Dissolved organic carbon in tropical watersheds : Linking field observation and eco-hydrological modelling

Dissolved organic carbon (DOC) is a general description of the organic material dissolved in water. DOC is an important source of energy, carbon, and nutrient transfers from terrestrial to aquatic ecosystems. The export of DOC into aquatic ecosystems may contribute to the carbon balance of terrestrial ecosystems and to water degradation. Ongoing climate and land cover changes will affect both DOC generation and transport, with implications for both terrestrial and aquatic ecosystems. An assessment of land use land cover and climate variability’s impacts on DOC export is needed for better management of ecosystems. Watersheds are fundamental units of ecosystem functioning and are therefore an interesting organizational unit when used to understand the combined effects of land use land cover and climate variability on DOC export. Some studies have been conducted to explore this impact of land cover and climate variability on DOC, but most were conducted in a temperate environment and few in a tropical environment. In this regard, this dissertation focused on the impact of land use land cover and climate variability on DOC mobilization and export in the Rukarara River Watershed (RRW), Rwanda. The main aim is to determine how different carbon input and output processes interact under climate and land cover variability to impact DOC emanating from tropical watersheds. Data used for this study include land cover maps produced from satellite imagery, daily air temperature and precipitation, digital elevation models (DEMs), water stage, flow, net primary productivity (NPP), soil properties such as total organic carbon (TOC), total nitrogen (TN), cation exchange capacity (CEC), aluminum (Al), iron (Fe), and soil texture within the RRW. Field observations were used to quantify riverine DOC loads, soil water extractable organic carbon (WEOC), DOC in percolation water (pDOC) and leached DOC (LDOC) and to describe their spatial variation and relationships with the aforementioned factors. Statistical models (including simple and quadratic regressions, general linear model, linear mixed effect models) were used to predict DOC within the study area. An eco-hydrological model, the Regional Hydro-Ecological Simulation System (RHESSys), was used to simulate streamflow and link it with stream DOC within the study area. The results of this study show that land use land cover and climate change interact to produce soil WEOC, from which a significant fraction is transported into streams, mainly through overland flow and loaded by the Rukarara River. The riverine DOC loss was low compared to the NPP of the RRW, but may affect the function of both land and water resources with the study area. The RHESSys model detected the response of the watershed to climate variability within the RRW and captured the significant monthly variability in streamflow within the RRW. This result indicates the potential use of RHESSys to estimate streamflow in the RRW and similar tropical watersheds. Stream DOC concentration was explained by simulated streamflow in the natural forest, indicating the potential use of RHESSys model simulated streamflow to predict stream DOC in the study watershed and similar ecosystems. Further studies should evaluate the performance of the RHESSys model to simulate other hydroecological processes in the tropical environment

Sources of soil dissolved organic carbon in a mixed agricultural and forested watershed in Rwanda

Dissolved organic carbon (DOC) plays a key role in linking terrestrial and aquatic carbon cycles. Most of the work on soil and water DOC has been conducted in temperate watersheds. There is still a gap in knowledge on DOC dynamics within the tropics. This study assesses water extractable organic carbon (WEOC) in topsoils and describes the relationship between WEOC and land use/land cover (LULC), slope position, curvature and soil properties using linear regression in the Rukarara River Watershed (RRW) in Rwanda. The study analyzes DOC concentration in soil percolation water (pDOC) and describes its relationship with antecedent precipitation index (API) and mean antecedent temperature (MAT) within the watershed using quadratic regression. Generalized linear model (GLM) and linear mixed effect model (LME) with site and/or LULC random effects are used to predict WEOC within the watershed. WEOC concentrations range from 124 to 855 mgC/L in the study area. The highest WEOC concentrations were observed in natural forest, followed by tree plantations, tea plantations and croplands. t-test results did not reveal a significant difference between concentrations of WEOC in valleys, upper slopes, ridges, flat, concave and convex areas in the Rukarara River Watershed (RRW). Considering the relationship between WEOC and soil properties, significant positive correlation coefficients were 0.60, 0.53, 0.50, and 0.36 respectively for the total organic carbon (TOC), the total nitrogen (TN), the cation exchange capacity (CEC), and the aluminum (Al). The best predictor WEOC as a function of soil properties was the generalized linear model (GLM) and indicated soil TOC as the overarching soil factor of WEOC in the RRW by 71%. The pDOC concentration ranges between 0.34 and 10.03 mgC/L and its relationship with both API and MAT was concave upward. APIs explained 12 to 17% of the pDOC variation in the RRW whereas MATs explained 8 to 25%, the natural forest site showing the highest values and the cropland site the lowest values. This result means that a conversion from forest to cropland within the RRW could decrease DOC in both soils and in percolation water. Also, an increase of precipitation and temperature up to respective optima in the RRW, could increase DOC in percolation water and consequently in streams. This increase of instream DOC can impact the water quality of the Rukarara River and its streams, with implications for their ecological function. Strategies of land management and water resources should be enhanced to preserve soil and water quality in the RRW

