The rate and characteristics of land use change in tropical watersheds due to changing demographic, economic and policy factors have important consequences for catchment health and environmental services. Few tropical watershed studies have lasted long enough to facilitate a credible analysis of the long-term effects of land use change on the environmental services provided by watersheds. This paper examines the driving forces and patterns of historical land use change in two long-term watershed studies in Kenya and Sri Lanka and their hydrological impacts. The upper Ewaso Ng’iro north basin is located to the north and west of Mt. Kenya, and has experienced dramatic changes in both land ownership and land use due to rapid population growth of 7–8% per annum. The upper Nilwala basin is located in the south of Sri Lanka and the area has undergone serious deforestation over the last 50 years, for agricultural land uses mainly for tea and home gardens. The loss of watershed functions associated with the impacts of land use change and their socio-economic dimensions are discussed, along with lessons that can be drawn from these studies. These case studies confirm the importance of long-term monitoring of the interaction between land use changes and catchment health. Moreover, the involvement of all stakeholders is crucial for problem identification through to the research and the search for any viable ecological, social and economical solutions. A holistic approach involving relevant disciplines in watershed studies is vital. The paper concludes that use of models that integrate both biophysical and socio-economic data should be encouraged to derive decision support tools for farmers and managers alike who are faced with resolving conflicts and other issues related to limited land and water resources.
Tag: catchment hydrology
Roof Catchment Rainwater Harvesting Techniques
In the household approach, water is harvested from the roof of the household and stored in tanks made of different materials, namely Ferro-cement, bricks and or polyethylene.
Expanding Rubber Plantations in Southern China: Evidence for Hydrological Impacts
While there is increasing evidence concerning the detrimental effects of expanding rubber plantations on biodiversity and local water balances, their implications on regional hydrology remain uncertain. We studied a mesoscale watershed (100 km2) in the Xishuangbanna prefecture, Yunnan Province, China. The influence of land-cover change on streamflow recorded since 1992 was isolated from that of rainfall variability using cross-simulation matrices produced with the monthly lumped conceptual water balance model GR2M. Our results indicate a statistically significant reduction in wet and dry season streamflow from 1992 to 2002, followed by an insignificant increase until 2006. Analysis of satellite images from 1992, 2002, 2007, and 2010 shows a gradual increase in the areal percentage of rubber tree plantations at the watershed scale. However, there were marked heterogeneities in land conversions (between forest, farmland, grassland, and rubber tree plantations), and in their distribution across elevations and slopes, among the studied periods. Possible effects of this heterogeneity on hydrological processes, controlled mainly by infiltration and evapotranspiration, are discussed in light of the hydrological changes observed over the study period. We suggest pathways to improve the eco-hydrological functionalities of rubber tree plantations, particularly those enhancing dry-season base flow, and recommend how to monitor them.
Guiding community discussions on human-water-related challenges by serious gaming in the upper Ewaso Ng’iro river basin, Kenya
Water-related conflicts in river catchments occur due to both internal and external pressures that affect catchment water availability. Lack of shared understanding by catchment stakeholders increase the complexity of human-water issues at the river catchment scale. Among a range of participatory approaches, the development and use of serious games gained prominence as a tool to stimulate discussion and reflection among stakeholders about sustainable resource use and collective action. This study designed and implemented the ENGAGE game (Exploring New Gaming Approach to Guide and Enlighten), that mimics the dynamics observed during the dry season in the upper Ewaso Ng’iro catchment, North West of Mount Kenya. The purpose of this study was to explore the potential role of serious gaming in subsequent steps of strengthening stakeholder engagement (agenda setting, shared understanding, commitment to collective action, and means of implementation) toward addressing complex human-water-related challenges at the catchment scale. We assessed the type of decisions made during gameplay, the communication dynamics, active participation, and the implication of decisions made on water availability. The results of three game sessions show that the ENGAGE game raised awareness and provided a recognizable hydro-logic background to conflicts while guiding community discussions toward implementable decisions. The results revealed increasing active participation, knowledge gain, and use of plural pronouns, and decreasing individual interests and conflicts among game participants. This study presents important implications for creating a collective basis for water management and can inform human-water policies and modification of the process behind water allocation rules in a river catchment.
