Hydrology of the Sudanian Savannah in West Africa, Burkina Faso

As in all semi-arid regions, Tambarga, a small village surrounded by national parks in the landlocked country of Burkina Faso, is affected by the seasonality of the local hydrology. Seasonal and spatial variability of rainfall shapes the livelihoods of rural farmers who depend mainly on rain-fed agriculture. We instrumented the Tambarga catchment (area=~4 km2) to investigate the rainfall patterns, the resulting hydrologic processes (surface runoff, base flow) and the evaporation effects on the local water balance. Thus, we have measured hydrological, meteorological and soil parameters at high spatial and temporal resolution over the catchment since 2009. Data for this research were acquired from the 2010 to 2012 rainy seasons using a network of automatic wireless weather stations (SensorScope stations), two weirs, eight piezometric wells, and two surface energy balance stations. Rain at Tambarga is triggered by the convective mechanism, which is mainly controlled by the sensible heat flux. The daytime rain events at Tambarga are convective and are characterized by their high intensities and short durations. An overall increase in rainfall of 10-30% is observed in the savannah forest when compared to the agricultural field from 2010 to 2012. Biotic (leaf area index) and abiotic (well drained soil) factors, causing enhanced sensible heat flux at the savannah site, lead to an increased predisposition toward convective rainfall. The hydrologic processes concomitant to the rain events were identified over the basin and intermittent responses of streamflow were isolated. The rise of the perched water tables located in the upstream and downstream regions of the basin generated two separate flows in the riverbed after the first rain. The intermediate zone operates like a deep storage tank, and by filling, it creates a connection between the two perched water tables. This connection induces a continuous flow throughout the 2.8 km of the stream. The hydrologic response at the basin outlet is observed to alternate between two different states over the course of the season. At the start of the rainy season, when the soils are very dry and the groundwater level is deep, a typical single-peak hydrograph is observed. This evolves into a double-peak hydrograph when the rainy season is completely established. The single-peak hydrograph is correlated with rainfall intensity, while the double-peak hydrograph is also correlated with the antecedent soil moisture condition. The baseflow, meanwhile, occurs when the groundwater level is higher than the riverbed at certain locations in the basin. In contrast, during dry days (no rain) the baseflow exhibits a diurnal flow pattern. This diurnal pattern consists of decreased flow rate from sunrise (6:00 A.M.) to 1:00 P.M. and flow rate recovery to its previous level from 1:00 P.M. to sunset (6:00 P.M.). The diurnal pattern of the streamflow was found to be derived both from infiltration in the riverbed and evaporation over the river edges. The infiltration process dominates during the beginning of the season. While late in the season, when the groundwater network is interconnected and its overall level is higher than the riverbed elevation, evaporation controls the diurnal pattern. An evaporation contributing area is defined when the diurnal pattern is controlled by the evaporation. This area is about 0.6% of the basin area and could be rationally represented by the riparian area and the outlet wetland. Given the importance of rainfall-runoff processes over this catchment, a water balance was completed based on a simple lumped model. This model allowed for testing some water management strategies to improve agricultural production. Its implementation suggests that storing water underground for irrigation purpose with deeper wells is a way to achieve better agricultural productivity and therefore, reduce the vulnerability of the local people

Depletion of Stem Water of Sclerocarya birrea Agroforestry Tree Precedes Start of Rainy Season in West African Sudanian Zone

Understanding water use by agroforestry trees in dry-land ecosystems is essential for improving water management. Agroforestry trees are valued and promoted for many of their ecologic and economic benefits but are often criticized as competing for valuable water resources. In order to understand the seasonal patterns of source water used by agroforestry trees, samples from rain, ground, and surface water were collected weekly in the subcatchment of the Singou watershed that is part of the Volta Basin. Soil and vegetation samples were collected from and under a Sclerocarya birrea agroforestry trees located in this catchment in sealed vials, extracted, and analyzed with a Picarro L2130-i CRDS to obtain both O18 and DH fractions. Meteorological measurements were taken with a network of wireless, autonomous stations that communicate through the GSM network (Sensorscope) and two complete eddy-covariance energy balance stations, in addition to intense monitoring of sub-canopy solar radiation, throughfall, stemflow, and soil moisture. Examination of the time series of O18 concentrations confirm that values in soil and xylem water are coupled, both becoming enriched during the dry season and depleted during the rainy season. Xylem water O18 levels drops to groundwater O18 levels in early March when trees access groundwater for leafing out, however soil water does not reach this level until soil moisture increases in mid-June. The relationship between the DH and O18 concentrations of water extracted from soil and tree samples do not fall along the global meteoric water line. In order to explore whether this was a seasonally driven, we grouped samples into an “evaporated” group or a “meteoric” group based on the smaller residual to the respective lines. Although more soil samples were found along the m-line during the rainy season than tree samples or dry season soil samples, there was no significant difference in days since rain for any group This suggests that xylem water is always under stress from evapotranspiration and soil water underwent evaporation soon after a rain event. Visual observation of tree confirms conclusion that trees access deep ground water in March and April, before rain begins and before soil is connected to groundwater. Results from the research are being integrated into a local outreach project to improve use of agroforestry