Spatial Pattern Analysis of Tarenna Rwandensis (Rubiaceae), an Endemic Tree Species of Nyungwe National Park (Rwanda)

The spatial pattern of individuals of T. rwandensis, an endemic ligneous species of Nyungwe National Park was analyzed. Individual was classified into three age-based categories: seedlings, saplings and adults to detect intraspecific interactions. Three rectangular plots of 21´9m each were made at Uwinka and each individual T. rwandensis identified was mapped using cartesian coordinates (x,y). The point pattern generated was analyzed by means of Ripley’s K function for univariate analysis and O-ring statistics for bivariate analysis to compare observed sets of values from the data. Our study revealed that T. rwandensis doesn’t regenerate adequately; we observed a very low number of seedlings compared to other age categories. The univariate analysis of the observed individuals showed that seedlings are clumped from the scale of 0.5 m up to 4m.  Saplings and adults show contrasting spatial patterns in different plots due to competition and environmental heterogeneity. Concerning bivariate analysis, the study showed neutral interactions between adults and seedlings for all considered scales in plot 1 whereas saplings exhibit neutral interactions around adults only at the scale of 0.5 up to 1.5m. In this plot 1, we found that seedlings and saplings have clear spatial neutral interactions for the scale of 0.5 up to 2.5m scale. In plot 1, seedlings show neutral interactions around adults for all considered whereas saplings exhibit neutral interactions around adults only at the scale of 0.5 up to 1.5m. Seedlings exhibit a clear spatial neutral interaction around saplings from 0.5 up to 2.5m scale in this plot 1. In the plots 2 and 3, spatial neutral interactions have been observed between adults and saplings for all scales considered. Key words: Spatial pattern, Tarenna rwandensis, regeneration, spatial interactions and Nyungwe National Park

Data for assessment of soil water extractable and percolation water dissolved organic carbon in watersheds

(“Sources of soil dissolved organic carbon in a mixed agricultural and forested watershed in Rwanda”, [6]) This data article presents water extractable organic carbon (WEOC), percolation water dissolved organic carbon (pDOC), and mean antecedent precipitation indices (API) and mean antecedent temperature (MAT) data. The article also presents edaphic properties such soil texture elements, total organic carbon (TOC), total nitrogen (TN), cation exchange capacity (CEC), iron (Fe), and aluminum (Al). Additionally, the article presents topography attributes such including topographic position index (TPI) and curvature. All these data were used to analyze both WEOC and pDOC dynamics in the Rukarara River Watershed (RRW), Rwanda. WEOC and soil properties data were analyzed from sampled 52 soil composites samples collected during from October to December 2016 using 53 × 50 mm rings. Data of pDOC were analyzed from percolation water samples collected using a zero tension lysimeters on various dates during the period from Jun 2015 to Jun 2017. API and MAT data for various antecedent days were calculated on basis of rainfall and air temperature data recorded at three stations within the RRW using respectively tipping bucket rain gauges and sensors installed at three sites located representing the main land use land cover classes within the RRW

Dissolved organic carbon leaching flux in a mixed agriculture and forest watershed in Rwanda

