S22

The paramo biome, located above 3300 m a.s.l. and covered mainly by tussock grasslands, provides ecosystem services for Andean cities, especially water resources used for drinking water, agriculture, hydropower generation and sustaining aquatic ecosystems. Even though research about the main components of the water cycle has increased substantially in the last decade, evaporation has remained unknown. In this study, we quantified for the first time daily, monthly and annual evaporation, its components (i.e. interception and transpiration) at event scale and its climatic drivers at a representative páramo catchment in Southern Ecuador (Figure 1). We used the eddy-covariance method to quantify evaporation. We additionally compared those measurements with lysimeters, water balance, energy balance, hydrological models (HBV-light and PDM) and the calibration of the Penman-Monteith equation in order to find easier and cheaper alternatives for estimating evaporation at the páramo (Figure 2).

Our results show that the paramo biome had a relatively low evaporation rate (annual ETa/P = 0.5, 1.7 mm/day) and is an energy-limited site, where net radiation is the primary control of evaporation (annual ETa/Rn = 0.47) (Figure 3). The secondary controls were wind speed, surface and aerodynamic conductance, especially important during dry periods. The maximum capacity of tussock grasslands to intercept water was 2 mm. During small events (P < 2 mm), between 100 and 80 % of precipitation was intercepted and released back to the atmosphere; while during large events (P > 2 mm), interception decreased from 80 to 10 %. During dry periods, transpiration rates ranged between 0.7 and 2.7 mm/day and on top, the fog and dew harvested by the vegetation contributed to the evaporation in around 30 %. When comparing eddy-covariance measurements with other methods, we found that the two hydrological models and the calibrated Penman-Monteith equation are robust alternatives for daily estimation of evaporation at the páramo. This study contributed to the understanding of the water cycle at the most important Andean ecosystem for its water resources.

Figure 1. Zhurucay ecohydrological observatory.

Figure 2. Methodology undertaken in the study.

Figure 3. Evaporation (ETa) and climatic variables at the study site.

How to cite: Ochoa-Sánchez, A., Crespo, P., Carrillo-Rojas, G., Sucozhañay, A., Marín, F., and Célleri, R.: Evaporation dynamics and partitioning in Andean tussock grasslands, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-10, https://doi.org/10.5194/iahs2022-10, 2022.

In this work, the simulation of the water supply has been carried out using the SWAT hydrological model to generate streamflows throughout Peru. For this purpose, 35 hydrological stations distributed in the 3 drainages of Peru have been used, simulating the entire country for the first time considering 4,355 sub-basins and 168 hydrographic units (HU); obtaining time series of streamflows for the period 1981-2016.
To evaluate the impact of climate change on water supply of Peru, three regional climate models based on dynamic regionalization have been used, obtaining time series from 1981 to 2065 with a spatial resolution of ~ 10 km. Based on these data, and using the delta change method, the streamflows generated in the period 2035-2065 versus 1981-2016 have been compared.
The results of the impact of climate change on water supply showed different results for each one of the 14 hydrological regions that Peru was divided. Spatially, the HUs with conditions towards a more marked decrease (increase) in flows are those located towards the center-south of the Pacific drainage and HUs dispersed in the Amazon drainage (north of the Pacific drainage). The uncertainties of hydrological modeling, as well as climate scenarios, are discussed in detail. All the results of this work can be accessed on the web portal https://idesep.senamhi.gob.pe/balance-hidrico/.

How to cite: Lavado-Casimiro, W.: Assessment of climate Change impact on water supply in Peru, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-23, https://doi.org/10.5194/iahs2022-23, 2022.

The streamflow is important for planning et decision-making of development activities  related to water resource. The hydrological mathematical models are frequently used to simulate the transformation of rainfall into streamflow as well as Artificial Neural Networks (ANNs. In order to contribute to the establishment of an adequate framework for water resource management in the Davo river basin, this study presents an intercomparison between streamflow estimations from conventional hydrological modelling and ANN analysis. The Davo River basin is located in the southwestern forest of Côte d'Ivoire. The hydrological models used are the HEC-HMS model (Hydrologic Engineering Center-Hydrologic Modeling System) and the GR4J model (Rural Engineering with four daily parameters), while the data-driven ANN model is developed in MATLAB. The three tools receive the same data and their parameters are calibrated using the same objective functions. The quality of the simulations is measured in the control phase using several statistical criteria. For the assesment of the impacts of climate change on hydrological regime of the Davo river basin, the AFRICA-CORDEX (Coordinated Downscaling Experiment for Africa) data under two RCP (representative concentration pathway) scenarios   RCP4.5 and RCP8.5 are integrated in the three tools of modelling. The approach developed in this study allows a considerable improvement of the outils performances developed in calibration and validation. In addition, the results of the current study provide valuable feedback for water resources’ planners in Côte d’Ivoire and the developing regions.

