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.

The effect of seasonal soil frost on the alpine groundwater recharge including climate change aspects

Daniel Bayard

In alpine areas, the snow cover plays an important role as a water reservoir. Water is stored as snow over the winter and released in spring, recharging mountain aquifers through infiltration. These aquifers are essential, especially for supplying water for human activities during dry seasons. Numerous studies have shown that locally soil frost can drastically reduce the water infiltration. However, we know much less about the hydrological impact of soil frost at a larger scale, in particular with regard to groundwater recharge. This was our motivation for initiating an extensive field experiment in the southern Swiss Alps. Several methods at different spatial scales were adapted to explore (a) the local effect of a partially frozen ground on the snowmelt discharge in an alpine area, (b) the key processes influencing groundwater recharge during snowmelt periods, such as snow cover and soil frost evolution, snowmelt water runoff types, (c) the large-scale effect of soil frost on the aquifer recharge, and (d) winter situations that are critical with respect to flooding. In order to take into account spatial, altitudinal and climatic differences, the field study was run for two winter seasons at Gd St Bernard (2500 m), a site with very high winter precipitation and strong winds, and at Hannigalp above Grächen (2100 m), a ski resort with a rather dry winter climate. The different components of the soil water balance (lateral runoff, deep percolation, liquid soil water content) were measured on delimited plots of 5 m2. Additionally, a dye tracer experiment visualized the snowmelt infiltration patterns into the frozen, respectively non-frozen ground. At Gd St Bernard, investigations of the liquid water content and the soil temperature were additionally conducted at two secondary sites, differing in their orientation, so as to gain a better insight of the spatial soil frost expansion. Finally, an analysis of the water-table variation at the village of Grächen (some 500 m below Hannigalp) during the snowmelt was carried out for a 10-year time period, including our experimental seasons. We complemented our field investigations by using a numerical soil-snow-atmosphere transfer model that simulates the seasonal development of the soil frost and the snow cover, as well as lateral and vertical water flow in the top soil. This model was used as an upper boundary condition to a 2-D groundwater model, to calculate the outlet flow at the base of a simplified alpine aquifer. The two winters investigated had contrasting meteorological conditions, which resulted in intriguing differences and similarities with respect to the soil physical conditions: the first one having a thick snowpack and hardly any soil frost, and the second one with hardly any snow until January and a deep and persistent soil frost. In spite of a very thick snowpack, the water balance measurements from winter 2000/2001 showed for both sites zero to low surface and subsurface flow due to the fact that soil frost was local and the soil permeability high. During the next winter, approximately 25% of the total meltwater run off laterally. The soil infiltration capacity was mainly reduced by the presence of a basal ice sheet, caused by the freezing of meltwater after a mid-winter snowmelt event. The field data were used to calibrate and validate a physically-based one-dimensional model. To allow an accurate parameter setting for the water repellent soil of Hannigalp, laboratory infiltration experiments on soil columns were additionally carried out. Using the resultant information, the model predicted correctly most measured soil parameters, in particular good matching was observed between measured and simulated snowmelt discharge. At Gd St Bernard, the simulated results were less satisfactory, especially during the first winter, when large discrepancies were noted between measured and simulated lateral runoff. Apart from the shortage in the soil parameter characterization, the deviation was a result of the simplified description of the experimental field. The model was, for example, not able to simulate surface runoff generated by a steep slope under unfrozen conditions, as the soil surface is considered by the model to be flat. Nevertheless, results were accurate enough to reproduce the occurrences of snowmelt events and the accumulated values of snow-melt discharge pathways under frozen conditions. Long-term weather data were used to simulate the soil frost depth at both sites. The simulation results showed that the probability of the occurrence of frozen soils was lowest on slopes with south or north exposure. On southerly exposed plots, the heat stored in the soil during the summer inhibited the development of deep and persistent soil frost, while most pore ice thawed due to underneath heating. On north plots, the building of soil frost was rare, as early snowfall insulated the ground from the atmosphere. However, during snow-poor winters, the frost penetrated deep into the ground, and the low underneath heat flux at that location hardly affected the frost depth until snowmelt. The snow depth and average soil temperature were hence the two components affecting the frost depth extent. The altitude soil frost variation was related to the spatial extent of these two components. The higher the altitude, the higher the precipitation and the lower the mean air/surface temperature. Simulating the whole Grächen recharge area, seasonal soil frost was rare between 1800 m and 2000 m, as the snowpack was thick enough to insulate the ground, and the soil was warm enough to inhibit deep soil frost. In lower areas, the shallow snowpack enabled the soil to freeze during most winters, whereas at higher areas, deep and persistent soil frost was simulated due to the low underneath heating. A 10-year water-table depth record was used to examine possible large-scale effects of seasonal soil frost. We noted that the water-table rise at snowmelt was lowest at the end of winters with extensive soil frost. However, such winters were characterized by low precipitation, and reduced snow cover. In contrast, winters with little soil frost were mostly snow rich. Consequently, only a reduced number of winters were directly comparable. Considering these winters, the reduction in the water-table rise at snowmelt due to seasonal soil frost varied only between 10 and 30%, as, during snowmelt, most meltwater was able to re-infiltrate into the very permeable ground in lower unfrozen areas. We may assume that a soil with a smaller infiltration capacity can increase the influence of the soil frost on the aquifer recharge. Finally, we investigated the effect of a changing climate on the hydraulic and thermic regimes at different altitudes in the area of Grächen. We assume an increase in the air temperature of 2°C generates deeper soil frost on lower areas, as the resultant smaller snow cover does not insulate the soil from the atmosphere any more. However, it does reduce the soil frost on higher, snow rich areas, due to the warmer air temperature. As a whole, minor changes were simulated on the mean groundwater recharge, as more/less meltwater was able to infiltrate in higher/lower areas. By increasing additionally the precipitation by 15%, the soil frost diminished slightly over the whole catchment. By combining both climate change scenarios, we may expect an increase in extreme events, as, on the one hand, the rainfall intensity increases, and, on the other hand, rain on snow events over frozen soils occurs more often in lower areas. In practice, this work shows the importance of better investigating the hydrothermal behaviour of alpine regions. Such a knowledge is necessary, as more and more efforts are put into managing efficiently, durably and globally the available water resources. This work stresses also the necessity to consider seasonally soil frost in alpine risk management. As an example, flooding is mostly a consequence of strong rain precipitation at high altitudes combined with heavy snowmelt. Obviously, the risk of extreme runoff events increases during frozen winters, when the soil infiltration capacity is further reduced by the soil ice. Also, it has been shown that unstable slopes are mostly related to specific hydrogeological situations, like strong precipitation, which induces a sharp increase in the underneath water circulation. The melt water in spring is hence a potential activating factor, and a frozen soil may reduce the risk of sliding, as less water infiltrates into the ground.

