Conflicting interests in the use and management of wetlands have always resulted in their degradation. The degradation of wetlands affects their natural functioning, environmental health and livelihood of the people who depend on them. The eastern Usangu wetland has suffered a lot from multiple-use pressure arising from both national and local interests. As a result, the government banned the use of the wetland in 2006 to support its restoration process. The aim of the current study was to assess the restoration process of the eastern Usangu wetland using time-series Landsat images over a 20-year period, from 1995 to 2015. Cross-tabulation of composite NDVI images was used to examine the changes. The results indicate that the land cover declined by 20% between 1995 and 2005, and increased by more than 25% between 2005 and 2015. The size of the permanent swamp increased consistently, by more than 15% between 1995 and 2015. Wetland use has declined to about 15% over the 20 years. Wetland restoration seems to be a slow process that depends on multiple factors. It thus is important that wetlands are managed well for sustained benefits, rather than waiting to rescue them in a crisis. The well-being of the people depending on the wetlands should be considered when implementing measures to protect the wetlands. Awareness creation among the users, diversification of sources of income and enforcement of the laws and policies governing the use of wetlands by the government may improve status of wetlands.
Wetland restoration is designed to bring back or re-establish the natural functions and values of wetland ecosystems that have been altered or degraded through the removal of vegetation by cultivation, grazing, burning, construction, filling and grading, and by changes in water levels and drainage patterns. Several other processes taking place outside the wetland area, like deforestation, loss of recharge area, extreme weather conditions, land conversion and changes in local drainage patterns, can severely affect the natural functioning of wetlands. According to [
Remote sensing techniques have now become the most cost-effective method for monitoring and managing wetlands. Remote sensing involves the acquisition of information about the Earth’s surface at a remote distance, usually by airplane or satellite [
The Normalised Difference Vegetation Index (NDVI) is one of the most commonly used vegetation indexes for discriminating between vegetated and non-vegetated areas in environments with low to moderate vegetation cover on light soils or backgrounds [
Even though much effort are being done to restore wetlands, post-restoration monitoring is commonly underfunded, understaffed or short term, and the data collected are rarely published [
The Usangu wetland lies between longitudes 33˚00'E and 35˚00'E, and latitudes 8˚00'S and 9˚30'S (
The Usangu wetlands are amongst the most valuable ecosystems in Tanzania, providing habitat for over 400 bird species and numerous other flora and fauna [
Park, which attracts international tourism. Other small-scale economic activities, like fishing, macrophyte harvesting, handicrafts, sand mining and ritual activities, are also common in the wetlands [
The productivity of the Usangu wetlands and conflicting interests in its utilisation resulted in serious water shortages in various years. In December 1993, the Great Ruaha River upstream of Tanzania’s Mtera Dam stopped flowing for the first time in living memory [
The assessment of the land use and cover change (LULC) in the eastern Usangu wetland was done using the NDVI and three software i.e. ENVI (5.3), Erdas imagine (2011) and ArcGIS (10.3) were used for data analysis. NDVI is appropriate for assessing land use and cover of the wetland especially in areas that are not heavily vegetated. The study area has no thick vegetation, hence the index correlates linearly with land cover across
the area’s radiometric range [
Multi-temporal evaluation of LULC change could be done by using the pixel-based clustering and post- classification tabulation. This involves pixel clustering into themes or classes, and pairwise comparison of variable aerial coverage of the classes between the images. The thematic aerial change involves the transformation of the pairs of pixels from class to class between the image pairs, each representing land cover at a particular time of interest. The principle in this pairwise cross-tabulation is that the thematic maps must cover the same aerial extent, and the number of categories should be the same for each image. Eastman [
The main datasets used for this study were multispectral images, Landsat Thematic Mapper (TM) and Operational Land Imager (OLI) images of the years 1995, 2005 and 2015 (
The images were used to generate NDVI composites for each of the study years. Image composites were preferred to singular images for better representation of annual phenomena, and also for cloud masking. The images were re-projected to datum WGS 84 and Zone 36 South, and resampled to a spatial resolution of 30 m for each pixel. The respective red and near-infrared bands from each image were then radiometrically calibrated to obtain at-sensor radiance pixel brightness. This is important for improved consistency between pixel radiance and the respective phenomena on the surface of the ground. The NDVI (NIR-R/NIR+R) was then computed using the obtained radiance. It was not important to conduct atmospheric correction because the atmosphere in the study
Data type | Date | Path/row | Red, NIR & MIR bands Spatial resolution | Producer |
---|---|---|---|---|
Landsat TM | June-Nov. 1995 | 169/066 | Band 3 & 4, 30 m | USGS |
Landsat TM | June-Nov. 2005 | 169/066 | ||
Landsat OLI | June-Nov. 2015 | 169/066 | Band 4 & 5, 30 m |
area was clear. The respective NDVIs were made on condition that the resulting composites had to be made up of maximum intersecting pixel values. The resulting composites were delimited to fit to the aerial coverage of the study district, using an edited wetland boundary vector dataset. For editing the interpretation of the Aster GDEM V2 elevation, Landsat datasets, and those of Digital Globe on Google Earth were used.
