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Restoring the perennial Great Ruaha River using ecohydrology, engineering and governance methods in Tanzania
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Contents lists available at ScienceDirect
Ecohydrology & Hydrobiology journal homepage: www.elsevier.com/locate/ecohyd
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Original Research Article
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Restoring the perennial Great Ruaha River, Tanzania, using Q1 ecohydrology, engineering and Governance methods
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Q2 Emilian
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Kihwele a,*, Epaphras Muse b, Evance Magomba a, Bakari Mnaya c, Ahamed Nassoro b, Paul Banga b, Edimund Murashani b, Daniel Irmamasita b, Halima Kiwango b, Charon Birkett d, Eric Wolanski e
Q3 a Serengeti National Park, TANAPA, Tanzania b
Ruaha National Park, TANAPA, Tanzania TANAPA, PO Box 3134 Arusha, Tanzania ESSIC, University of Maryland, 5825 University Research Court, College Park, MD 20740, USA Q4 e Australian Centre for Tropical Freshwater Research, and Australian Institute of Marine Science, Townsville, QLD 4811, Australia c
d
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 February 2017 Received in revised form 17 October 2017 Accepted 20 October 2017 Available online xxx
The Great Ruaha River (GRR) in Tanzania was perennial before 1993. Its source, the Usangu wetlands, was also perennial. Since then, the GRR has started drying out during the dry season, with a trend towards earlier and longer periods of drying. This drying process degrades the surrounding ecosystems along the entire length of the GRR, including the Ruaha National Park (RNP) and impacts human livelihoods throughout its course; it also impairs the economy of Tanzania through reduced hydropower generation at the Mtera and Kidatu power plants. The Usangu wetlands dried up in 2000, 2002 and 2005 during the dry season and its areal extent has been shrinking. Intensive livestock grazing and both dry and wet season irrigated agriculture in the Usangu wetlands, were the main reasons for this water crisis. In 2006, the Government of Tanzania moved to address the crisis by removing livestock from the Usangu wetlands, attempting to regulate water use in the GRR catchment and expanding the RNP to include the Usangu wetlands. The present study uses satellite radar altimetry data dated 1992 to 2015 for the Usangu wetlands, verified using data from water level loggers, to assess the effects of these hydrological changes. The combined data sets show that the perennial Usangu wetlands have become re-established, though somewhat diminished in size, and that as a result of eutrophication from fertilizers leached from irrigated fields, 95% of their surface is now covered by floating vegetation, resulting in reduced loss of water by evaporation. This reduction has extended the water flow to the GRR by one to two months. These figures highlight the important role played by the wetlands vegetation in controlling the water budget. The hydrological model of the Usangu wetlands was further developed and verified, and then used to justify the construction of a low-level V-notch weir at the outlet of the Usangu wetlands; by slowing down the outflow from the wetlands, this construction would reduce the period of zero flow in the GRR during the dry season by one to two months. Additional ecohydrological solutions based on the improvement of water governance upstream of the
Keywords: Restoration Wetlands Irrigation Water levels Ecosystems Satellite radar altimetry
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Q5 * Corresponding author.
E-mail address: EmilianKihwelea*[email protected]">EmilianKihwelea*[email protected] ().
https://doi.org/10.1016/j.ecohyd.2017.10.008 1642-3593/ß 2017 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier Sp. z o.o. All rights reserved.
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Usangu wetlands, such as controlling rice and horticultural irrigation activities along the length of the GRR, should be integrated with this engineering solution; this would meet the hydrological needs of the water users all along the GRR. ß 2017 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier Sp. z o.o. All rights reserved.
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1. Introduction In Africa and throughout the world, humans have modified landscapes and freshwater habitats, with subsequent effects on hydrology, and ecosystem function and services for both wildlife and people (MEA, 2005; Birol et al., 2006; Elisa et al., 2010; McCartney and King, 2011). This is critically important as changes in the quality and quantity of freshwater impact on the ecological health of ecosystems (Gereta et al., 2002; SMUWC, 2002; Yawson et al., 2004; Wolanski et al., 2004; Mtahiko et al., 2006; Gereta and Wolanski, 2008; Gereta et al., 2009; WWF, Q6 2010; Wolanski and Gereta, 2001; Mnaya et al., 2011; McClain et al., 2013; Nikghalb et al., 2016). Freshwater is increasingly in short supply in Tanzania, partly as a result of water mismanagement and partly due to the rapid growth of the human population, livestock numbers and irrigated agriculture (de Villiers, 2000; Postel and Richter, 2003; Crisman et al., 2003; Mtahiko et al., 2006; Elisa et al., 2010; Mnaya et al., 2011; Kihwele et al., 2012). This shortage of water is of both ecological and economical concern to hydro-electricity producers, artisanal farmers and fishermen, together with other downstream water users, because generally the water governance system gives them no control over the decisions made by the water users upstream. This crisis has a destructive influence on the wildlife ecosystems of the Ruaha National Park (RNP) (Fig. 1) and the Katavi National Park, as these ecosystems respectively depend on the formerly perennial Great Ruaha and Katuma Rivers, which have now become seasonal as a result of non-managed irrigated agriculture and livestock grazing in the river catchments upstream of the protected areas (Fig. 1; Mtahiko et al., 2006; Elisa et al., 2010; Kihwele et al., 2012; McClain et al., 2013). While upstream users benefit from free use of the river water, the ecosystems and livelihoods downstream are suffering. If left unregulated and without ecohydrological intervention, it seems likely that these ecosystems will collapse, resulting in serious socio-economic and ecological consequences. Similarly, the Serengeti National Park is threatened by the increasing likelihood of the Mara River drying out as a result of deforestation, land degradation, damming and water abstraction for irrigation in Kenya (Gereta et al., 2009, 2009; Mati et al., 2008; Mnaya et al., 2011, 2017; O’Sullivan et al., 2016). The Great Ruaha River (GRR; Fig. 2) originates from the Usangu wetlands that drain several rivers originating in the Poroto and Kipengere mountain ranges of the southern highlands of Tanzania where rainfall is about 1.5 m year 1 with a high interannual variability (SMUWC, 2002; Mtahiko et al., 2006; Kashaigili et al., 2006; WWF, 2010; McClain et al., 2013). Before rice irrigation started in the early 1990s, the mean inflow into the Usangu wetlands
was about 73 m3 s 1 in the wet season and 17 m3 s 1 in the dry season (SMUWC, 2002). This flow has significantly reduced in the dry season since 1993 with a trend towards earlier and longer periods (SMUWC, 2002). The Usangu wetlands are the source of water for the GRR, which starts at the Ngiriama rock bar and flows through the Ruaha National Park (RNP) before providing water for two hydroelectricity plants further downstream (Mtera and Kidatu) which together generate about 50% of Tanzania’s electricity (SMUWC, 2002; Mtahiko et al., 2006; WWF, 2010; McClain et al., 2013). Further downstream, this water joins the Rufiji River to drain in the Selous Game Reserve before moving on to the Indian Ocean. Since 1993, the GRR has dried out during the dry season with a trend towards earlier and longer periods of drying. This degrades the surrounding ecosystems all along the GRR, including the RNP, and impacts human livelihoods all along the river. It also affects the economy of Tanzania by reducing hydropower generation at the Mtera and Kidatu power plants (Mtahiko et al., 2006). The two main causes of this lack of water during the dry season were unregulated rice irrigation and cattle invading the Usangu wetlands: the cattle destroyed the wetlands by trampling the vegetation, this was then burned by the shepherds and the area overgrazed to bare land by the cattle (Kihwele et al., 2012). The reinstatement of perennial flows in the GRR become an issue of concern to the Government of Tanzania and led to the pronouncement in 2002 by the then Prime Minister, His Excellence Frederick Sumaye, that the river must be reinstated and have year-round flows by 2010 (WWF, 2010). This commitment resulted in the implementation of a programme of resources planning, development and management so that human activity does not endanger the sustenance of the Great Ruaha ecosystems (SMUWC, 2002). Following the pronouncement of the Prime Minister, various measures have been implemented. Such measures included the eviction in 2006 of approximately one million livestock from the eastern Usangu wetland, the inclusion of this wetland into the RNP in 2008, and the implementation of various measures to better manage and use water resources along the rivers discharging into the Usangu wetlands. Kihwele et al. (2012) described the hydrological changes that resulted from removing cattle from the eastern Usangu wetlands. They showed that by 2011, the Usangu wetland was once again perennial, and that floating vegetation had regenerated and covered about 95% of the eastern Usangu wetland. They also suggested that the amount of water flowing out of Usangu to form the GRR had nearly doubled, but the discharge was still largely concentrated in the wet season and that the rivers still dried out annually during the dry season, although
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Fig. 1. Photographs describing the impacts in the Ruaha National Park of human activities upstream that have resulted in the Great Ruaha River (GRR) flow ceasing during the dry season. (Top left) Elephants digging holes in the sand in the GRR bed to find drinking water. (Top right) Hippos attempting to survive in an overcrowded, small, eutrophicated muddy water hole. (Bottom left) Overgrazing of the riparian vegetation by buffaloes within the riverine vegetation close to the few remaining water holes and consequent soil erosion. (D) Fish suffocating and dying from anoxia due to elevated water temperature and lack of oxygen.