Study region: Rukarara River Watershed (RRW), Rwanda. Study focus: DOC leaching has important environmental consequences for both terrestrial and aquatic ecosystems. This paper measures leached dissolved organic carbon (LDOC) and the factors controlling its variation in the RRW, a mixed agriculture and forest watershed in Rwanda. The study describes the relationship of LDOC with land use/land cover (LULC), soil properties, rainfall characteristics, and stream DOC using linear regression and linear mixed effects models. New hydrological insights: The annual LDOC flux from topsoil to deeper soil horizons was 13.62 tonnes of carbon for the study area. This annual LDOC flux represents approximately 2% of the net primary productivity and 0.02% of the soil carbon stock. The plantation forest and natural forest sites showed higher LDOC fluxes than other LULC classes. Soil TOC, TN, rainfall intensity and amount positively affected LDOC flux (7 ≤ R2 ≤ 30%) in the RRW. Cation exchange capacity, runoff, and rainfall storage negatively affected the RRW (7 ≤ R2 ≤ 23%). LDOC flux explained 14% of the variation of stream DOC in the RRW. Our results imply that, under ongoing climate and LULC changes, an increase of the proportion of plantation forest and rainfall intensity throughout the watershed will increase LDOC flux, and will have a weak positive effect on stream DOC

Data for assessment of leached dissolved organic carbon in watersheds

(“Dissolved organic carbon leaching flux in a mixed agriculture and forest watershed in Rwanda” [1]). This article presents data of leached dissolved organic carbon (LDOC), stream water dissolved organic carbon), rainfall amount (Ra), rainfall intensity (Ri), rainfall soil storage (S), runoff (Q), and soil properties such as total organic carbon (TOC), total nitrogen (TN), cation exchange capacity (CEC), and soil texture data collected in the Rukarara River Watershed (RRW), a tropical watershed. All these data were used to analyze leached dissolved organic carbon (LDOC) fluxes in the watershed and their relationship with stream DOC. LDOC and soil properties data were collected at three sites in multiple plots per site located in natural forest (NF), tea plantations (TP), plantation forests (PF), and croplands (CL). Twenty-three plots in total were sampled to collect LDOC data. Soil properties data were analyzed from soil samples collected nearby the plots. Soil texture elements data were used to calculate soil porosity and saturated hydraulic conductivity (Ks). Data of stream DOC were analyzed from water samples collected and analyzed in the laboratory using a TOC analyzer. Rainfall data were recorded within the RRW using tipping bucket rain gauges installed at three sites. These rainfall data were used to calculate rainfall intensity, potential surface runoff, and rainfall soil storage

Data for dynamics analysis of riverine dissolved organic in watersheds

This data article presents water stage, flow, and net primary productivity (NPP) data that were used to analyze the dynamics of the riverine dissolved organic carbon (DOC) dynamics in the Rukarara River watershed in Rwanda. We measured water stage data every 15 min and calculated daily averages used to estimate flow based on rating curves. The rating curves were produced using several measured contemporaneous water stage and flow data. Estimated flow data were used to calculate water dissolved organic carbon (DOC) loadings separate the total stream flow into quick and baseflow. Annual NPP data for a 15-year period were used to estimate the effect of proportion of stream DOC loading on carbon sequestration within the Rukarara River watershed

Riverine dissolved organic carbon in Rukarara River Watershed, Rwanda

Dissolved organic carbon (DOC) loading is rarely estimated in tropical watersheds. This study quantifies DOC loading in the Rukarara River Watershed (RRW), a Rwandan tropical forest and agricultural watershed, and evaluates its relationship with hydrological factors, land use and land cover (LULC), and topography to better understand the impact of stream DOC export on watershed carbon budgets. The annual average load for the study period was 977.80 kg C, which represents approximately 8.44% of the net primary productivity of the watershed. The mean daily exports were 0.37, 0.14, 0.075 and 0.32 kg C/m2 in streams located in natural forest, tea plantation, small farming areas, and at the outlet of the river, respectively. LULC is a factor that influences DOC loading. The quick flow was the main source of stream DOC at all study sites. Stream DOC increases with increasing water flow, indicating a positive relationship. Thus, the expectation is that a change in land cover and/or rainfall will result in a change of stream DOC dynamics within the watershed. Topography was also found to influence the dynamics of stream DOC through its effect on overland flow in terms of drainage area and total length of flow paths. Tea plantations were located in areas of high drainage density and projected increase of rainfall in the region, as a consequence of climate change, could increase stream DOC content and affect stream water quality, biodiversity, balance between autotrophy and heterotrophy, and bioavailability of toxic compounds within the RRW