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7 pages, 238 KiB

Open AccessEditorial

Special Issue “Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology”

by Gil Mahé, Luc Descroix, Alain Laraque, Olivier Ribolzi and Guillaume Lacombe

Water 2021, 13(4), 491; https://doi.org/10.3390/w13040491 - 14 Feb 2021

Cited by 1 | Viewed by 2304

Abstract

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(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

Research

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32 pages, 10102 KiB

Open AccessArticle

Are the Fouta Djallon Highlands Still the Water Tower of West Africa?

by Luc Descroix, Bakary Faty, Sylvie Paméla Manga, Ange Bouramanding Diedhiou, Laurent A. Lambert, Safietou Soumaré, Julien Andrieu, Andrew Ogilvie, Ababacar Fall, Gil Mahé, Fatoumata Binta Sombily Diallo, Amirou Diallo, Kadiatou Diallo, Jean Albergel, Bachir Alkali Tanimoun, Ilia Amadou, Jean-Claude Bader, Aliou Barry, Ansoumana Bodian, Yves Boulvert, Nadine Braquet, Jean-Louis Couture, Honoré Dacosta, Gwenaelle Dejacquelot, Mahamadou Diakité, Kourahoye Diallo, Eugenia Gallese, Luc Ferry, Lamine Konaté, Bernadette Nka Nnomo, Jean-Claude Olivry, Didier Orange, Yaya Sakho, Saly Sambou and Jean-Pierre Vandervaere Show full author list Hide full author list

Water 2020, 12(11), 2968; https://doi.org/10.3390/w12112968 - 22 Oct 2020

Cited by 8 | Viewed by 7205

Abstract

A large share of surface water resources in Sahelian countries originates from Guinea’s Fouta Djallon highlands, earning the area the name of “the water tower of West Africa”. This paper aims to investigate the recent dynamics of the Fouta Djallon’s hydrological functioning. The [...] Read more.

A large share of surface water resources in Sahelian countries originates from Guinea’s Fouta Djallon highlands, earning the area the name of “the water tower of West Africa”. This paper aims to investigate the recent dynamics of the Fouta Djallon’s hydrological functioning. The evolution of the runoff and depletion coefficients are analyzed as well as their correlations with the rainfall and vegetation cover. The latter is described at three different space scales and with different methods. Twenty-five years after the end of the 1968–1993 major drought, annual discharges continue to slowly increase, nearly reaching a long-term average, as natural reservoirs which emptied to sustain streamflows during the drought have been replenishing since the 1990s, explaining the slow increase in discharges. However, another important trend has been detected since the beginning of the drought, i.e., the increase in the depletion coefficient of most of the Fouta Djallon upper basins, as a consequence of the reduction in the soil water-holding capacity. After confirming the pertinence and significance of this increase and subsequent decrease in the depletion coefficient, this paper identifies the factors possibly linked with the basins’ storage capacity trends. The densely populated areas of the summit plateau are also shown to be the ones where vegetation cover is not threatened and where ecological intensification of rural activities is ancient. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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28 pages, 28887 KiB

Open AccessArticle

UAV and LiDAR Data in the Service of Bank Gully Erosion Measurement in Rambla de Algeciras Lakeshore

by Radouane Hout, Véronique Maleval, Gil Mahe, Eric Rouvellac, Rémi Crouzevialle and Fabien Cerbelaud

Water 2020, 12(10), 2748; https://doi.org/10.3390/w12102748 - 1 Oct 2020

Cited by 15 | Viewed by 2918

Abstract

The Rambla de Algeciras lake in Murcia is a reservoir for drinking water and contributes to the reduction of flooding. With a semi-arid climate and a very friable nature of the geological formations at the lakeshore level, the emergence and development of bank [...] Read more.