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the no-flow periods were shortened by between one and two months. Despite these efforts, the perennial flow of the GRR has not been restored, indicating that the developments in the Usangu catchment are unsustainable due to water governance being insufficient at the watershed scale. Integrating governance, ecohydrological and engineering solutions to manage water and ecosystems has now become a priority of concern and is the reason for our study. The objective of this study was twofold: firstly to quantify the hydrological changes that occurred within the Usangu wetlands following the decision of the government to evict livestock and annex the wetlands to the RNP, and secondly, to use the results to propose a partial solution to restore perennial flows. The paper re-analyzes earlier water level and rainfall data that has since been extended with data for the three most recent years. Based on these results, it proposes solutions based on a combination of Ecohydrology, aimed at decreasing the evaporation due to the eutrophication of the wetlands caused by rice irrigation, and engineering, by constructing a V-notch weir at the outlet of the wetlands. The solutions are intended to slow the outflow from the wetlands at the onset of the dry
season and thus decrease the no-flow period in the RNP by about two months, thereby providing water for wildlife.
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2. Methods
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Monthly precipitation was obtained from the Rufiji Basin Water Offices in Mbeya and Unilever tea plantation in Mufindi (Iringa catchment). The rainfall data from Mbeya represents the rainfall within the Mporoto and Kipengere mountain area, which is the source for the rivers to the southwest (Fig. 2). The data from the Unilever tea plantation represents the rainfall in the Ndembela River catchment. Data on the rice irrigation schemes/projects and inorganic fertilizer use within Mbarali District was obtained from Mbarali District Council through the Department of Agriculture, Livestock and Fishery Development. Maps of rice farms were obtained with Google Earth. Analysis of Google Earth images and general visual observations indicated that the areas surrounding the Usangu wetlands appeared to be heavily overgrazed by cattle and goats. To quantify this overgrazing, a survey of livestock (cattle and goats) was performed in 2015 in
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Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Fig. 2. Location map of the Usangu wetlands and the Great Ruaha River (GRR). The wetlands drain a number of rivers including the Gwiri, Mambi, Lunwa, Mbarali, Kioga and Ndembera rivers, and they form western and eastern wetlands. The GRR exits the Usangu wetlands at Ngiriama, which is a rock bar preventing water outflow, as long as the water level in the wetlands is below that of the rock bar. The GRR flows through Ruaha National Park (RNP) and then on to the Mtera and Kidatu dams (shown in the insert) to generate hydroelectricity. The map shows the general location of irrigated rice farms along all the rivers flowing into the Usangu wetlands, as well as the location of the old (before 2006) and new (after 2006) RNP boundaries. Also the Eastern Usangu wetland shows the location of the HOBO deployment sites at SMUC, Manjulwa and Ngiriama, and the portion of the TOPEX/Jason altimetry satellite track utilised for satellite-derived water level variations. The square box shows the area where overgrazing was monitored using MODIS images.
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33 villages on the fringe of the Usangu wetlands within the Mbarali District. The FAO method (http://www.fao.org/ Wairdocs/ILRI/x5443E/x5443e04.htm) was used to determine the degree of overgrazing in each of the areas of communal land owned by these villages assuming the average weight of a Tanzanian cow to be 258 kg (http://www.ajol.info/index.php/tvj/article/view/ 42029)and a goat to be 28 kg ( http://www.fao.org/ wairdocs/ilri/x5472b/x5472b1b.htm). Water levels within the eastern Usangu wetland were monitored from 2012 to 2013 using HOBO water level loggers at three sites: (1) Mwanjulwa, (2) SMUC marker, and (3) Ngiriama (Fig. 2). Time-series data of water level variation was acquired by satellite radar altimetry from the NASA/CNES TOPEX/ Jason suite of radar altimeters according to Birkett (1998). The material was obtained from a 1993–2015 data series derived for a portion of a satellite ground track crossing the Eastern Usangu wetland (Ihefu) region (Fig. 2). The methodology used repeat track gathering of height information with all-weather day/night capability advantages. The technique was limited to nadir-observations only, and the positioning of the ground tracks and the repeat frequency (in this case, 10 days) remained fixed for the mission duration. The altimetric surface water heights
were also not derived as spot heights but as an average within the instrument footprint; these were further averaged along the satellite ground track to improve height estimates and reduce uncertainty. The resulting time series were relative water height variations based on a section of the ground track (i.e., not a single location) with a profile (not mean spot height) datum that was based on one single overpass (date/time). This arbitrary datum was then translated to the geocentric datum of the satellite mission. The HOBO sites were not located directly underneath the satellite altimeter track, so the loggers and satellite were recording local data at different sites. To take this into account, the method of Kihwele et al. (2012) was adapted to combine and verify the satellite/in situ data sets by correcting the satellite altimetry data for the time lag between the water level at Ngiriama and at the satellite track attributable to the vegetation-induced friction on the water flow through the wetlands during flow recession. To correct the altimetry data for this wetland drainage time lag, the satellite altimeter data was compared with the HOBO data in the centre of the wetlands; an excellent fit was created between those two data sets (see Fig. 5 discussed later) by extending the flow recession curve from the satellite altimeter by three weeks, the correction
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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being linear in time. Also, a rating curve was constructed for the outflow of the Usangu wetlands at Ngiriama by using the SMUWC river gauging data at that point. When the water level at Ngiriama was zero, the GRR flow should be zero; this was checked against the 2009–2015 river flow data at Msembe, revealing a maximum error of six days in identifying the start of the no-flow period (5 days; n = 7). The resulting data allowed the history of the water level in the Usangu wetlands and of the GRR flow at Ngiriama from 1993 to 2015 to be obtained. The draining of Usangu wetlands in the dry season was then modelled using the open channel flow model of Brown et al. (1981). The net loss of water by evaporation/evapotranspiration in the Usangu wetlands was then calculated by comparing the 2012 water level data with the model predictions. To measure the percentage cover of the floating vegetation over the wetlands, a wetland vegetation survey was conducted during August 2014 at three locations in the Usangu wetlands: Mahongole (within the western Usangu wetland), Ikoga and Nyota (within the eastern Usangu wetland). A graduated wooden 1 m 1 m quadrant composed of 10 cm 10 cm sub-quadrants was used to estimate the percentage cover; a score of 1or 0 was assigned if the sub-quadrant was either fully covered or fully uncovered by the wetland vegetation. At each location, three square plots of 50 m 50 m were randomly established in each site. With the exception of the nature of the sampling quadrats, the nested sampling technique described by Stohlgren et al. (1995) was adopted with some modifications. In every 50 m 50 m area, 10 m 10 m quadrats were nested, followed by other 5 m 5 m quadrats. Finally, 1 m 1 m quadrats were nested within the 5 m 5 m quadrats.