Traditional food plants of the upper Aswa River catchment of northern Uganda—a cultural crossroads
In the parkland agroforestry system of northern Uganda, smallholder farming households rely on a diversity of plant species to fulfil their nutritional requirements, many of which also serve a range of medicinal, cultural, and livelihood functions. The purpose of the study was to assemble an inventory of indigenous plant species used as food in four districts within the Aswa River catchment of northern Uganda, and to document their utilization and management by rural communities. From July 1999 to August 2000, a series of 61 community-based focus group discussions on the utilization of plant biodiversity were conducted in the vernacular language at 34 sites in four districts of northern Uganda, with participation by key informants self-selected on basis of their technical knowledge and personal interest. Of these, 232 respondents subsequently contributed to a collection of herbarium specimens, which were submitted to the Makerere University Herbarium for identification. On receipt of each specimen collected, a structured interview was conducted to document the botanical, ecological, seasonal, and alimentary attributes of each identified taxon, and details of its processing and utilization by the community from which it was obtained. The data analysis was undertaken during 2019 and 2020, including statistical tests to assess the relative importance of the cited taxa using the Relative Importance Index (RI), and to determine the similarity of edible plant use between the four cultures using the Jaccard Index of similarity (JI). Key informant interviews yielded 1347 use reports (URs) for 360 identified specimens of 88 indigenous edible plant species. The data describes patterns of use of indigenous edible plants of four cultures of the Aswa River catchment of northern Uganda. RI scores ranged from 0.93 to 0.11, with fruit trees occupying the top 25 taxa (RI 0.45 and above). Jaccard similarity scores ranged from 25.8% between Lango and Acholi, to 15.8% between Acholi and Ethur, indicating that cultural factors appear to be more significant than shared ancestry as determinants of cultural similarity of plant use. The data constitute an inventory of on-farm plant species, including cultivated, semi-cultivated, and wild plants, integrated into a parkland agroforestry system in which useful trees and other plant species are sustained and managed under cultivation. Agricultural and on-farm plant biodiversity may be seen as a food security resource, and a nutritional buffer against increasing risks and stressors on low-input smallholder agriculture. Further studies should assess the intra-species biodiversity of these resources, with respect to farmer-valued traits and vernacular (folk) classification systems.
Particulate macronutrient exports from tropical African montane catchments point to the impoverishment of agricultural soils
Agricultural catchments in the tropics often generate high concentrations of suspended sediments following the conversion of natural ecosystems. The eroded fine particles are generally enriched with carbon (TC) and nutrients (TN and TP) originating from the topsoil of agricultural land. Sediment-associated TC, TN and TP are an important loss to the terrestrial ecosystem and tightly connected to an increase in riverine particulate TC and nutrient export. Soil nutrient depletion can limit crop growth and yields, whereas an excess of nutrients in streams can cause eutrophication in freshwater systems. Streams in East Africa, with widespread land conversion from forests to agriculture, are expected to receive high loads of sediment-associated TC, TN and TP. In this study, we assess the effect of land use on particulate TC, TN and TP concentrations. Suspended sediments (time-integrated, manual-event-based and automatic-event-based sediment samples) were analysed for TC, TN and TP concentrations collected at the outlet of a natural montane forest (35.9), a tea-tree plantation (33.3) and a smallholder agriculture (27.2 km2) catchment in western Kenya during a wet sampling period in 2018 and a drier sampling period in 2019. Particulate TC, TN and TP concentrations were up to 3-fold higher (p<0.05) in the natural forest catchment compared to fertilized agricultural catchments. However, because of higher sediment loads from the smallholder agriculture catchment, the total sediment-associated loads of TC, TN and TP were higher compared to the natural forest and tea-tree plantation catchment. The higher C:N ratios in the natural forest suggest that the particulate nutrients are of organic origin due to tighter nutrient cycles, whereas lower ratios in both agricultural catchments suggest a mixture of mineral and organic sediment sources. The findings of this study imply that with the loss of natural forest, the inherent soil fertility is progressively lost under the current low fertilization rates and soil management strategies.