The Rambla de Algeciras lake in Murcia is a reservoir for drinking water and contributes to the reduction of flooding. With a semi-arid climate and a very friable nature of the geological formations at the lakeshore level, the emergence and development of bank gullies is favored and poses a problem of silting of the dam. A study was conducted on these lakeshores to estimate the sediment input from the bank gullies. In 2018, three gullies of different types were the subject of three UAV photography missions to model in high resolution their low topographic change, using the SfM-MVS photogrammetry method. The combination of two configurations of nadir and oblique photography allowed us to obtain a complete high-resolution modeling of complex bank gullies with overhangs, as it was the case in site 3. To study annual lakeshore variability and sediment dynamics we used LiDAR data from the PNOA project taken in 2009 and 2016. For a better error analysis of UAV photogrammetry data we compared spatially variable and uniform uncertainty models, while taking into account the different sources of error. For LiDAR data, on the other hand, we used a spatially uniform error model. Depending on the geomorphology of the gullies and the configuration of the data capture, we chose the most appropriate method to detect geomorphological changes on the surfaces of the bank gullies. At site 3 the gully topography is complex, so we performed a 3D distance calculation between point clouds using the M3C2 algorithm to estimate the sediment budget. On sites 1 and 2 we used the DoD technique to estimate the sediment budget as it was the case for the LiDAR data. The results of the LiDAR and UAV data reveal significant lakeshore erosion activity by bank gullies since the annual inflow from the banks is estimated at 39 T/ha/year. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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24 pages, 4364 KiB

Open AccessArticle

Anthropization and Climate Change: Impact on the Discharges of Forest Watersheds in Central Africa

by Valentin Brice Ebodé, Gil Mahé, Jean Guy Dzana and Joseph Armathé Amougou

Water 2020, 12(10), 2718; https://doi.org/10.3390/w12102718 - 29 Sep 2020

Cited by 16 | Viewed by 3273

Abstract

Climate change and anthropization are major drivers of river flows variability. However, understanding their simultaneous impact on discharges is limited. As a contribution to address this limitation, the objective of this study is to assess the impact of climate change and anthropization on [...] Read more.

Climate change and anthropization are major drivers of river flows variability. However, understanding their simultaneous impact on discharges is limited. As a contribution to address this limitation, the objective of this study is to assess the impact of climate change and anthropization on the discharges of two watersheds of Central Africa (Nyong and Ntem) over a recent period. For this, the hydropluviometric data of the watersheds concerned were analyzed using the Pettitt test. Similarly, the dynamics of the main land use modes (LUM) have been assessed, through classifications obtained from the processing of Landsat satellite images of the watersheds studied on two dates. The results of this study show that in Central Africa, annual discharges have decreased significantly since the 1970s, and yet the decline in annual rainfall does not become significant until the 2000s. The discharges of the rainy seasons (spring and autumn) recorded the most important changes, following variations in the rainfall patterns of the dry seasons (winter and summer) that precede them. Winters experienced a significant decrease in precipitation between the 1970s and 1990s, which caused a drop in spring flows. Their rise, which began in the 2000s, is also accompanied by an increase in spring flows, which nevertheless seems rather slight in the case of the Nyong. Conversely, between the 1970s and 1990s, there was a joint increase in summer rainfall and autumn flows. A decrease of summer rainfall was noted since the 2000s, and is also noticeable in autumn flows. Maximum flows have remained constant on the Nyong despite the slight drop in rainfall. This seems to be the consequence of changes in land use patterns (diminution of forest and increasing of impervious areas). The decrease in maximums flows noted on the Ntem could be linked to the slight drop in precipitation during the rainy seasons that generates it. Factors such as the general decrease in precipitation during the winter and the reduction in the area occupied by water bodies could justify the decrease in minimum flows observed in the two watersheds. These findings would be vital to enhance water management capabilities in the watersheds concerned and in the region. They can also give some new elements to study and understand the seasonal variation and fresh water availability in downstream, estuaries and coastal areas of the regional rivers. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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25 pages, 6793 KiB

Open AccessArticle

Seasonal Variations of the Depletion Factor during Recession Periods in the Senegal, Gambia and Niger Watersheds

by Jean-Claude Bader, Honoré Dacosta and Jean-Christophe Pouget

Water 2020, 12(9), 2520; https://doi.org/10.3390/w12092520 - 9 Sep 2020

Cited by 2 | Viewed by 2168

Abstract

The daily depletion factor K describes the discharge decrease of rivers only fed by groundwater in the absence of rainfall. In the Senegal, Gambia and Niger river basins in West Africa, the flow recession can exceed 6 months and the precise knowledge of [...] Read more.