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3. Results
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3.1. Overgrazing
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The relative degree of overgrazing was found to be 221% based on total biomass (301%, n = 33), and 315% based on Total Livestock Units (TLU) (427%, n = 33). The reason for the higher standard deviation could possibly be attributed to the overgrazing that occurs everywhere outside the protected area, and even in the protected area in the western Usangu wetland which was not been cleared. Hence, the extensive land clearing and even a trend towards dry season desertification can be attributed to the livestock grazing around Usangu, as observed visually. This overgrazing is also visually indicated by the MODIS data in the SW Usangu box (shown in Fig. 2), from where cattle have not been removed, and furthermore, the data reveals that this level actually increased between 2009 and 2017 (not shown).
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3.2. Increased abstraction of water and use of fertilisers
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There has been a large increase in large-scale irrigated rice farming, with the trend projected to increase in the near future under the auspices of the agriculture first policy, or ‘‘Kilimo Kwanza’’ in Swahili, implemented to address food security in Tanzania. Within the Usangu landscape, large-scale farms occupy about 25% of the
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Table 1 Large-scale national related irrigation schemes/projects within the Usangu landscape that consume water and require huge amounts of fertilizers. Name of the rice scheme
Area/acreage (Ha)
Established year
Kimani rice farm Mbarali rice farm Kapunga rice farm Madibira rice farm Total acreage
1500 3200 3200 3000 10,900
1950 1976 1997 2000
rice cultivation areas (Table 1) and have water user rights, while the remaining 75% use water illegally. According to the allowable water abstraction and flow data (Table 2), the anticipated increase in onion and tomato farming is expected to reduce the river flows below 25–30% that currently reaches the wetlands. A dry season survey of 2015 recorded 100 water pumps along a 1 km stretch of the Mbarali River, illegally pumping water from the river (Fig. 3).
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3.3. Rainfall
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Rainfall data from Mbeya showed a marked seasonal and interannual variability (Fig. 4A). Annual rainfall at Unilever Tea decreased at a rate of 3.88 mm/year (Fig. 4B) while that at Mbeya increased slightly at a rate of 2.28 mm/ year (Fig. 4C). These estimates of the net increase/decrease in annual rainfall appear unreliable in view of the much larger interannual variability.
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3.4. Usangu water budget
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The corrected satellite altimetry data closely matches the observations from the HOBO water level loggers. They show that the wetlands initially fill from the Ruaha River near the centre, and then empty from there, resulting in the creation of small water slopes typically of less than 20 cm across the wetlands. This slope is the difference between the uncorrected altimetry water level from the edge of the wetlands and the water level at the SMUWC side, in the middle of the wetlands, during flow recession. Before the removal of cattle from Usangu wetlands (1992–2005), the water depth at the Ngiriama rock bar commonly reached peaks of about 1.5–2 m, with mean annual depth decreasing at the rate of 3.65 cm/year (Fig. 6A). However, following the removal of cattle from the wetlands (2006–2015), the water peaked at about 0.6– 1.3 m. With mean annual water depth decreasing at rate of only 0.04 cm/year, which is probably too small to be statistically significant (Fig. 6B), our findings suggest that the Usangu wetlands are now stabilised but receiving much less water as a result of water abstraction for irrigation. It is important to note that there are still extended periods of zero flow in the GRR; in Fig. 6B, these are the periods when the water depth at the Ngiriama rock bar was zero. From this data, it was possible to calculate the yearly water flows for each hydrologic year starting in 1993. When plotted against the annual rainfall, the data
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Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Table 2 Allowable water abstraction within rivers of the Usangu catchment. Name of the River
Season
Allowable daily flow abstraction (M3/S)
Flow abstraction from SMUWC (2002) (M3/S)
Abstraction percentage
Chimala
Wet Dry Wet Dry Wet Dry Wet Dry
4.17 1.14 7.19
7 1 5.2
59.6 113.8 138.2
8.4 0.01 3.04 0.54
12.8
65.6
3.73 0.6
81.6 90.3
Kimani Mbarali Ndembela
Fig. 3. Photographs showing water illegally abstracted during the dry season to irrigate onion and tomato farms along the Mbarali River.
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suggests (Fig. 7) that the total volume of water flowing annually in the GRR increased by 200 GL after the removal of the cattle and restoration of the wetlands.
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3.5. Wetlands vegetation cover
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In 2014, the percentage cover by floating vegetation of three sampling sites in the Usangu wetlands was 65% + SE for Mahongole, 98% + SE for Ikoga and 85% + SE for Nyota; these are in general agreement with Kihwele et al. (2012), who reported a 95% cover in 2011. The lower wetland vegetation cover in Mahongole can be explained by the existence of few settlements and cattle in that area; however, no data on their density is available. The most dominant species encountered during the survey were
Echinochloa scabra, Ludwigia jusiaoides and Panicum maximum.
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4. Discussion
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The water volume in the Usangu wetland has significantly increased following Government intervention. The increase in water has beneficial ecological and socio-economic implications for water users downstream of Usangu including RNP, hydroelectricity production for Tanzania, and the fisheries of Lake Mtera. Although the water level in the wetland has now stabilised, perennial water flow in the GRR has still not yet been achieved, with dry-outs occurring for at least one month each year.
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Fig. 4. (A) Monthly rainfall at Mbeya showing pronounced seasonal and interannual variation; (B) Annual rainfall at Unilever tea plantation with annual rainfall showing a decreasing trend of 3.9 mm/year; (C) Annual rainfall at Mbeya with mean annual rainfall increasing at the rate of about 2 mm/year.
Fig. 5. Time-series plots of the water level in the Usangu wetlands at the Mwanjulwa and SMUC HOBO loggers, as measured by satellite altimetry in 2012 with and without correction for the time lag effect during recession. The findings were further verified against the occurrence of zero flow from river gauging data at Msembe.