Keywords: Climate change, Streamflow, Artificial Neural Networks, HEC-HMS, GR4J, Davo river basin, Côte d’Ivoire

How to cite: Kouamé, Y. M., Meledje, N. E. H., Soro, T. I., Soro, G. E., Kombyla, D. V., Kouassi, K. L., Goula, B. T. A., Paturel, J. E., Diedhiou, A., and Amoussou, E.: Artificial Neural Networks (ANNs) and Hydrological Models to Simulate Streamflow in a context of climate change: Case of Davo river basin in Côte d’Ivoire, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-89, https://doi.org/10.5194/iahs2022-89, 2022.

The objective of this study is to analyze the links between climate indices and the variability of precipitation and flow series. In order to better understand the non-stationarity of the different stations, the flow and rainfall data used concern the Tortiya station (1960-1996) and the Lafigué station (1977-1996). The climate indices coupled to these series are the NAO (North Atlantic Oscillation) and ENSO (El Niño - Southern Oscillation) over the same study period. The methodology of this work consisted in applying wavelet analysis and wavelet coherence on the different time series. These methods highlighted the different modes of variability occurring in the time series, namely : the sub-annual mode (< 1 year), the annual mode (1 year) and the interannual mode (1-2 years ; 2-4 years; 4-8 years). Firstly, the results of the present analysis show that the variability of the signal is explained at high frequencies (6 months to 1 year) in the different time series. At this frequency, it is the annual mode (1 year) that records all the signal variability between 30% and 70%. On the other hand, this work presents signals on other frequencies and periods but fairly, especially 2-year frequency, in the 1990s at the Lafigué station (8.2%). Wavelet analysis revealed that the dominant signal is very broadly significant  at the annual cycle level. In addition, the use of wavelet coherence between climate indices (ENSO, NAO) and precipitation, flow indicates a strong influence of NAO on rainfall and flow series.

Key words : Wavelet, coherence, periodicity, variability, upstream Tortiya watershed 

How to cite: Amalaman, M. A., Mahe, G., Diomande, I. B., Tra Bi, Z. A., Rouche, N., Nouaceur, Z., and Laignel, B.: WAVELET ANALYSIS OF TIME SERIES AT RAINFALL AND FLOW STATIONS IN THE UPSTREAM TORTIYA WATERSHED (Northern Ivory Coast), IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-134, https://doi.org/10.5194/iahs2022-134, 2022.

Low flows and irrigation in Mono River basin at Athiéme outlet (West -Africa)

¹Coffi Justin Noumon, ²Domiho Japhet Kodja, ²Ernest Amoussou, ¹Luc O. Sintondji,¹ Euloge K. Agbossou.

¹Laboratoire d’Hydraulique et de Maîtrise de l’Eau (LHME), Institut National de l’Eau, Université d’Abomey Calavi 01 BP 526 Cotonou (Bénin).

²Laboratoire Pierre Pagney Climat Eau Ecosystèmes et Développement (LACEEDE), Institut de Géographie et Aménagement du Territoire et de l’Environnement, Université d’Abomey Calavi 01 BP 526 Cotonou (Bénin).

Auteur correspondant: ¹Coffi Justin NOUMON (justinoum2001@yahoo.fr)

Abstract

The basic goal of this survey is the assessment of water resources of Mono River at Athiémé outlet in low flow period where it availability for agriculture uses is problematic. Therefore, daily data (precipitations, ETP, flows) from 1971 to 2016 are collected. It allows to simulate low discharge with GR4J model. Field survey permits the characterization of agriculture uses and the assessment of water requirement for irrigation. Low flows of Mono River at Athiémé outlet simulation with GR4J showed a good performance of the model to simulate low discharges before the construction of Nangbéto dam in 1988. Criteria values of Nash-Sutcliffe before dam establishment are 69% (calibration) and 71% (validation). The performance of the model is less for the period after the dam establishment: The change in the hydrological regime of the river due to water jets from Nangbéto dam can explain this low performance. Water requirement for irrigated areas estimation has given 42 587 596 m3 in 2020 and 73 007 308 m3 in 2030 mainly for the constraint period production which came from february to may. The comparison of water availability/water requirement shows that the water needs are closely covered monthly but a fine decade period analysis revealed some deficit decades. These results involve a great attention and consequent investments for deep knowledge of the resource in order to plan it sustainable management.