The daily depletion factor K describes the discharge decrease of rivers only fed by groundwater in the absence of rainfall. In the Senegal, Gambia and Niger river basins in West Africa, the flow recession can exceed 6 months and the precise knowledge of K thus allows discharge forecasts to be made over several months, and is hence potentially interesting for hydraulic structure managers. Seasonal flow recession observed at 54 gauging stations in these basins from 1950 to 2016 is represented by empirical and usual conceptual models that express K. Compared to conventional conceptual models, an empirical model representing K as a polynomial of the decimal logarithm of discharge Q gives better representations of K and better discharge forecasting at horizons from 1 to 120 days for most stations. The relationship between specific discharge Qs and K, not monotonous, is highly homogeneous in some sub-basins but differs significantly between the Senegal and Gambia basins on the one hand and the Niger basin on the other. The relationship K(Q) evolves slightly between three successive periods, with values of K generally lower (meaning faster discharge decrease) in the intermediate period centered on the years 1970–1980. These climate-related interannual variations are much smaller than the seasonal variations of K. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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23 pages, 9244 KiB

Open AccessFeature PaperArticle

Understory Limits Surface Runoff and Soil Loss in Teak Tree Plantations of Northern Lao PDR

by Layheang Song, Laurie Boithias, Oloth Sengtaheuanghoung, Chantha Oeurng, Christian Valentin, Bounthan Souksavath, Phabvilay Sounyafong, Anneke de Rouw, Bounsamay Soulileuth, Norbert Silvera, Bounchanh Lattanavongkot, Alain Pierret and Olivier Ribolzi

Water 2020, 12(9), 2327; https://doi.org/10.3390/w12092327 - 19 Aug 2020

Cited by 16 | Viewed by 5734

Abstract

Many mountainous regions of the humid tropics experience serious soil erosion following rapid changes in land use. In northern Lao People’s Democratic Republic (PDR), the replacement of traditional crops by tree plantations, such as teak trees, has led to a dramatic increase in [...] Read more.

Many mountainous regions of the humid tropics experience serious soil erosion following rapid changes in land use. In northern Lao People’s Democratic Republic (PDR), the replacement of traditional crops by tree plantations, such as teak trees, has led to a dramatic increase in floods and soil loss and to the degradation of basic soil ecosystem services such as water filtration by soil, fertility maintenance, etc. In this study, we hypothesized that conserving understory under teak trees would protect soil, limit surface runoff, and help reduce soil erosion. Using 1 m² microplots installed in four teak tree plantations in northern Lao PDR over the rainy season of 2017, this study aimed to: (1) assess the effects on surface runoff and soil loss of four understory management practices, namely teak with no understory (TNU; control treatment), teak with low density of understory (TLU), teak with high density of understory (THU), and teak with broom grass, Thysanolaena latifolia (TBG); (2) suggest soil erosion mitigation management practices; and (3) identify a field visual indicator allowing a rapid appraisal of soil erosion intensity. We monitored surface runoff and soil loss, and measured teak tree and understory characteristics (height and percentage of cover) and soil surface features. We estimated the relationships among these variables through statistics and regression analyses. THU and TBG had the smallest runoff coefficient (23% for both) and soil loss (465 and 381 g·m⁻², respectively). The runoff coefficient and soil loss in TLU were 35% and 1115 g·m⁻², respectively. TNU had the highest runoff coefficient and soil loss (60%, 5455 g·m⁻²) associated to the highest crusting rate (82%). Hence, the soil loss in TBG was 14-times less than in TNU and teak tree plantation owners could divide soil loss by 14 by keeping understory, such as broom grass, within teak tree plantations. Indeed, a high runoff coefficient and soil loss in TNU was explained by the kinetic energy of rain drops falling from the broad leaves of the tall teak trees down to bare soil, devoid of plant residues, thus leading to severe soil surface crusting and soil detachment. The areal percentage of pedestal features was a reliable indicator of soil erosion intensity. Overall, promoting understory, such as broom grass, in teak tree plantations would: (1) limit surface runoff and improve soil infiltrability, thus increase soil water stock available for both root absorption and groundwater recharge; and (2) mitigate soil loss while favoring soil fertility conservation. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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27 pages, 4594 KiB