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The floating vegetation that now covers the wetlands (Fig. 8a) has most likely arisen following eutrophication from fertilizers leached from irrigated farms. In order for an acre of a rice to have maximum yields in large-scale
farms, about 250 kg of fertilizers are required per year. Thus approximately 6812 tonnes of fertilizers are used every year in the Usangu catchment, which ultimately find their way into the wetland, and this does not account for other agricultural chemicals used for agricultural production. This floating vegetation has reduced the potential evaporation rate; the hydraulics model accurately reproduces the recession phase of Usangu in 2012 (Fig. 8b), provided that the net loss of water by evaporation/ evapotranspiration over the wetlands is 0.3 cm/day. Hence, the water budget of the Usangu wetland is strongly controlled by the floating vegetation because, to fit the predicted water level with the data, the net evaporation rate has to be half the value observed in the absence of the floating vegetation. Therefore, the physico-biological feedback pathways between the inflows and outflows of water in the wetland and the floating vegetation cannot be neglected in eco-hydrological processes. A similar effect has been reported for the Okavango swamp in the Botswana and the Sudd wetlands in the Sudan (Bauer et al., 2004; Mohamed et al., 2005). The Usangu wetlands have once again become perennial and in doing so have regained their sponge effect of retaining water for a longer period (Kihwele et al., 2012). However, due to the lack of large floods, the floating plants settle on the bottom and are not flushed out when they die; instead, they form an organic layer decaying on the
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Fig. 6. (A) Time-series plot of the water depth at the Ngiriama rock bar, the outlet of the Usangu wetlands forming the GRR, before 2006. (B) Time-series plot of the water depth at Ngiriama rock bar, the outlet of Usangu wetlands forming the GRR, after 2006. In 2006, the government evicted approximately one million cattle from the Usangu wetlands.
Fig. 7. Scatter plot of the yearly water flow from the Usangu wetlands to the GRR against yearly rainfall at Mbeya from 1992 to 2012. ‘‘Pre-cattle’’ indicates years when the wetlands were not invaded by cattle, ‘‘Cattle removed’’ indicates years (after 2006) when livestock were removed from the wetlands. The dotted lines ‘‘no cattle’’ and ‘‘with cattle’’ show trends of flows in GRR after and before 2006. Modified from Kihwele et al. (2012).
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bottom. Our findings indicate that in 2015, nine years since the cattle were removed, a 0.5 m thick unconsolidated organic ooze layer had built up on the bottom. As this material is still not consolidated, it is not known how long
it will take it to fill the wetlands and transform them to a seasonal flood plain. This is an important topic for further research. Climate change may exacerbate the effects of reduced flow levels to RNP through changes in rainfall. The annual rainfall itself may not necessarily increase or decrease, indeed no clear trend has been found (Fig. 4), but annual variability may increase, leading to a greater possibility of more common or more severe droughts (Wolff et al., 2011). The increase of the GRR flow from the Usangu wetlands following cattle removal is not enough to ensure perennial water flow in the GRR because water abstraction by irrigators is higher, and that cannot allow Usangu wetlands to completely fill to historical levels of 1.5–2 m (Fig. 6). Like wetlands elsewhere in the world (Zalewski, 2002; Wolanski et al., 2004; Plater and Kirby, 2006), the Usangu wetlands are important ecohydrological tools for regulating hydrology, while reducing sediments, nutrient enrichment to GRR and pollutant sequestration. The lack of flow within the park has severe ecological and environmental consequences for biodiversity conservation (Fig. 1). Importantly, other stakeholders downstream of Usangu who depend on the river are equally affected. Thus the water crisis within the catchment basin remains unresolved. There is an urgent need to prolong the flow duration during the dry season. For this to happen, we suggest that engineering solutions have to be integrated with Eco-hydrological solutions. The solution that we propose consists of construction of a 1.5 m high V-notch weir at the Ngiriama rock bar (Fig. 8c) to slow the outflow from Usangu, so that while the same amount of water will still flow out of Usangu, it will be spread over an extra 64 days
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Fig. 8. (a) Photograph of the floating vegetation covering the Usangu wetlands; the floating vegetation was cut manually to create a channel for navigation. (b) The observed and predicted water level at the Ngiriama rock bar at the outlet of Usangu wetlands during the flow recession in 2012. The predicted water level at Ngiriama in 2012 in the presence of the small V-notch weir proposed in this study. (c) Aerial Photograph of the Ngiriama rock bar showing the location of the proposed V-notch weir; the Usangu wetlands are visible in the background. The water level was 0.6 m at the time of this photograph, indicating that the V-notch weir would only need to be built in the channels: the rest of the perimeter is dry.
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(Fig. 8b). In an average hydrological year, this approach will ensure that there will be water flowing through the GRR throughout the year, which will provide water all year round in the top half of the GRR in RNP, where the river bed is rocky; however, near the end of the recession, this flow will probably not provide surface water further downstream, where the river bed is sandy, as the water will infiltrate underground (Mtahiko et al., 2006).
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In summary, it may not be possible to restore the full ecosystem services provided by the Great Ruaha River (GRR) in Tanzania due to the irrigation farming that has expanded without governance; however, our findings demonstrate that the Ecohydrology solution adopted by the Government of Tanzania to restore the Usangu wetlands has been greatly beneficial for the water budget of the GRR and the environment. We argue that governance is desperately needed to prevent further deterioration of the water budget of the GRR. Water governance at the basin scale has previously been lacking, and is now urgently needed for the aim of managing water resource
use and development; this will require the allocation of water between competing uses, enforcing water quality and quantity standards, and protecting and conserving ecosystems, while considering social, institutional and political factors. With the large overgrazing taking place in the communal lands around the Usangu wetlands, there is great pressure from local politicians to allow cattle to invade these protected wetlands. As records have shown, this will halve the GRR annual water flow, and increase the GRR no-flow period during the dry season by one to two months. Such a move must be resisted. Finally we argue that with the little remaining water that irrigators have not yet captured, it is possible to restore perennial flow in the GRR in RNP by constructing a small, 1.5 m high V-notch weir at the outlet of the Usangu wetlands. The radar altimetry data was found to play a very useful and productive role in monitoring water level within the Usangu wetlands. The use of this altimetry data provided long-term hydrological time-series data on water level that was successfully verified by the data from the HOBO water level loggers. This remote sensing monitoring technology is an effective method for monitoring large water bodies and it should be widely adopted by Tanzania.
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Conflict of interest
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None declared.
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Ethical statement
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Authors state that the research was conducted accordQ7 ing to ethical standards.
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Acknowledgements
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This study was funded by Tanzania National Parks and facilitated by Ruaha National Parks and by the NASA grant NNX13AH15G. Our due appreciation should go to the Director General for Tanzania National Park, and the Chief Park Warden for Ruaha National Park.
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Funding body
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This study was funded by Tanzania National Parks and facilitated by Ruaha National Parks and by the NASA grant NNX13AH15G. Our due appreciation should go to the Director General for Tanzania National Park, and the Chief Park Warden for Ruaha National Park.