Hydrologic Activity of Deciduous Agroforestry Tree Observed through Monitoring of Stable Isotopes in Stem Water, Solar Radiation Attenuation, and Sapflow

The net benefit of agroforestry trees for small scale farmers in dryland agricultural systems is debatable because while they provide significant direct and indirect services, they also consume considerable amounts of scare water resources. In this study we monitor the stable isotopes of water to improve a water budget of a Sclerocarya birrea tree in a millet field in South Eastern Burkina Faso. Data obtained from air temperature and humidity, surface temperature, solar radiation, and soil moisture sensors attached to a wireless sensor network uniquely configured around the agroforestry tree provided the initial calculation of the local water balance. Isotopic ratios were determined from water extracted from stems and sub canopy soil, and from nearby ground water, precipitation, and surface water that was sampled weekly. A linear mixing model is used to predict when the tree switched between water sources. The results from the linear mixing model coupled with a tree water balance demonstrate the extreme seasonality of the annual cycle of water use by this deciduous species

Hydrology and density feedbacks control the ecology of intermediate hosts of schistosomiasis across habitats in seasonal climates

We report about field and theoretical studies on the ecology of the aquatic snails (Bulinus spp. and Biomphalaria pfeifferi) that serve as obligate intermediate hosts in the complex life cycle of the parasites causing human schistosomiasis. Snail abundance fosters disease transmission, and thus the dynamics of snail populations are critically important for schistosomiasis modeling and control. Here, we single out hydrological drivers and density dependence (or lack of it) of ecological growth rates of local snail populations by contrasting novel ecological and environmental data with various models of host demography. Specifically, we study various natural and man-made habitats across Burkina Faso's highly seasonal climatic zones. Demographic models are ranked through formal model comparison and structural risk minimization. The latter allows us to evaluate the suitability of population models while clarifying the relevant covariates that explain empirical observations of snail abundance under the actual climatic forcings experienced by the various field sites. Our results link quantitatively hydrological drivers to distinct population dynamics through specific density feedbacks, and show that statistical methods based on model averaging provide reliable snail abundance projections. The consistency of our ranking results suggests the use of ad hoc models of snail demography depending on habitat type (e.g., natural vs. man-made) and hydrological characteristics (e.g., ephemeral vs. permanent). Implications for risk mapping and space-time allocation of control measures in schistosomiasisendemic contexts are discussed

A Theoretical Analysis of the Geography of Schistosomiasis in Burkina Faso Highlights the Roles of Human Mobility and Water Resources Development in Disease Transmission

We study the geography of schistosomiasis across Burkina Faso by means of a spatially explicit model of water-based disease dynamics. The model quantitatively addresses the geographic stratification of disease burden in a novel framework by explicitly accounting for drivers and controls of the disease, including spatial information on the distributions of population and infrastructure, jointly with a general description of human mobility and climatic/ecological drivers. Spatial patterns of disease are analysed by the extraction and the mapping of suitable eigenvectors of the Jacobian matrix subsuming the stability of the disease-free equilibrium. The relevance of the work lies in the novel mapping of disease burden, a byproduct of the parametrization induced by regional upscaling, by model-guided field validations and in the predictive scenarios allowed by exploiting the range of possible parameters and processes. Human mobility is found to be a primary control at regional scales both for pathogen invasion success and the overall distribution of disease burden. The effects of water resources development highlighted by systematic reviews are accounted for by the average distances of human settlements from water bodies that are habitats for the parasite's intermediate host. Our results confirm the empirical findings about the role of water resources development on disease spread into regions previously nearly disease-free also by inspection of empirical prevalence patterns. We conclude that while the model still needs refinements based on field and epidemiological evidence, the proposed framework provides a powerful tool for large-scale public health planning and schistosomiasis management

Workshop Presentation: Sensors for Sustainability, Info4dourou.