Open AccessEditor’s ChoiceArticle

Comparison of the MUSLE Model and Two Years of Solid Transport Measurement, in the Bouregreg Basin, and Impact on the Sedimentation in the Sidi Mohamed Ben Abdellah Reservoir, Morocco

by Mohamed Abdellah Ezzaouini, Gil Mahé, Ilias Kacimi and Abdelaziz Zerouali

Water 2020, 12(7), 1882; https://doi.org/10.3390/w12071882 - 1 Jul 2020

Cited by 33 | Viewed by 4038

Abstract

The evaluation and quantification of solids transport in Morocco often uses the Universal Soil Loss Model (USLE) and the revised version RUSLE, which presents a calibration difficulty. In this study, we apply the MUSLE model to predict solid transport, for the first time [...] Read more.

The evaluation and quantification of solids transport in Morocco often uses the Universal Soil Loss Model (USLE) and the revised version RUSLE, which presents a calibration difficulty. In this study, we apply the MUSLE model to predict solid transport, for the first time on a large river basin in the Kingdom, calibrated by two years of solid transport measurements on four main gauging stations at the entrance of the Sidi Mohamed Ben Abdellah dam. The application of the MUSLE on the basin demonstrated relatively small differences between the measured values and those expected for the calibrated version, these differences are, for the non-calibrated version, +5% and +102% for the years 2016/2017 and 2017/2018 respectively, and between −33% and +34% for the calibrated version. Besides, the measured and modeled volumes that do not exceed 1.78 × 10⁶ m³/year remain well below the dam’s siltation rate of 9.49 × 10⁶ m³/year, which means that only 18% of the dam’s sediment comes from upstream. This seems very low because it is calculated from only two years. The main hypothesis that we can formulate is that the sediments of the dam most probably comes from the erosion of its banks. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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27 pages, 6449 KiB

Open AccessArticle

Climate and Extreme Rainfall Events in the Mono River Basin (West Africa): Investigating Future Changes with Regional Climate Models

by Ernest Amoussou, Hervé Awoye, Henri S. Totin Vodounon, Salomon Obahoundje, Pierre Camberlin, Arona Diedhiou, Kouakou Kouadio, Gil Mahé, Constant Houndénou and Michel Boko

Water 2020, 12(3), 833; https://doi.org/10.3390/w12030833 - 16 Mar 2020

Cited by 17 | Viewed by 4733

Abstract

This study characterizes the future changes in extreme rainfall and air temperature in the Mono river basin where the main economic activity is weather dependent and local populations are highly vulnerable to natural hazards, including flood inundations. Daily precipitation and temperature from observational [...] Read more.

This study characterizes the future changes in extreme rainfall and air temperature in the Mono river basin where the main economic activity is weather dependent and local populations are highly vulnerable to natural hazards, including flood inundations. Daily precipitation and temperature from observational datasets and Regional Climate Models (RCMs) output from REMO, RegCM, HadRM3, and RCA were used to analyze climatic variations in space and time, and fit a GEV model to investigate the extreme rainfalls and their return periods. The results indicate that the realism of the simulated climate in this domain is mainly controlled by the choice of the RCMs. These RCMs projected a 1 to 1.5 °C temperature increase by 2050 while the projected trends for cumulated precipitation are null or very moderate and diverge among models. Contrasting results were obtained for the intense rainfall events, with RegCM and HadRM3 pointing to a significant increase in the intensity of extreme rainfall events. The GEV model is well suited for the prediction of heavy rainfall events although there are uncertainties beyond the 90th percentile. The annual maxima of daily precipitation will also increase by 2050 and could be of benefit to the ecosystem services and socioeconomic activities in the Mono river basin but could also be a threat. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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26 pages, 6710 KiB

Open AccessFeature PaperArticle

Inverse Estuaries in West Africa: Evidence of the Rainfall Recovery?

by Luc Descroix, Yancouba Sané, Mamadou Thior, Sylvie-Paméla Manga, Boubacar Demba Ba, Joseph Mingou, Victor Mendy, Saloum Coly, Arame Dièye, Alexandre Badiane, Marie-Jeanne Senghor, Ange-Bouramanding Diedhiou, Djiby Sow, Yasmin Bouaita, Safietou Soumaré, Awa Diop, Bakary Faty, Bamol Ali Sow, Eric Machu, Jean-Pierre Montoroi, Julien Andrieu and Jean-Pierre Vandervaere Show full author list Hide full author list

Water 2020, 12(3), 647; https://doi.org/10.3390/w12030647 - 28 Feb 2020

Cited by 26 | Viewed by 6671

Abstract

In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian [...] Read more.