EPFL2003

Observation of Sensible and Latent Heat Fluxes Over a Mixed Savanna – Agricultural Catchment in the Semi-Arid Sudanian Savanna, Burkina Faso

Natalie Claire Ceperley, Theophile Mande, Marc Parlange, Andrea Rinaldo, Scott W. Tyler, Nicolaas Cornelis van de Giesen

Evaporation and the surface energy balance over two land cover types in Sudanian savanna were investigated in terms of their seasonal and interannual variability using four years of eddy covariance measurement (2009-2013). The observations demonstrate the strong controls on evaporation by vegetation through modification of the surface roughness, albedo, and available moisture related to seasonal variations in land cover and topography. Two characteristic land uses of semi-arid West Africa (Burkina Faso) were monitored: agricultural fields and gallery forest. The sites receive around 800 mm of rain most years, typically between the months of May and October, with up to approximately 20% being transferred to runoff. Seasonal variation in all components of the energy balance was found to be greater over the agricultural landscape, the total latent energy flux over the gallery forest was higher by approximately 30%, and the evaporative fraction was more dependent on soil moisture in the agricultural fields. According to the diurnal behavior of the evaporative fraction, we isolate constant periods that we compare with measures of cloudy sky, NDVI and soil moisture. We identify predictors of diurnal variation of the evaporative fraction according to land cover, which allows incorporation of remote sensing and satellite data. Improved understanding of the variability of fluxes over diverse land cover will allow us to improve the estimation of evaporation to the atmosphere over the entire watershed. Ongoing outreach projects accompany this research to integrate findings with local knowledge to improve resilience of small-scale rain fed agriculture.

2013

Use of a Distributed Sensor Network to Parameterize a Model of Flows between the Soil, Vegetation, and Atmosphere in a Mixed Savanna-Agricultural Catchment in South Eastern Burkina Faso.

Natalie Claire Ceperley

The hydrologic processes underlying a natural savanna in a semi-arid environment is altered when savanna is converted to agriculture. Inﬁltration is often observed to decrease while rapid runoff increases to the detriment of crop productivity, downstream populations and local water sources. Planting a mix of vegetation in cultivated fields, mimicking the dispersal of trees in a savanna has been proposed as a solution to help buffer these consequences. However the hydrologic behavior of savanna, agricultural, and agroforestry land cover is not fully understood for the Sudanian vegetation zone of West Africa and the consequences of land use changes are debatable. We use a distributed sensor network of seventeen wireless meteorological stations including soil moisture sensors in addition to two eddy covariance stations and sap ﬂow probes dispersed across cultivated rice and millet ﬁelds, natural savanna, fallow ﬁelds, and around agroforestry trees, specifically Sclerocarya birrea, to understand land cover controls on variations in evapotranspiration and infiltration over a semi arid watershed in Southeastern Burkina Faso. N ormalized difference vegetation indexes taken from weekly MODIS images as well as personal field observations were used to inform seasonal and spatial variations in albedo, rate of transfer to ground heat flux, and roughness length. Measurements are used parameterize a distributed model of energy and hydrologic fluxes between the soil and atmosphere controlled by the vegetated surface and presence of agroforestry trees. We use this model to further understanding of the effect of Sudanian land cover on the water and energy balances by exploring the effects of modifications in the arrangement and dispersion of vegetation types. Outreach work in the community is on-going and our hope is that the results of our modeling will inform local farming practices.