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References
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Contents lists available at ScienceDirect
Ecohydrology & Hydrobiology journal homepage: www.elsevier.com/locate/ecohyd
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Original Research Article
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Restoring the perennial Great Ruaha River, Tanzania, using Q1 ecohydrology, engineering and Governance methods
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Q2 Emilian
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Kihwele a,*, Epaphras Muse b, Evance Magomba a, Bakari Mnaya c, Ahamed Nassoro b, Paul Banga b, Edimund Murashani b, Daniel Irmamasita b, Halima Kiwango b, Charon Birkett d, Eric Wolanski e
Q3 a Serengeti National Park, TANAPA, Tanzania b
Ruaha National Park, TANAPA, Tanzania TANAPA, PO Box 3134 Arusha, Tanzania ESSIC, University of Maryland, 5825 University Research Court, College Park, MD 20740, USA Q4 e Australian Centre for Tropical Freshwater Research, and Australian Institute of Marine Science, Townsville, QLD 4811, Australia c
d
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 February 2017 Received in revised form 17 October 2017 Accepted 20 October 2017 Available online xxx
The Great Ruaha River (GRR) in Tanzania was perennial before 1993. Its source, the Usangu wetlands, was also perennial. Since then, the GRR has started drying out during the dry season, with a trend towards earlier and longer periods of drying. This drying process degrades the surrounding ecosystems along the entire length of the GRR, including the Ruaha National Park (RNP) and impacts human livelihoods throughout its course; it also impairs the economy of Tanzania through reduced hydropower generation at the Mtera and Kidatu power plants. The Usangu wetlands dried up in 2000, 2002 and 2005 during the dry season and its areal extent has been shrinking. Intensive livestock grazing and both dry and wet season irrigated agriculture in the Usangu wetlands, were the main reasons for this water crisis. In 2006, the Government of Tanzania moved to address the crisis by removing livestock from the Usangu wetlands, attempting to regulate water use in the GRR catchment and expanding the RNP to include the Usangu wetlands. The present study uses satellite radar altimetry data dated 1992 to 2015 for the Usangu wetlands, verified using data from water level loggers, to assess the effects of these hydrological changes. The combined data sets show that the perennial Usangu wetlands have become re-established, though somewhat diminished in size, and that as a result of eutrophication from fertilizers leached from irrigated fields, 95% of their surface is now covered by floating vegetation, resulting in reduced loss of water by evaporation. This reduction has extended the water flow to the GRR by one to two months. These figures highlight the important role played by the wetlands vegetation in controlling the water budget. The hydrological model of the Usangu wetlands was further developed and verified, and then used to justify the construction of a low-level V-notch weir at the outlet of the Usangu wetlands; by slowing down the outflow from the wetlands, this construction would reduce the period of zero flow in the GRR during the dry season by one to two months. Additional ecohydrological solutions based on the improvement of water governance upstream of the
Keywords: Restoration Wetlands Irrigation Water levels Ecosystems Satellite radar altimetry
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Q5 * Corresponding author.
E-mail address: EmilianKihwelea*[email protected]">EmilianKihwelea*[email protected] ().
https://doi.org/10.1016/j.ecohyd.2017.10.008 1642-3593/ß 2017 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier Sp. z o.o. All rights reserved.
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Usangu wetlands, such as controlling rice and horticultural irrigation activities along the length of the GRR, should be integrated with this engineering solution; this would meet the hydrological needs of the water users all along the GRR. ß 2017 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier Sp. z o.o. All rights reserved.
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1. Introduction In Africa and throughout the world, humans have modified landscapes and freshwater habitats, with subsequent effects on hydrology, and ecosystem function and services for both wildlife and people (MEA, 2005; Birol et al., 2006; Elisa et al., 2010; McCartney and King, 2011). This is critically important as changes in the quality and quantity of freshwater impact on the ecological health of ecosystems (Gereta et al., 2002; SMUWC, 2002; Yawson et al., 2004; Wolanski et al., 2004; Mtahiko et al., 2006; Gereta and Wolanski, 2008; Gereta et al., 2009; WWF, Q6 2010; Wolanski and Gereta, 2001; Mnaya et al., 2011; McClain et al., 2013; Nikghalb et al., 2016). Freshwater is increasingly in short supply in Tanzania, partly as a result of water mismanagement and partly due to the rapid growth of the human population, livestock numbers and irrigated agriculture (de Villiers, 2000; Postel and Richter, 2003; Crisman et al., 2003; Mtahiko et al., 2006; Elisa et al., 2010; Mnaya et al., 2011; Kihwele et al., 2012). This shortage of water is of both ecological and economical concern to hydro-electricity producers, artisanal farmers and fishermen, together with other downstream water users, because generally the water governance system gives them no control over the decisions made by the water users upstream. This crisis has a destructive influence on the wildlife ecosystems of the Ruaha National Park (RNP) (Fig. 1) and the Katavi National Park, as these ecosystems respectively depend on the formerly perennial Great Ruaha and Katuma Rivers, which have now become seasonal as a result of non-managed irrigated agriculture and livestock grazing in the river catchments upstream of the protected areas (Fig. 1; Mtahiko et al., 2006; Elisa et al., 2010; Kihwele et al., 2012; McClain et al., 2013). While upstream users benefit from free use of the river water, the ecosystems and livelihoods downstream are suffering. If left unregulated and without ecohydrological intervention, it seems likely that these ecosystems will collapse, resulting in serious socio-economic and ecological consequences. Similarly, the Serengeti National Park is threatened by the increasing likelihood of the Mara River drying out as a result of deforestation, land degradation, damming and water abstraction for irrigation in Kenya (Gereta et al., 2009, 2009; Mati et al., 2008; Mnaya et al., 2011, 2017; O’Sullivan et al., 2016). The Great Ruaha River (GRR; Fig. 2) originates from the Usangu wetlands that drain several rivers originating in the Poroto and Kipengere mountain ranges of the southern highlands of Tanzania where rainfall is about 1.5 m year 1 with a high interannual variability (SMUWC, 2002; Mtahiko et al., 2006; Kashaigili et al., 2006; WWF, 2010; McClain et al., 2013). Before rice irrigation started in the early 1990s, the mean inflow into the Usangu wetlands
was about 73 m3 s 1 in the wet season and 17 m3 s 1 in the dry season (SMUWC, 2002). This flow has significantly reduced in the dry season since 1993 with a trend towards earlier and longer periods (SMUWC, 2002). The Usangu wetlands are the source of water for the GRR, which starts at the Ngiriama rock bar and flows through the Ruaha National Park (RNP) before providing water for two hydroelectricity plants further downstream (Mtera and Kidatu) which together generate about 50% of Tanzania’s electricity (SMUWC, 2002; Mtahiko et al., 2006; WWF, 2010; McClain et al., 2013). Further downstream, this water joins the Rufiji River to drain in the Selous Game Reserve before moving on to the Indian Ocean. Since 1993, the GRR has dried out during the dry season with a trend towards earlier and longer periods of drying. This degrades the surrounding ecosystems all along the GRR, including the RNP, and impacts human livelihoods all along the river. It also affects the economy of Tanzania by reducing hydropower generation at the Mtera and Kidatu power plants (Mtahiko et al., 2006). The two main causes of this lack of water during the dry season were unregulated rice irrigation and cattle invading the Usangu wetlands: the cattle destroyed the wetlands by trampling the vegetation, this was then burned by the shepherds and the area overgrazed to bare land by the cattle (Kihwele et al., 2012). The reinstatement of perennial flows in the GRR become an issue of concern to the Government of Tanzania and led to the pronouncement in 2002 by the then Prime Minister, His Excellence Frederick Sumaye, that the river must be reinstated and have year-round flows by 2010 (WWF, 2010). This commitment resulted in the implementation of a programme of resources planning, development and management so that human activity does not endanger the sustenance of the Great Ruaha ecosystems (SMUWC, 2002). Following the pronouncement of the Prime Minister, various measures have been implemented. Such measures included the eviction in 2006 of approximately one million livestock from the eastern Usangu wetland, the inclusion of this wetland into the RNP in 2008, and the implementation of various measures to better manage and use water resources along the rivers discharging into the Usangu wetlands. Kihwele et al. (2012) described the hydrological changes that resulted from removing cattle from the eastern Usangu wetlands. They showed that by 2011, the Usangu wetland was once again perennial, and that floating vegetation had regenerated and covered about 95% of the eastern Usangu wetland. They also suggested that the amount of water flowing out of Usangu to form the GRR had nearly doubled, but the discharge was still largely concentrated in the wet season and that the rivers still dried out annually during the dry season, although
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Fig. 1. Photographs describing the impacts in the Ruaha National Park of human activities upstream that have resulted in the Great Ruaha River (GRR) flow ceasing during the dry season. (Top left) Elephants digging holes in the sand in the GRR bed to find drinking water. (Top right) Hippos attempting to survive in an overcrowded, small, eutrophicated muddy water hole. (Bottom left) Overgrazing of the riparian vegetation by buffaloes within the riverine vegetation close to the few remaining water holes and consequent soil erosion. (D) Fish suffocating and dying from anoxia due to elevated water temperature and lack of oxygen.