In West Africa, as in many other estuaries, enormous volumes of marine water are entering the continent. Fresh water discharge is very low, and it is commonly strongly linked to rainfall level. Some of these estuaries are inverse estuaries. During the Great Sahelian Drought (1968–1993), their hyperhaline feature was exacerbated. This paper aims to describe the evolution of the two main West African inverse estuaries, those of the Saloum River and the Casamance River, since the end of the drought. Water salinity measurements were carried out over three to five years according to the sites in order to document this evolution and to compare data with the historical ones collected during the long dry period at the end of 20th century. The results show that in both estuaries, the mean water salinity values have markedly decreased since the end of the drought. However, the Saloum estuary remains a totally inverse estuary, while for the Casamance River, the estuarine turbidity maximum (ETM) is the location of the salinity maximum, and it moves according to the seasons from a location 1–10 km downwards from the upstream estuary entry, during the dry season, to a location 40–70 km downwards from this point, during the rainy season. These observations fit with the functioning of the mangrove, the West African mangrove being among the few in the world that are markedly increasing since the beginning of the 1990s and the end of the dry period, as mangrove growth is favored by the relative salinity reduction. Finally, one of the inverse estuary behavior factors is the low fresh water incoming from the continent. The small area of the Casamance and Saloum basins (20,150 and 26,500 km² respectively) is to be compared with the basins of their two main neighbor basins, the Gambia River and the Senegal River, which provide significant fresh water discharge to their estuary. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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22 pages, 4763 KiB

Open AccessArticle

A Comparative Study of Statistical Methods for Daily Streamflow Estimation at Ungauged Basins in Turkey

by Mustafa Utku Yilmaz and Bihrat Onoz

Water 2020, 12(2), 459; https://doi.org/10.3390/w12020459 - 9 Feb 2020

Cited by 24 | Viewed by 3805

Abstract

In this study, a comparative evaluation of the statistical methods for daily streamflow estimation at ungauged basins is presented. The single donor station drainage area ratio (DAR) method, the multiple-donor stations drainage area ratio (MDAR) method, the inverse similarity weighted (ISW) method, and [...] Read more.

In this study, a comparative evaluation of the statistical methods for daily streamflow estimation at ungauged basins is presented. The single donor station drainage area ratio (DAR) method, the multiple-donor stations drainage area ratio (MDAR) method, the inverse similarity weighted (ISW) method, and its variations with three different power parameters (1, 2, and 3) are applied to the two main subbasins of the Euphrates Basin in Turkey to estimate daily streamflow data. Each station in each basin is considered in turn as the target station where there are no streamflow data. The donor stations are selected based on the physical similarities between the donor and target stations. Then, streamflow data from the most physically similar donor station(s) is transferred to the target station using the statistical methods. In addition, the effect of data preprocessing on the estimation performance of the statistical methods is investigated. The preprocessing discussed in this study is streamflow data smoothing using the two-sided moving average (MA). Three statistical methods using the smoothed data by the MA, named as DAR-MA, MDAR-MA, and ISW-MA, are proposed. The estimation performance of the statistical methods is compared by using daily streamflow data with preprocessing and without preprocessing. The Nash–Sutcliffe efficiency (NSE), the ratio of the root mean square error (RMSE) to the standard deviation of the observed data (RSR), the percent bias (PBIAS), and the coefficient of determination (R²) are used to evaluate the performance of the statistical methods. The results show that MDAR and ISW give improved performances compared to DAR to estimate daily streamflow for 7 out of 8 target stations in the Middle Euphrates Basin and for 4 out of 7 target stations in the Upper Euphrates Basin. Higher NSE values for both MDAR and ISW are mostly obtained with the three most physically similar donor stations in the Middle Euphrates Basin and with the two most physically similar donor stations in the Upper Euphrates Basin. The best statistical method for each target station exhibits slightly greater NSE when the smoothed data by the MA is used for all target stations in the Middle Euphrates Basin and for 6 out of 7 target stations in the Upper Euphrates Basin. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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12 pages, 4302 KiB