A hybrid approach combining unsupervised classification and manual aggregation was used. The approach takes advantages of both supervised and unsupervised classification [
Six main land use and cover categories were identified in the study area (
The coverage of each LULC category varied each year (
In 2005 as indicated in
The LULC in 2015, as indicated in
An overview of the changes detected between 1995 (T0) and 2005 (T1), as well as between T1 and 2015 (T2), is presented in
Basically, three different categories of change in T0, T1 and T2 can be seen from the table, i.e. increasing, declining and stable. Between T0 and T1, land cover that increased in surface area was that induced by anthropogenic factors. These included settlements and other open land, scattered cropland, and grassland with scattered cropland. The increase is not very stable, however, because scattered croplands, for example, declined significantly between T1 and T2.Settlements and other open land seemed to increase between T0 and T1 also T1 and T2.Open bushland cover was dynamic; it suffered a significant loss between T0 and T1and begun to recover between T1 and T2. Seasonally inundated grassland showed a dynamic trend increasing and then declining. The permanent swamp increased steadily throughout the period covered, i.e. T0 and T1 and T1 and T2, and from T0
Category | Category name | Category description |
---|---|---|
1 | Settlement and other open land | Scattered, mainly semi-permanent housing structures made of mud and with thatch roofs, associated with bare soils covered by scattered patches of dry grass |
2 | Scattered croplands | Cultivated areas planted with various crops like maize, but widely spaced with no clear pattern |
3 | Grassland with scattered croplands | Naturally mixed herbaceous vegetation and grass. The vegetation is dense and high, although scattered farming and extensive grazing takes place |
4 | Open bushland | Areas covered by shrubs and grass; in extended dry season, farming and grazing may take place |
5 | Seasonally inundated grassland | Transitional area between permanent swamp and open bushland usually covered with wetland vegetation species like cyperus. Normally floods in wet season |
6 | Perennial swamp | Permanently flooded swamp |
Land cover/use category | 1995 | 2005 | 2015 |
---|---|---|---|
Settlement and other open land | 3.2 | 6.9 | 7.8 |
Scattered croplands | 6.8 | 15.5 | 5.1 |
Grassland with scattered croplands | 4.4 | 8.9 | 3.2 |
Open bushland | 46.4 | 20.5 | 40.7 |
Seasonally inundated grassland | 18.5 | 21.8 | 6.4 |
Perennial swamp | 20.8 | 26.4 | 36.7 |
Land use/cover category | 1995 (T1) | 2005 (T2) | 2015 (T3) | Diff T1-T0 | Diff T2-T1 | Diff T2-T0 |
---|---|---|---|---|---|---|
Area in km2 | Area in km2 | Area in km2 | ||||
Settlement and other open land | 42.38 | 92.26 | 104.18 | 49.88 | 11.92 | 61.79 |
Scattered croplands | 89.98 | 206.97 | 67.95 | 116.69 | −139.01 | −22.03 |
Grassland with scattered croplands | 58.52 | 118.28 | 42.81 | 59.76 | −75.47 | −15.72 |
Open bushland | 618.52 | 273.46 | 543.06 | −345.06 | 269.61 | −213.10 |
Seasonally inundated grassland | 246.95 | 290.75 | 85.69 | 43.80 | −205.06 | −161.26 |
Perennial swamp | 276.93 | 351.42 | 489.73 | 74.49 | 138.31 | 212.80 |
Total | 1333.29 | 1333.13 | 1333.42 |
to T2 generally the swamp gained about 15% from other uses/covers. This indicates some stable recovery and it is a good sign that the wetland is restoring.
Land use class | Gross gain | Gross loss | Total change (Gain + Loss) | Swap (Total change - difference of gain and loss) | Absolute net change (Total change-Swap) |
---|---|---|---|---|---|
Settlement and other open land | 77.34 | 27.40 | 104.74 | 54.80 | 49.94 |
Scattered croplands | 183.21 | 66.23 | 249.44 | 132.45 | 116.98 |
Grassland with scattered croplands | 107.16 | 47.42 | 154.58 | 94.85 | 59.73 |
Open bushland | 53.87 | 398.85 | 452.72 | 107.73 | 344.98 |
Seasonally inundated grassland | 210.49 | 166.67 | 377.17 | 333.34 | 43.82 |
Perennial swamp | 119.86 | 45.36 | 165.23 | 90.73 | 74.50 |
Total | 751.93 | 751.94 | 1503.87 | 813.91 | 689.96 |
place within the bushlands. The grassland with scattered cropland gained 107.16 km2, which is directly related to farming and grazing. Considering that the analysis was based on the dry season, when most of the areas are dry, it is not surprising to find increasing wetland farming due to the availability of moisture in the soil [
disturbance was decreasing slowly.