Presentation of Info4Dourou Projec

Space and time predictions of schistosomiasis snail host population dynamics across hydrologic regimes in Burkina Faso

The ecology of the aquatic snails that serve as obligatory intermediate hosts of human schistosomiasis is driven by climatic and hydrological factors which result in specific spatial patterns of occurrence and abundance. These patterns in turn affect, jointly with other determinants, the geography of the disease and the timing of transmission windows, with direct implications for the success of control and elimination programmes in the endemic countries. We address the spatial distribution of the intermediate hosts and their seasonal population dynamics within a predictive ecohydrological framework developed at the national scale for Burkina Faso, West Africa. The approach blends river network-wide information on hydrological ephemerality which conditions snail habitat suitability together with ensembles of discrete time ecological models forced by remotely sensed estimates of temperature and precipitation. The models were validated against up to four years of monthly snail abundance data. Simulations of model ensembles accounting for the uncertainty in remotely sensed products adequately reproduce observed snail demographic fluctuations observed in the field across habitat types, and produce national scale predictions by accounting for spatial patterns of hydrological conditions in the country. Geospatial estimates of seasonal snail abundance underpin large-scale, spatially explicit predictions of schistosomiasis incidence. This work can therefore contribute to the development of disease control and elimination programmes

Info4Dourou: sustainable management of the natural resources of the Singou watershed, Burkina Faso

Presentation of the combined research development project

Comparative analysis of time-based and quadrat sampling in seasonal population dynamics of intermediate hosts of human schistosomes

Background Despite their importance for designing and evaluating schistosomiasis control trials, little attention in the literature has been dedicated to sampling protocols for the parasite's snail intermediate hosts since their first development. We propose a comparative analysis of time-based and quadrat sampling protocols to quantify the seasonal variations in the abundance of these aquatic snail species of medical importance. Methodology/Principal findings Snail populations were monitored during 42 consecutive months in three types of habitats (ephemeral pond, ephemeral river and permanent stream) in two sites covering different climatic zones in Burkina Faso. We employed both a widely used time-based protocol of 30min of systematic collection at a weekly interval, and a quadrat protocol of 8 replicates per sample at a monthly interval. The correspondence between the two protocols was evaluated using an ensemble of statistical models including linear and saturating-type functional forms as well as allowing for count zero-inflation. The quadrat protocol yielded on average a relative standard error of 40%, for a mean snail density of 16.7 snails/m2 and index of dispersion of 1.51. Both protocols yielded similar seasonal patterns in snail abundance, confirming the asynchrony between permanent and ephemeral habitats with respect to the country's seasonal rainfall patterns. Formal model comparison of the link between time vs. quadrat counts showed strong support of saturation for the latter and measurement zero-inflation, providing important evidence for the presence of density feedbacks in the snail's population dynamics, as well as for spatial clustering. Conclusions/Significance In addition to the agreement with the time-based method, quadrat sampling provided insight into snail population dynamics and comparable density estimates across sites. The re-evaluation of these "traditional" sampling protocols, as well as the correspondence between their outputs, is of practical importance for the design and evaluation of schistosomiasis control trials

Classification and prediction of river network ephemerality and its relevance for waterborne disease epidemiology

The transmission of waterborne diseases hinges on the interactions between hydrology and ecology of hosts, vectors and parasites, with the long-term absence of water constituting a strict lower bound. However, the link between spatio-temporal patterns of hydrological ephemerality and waterborne disease transmission is poorly understood and difficult to account for. The use of limited biophysical and hydroclimate information from otherwise data scarce regions is therefore needed to characterize, classify, and predict river network ephemerality in a spatially explicit framework. Here, we develop a novel large-scale ephemerality classification and prediction methodology based on monthly discharge data, water and energy availability, and remote-sensing measures of vegetation, that is relevant to epidemiology, and maintains a mechanistic link to catchment hydrologic processes. Specifically, with reference to the context of Burkina Faso in sub-Saharan Africa, we extract a relevant set of catchment covariates that include the aridity index, annual runoff estimation using the Budyko framework, and hysteretical relations between precipitation and vegetation. Five ephemerality classes, from permanent to strongly ephemeral, are defined from the duration of 0-flow periods that also accounts for the sensitivity of river discharge to the long-lasting drought of the 70's-80's in West Africa. Using such classes, a gradient-boosted tree-based prediction yielded three distinct geographic regions of ephemerality. Importantly, we observe a strong epidemiological association between our predictions of hydrologic ephemerality and the known spatial patterns of schistosomiasis, an endemic parasitic waterborne disease in which infection occurs with human-water contact, and requires aquatic snails as an intermediate host. The general nature of our approach and its relevance for predicting the hydrologic controls on schistosomiasis occurrence provides a pathway for the explicit inclusion of hydrologic drivers within epidemiological models of waterborne disease transmission