Open AccessArticle

Recent Trend in Hydroclimatic Conditions in the Senegal River Basin

by Ansoumana Bodian, Lamine Diop, Geremy Panthou, Honoré Dacosta, Abdoulaye Deme, Alain Dezetter, Pape Malick Ndiaye, Ibrahima Diouf and Théo Vischel

Water 2020, 12(2), 436; https://doi.org/10.3390/w12020436 - 6 Feb 2020

Cited by 42 | Viewed by 1042726

Abstract

Analyzing trends of annual rainfall and assessing the impacts of these trends on the hydrological regime are crucial in the context of climate change and increasing water use. This research investigates the recent trend of hydroclimatic variables in the Senegal River basin based [...] Read more.

Analyzing trends of annual rainfall and assessing the impacts of these trends on the hydrological regime are crucial in the context of climate change and increasing water use. This research investigates the recent trend of hydroclimatic variables in the Senegal River basin based on 36 rain gauge stations and three hydrometric stations not influenced by hydraulic structures. The Man Kendall and Pettitt’s tests were applied for the annual rainfall time series from 1940 to 2013 to detect the shift and the general trend of the annual rainfall. In addition, trends of average annual flow rate (AAFR), maximum daily flow (MADF), and low flow rate (LFR) were evaluated before and after annual rainfall shift. The results show that the first shift is situated on average at 1969 whereas the second one is at 1994. While the first shift is very consistent between stations (between 1966 and 1972), there is a significant dispersion of the second change-point between 1984 and 2002. After the second shift (1994), an increase of annual rainfall is noticed compared to the previous period (1969–1994) which indicates a not significant, partial rainfall recovery at the basin level. The relative changes of hydrologic variables differ based on the variables and the sub-basin. Relative changes before and after first change-point are significantly negative for all variables. The highest relative changes are observed for the AAFR. Considering the periods before and second shifts, the relative changes are mainly significantly positive except for the LFR. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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17 pages, 6636 KiB

Open AccessArticle

Water and Sediment Budget of Casiquiare Channel Linking Orinoco and Amazon Catchments, Venezuela

by Alain Laraque, Jose Luis Lopez, Santiago Yepez and Paul Georgescu

Water 2019, 11(10), 2068; https://doi.org/10.3390/w11102068 - 3 Oct 2019

Cited by 3 | Viewed by 8799

Abstract

The Casiquiare River is a natural channel that connects two of the greatest rivers in the world, the Orinoco and the Amazon in the South American continent. The aim of this paper is to present a review and synthesis of the hydrological and [...] Read more.

The Casiquiare River is a natural channel that connects two of the greatest rivers in the world, the Orinoco and the Amazon in the South American continent. The aim of this paper is to present a review and synthesis of the hydrological and sedimentological knowledge of the Casiquiare River, including the first hydro-sedimentary balance of the Casiquiare fluvial system conducted 9–12 September 2000 at the bifurcation and mouth during the expedition ‘Humboldt-Amazonia 2000’. Bathymetric flow discharge and physico-chemical measurements were made at the inlet and outlet of the Casiquiare Channel. The main conclusions of this study indicate that Casiquiare is taking a significant proportion of flow (20% to 30%) from the Upper Orinoco basin to the Amazon basin. Throughout its 356 km-course, this chameleon channel undergoes significant morphological, hydrological, and bio-geochemical variations between the inlet and outlet, whose most visible witnesses are the increase in its width (3 to 4 times), flow (7 to 9 times), and its change in water color (white to black water), under the influence of tributaries coming from vast forest plains. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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18 pages, 6355 KiB

Open AccessArticle

Spatial Drought Characterization for Seyhan River Basin in the Mediterranean Region of Turkey

by Yonca Cavus and Hafzullah Aksoy

Water 2019, 11(7), 1331; https://doi.org/10.3390/w11071331 - 27 Jun 2019

Cited by 56 | Viewed by 7180

Abstract

Drought is a natural phenomenon that has great impacts on the economy, society and environment. Therefore, the determination, monitoring and characterization of droughts are of great significance in water resources planning and management. The purpose of this study is to investigate the spatial [...] Read more.