Significant changes were observed between T1 and T2 (
The consistent increase in perennial swamp by more than 5% every ten years is proof that restoration is taking place naturally. The permanent swamp has been increasing steadily without a significant loss from 1995. On the other hand, the declining trend of wetland uses is an indication that the measures taken to protect the wetland are becoming effective. From the results it can be observed that the process of wetlands restoration is slow, as in the period of 20 years only 16% of the lost parts of permanent swamp were restored. The restoration process depends on a multiplicity of factors, with climate being one of them, thus it is not a linear process. The importance of wetlands management and conservation cannot be overemphasised. The impact of mismanagement of wetlands has been more obvious in eastern Usangu, and without deliberate efforts by the government to protect the wetland it could have been completely depleted by now. The eastern Usangu wetland is not the only wetland under threat in Tanzania. The Lake Victoria wetlands, for instance, have suffered significantly from degradation. Infestation of water hyacinth since the 1990s, resulting from increased nutrients due to pollution from various sources in the lake, illegal fishing practices, farming in the fringes of the wetland and the ever-growing needs of the population continue to threaten the lake ecosystem [
Because of the ongoing global climate change, wetlands are becoming even more vulnerable to change [
Land use class | Gross gain | Gross loss | Total change (Gain + Loss) | Swap (Total change - Difference of gain and loss) | Absolute net change (Total change-Swap) |
---|---|---|---|---|---|
Settlement and other open land | 83.22 | 71.32 | 154.55 | 142.64 | 11.90 |
Scattered croplands | 52.64 | 191.66 | 244.30 | 105.28 | 139.02 |
Grassland with scattered croplands | 34.45 | 109.96 | 144.41 | 68.91 | 75.51 |
Open bushland | 342.81 | 73.31 | 416.11 | 146.61 | 269.50 |
Seasonally inundated grassland | 50.93 | 256.07 | 307.00 | 101.86 | 205.14 |
Perennial swamp | 174.15 | 35.89 | 210.04 | 71.79 | 138.25 |
Total | 738.20 | 738.20 | 1476.40 | 637.08 | 839.32 |
will have a pronounced effect on wetlands through alterations in hydrological regimes with great global variability. It is therefore important to strengthen measures to conserve and restore wetlands in order to enhance their function and their continued support of people’s needs. Efforts to restore wetlands can succeed if there is good co-operation by the local community and government leaders [
Since the laws and policies governing environmental resource use and management are in place, the government should enforce them, the Usangu wetland sets an example in this case. Poor enforcement of policies and laws is one of the factors that has significantly accelerated the degradation of resources in Tanzania. There are many wetland users who do not know the importance of wetlands and how to manage them without jeopardizing their future existence. Awareness creation on wetlands through various media such as radio and Television programs, newspapers and use of extension officers to educate farmers and livestock keepers can also contribute to active involvement and greater public participation in issues related to the conservation and management of wetlands. It also is important to consider diversifying people’s means of earning income, especially in the rural areas, in order to reduce their dependence on natural resources. In the case of eviction, as in the case of Usangu, the government should ensure that the destination to which people are being moved is well prepared to receive the incoming population to reduce the risks of the new environment being degraded in the longrun.
This study investigated the wetland restoration process by using remote sensing to assess the LULC of the eastern Usangu wetland between 1995 and 2015. The results indicate that the restoration is taking place, although very slowly. Between 1995 and 2005, only 5% of the permanent swamp was restored and wetland encroachment was evident because grazing and farming increased. The permanent swamp continued to increase by 10% between 2005 and 2015 and at the same period the wetland uses declined tremendously. The vegetation cover increased to for instance open bushland from 21% in 2005 to 41% in 2015. These changes would not have happened without deliberate efforts by the government to rescue the wetland after a number of scientific warnings from various scholars on the dangers the wetland was facing. Many wetlands in the country are also threatened by increased and conflicting uses that alter their functioning and reduce their ability to support the environment and people’s needs. It is important to ensure that the wetlands are managed wisely for sustained benefit. The role of government in the management of wetland resources is clear, and thus the government is urged to play its part, rather than waiting until conditions become critical.
The current study covered the eastern part of the Usanga wetland and used remote sensing techniques only in assessing the situation based on LULC changes. It will be of interest if further research could be done to assess the restoration process closely by using ecological approaches, including physical field visits. The assessment of other environmental variables that may indicate the trends in changes would be important to inform policy and decision makers, and influence similar efforts in other wetlands encountering similar challenges as the Usangu wetland.
Emiliana John Mwita, (2016) Monitoring Restoration of the Eastern Usangu Wetland by Assessment of Land Use and Cover Changes. Advances in Remote Sensing,05,145-156. doi: 10.4236/ars.2016.52012