone in the Sokoto-Rima basin have shown significant degradation of the river networks and vegeta­ tion (Omojola, 1998; Soneye, 2000). Some losses of plant species had also occurred. The causes are attributed to more intense cultivation of the fadama and even part of the upland areas and the desiccation resulting from the accumulation of water deficits over the decades of droughts (Omojola, 1998; Soneye, 2000). The land-use survey of 1996 (Yelwa, 1996) in the Yobe basin is expected to reflect similar changes that had taken place when compared with any land-use survey undertaken some decades earlier.

Reed beds and siltation

The evolution of the weeds has been associated with the two consecutive years of low runoff driven by the drought of 1972/73 and followed in 1974 by the closure of the Tiga dam and subsequent development of more large dam reservoirs upstream. The resulting flow reduction aggravated the rich siltation materials deposited in the aggrading flood plain. For example, peak reduction between two periods (pre-Tiga, 1964-1973 and post-Tiga, 1974-1989) have been calculated for indicated stations as follows: Hadejia at Wudil (64%), Hadejia at Hadejia (34%), Jama'are at Bunga (27%), and Yobe at Gashua (21%>). The flow reduction, especially the depth of flow, promoted rapid decline of the Hadejia River through weed growth and further silting. It is known that these reeds germinate in waterlogged soils with a thin layer (<30 cm and sometimes as little as 3 cm) of water and the seedlings only grow under these near-dry conditions. The reeds have continued to extend their coverage in the Hadejia river system. Apart from hampering river flow, the Typha reed beds are also known to serve as breeding and roosting grounds for Quela birds. The birds are pests to farmers because they eat seeds of crops, particularly millet and sorghum.

Fish degradation

Fish ecology and the fishing industry have been hard hit by the changes in the river regime and the flooding cycle. As a general rule, the catch of fish from African flood- 110 Lekan Oyebande plain fisheries is proportional to the area of inundation, and yields of 40-60 kg ha"1 of flooded area are expected from most waters. Studies of the relationship between fish yield and flooded area in these wetlands has been represented by the equation:

C = 4.23,41005 (1) where C = catch in tonnes and A = area in km2 (Thomas, 1996). Thus as the area of wetlands declines (Table 2), it results in a virtually proportional decline in the yield of fish from the wetlands. Also in the dry season as the flood recedes, fish become confined to smaller areas and eventually to pools and lakes. During this phase, fish begin to move from the fadamas back into the river channels Large populations of migrating fish are usually encountered during the period. Fishermen reported catching up to 450 kg of fish, mainly Alestes (Thomas et al, 1993), in one of the villages every day for a five-day period when this species was migrating (Hollis et al, 1993.). On the other hand, during the wet season, fish move out of the river channels into the fadamas to breed. It has been observed that the quality of fish in ox-bow lakes has declined in the last 20 years due to siltation that has made the lakes too shallow. In addition perhaps more than five species are no longer found in different parts of the flood plain. The decline in fish species diversity is blamed on reduced flooding and changes in its regime such as extent or area, duration and depth. High fishing intensity leading to over-fishing could also be a factor. In general, a minimum productive depth of 30 cm is required, and this must be maintained long enough to allow adult fish to feed and reproduce and for the fry to develop. Fish species such as Alestes and Shilbe spp. whose pattern of migration and spawning is triggered by the rising flood are more severely affected by the change in the flood cycle, much more than Claris and Tilapia, for example (Drijver & van Wetten, 1992). A comparison of flooding extents and fish catches in 1992, 1993, 1994 and 1996 (the last two years being regarded as good years) together with the information obtained from a questionnaire of the fishermen's perception of the flood impact led to the conclusion that the minimum annual flooding extent required to sustain the fish ecosystem and fishing industry is 800 km".

Birds

The open water bodies (land-use type 5) of the Hadejia-Nguru Wetlands are known internationally for their large number of waterfowl (Polet et al, 1997), while the mudflats host large numbers of waders. Altogether the open-water bodies contain over 300 species of water-related birds, largely made up of palaeo arctic migrants, but also including Afro-tropical migrants as well as resident species (Aminu-Kano, 1994). Table 4 shows the number of birds enumerated in recent years in the various sections of the wetlands. The number of birds and the extent of the flood are highly correlated. The relationship is however not exact because in addition to flood extent, flow regime characteristics such as frequency, depth of water and duration of specified flows are important. It is clear from the table that poor flooding results in low number of waterfowl. Streamflow regime change and ecological response in the Lake Chad basin in Nigeria 111

CONCLUSION

The effect of decreased flooding on the ecology of traditional agricultural production, water-related birds and fisheries as well as groundwater recharge has been significant if not severe. It is concluded that peak flows that ensure a minimum inundation of 800 km2 annually (including at least 300 km2 in the Marma Channel and Nguru Lake) is required to sustain the wetlands' rich ecology. There is need for further research into ways of accurately quantifying the water requirements of certain components of the Yobe basin ecosystems.

REFERENCES

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