Drought is a natural phenomenon that has great impacts on the economy, society and environment. Therefore, the determination, monitoring and characterization of droughts are of great significance in water resources planning and management. The purpose of this study is to investigate the spatial drought characterizations of Seyhan River basin in the Eastern Mediterranean region of Turkey. The standardized precipitation index (SPI) was calculated from monthly precipitation data at 12-month time scale for 19 meteorological stations scattered over the river basin. Drought with the largest severity in each year is defined as the critical drought of the year. Frequency analysis was applied on the critical drought to determine the best-fit probability distribution function by utilizing the total probability theorem. The sole frequency analysis is insufficient in drought studies unless it is numerically related to other factors such as the severity, duration and intensity. Also, SPI is a technical tool and thus difficult to understand at first glance by end-users and decision-makers. Precipitation deficit defined as the difference between precipitation threshold at SPI = 0 and critical precipitation is therefore more preferable due to its usefulness and for being physically more meaningful to the users. Precipitation deficit is calculated and mapped for 1-, 3-, 6- and 12-month drought durations and 2-, 5-, 10-, 25-, 50- and 100-year return periods at 12-month time scale from the frequency analysis of the critical drought severity. The inverse distance weighted (IDW) interpolation technique is used for the spatial distribution of precipitation deficit over the Seyhan River basin. The spatial and temporal characteristics of drought suggest that the Seyhan River Basin in the Eastern Mediterranean region of Turkey experiences quite mild and severe droughts in terms of precipitation deficit. The spatial distribution would alter greatly with increasing return period and drought duration. While the coastal part of the basin is vulnerable to droughts at all return periods and drought durations, the northern part of the basin would be expected to be less affected by the drought. Another result reached in this study is that it could be common for one point in the basin to suffer dry conditions, whilst surrounding points in the same basin experience normal or even humid conditions. This reinforces the importance of spatial analysis over the basin under investigation instead of the point-scale temporal analysis made in each of the meteorological stations. With the use of spatial mapping of drought, it is expected that the destructive and irreversible effects of hydrological droughts can be realized in a more physical sense. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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15 pages, 2319 KiB

Open AccessArticle

Expanding Rubber Plantations in Southern China: Evidence for Hydrological Impacts

by Xing Ma, Guillaume Lacombe, Rhett Harrison, Jianchu Xu and Meine van Noordwijk

Water 2019, 11(4), 651; https://doi.org/10.3390/w11040651 - 29 Mar 2019

Cited by 15 | Viewed by 5404

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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17 pages, 3589 KiB

Open AccessArticle

A New 60-Year 1940/1999 Monthly-Gridded Rainfall Data Set for Africa

by Claudine Dieulin, Gil Mahé, Jean-Emmanuel Paturel, Soundouss Ejjiyar, Yves Tramblay, Nathalie Rouché and Bouabid EL Mansouri

Water 2019, 11(2), 387; https://doi.org/10.3390/w11020387 - 22 Feb 2019

Cited by 35 | Viewed by 4864

Abstract

The African continent has a very low density of rain gauge stations, and long time-series for recent years are often limited and poorly available. In the context of global change, it is very important to be able to characterize the spatio-temporal variability of [...] Read more.

The African continent has a very low density of rain gauge stations, and long time-series for recent years are often limited and poorly available. In the context of global change, it is very important to be able to characterize the spatio-temporal variability of past rainfall, on the basis of datasets issued from observations, to correctly validate simulations. The quality of the rainfall data is for instance of very high importance to improve the efficiency of the hydrological modeling, through calibration/validation experiments. The HydroSciences Montpellier Laboratory (HSM) has a long experience in collecting and managing hydro-climatological data. Thus, HSM had initiated a program to elaborate a reference dataset, in order to build monthly rainfall grids over the African continent, over a period of 60 years (1940/1999). The large quantity of data collected (about 7000 measurement points were used in this project) allowed for interpolation using only observed data, with no statistical use of a reference period. Compared to other databases that are used to build the grids of the Global Historical Climatology Network (GHCN) or the Climatic Research Unit of University of East Anglia, UK (CRU), the number of available observational stations was significantly much higher, including the end of the century when the number of measurement stations dropped dramatically, everywhere. Inverse distance weighed (IDW) was the chosen method to build the 720 monthly grids and a mean annual grid, from rain gauges. The mean annual grid was compared to the CRU grid. The grids were significantly different in many places, especially in North Africa, Sahel, the horn of Africa, and the South Western coast of Africa, with HSM_SIEREM data (database HydroSciences Montpellier_Système d’Information Environnementales pour les Ressources en Eau et leur Modélisation) being closer to the observed rain gauge values. The quality of the grids computed was checked, following two approaches—cross-validation of the two interpolation methods, ordinary kriging and inverse distance weighting, which gave a comparable reliability, with regards to the observed data, long time-series analysis, and analysis of long-term signals over the continent, compared to previous studies. The statistical tests, computed on the observed and gridded data, detected a rupture in the rainfall regime around 1979/1980, on the scale of the whole continent; this was congruent with the results in the literature. At the monthly time-scale, the most widely observed signal over the period of 1940/1999, was a significant decrease of the austral rainy season between March and May, which has not earlier been well-documented. Thus, this would lead to a further detailed climatological study from this HSM_SIEREM database. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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Open AccessReview