Key words: Mono basin, Low flow, GR4J, Irrigation.

How to cite: Noumon, C. J., Kodja, D. J., Amoussou, E., Sintondji, L. O., and Agbossou, E. K.: Low flows and irrigation in Mono River basin at Athiéme outlet (West -Africa)., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-19, https://doi.org/10.5194/iahs2022-19, 2022.

In recent decades, floods are recurrent and intensifying in African major cities. Every year, rains cause material damage and human losses, in Abidjan (Ivory Coast). This study contributes to the characterization of the rain hazard in the District of Abidjan. The available data are made up of daily rainfall from a historical station "Abidjan airport" (1961-2014) and an academic network of 21 rain gauges progressively implemented in Abidjan since 2016.

 The annual rainfall observed from 2016 to 2021 on the sites of this network are representative of the interannual variability of the historical station. An analysis (date of occurrence, rainfall depth, mean intensity of wet days and number of rainy days) on the Highest Cumulative Rainfall Periods (HCRP:15, 30 and 60 days) is conducted on the long-term station, in order to characterize the periods of highest risk of flooding occurrence during the long and short rainy seasons. Date of HCRP occurrence during long rainy season is relatively stable over time. It tends to occur later in the short rainy season. In recent years, the mean intensity of wet days remains steady despite intensifying flooding.

The current network of rain gauges permits to discuss the spatial variability of the date of HCRP over Abidjan. It can be seen that the date of HCRP occurrence in the long rainy season is uniform across the area of Abidjan, whereas it appears more variable during the short rainy season. At the daily scale and for 60 days- HCRP of the two rainy seasons, the extreme quantiles are determined and their evolution analysed over the historical period.  With the current network, the experimental variograms of extreme rainfalls allows to evaluate their spatial extensions according to the rainy season.

Beyond this analysis of daily rainfall, the next step will consist in the characterisation of rainfall events at the sub-daily scale in order to update the intensity-duration-frequency curves and the design storm, which is a prerequisite for a better estimation of floods and sizing of structures.

How to cite: Yao, K. C. J., Kacou, M., Koffi, E. S., Dao, A., Dutremble, C., Guilliod, M., Kamagaté, B., Neppel, L., Paturel, J.-E., Perrin, J.-L., Salles, C., Séguis, L., and Zahiri, E. P.: Rainfall risk over the city of Abidjan (Ivory Coast): First contribution to the joint analysis of daily rainfall from a historical record and a recent network of rain gauges., IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-59, https://doi.org/10.5194/iahs2022-59, 2022.

In Africa, floods have become “classic” hazards (David, 2004) due to climate change. If this trend continues, modeling predicts that in 2050 West Africa will experience a warming of 0.5 to 2 ° C with dramatic consequences. Galloping and poorly regulated urbanization amplifies urban runoff, exposure of the most precarious habitats and modifies the water cycle through massive deforestation and the modification of the destination of associated land uses (Montanari et al. 2013) , (Jourdan, 2019). Over a million and a half people are affected each year in Africa by flooding following heavy rains. Over the period from 2009 to 2014, there were a total of 74 deaths caused by flooding in the District of Abidjan.

In the framework of operational and scientific research actions around the updating of hydrological standards in West Africa (ActNAO Project) funded by the Institute of Research for Development, the University of Montpellier and the UNESCO ICIREWARD center, and in response to the WMO call, we carried out a study the aim of which is to define a method for characterizing the hydrological response of the watersheds of Abidjan in the face of intense rain, interpreting the states of surface and d 'land use, applicable at the level of the municipality and the district, and adapted to the nature and availability of data. The impact of past and future climate and land uses changes are studied. Ultimately, this study should make it possible to determine the runoff scenarios under various constraints of global changes and the conditions of resilience of the territory.

How to cite: Borrell, V., Kouacou, E., Abdoulaye, A., Kouame, B. K., Zile Kouadio, A., Paturel, J.-E., Dje, B., Ogilvie, A., and Servat, E.: Floods in Abidjan and global changes at the neighborhood level, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-690, https://doi.org/10.5194/iahs2022-690, 2022.