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the no-flow periods were shortened by between one and two months. Despite these efforts, the perennial flow of the GRR has not been restored, indicating that the developments in the Usangu catchment are unsustainable due to water governance being insufficient at the watershed scale. Integrating governance, ecohydrological and engineering solutions to manage water and ecosystems has now become a priority of concern and is the reason for our study. The objective of this study was twofold: firstly to quantify the hydrological changes that occurred within the Usangu wetlands following the decision of the government to evict livestock and annex the wetlands to the RNP, and secondly, to use the results to propose a partial solution to restore perennial flows. The paper re-analyzes earlier water level and rainfall data that has since been extended with data for the three most recent years. Based on these results, it proposes solutions based on a combination of Ecohydrology, aimed at decreasing the evaporation due to the eutrophication of the wetlands caused by rice irrigation, and engineering, by constructing a V-notch weir at the outlet of the wetlands. The solutions are intended to slow the outflow from the wetlands at the onset of the dry
season and thus decrease the no-flow period in the RNP by about two months, thereby providing water for wildlife.
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Monthly precipitation was obtained from the Rufiji Basin Water Offices in Mbeya and Unilever tea plantation in Mufindi (Iringa catchment). The rainfall data from Mbeya represents the rainfall within the Mporoto and Kipengere mountain area, which is the source for the rivers to the southwest (Fig. 2). The data from the Unilever tea plantation represents the rainfall in the Ndembela River catchment. Data on the rice irrigation schemes/projects and inorganic fertilizer use within Mbarali District was obtained from Mbarali District Council through the Department of Agriculture, Livestock and Fishery Development. Maps of rice farms were obtained with Google Earth. Analysis of Google Earth images and general visual observations indicated that the areas surrounding the Usangu wetlands appeared to be heavily overgrazed by cattle and goats. To quantify this overgrazing, a survey of livestock (cattle and goats) was performed in 2015 in
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Fig. 2. Location map of the Usangu wetlands and the Great Ruaha River (GRR). The wetlands drain a number of rivers including the Gwiri, Mambi, Lunwa, Mbarali, Kioga and Ndembera rivers, and they form western and eastern wetlands. The GRR exits the Usangu wetlands at Ngiriama, which is a rock bar preventing water outflow, as long as the water level in the wetlands is below that of the rock bar. The GRR flows through Ruaha National Park (RNP) and then on to the Mtera and Kidatu dams (shown in the insert) to generate hydroelectricity. The map shows the general location of irrigated rice farms along all the rivers flowing into the Usangu wetlands, as well as the location of the old (before 2006) and new (after 2006) RNP boundaries. Also the Eastern Usangu wetland shows the location of the HOBO deployment sites at SMUC, Manjulwa and Ngiriama, and the portion of the TOPEX/Jason altimetry satellite track utilised for satellite-derived water level variations. The square box shows the area where overgrazing was monitored using MODIS images.
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33 villages on the fringe of the Usangu wetlands within the Mbarali District. The FAO method (http://www.fao.org/ Wairdocs/ILRI/x5443E/x5443e04.htm) was used to determine the degree of overgrazing in each of the areas of communal land owned by these villages assuming the average weight of a Tanzanian cow to be 258 kg (http://www.ajol.info/index.php/tvj/article/view/ 42029)and a goat to be 28 kg ( http://www.fao.org/ wairdocs/ilri/x5472b/x5472b1b.htm). Water levels within the eastern Usangu wetland were monitored from 2012 to 2013 using HOBO water level loggers at three sites: (1) Mwanjulwa, (2) SMUC marker, and (3) Ngiriama (Fig. 2). Time-series data of water level variation was acquired by satellite radar altimetry from the NASA/CNES TOPEX/ Jason suite of radar altimeters according to Birkett (1998). The material was obtained from a 1993–2015 data series derived for a portion of a satellite ground track crossing the Eastern Usangu wetland (Ihefu) region (Fig. 2). The methodology used repeat track gathering of height information with all-weather day/night capability advantages. The technique was limited to nadir-observations only, and the positioning of the ground tracks and the repeat frequency (in this case, 10 days) remained fixed for the mission duration. The altimetric surface water heights
were also not derived as spot heights but as an average within the instrument footprint; these were further averaged along the satellite ground track to improve height estimates and reduce uncertainty. The resulting time series were relative water height variations based on a section of the ground track (i.e., not a single location) with a profile (not mean spot height) datum that was based on one single overpass (date/time). This arbitrary datum was then translated to the geocentric datum of the satellite mission. The HOBO sites were not located directly underneath the satellite altimeter track, so the loggers and satellite were recording local data at different sites. To take this into account, the method of Kihwele et al. (2012) was adapted to combine and verify the satellite/in situ data sets by correcting the satellite altimetry data for the time lag between the water level at Ngiriama and at the satellite track attributable to the vegetation-induced friction on the water flow through the wetlands during flow recession. To correct the altimetry data for this wetland drainage time lag, the satellite altimeter data was compared with the HOBO data in the centre of the wetlands; an excellent fit was created between those two data sets (see Fig. 5 discussed later) by extending the flow recession curve from the satellite altimeter by three weeks, the correction
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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being linear in time. Also, a rating curve was constructed for the outflow of the Usangu wetlands at Ngiriama by using the SMUWC river gauging data at that point. When the water level at Ngiriama was zero, the GRR flow should be zero; this was checked against the 2009–2015 river flow data at Msembe, revealing a maximum error of six days in identifying the start of the no-flow period (5 days; n = 7). The resulting data allowed the history of the water level in the Usangu wetlands and of the GRR flow at Ngiriama from 1993 to 2015 to be obtained. The draining of Usangu wetlands in the dry season was then modelled using the open channel flow model of Brown et al. (1981). The net loss of water by evaporation/evapotranspiration in the Usangu wetlands was then calculated by comparing the 2012 water level data with the model predictions. To measure the percentage cover of the floating vegetation over the wetlands, a wetland vegetation survey was conducted during August 2014 at three locations in the Usangu wetlands: Mahongole (within the western Usangu wetland), Ikoga and Nyota (within the eastern Usangu wetland). A graduated wooden 1 m 1 m quadrant composed of 10 cm 10 cm sub-quadrants was used to estimate the percentage cover; a score of 1or 0 was assigned if the sub-quadrant was either fully covered or fully uncovered by the wetland vegetation. At each location, three square plots of 50 m 50 m were randomly established in each site. With the exception of the nature of the sampling quadrats, the nested sampling technique described by Stohlgren et al. (1995) was adopted with some modifications. In every 50 m 50 m area, 10 m 10 m quadrats were nested, followed by other 5 m 5 m quadrats. Finally, 1 m 1 m quadrats were nested within the 5 m 5 m quadrats.