Tropical Montane Forest Conversion Is a Critical Driver for Sediment Supply in East African Catchments
Land use change is known to affect suspended sediment fluxes in headwater catchments. There is however limited empirical evidence of the magnitude of these effects for montane catchments in East Africa. We collected a unique 4-year high-frequency data set and assessed seasonal sediment variation, water pathways, and sediment response to hydrology in three catchments under contrasting land use in the Mau Forest Complex, Kenya’s largest tropical montane forest. Annual suspended sediment yield was significantly higher in a smallholder agriculture-dominated catchment (131.5 ± 90.6 t km−2 yr−1) than in a tea-tree plantation catchment (42.0 ± 21.0 t km−2 yr−1) and a natural forest catchment (21.5 ± 11.1 t km−2 yr−1) (p < 0.05). Transfer function models showed that in the natural forest and the tea-tree plantations subsurface flow pathways delivered water to the stream, while in the smallholder agriculture shallow subsurface and surface runoff were dominant. There was a delayed sediment response to rainfall for the smallholder agriculture and the tea-tree plantations. A slow depletion in sediment supply suggests that the wider catchment area supplies sediment, especially in the catchment dominated by smallholder farming. In contrast, a fast sediment response and depletion in sediment supply in the natural forest suggests a dominance of temporarily stored and nearby sediment sources. This study shows that the vegetation cover of a forest ecosystem is very effective in conserving soil, whereas catchments with more bare soil and poor soil conservation practices generated six times more suspended sediment yield. Catchment connectivity through unpaved tracks is thought to be the main explanation for the difference in sediment yield.
Agricultural land is the main source of stream sediments after conversion of an African montane forest
In many parts of Africa, soil erosion is an important problem, which is evident from high sediment yields in tropical montane streams. Previous studies in Kenya pointed to a large contribution from catchments cultivated by smallholder farmers. This led to the hypothesis that unpaved tracks and gullies are the main sediment sources in smallholder agriculture catchments of the highlands of Kenya. The aim of this study was to investigate the sediment sources with sediment fingerprinting to generate the knowledge base to improve land management and to reduce sediment yields. Four main sediment sources (agricultural land, unpaved tracks, gullies and channel banks) and suspended sediments were analysed for biogeochemical elements as potential tracers. To apportion the catchments target sediment to different sources, we applied the MixSIAR un-mixing modelling under a Bayesian framework. Surprisingly, the fingerprinting analysis showed that agricultural land accounted for 75% (95% confidence interval 63–86%) of the total sediment. Channel banks contributed 21% (8–32%), while the smallest contributions to sediment were generated by the unpaved tracks and gullies with 3% (0–12%) and 1% (0–4%), respectively. Erosion management strategies should target agricultural lands with an emphasis on disconnecting unpaved tracks form hillslope source areas to reduce sediment yields to Lake Victoria.
Monitoring of Suspended Sediments in a Tropical Forested Landscape With Citizen Science
Catchments are complex systems, which require regular monitoring of hydro-chemical parameters in space and time to provide comprehensive datasets. These are needed to characterize catchment behavior on a local level, make future projections based on models, implement mitigation measures and meet policy targets. However, many developing countries lack a good infrastructure for hydrological monitoring since its establishment is costly and the required resources are often not available. To overcome such challenges in data scarce regions like Kenya, a participatory citizen science approach can be a promising strategy for monitoring water resources. This study evaluates the potential of using a contributory citizen science approach to explore spatiotemporal turbidity and suspended sediment dynamics in the Sondu-Miriu river basin, western Kenya. A group of 19 citizen scientists was trained to monitor turbidity using turbidity tubes and water levels with water level gauges in six nested subcatchments of the Sondu-Miriu river basin. Over the course of the project, a total of 37 citizen scientists participated and contributed to the overall dataset of turbidity. The sampling effort and data contribution varied from year to year and among participants with the majority of the data (72%) originating from 8 (22%) citizen scientists. Comparison between citizen-scientist collected suspended sediment data and measurements from automated stations showed high correlation (R2 > 0.9) which demonstrates that data collected by citizen scientists can be comparable to data collected using expensive monitoring equipment. However, there was reduced precision of the measurements of suspended sediment concentrations at low and high levels attributed largely to the detection limitations of the turbidity tubes and citizen scientists not capturing major sediment export events. Suspended sediment concentrations were significantly higher downstream (109 ± 94 mg L−1), a subcatchment dominated by agriculture and rangeland with low forest vegetation cover, as compared to a subcatchment with high forest cover (50 ± 24.7 mg L−1). This finding indicates that forest cover is a key landscape feature to control suspended sediment concentrations in the region. Future citizen science projects should focus on motivation and engagement strategies and the application of robust methods with improved detection limits and resolution to advance hydrological monitoring.