Recent Budget of Hydroclimatology and Hydrosedimentology of the Congo River in Central Africa

by Alain Laraque, Guy D. Moukandi N’kaya, Didier Orange, Raphael Tshimanga, Jean Marie Tshitenge, Gil Mahé, Cyriaque R. Nguimalet, Mark A. Trigg, Santiago Yepez and Georges Gulemvuga

Water 2020, 12(9), 2613; https://doi.org/10.3390/w12092613 - 18 Sep 2020

Cited by 22 | Viewed by 5784

Abstract

Although the Congo Basin is still one of the least studied river basins in the world, this paper attempts to provide a multidisciplinary but non-exhaustive synthesis on the general hydrology of the Congo River by highlighting some points of interest and some particular [...] Read more.

Although the Congo Basin is still one of the least studied river basins in the world, this paper attempts to provide a multidisciplinary but non-exhaustive synthesis on the general hydrology of the Congo River by highlighting some points of interest and some particular results obtained over a century of surveys and scientific studies. The Congo River is especially marked by its hydrological regularity only interrupted by the wet decade of 1960, which is its major anomaly over nearly 120 years of daily observations. Its interannual flow is 40,500 m³ s⁻¹. This great flow regularity should not hide important spatial variations. As an example, we can cite the Ubangi basin, which is the most northern and the most affected by a reduction in flow, which has been a cause for concern since 1970 and constitutes a serious hindrance for river navigation. With regard to material fluxes, nearly 88 × 10⁶ tonnes of material are exported annually from the Congo Basin to the Atlantic Ocean, composed of 33.6 × 10⁶ tonnes of TSS, 38.1 × 10⁶ tonnes of TDS and 16.2 × 10⁶ tonnes of DOC. In this ancient flat basin, the absence of mountains chains and the extent of its coverage by dense rainforest explains that chemical weathering (10.6 t km⁻² year⁻¹ of TDS) slightly predominates physical erosion (9.3 t km⁻² year⁻¹ of TSS), followed by organic production (4.5 t km⁻² year⁻¹ of DOC). As the interannual mean discharges are similar, it can be assumed that these interannual averages of material fluxes, calculated over the longest period (2006–2017) of monthly monitoring of its sedimentology and bio-physical-chemistry, are therefore representative of the flow record available since 1902 (with the exception of the wet decade of 1960). Spatial heterogeneity within the Congo Basin has made it possible to establish an original hydrological classification of right bank tributaries, which takes into account vegetation cover and lithology to explain their hydrological regimes. Those of the Batéké plateau present a hydroclimatic paradox with hydrological regimes that are among the most stable on the planet, but also with some of the most pristine waters as a result of the intense drainage of an immense sandy-sandstone aquifer. This aquifer contributes to the regularity of the Congo River flows, as does the buffer role of the mysterious “Cuvette Centrale”. As the study of this last one sector can only be done indirectly, this paper presents its first hydrological regime calculated by inter-gauging station water balance. Without neglecting the indispensable in situ work, the contributions of remote sensing and numerical modelling should be increasingly used to try to circumvent the dramatic lack of field data that persists in this basin. Full article

(This article belongs to the Special Issue Multiscale Impacts of Anthropogenic and Climate Changes on Tropical and Mediterranean Hydrology)

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