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3.1. Overgrazing
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The relative degree of overgrazing was found to be 221% based on total biomass (301%, n = 33), and 315% based on Total Livestock Units (TLU) (427%, n = 33). The reason for the higher standard deviation could possibly be attributed to the overgrazing that occurs everywhere outside the protected area, and even in the protected area in the western Usangu wetland which was not been cleared. Hence, the extensive land clearing and even a trend towards dry season desertification can be attributed to the livestock grazing around Usangu, as observed visually. This overgrazing is also visually indicated by the MODIS data in the SW Usangu box (shown in Fig. 2), from where cattle have not been removed, and furthermore, the data reveals that this level actually increased between 2009 and 2017 (not shown).
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3.2. Increased abstraction of water and use of fertilisers
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There has been a large increase in large-scale irrigated rice farming, with the trend projected to increase in the near future under the auspices of the agriculture first policy, or ‘‘Kilimo Kwanza’’ in Swahili, implemented to address food security in Tanzania. Within the Usangu landscape, large-scale farms occupy about 25% of the
5
Table 1 Large-scale national related irrigation schemes/projects within the Usangu landscape that consume water and require huge amounts of fertilizers. Name of the rice scheme
Area/acreage (Ha)
Established year
Kimani rice farm Mbarali rice farm Kapunga rice farm Madibira rice farm Total acreage
1500 3200 3200 3000 10,900
1950 1976 1997 2000
rice cultivation areas (Table 1) and have water user rights, while the remaining 75% use water illegally. According to the allowable water abstraction and flow data (Table 2), the anticipated increase in onion and tomato farming is expected to reduce the river flows below 25–30% that currently reaches the wetlands. A dry season survey of 2015 recorded 100 water pumps along a 1 km stretch of the Mbarali River, illegally pumping water from the river (Fig. 3).
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3.3. Rainfall
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Rainfall data from Mbeya showed a marked seasonal and interannual variability (Fig. 4A). Annual rainfall at Unilever Tea decreased at a rate of 3.88 mm/year (Fig. 4B) while that at Mbeya increased slightly at a rate of 2.28 mm/ year (Fig. 4C). These estimates of the net increase/decrease in annual rainfall appear unreliable in view of the much larger interannual variability.
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3.4. Usangu water budget
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The corrected satellite altimetry data closely matches the observations from the HOBO water level loggers. They show that the wetlands initially fill from the Ruaha River near the centre, and then empty from there, resulting in the creation of small water slopes typically of less than 20 cm across the wetlands. This slope is the difference between the uncorrected altimetry water level from the edge of the wetlands and the water level at the SMUWC side, in the middle of the wetlands, during flow recession. Before the removal of cattle from Usangu wetlands (1992–2005), the water depth at the Ngiriama rock bar commonly reached peaks of about 1.5–2 m, with mean annual depth decreasing at the rate of 3.65 cm/year (Fig. 6A). However, following the removal of cattle from the wetlands (2006–2015), the water peaked at about 0.6– 1.3 m. With mean annual water depth decreasing at rate of only 0.04 cm/year, which is probably too small to be statistically significant (Fig. 6B), our findings suggest that the Usangu wetlands are now stabilised but receiving much less water as a result of water abstraction for irrigation. It is important to note that there are still extended periods of zero flow in the GRR; in Fig. 6B, these are the periods when the water depth at the Ngiriama rock bar was zero. From this data, it was possible to calculate the yearly water flows for each hydrologic year starting in 1993. When plotted against the annual rainfall, the data
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Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Table 2 Allowable water abstraction within rivers of the Usangu catchment. Name of the River
Season
Allowable daily flow abstraction (M3/S)
Flow abstraction from SMUWC (2002) (M3/S)
Abstraction percentage
Chimala
Wet Dry Wet Dry Wet Dry Wet Dry
4.17 1.14 7.19
7 1 5.2
59.6 113.8 138.2
8.4 0.01 3.04 0.54
12.8
65.6
3.73 0.6
81.6 90.3
Kimani Mbarali Ndembela
Fig. 3. Photographs showing water illegally abstracted during the dry season to irrigate onion and tomato farms along the Mbarali River.
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suggests (Fig. 7) that the total volume of water flowing annually in the GRR increased by 200 GL after the removal of the cattle and restoration of the wetlands.
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3.5. Wetlands vegetation cover
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In 2014, the percentage cover by floating vegetation of three sampling sites in the Usangu wetlands was 65% + SE for Mahongole, 98% + SE for Ikoga and 85% + SE for Nyota; these are in general agreement with Kihwele et al. (2012), who reported a 95% cover in 2011. The lower wetland vegetation cover in Mahongole can be explained by the existence of few settlements and cattle in that area; however, no data on their density is available. The most dominant species encountered during the survey were
Echinochloa scabra, Ludwigia jusiaoides and Panicum maximum.
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4. Discussion
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The water volume in the Usangu wetland has significantly increased following Government intervention. The increase in water has beneficial ecological and socio-economic implications for water users downstream of Usangu including RNP, hydroelectricity production for Tanzania, and the fisheries of Lake Mtera. Although the water level in the wetland has now stabilised, perennial water flow in the GRR has still not yet been achieved, with dry-outs occurring for at least one month each year.
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Fig. 4. (A) Monthly rainfall at Mbeya showing pronounced seasonal and interannual variation; (B) Annual rainfall at Unilever tea plantation with annual rainfall showing a decreasing trend of 3.9 mm/year; (C) Annual rainfall at Mbeya with mean annual rainfall increasing at the rate of about 2 mm/year.
Fig. 5. Time-series plots of the water level in the Usangu wetlands at the Mwanjulwa and SMUC HOBO loggers, as measured by satellite altimetry in 2012 with and without correction for the time lag effect during recession. The findings were further verified against the occurrence of zero flow from river gauging data at Msembe.