This study was conducted in the White Bandama watershed (WBW) in Côte d’Ivoire (West Africa). The objective is to assess the impacts of future climate change (CC) on the hydropower potential (HPP) of the WBW. The methodology is based on coupling the SWAT (Soil and Water Assessment Tool) hydrological model with the Geographic Information System (GIS) QGIS to assess HPP on streams and evaluate the impacts of future CC on HPP of the watershed. Historical and climate projection data (precipitation, minimum and maximum temperature) for a set of three Regional Climate Models (RCM) from CORDEX-AFRICA (CCCma-CanRCM4, CCLM4-8-17 and REMO 2009) under RCP 4.5 were used. The biases of the ensemble mean were corrected by the Delta-change method. The relative change of streamflow discharge and HPP was assessed as the relative difference between the projection periods (2041-2070 and 2071-2100) and the reference period (1976-2005). The results showed a total of 22 future hydropower potential sites in the watershed. These sites were identified, geolocated and classified according to their potential capacity of generation in 82% as small (1 MW-25 MW potential capacity), 9% as medium (25 MW-100 MW potential capacity) and 9% as large (more than 100 MW potential capacity) hydropower. The climate models’ ensemble projected an upward trend for both the annual mean discharge of rivers and HPP of the WBW according to RCP 4.5 for the periods 2041-2070 and 2071-2100. The variation in flow rates is projected to have a positive impact on the HPP of the WBW. On the annual cycle, the months of August and September will record the highest monthly mean flows between 150 and 200 m³/s while the months from November to April will record low monthly mean flows on the WBW.

How to cite: Kouadio, K. C. A., Amoussou, E., Diedhiou, A., Coulibaly, T. J. H., Coulibaly, H., Didi, R., and Savané, I.: Using of hydrological model and geospatial tool to assess climate change impact on the hydropower potential of the White Bandama watershed in Côte d’Ivoire (West Africa), IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-274, https://doi.org/10.5194/iahs2022-274, 2022.

Analysis of reference evapotranspiration (ET₀) trends is essential for understanding the impacts of climate change on water resources. Thus, despite the continuous rise in temperatures, a decrease in evapotranspiration is noted in some parts of the world. This contrast is called the "evaporation paradox" and is thought to be related to the variation in wind speed, relative humidity and solar radiation. The objective of this work is to analyze, based on the Reanalysis weather data from the NASA Langley Research Center (LaRC) POWER project, the annual and seasonal trends of ET₀ and climate variables at the scale of the Senegal, Gambia and Casamance rivers basins over the period 1984-2019. Mann Kendall's test and Sen slope were used to analyze trends in ET₀ and climate variables. Results show that on annual scale, ET₀ increases significantly in 32% of the Senegal basin and decreases in less than 1% of it. In contrast, in the Casamance and Gambia basins, the annual ET₀ drops by 65% and 18%, respectively. On an annual scale, temperature and relative humidity show an increasing trend over all basins while wind speed and radiation decrease significantly. This confirms the existence of the "evaporation paradox" in the three basins. This phenomenon is explained by the increase in relative humidity and the decrease in wind speed.

Keywords: Reference evapotranspiration, Penman-Monteith, Mann Kendall test, Senegal, Gambia and Casamance River Basins.

How to cite: Ndiaye, P. M., Bodian, A., Diop, S. B., Diop, L., Dezetter, A., Ogilvie, A., and Djaman, K.: Trend analysis of reference evapotranspiration and climate variables in the main hydrosystems of Senegal: Senegal, Gambia and Casamance River Basins, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-149, https://doi.org/10.5194/iahs2022-149, 2022.

Reference evapotranspiration (ET₀) is an essential parameter for hydrological modeling, irrigation planning and for studying the impacts of climate change on water resources. The Penman-Monteith method is recommended for estimating ET₀ under all climatic conditions. But its application is limited in regions where data are difficult to access. The alternative is to use methods that incorporate fewer climatic parameters. The objective of this work is to evaluate twenty alternative methods of estimating ET₀ and to calibrate and validate the best ones in order to adapt them to the climatic context of the basins of Senegal, Gambia and Casamance. This tree basins are the main hydrosystems of Senegal. The KGE and the PBIAS were used to evaluate the performance of the methods. The results show that the methods of Valiantzas 1, Doorenboss & Pruitt and Penman are the most robust for the estimation of ET₀ in the three basins. The calibration/validation allowed the improvement of the methods of Trabert, Valiantzas 2 and Hargreaves & Samani for the estimation of the reference evapotranspiration in the basins of Senegal, Gambia and Casamance.

