Gérard Cochonneau

Heterogeneous Distribution of Rainfall and Discharge Regimes in the Ecuadorian Amazon Basin

The hydrology of the Ecuadorian Amazon basin is still poorly documented. The research developed between the Institute for Research and Development (IRD) and the National Institute of Meteorology and Hydrology (INAMHI) of Ecuador takes advantage of a newly available rainfall and discharge dataset to explore spatial distribution and regimes in this region (135 600 km 2 ). Forty-seven rainfall and 27 discharge stations were retained over a 30-yr period (1965-94). A new annual isoyets map is proposed for the Amazon basin of Ecuador. The most striking result is a high spatial regime variability, to the extent that out-of-phase regimes are found in nearby stations. Indeed, in high intra-Andean basins there is a marked bimodal rainfall regime, with maxima in March-April and in October and a minimum from June to August. On the contrary, as moisture transport reaches a maximum in June, a rainfall peak can also be observed at the same time on slopes to the east due to the moist easterlies. In the lowlands, a bimodal regime is observed as in the intra-Andean basin; as water vapor is abundant, seasonal variability is not as strong as in the Andes. Discharge regimes are related to rainfall regimes. However, even if no delay is observed between maximum rainfall and maximum discharge in the inner Andean basins, a delay varying from one to two months is observed in larger basins. Soil conditions also affect the discharge regime in the upper and inner Andes: paramo soils (a kind of tundra formation) release water, in particular after the March-April rainfall peak, and sustain the dry season runoff.

Monitoring water level in large trans-boundary ungauged basins with altimetry: the example of ENVISAT over the Amazon basin

Brasil and Bolivia have water plans projects on the Beni-Madeira river, a major tributary of the Amazon. There are four main tributaries to the Rio Madeira: the Guapore, the Mamore and the Beni rivers into the Bolivian territory, and the Madre de Dios River crossing the North of Bolivia, coming from Peru. Most parts of these rivers are very far from the Andean capital cities of Bolivia and Peru, unreachable for long periods of time. Very few gauging stations are in operation, either for the Bolivian or the Peruvian part, most of them being located at the Andes piedmont or near the confluence at the Brazilian border as they form the Madeira river. This situation is exemplary of large transboundary basins in the tropical part of the world. We have computed 39 water level time series using ENVISAT altimetry data over the four tributaries of the Madeira and the Madeira itself. We present a preliminary study mostly conducted onto the Guapore river, in order to assess the quality of these time series for a variety of situations, but mostly narrow and meandering riverbeds. Comparison between water levels variation in the mainstream and within the inundations plains and lakes are drawn. We conclude by the perspectives offered by the combined use of radar altimetry and SAR imagery for the global monitoring of water resources, in large tropical transboundary basins.

Monitoring water turbidity and surface suspended sediment concentration of the Bagre Reservoir (Burkina Faso) using MODIS and field reflectance data

Abstract Monitoring turbidity and Surface Suspended Sediment Concentration (SSSC) of inland waters is essential to address several important issues: erosion, sediment transport and deposition throughout watersheds, reservoir siltation, water pollution, human health risks, etc. This is especially important in regions with limited conventional monitoring capacities such as West Africa. In this study, we explore the use of Moderate Resolution Imaging Spectroradiometer data (MODIS, MOD09Q1 and MYD09Q1 products, red (R) and near infrared (NIR) bands) to monitor turbidity and SSSC for the Bagre Reservoir in Burkina Faso. High values ​​of these parameters associated with high spatial and temporal variability potentially challenge the methodologies developed so far for less turbid waters. Field measurements (turbidity, SSSC, radiometry) are used to evaluate different radiometric indices. The NIR/R ratio is found to be the most suited to retrieve SSSC and turbidity for both in-situ spectoradiometer measurements and satellite reflectance from MODIS. The spatio temporal variability of MODIS NIR/R together with rainfall estimated by the Tropical Rainforest Measuring Mission (TRMM) and altimetry data from Jason-2 is analyzed over the Bagre Reservoir for the 2000–2015 period. It is found that rain events of the early rainy season (February-March) through mid-rainy season (August) are decisive in triggering turbidity increase. Sediment transport is observed in the reservoir from upstream to downstream between June and September. Furthermore, a significant increase of 19% in turbidity values is observed between 2000 and 2015, mainly for the July to December period. It is especially well marked for August, with the central and downstream areas showing the largest increase. The most probable hypothesis to explain this evolution is a change in land use, and particularly an increase in the amount of bare soils, which enhances particle transport by runoff.

Fluctuations in the monthly discharge of Guyana Shield rivers, related to Pacific and Atlantic climate variability

Abstract The discharge variability of the main rivers that drain the Guyana Shield is analysed over the last 50 years using cross-wavelet, coherence and composite analysis involving oceanic and atmospheric variables. We highlight the overall hydro-climatological homogeneity of this region that allowed us to focus on the longest discharge time series available. Therefore, a wavelet cross-analysis was carried out between monthly and seasonal Maroni River discharge at the Langa Tabiki station and selected climate indices. This confirms a strong relationship between the hydrology of the Guyana Shield and the Pacific sea-surface temperature (SST) fluctuations. There is evidence of intermittent influence, of between inter-annual and near decadal scales, of the Atlantic SST fluctuations, in particular around 1970 and 1990. Finally, we show that the links between oceanic regions and high discharge in the rivers of Guyana are realized through the reinforcement of the Walker and Hadley cells between the Amazon and the adjacent oceans and through decreased trade winds and monsoon flux that favour the persistence of humidity over the Guyana Shield. Editor Z.W. Kundzewicz; Associate editor D. Hughes Citation Labat, D., Espinoza, J.-C., Ronchail, J., Cochonneau, G., de Oliveira, E., Doudou, J.C. and Guyot, J.-L., 2012. Fluctuations in the monthly discharge of Guyana Shield rivers, related to Pacific and Atlantic climate variability. Hydrological Sciences Journal, 57 (6), 1081–1091.

Inundation Risk in Large Tropical Basins and Potential Survey from Radar Altimetry: Example in the Amazon Basin

Altimetry turns out to be a highly valuable source of data in remote areas such as the unmonitored wetlands at the upstream part of contributors in continental-scale basins. We illustrate this for the Amazon basin, in the area of upper Rio Madeira, where the inundation risk increased recently on populations downstream because of changes in the local economy. With use of satellite altimetry, the hydrological relationship between the main stem and the related water bodies that cyclically retain and liberate large amounts of water can be explained with sufficient details to aid in risk mitigation.

Surface water quality monitoring in large rivers with MODIS data application to the amazon basin

In this study, we propose to assess the use of MODIS images to monitor interannual variation of sediment flow by comparing surface reflectance data and field measurements of suspended sediment (SS) concentration collected every 10 days in surface waters of different locations of the Amazon River in Brazil. These data come from the HYBAM measurement network that measures the hydrology and geochemistry of the main rivers of the Amazon basin. In particular, we assess the effect of river width by analyzing different locations corresponding to river widths going from 500 meters to several kilometers. Several MODIS products are considered that include 250-meter and 500-meter resolution images and 8-day composites from Terra and Aqua on-board MODIS sensors. We show that the SS concentration can be monitored accurately over different hydrological cycles.