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The floating vegetation that now covers the wetlands (Fig. 8a) has most likely arisen following eutrophication from fertilizers leached from irrigated farms. In order for an acre of a rice to have maximum yields in large-scale
farms, about 250 kg of fertilizers are required per year. Thus approximately 6812 tonnes of fertilizers are used every year in the Usangu catchment, which ultimately find their way into the wetland, and this does not account for other agricultural chemicals used for agricultural production. This floating vegetation has reduced the potential evaporation rate; the hydraulics model accurately reproduces the recession phase of Usangu in 2012 (Fig. 8b), provided that the net loss of water by evaporation/ evapotranspiration over the wetlands is 0.3 cm/day. Hence, the water budget of the Usangu wetland is strongly controlled by the floating vegetation because, to fit the predicted water level with the data, the net evaporation rate has to be half the value observed in the absence of the floating vegetation. Therefore, the physico-biological feedback pathways between the inflows and outflows of water in the wetland and the floating vegetation cannot be neglected in eco-hydrological processes. A similar effect has been reported for the Okavango swamp in the Botswana and the Sudd wetlands in the Sudan (Bauer et al., 2004; Mohamed et al., 2005). The Usangu wetlands have once again become perennial and in doing so have regained their sponge effect of retaining water for a longer period (Kihwele et al., 2012). However, due to the lack of large floods, the floating plants settle on the bottom and are not flushed out when they die; instead, they form an organic layer decaying on the
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Fig. 6. (A) Time-series plot of the water depth at the Ngiriama rock bar, the outlet of the Usangu wetlands forming the GRR, before 2006. (B) Time-series plot of the water depth at Ngiriama rock bar, the outlet of Usangu wetlands forming the GRR, after 2006. In 2006, the government evicted approximately one million cattle from the Usangu wetlands.
Fig. 7. Scatter plot of the yearly water flow from the Usangu wetlands to the GRR against yearly rainfall at Mbeya from 1992 to 2012. ‘‘Pre-cattle’’ indicates years when the wetlands were not invaded by cattle, ‘‘Cattle removed’’ indicates years (after 2006) when livestock were removed from the wetlands. The dotted lines ‘‘no cattle’’ and ‘‘with cattle’’ show trends of flows in GRR after and before 2006. Modified from Kihwele et al. (2012).
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bottom. Our findings indicate that in 2015, nine years since the cattle were removed, a 0.5 m thick unconsolidated organic ooze layer had built up on the bottom. As this material is still not consolidated, it is not known how long
it will take it to fill the wetlands and transform them to a seasonal flood plain. This is an important topic for further research. Climate change may exacerbate the effects of reduced flow levels to RNP through changes in rainfall. The annual rainfall itself may not necessarily increase or decrease, indeed no clear trend has been found (Fig. 4), but annual variability may increase, leading to a greater possibility of more common or more severe droughts (Wolff et al., 2011). The increase of the GRR flow from the Usangu wetlands following cattle removal is not enough to ensure perennial water flow in the GRR because water abstraction by irrigators is higher, and that cannot allow Usangu wetlands to completely fill to historical levels of 1.5–2 m (Fig. 6). Like wetlands elsewhere in the world (Zalewski, 2002; Wolanski et al., 2004; Plater and Kirby, 2006), the Usangu wetlands are important ecohydrological tools for regulating hydrology, while reducing sediments, nutrient enrichment to GRR and pollutant sequestration. The lack of flow within the park has severe ecological and environmental consequences for biodiversity conservation (Fig. 1). Importantly, other stakeholders downstream of Usangu who depend on the river are equally affected. Thus the water crisis within the catchment basin remains unresolved. There is an urgent need to prolong the flow duration during the dry season. For this to happen, we suggest that engineering solutions have to be integrated with Eco-hydrological solutions. The solution that we propose consists of construction of a 1.5 m high V-notch weir at the Ngiriama rock bar (Fig. 8c) to slow the outflow from Usangu, so that while the same amount of water will still flow out of Usangu, it will be spread over an extra 64 days
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Fig. 8. (a) Photograph of the floating vegetation covering the Usangu wetlands; the floating vegetation was cut manually to create a channel for navigation. (b) The observed and predicted water level at the Ngiriama rock bar at the outlet of Usangu wetlands during the flow recession in 2012. The predicted water level at Ngiriama in 2012 in the presence of the small V-notch weir proposed in this study. (c) Aerial Photograph of the Ngiriama rock bar showing the location of the proposed V-notch weir; the Usangu wetlands are visible in the background. The water level was 0.6 m at the time of this photograph, indicating that the V-notch weir would only need to be built in the channels: the rest of the perimeter is dry.
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(Fig. 8b). In an average hydrological year, this approach will ensure that there will be water flowing through the GRR throughout the year, which will provide water all year round in the top half of the GRR in RNP, where the river bed is rocky; however, near the end of the recession, this flow will probably not provide surface water further downstream, where the river bed is sandy, as the water will infiltrate underground (Mtahiko et al., 2006).
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5. Conclusions
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In summary, it may not be possible to restore the full ecosystem services provided by the Great Ruaha River (GRR) in Tanzania due to the irrigation farming that has expanded without governance; however, our findings demonstrate that the Ecohydrology solution adopted by the Government of Tanzania to restore the Usangu wetlands has been greatly beneficial for the water budget of the GRR and the environment. We argue that governance is desperately needed to prevent further deterioration of the water budget of the GRR. Water governance at the basin scale has previously been lacking, and is now urgently needed for the aim of managing water resource
use and development; this will require the allocation of water between competing uses, enforcing water quality and quantity standards, and protecting and conserving ecosystems, while considering social, institutional and political factors. With the large overgrazing taking place in the communal lands around the Usangu wetlands, there is great pressure from local politicians to allow cattle to invade these protected wetlands. As records have shown, this will halve the GRR annual water flow, and increase the GRR no-flow period during the dry season by one to two months. Such a move must be resisted. Finally we argue that with the little remaining water that irrigators have not yet captured, it is possible to restore perennial flow in the GRR in RNP by constructing a small, 1.5 m high V-notch weir at the outlet of the Usangu wetlands. The radar altimetry data was found to play a very useful and productive role in monitoring water level within the Usangu wetlands. The use of this altimetry data provided long-term hydrological time-series data on water level that was successfully verified by the data from the HOBO water level loggers. This remote sensing monitoring technology is an effective method for monitoring large water bodies and it should be widely adopted by Tanzania.
Please cite this article in press as: Kihwele, E., et al., Restoring the perennial Great Ruaha River, Tanzania, using ecohydrology, engineering and Governance methods. Ecohydrol. Hydrobiol. (2017), https://doi.org/10.1016/ j.ecohyd.2017.10.008
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Conflict of interest
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None declared.
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Ethical statement
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Authors state that the research was conducted accordQ7 ing to ethical standards.
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Acknowledgements
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This study was funded by Tanzania National Parks and facilitated by Ruaha National Parks and by the NASA grant NNX13AH15G. Our due appreciation should go to the Director General for Tanzania National Park, and the Chief Park Warden for Ruaha National Park.
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Funding body
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This study was funded by Tanzania National Parks and facilitated by Ruaha National Parks and by the NASA grant NNX13AH15G. Our due appreciation should go to the Director General for Tanzania National Park, and the Chief Park Warden for Ruaha National Park.
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