Keywords: Reference evapotranspiration, Penman-Monteith, Alternative Methods, Calibration/validation, Senegal, Gambia and Casamance Basins.

How to cite: Bodian, A., Ndiaye, P. M., Diop, S. B., Diop, L., Dezetter, A., Ogilvie, A., and Djaman, K.: Evaluation and calibration of Twenty alternative methods for estimating reference evapotranspiration in main hydrosystems of Senegal: Senegal, Gambia and Casamance River Basins, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-152, https://doi.org/10.5194/iahs2022-152, 2022.

The tropical terrestrial ecosystems naturally exist as alternative stable states, commonly referred to as forest and savanna ecosystems. However, these ecosystems, especially forests, are currently threatened by the risk of drought-induced forest-to-savanna transitions across the tropics and subtropics. Therefore, a better understanding of ecosystem dynamics and characteristics behind these alternative stable states is crucial in predicting their response to future hydroclimatic changes. Previous studies have analyzed these alternative stable states against precipitation predominantly based on space-for-time substitution. However, such a substitution provides a partial picture of ecosystem adaptation dynamics and associated ecosystem structural change over time.

Here, we empirically study the transient state of tropical ecosystems and their hydroclimatic adaptations by examining remotely sensed tree cover and root zone storage capacity over the last two decades in South America and Africa. Tree cover represents the above-ground ecosystem structure's density, and is derived directly from MODIS satellite data. Whereas root zone storage capacity is the maximum amount of soil moisture that the vegetation can access for transpiration is derived using daily precipitation and evaporation data.

We found that ecosystems at high (>75%) and low (<10%) tree cover adapt to changing precipitation by instigating considerable subsoil investment while experiencing limited tree cover change over time. For these ecosystems, the below-ground investment does not come at the cost of changing the above-ground ecosystem structure. Thus, we deem these ecosystems as stable since ecosystems' adaptive dynamics keep the structural characteristics intact. In contrast, unstable ecosystems at intermediate (30-60%) tree cover were unable to exploit the same level of adaptation as stable ecosystems, thus showing considerable changes to their above-ground ecosystem structure. We also found that ignoring this adaptive capacity of the ecosystem can underestimate the resilience of the forest ecosystems, which we find is largely underestimated in the case of the Congo rainforests. The results from this study emphasize the importance of the ecosystem's temporal dynamics and adaptation in inferring and assessing the risk of forest-savannah transitions under rapid hydroclimatic change.

How to cite: Singh, C., Wang-Erlandsson, L., Fetzer, I., and van der Ent, R.: Transient ecohydrology of the rainforests under changing climate, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-212, https://doi.org/10.5194/iahs2022-212, 2022.

Temporal distribution of rainfall is a crucial climatic factor that controls seasonal biomass productivity and crop yield, particularly in tropical semi-arid and sub-humid climates where agriculture is primarily rainfed. This study explores the relationship between temporal rainfall attributes – seasonality, timing, and duration of the wet season -- and cereal crop productivity in the rainfed agriculture (RFA) areas of Ethiopia using the CHIRPS rainfall. In the first step, we determined the rainfall seasonality using an entropy-based dimensionless seasonality index (DSI), defined the dates of onset and cessation from the cumulative rainfall anomalies, computed the duration of the wet season, and assessed the anomalies and trends in these attributes and their associations to El Niño–Southern Oscillation (ENSO) over the period 1981-2010. In the second step, we analyzed the correlation between de-trended total cereal production (the sum of maize, teff, sorghum, wheat, barley, millet, oats, and rice productions) during the main growing season (April - September), and anomalies of the temporal rainfall attributes during 1995-2010. We also estimated the effects of variabilities in the rainfall timing attributes on cereal production using a univariate linear regression model. We show that rainfall in the northern RFA areas of Ethiopia is highly seasonal and unimodal whereas the southern and southeastern parts are characterized by low seasonality and a bimodal regime. Anomalies in the onset dates and DSI are meaningfully related with ENSO: 54% of late-onset events and 50% of negative DSI anomalies during 1981-2010 were observed in El Niño years. The response of crop production to changes in the temporal rainfall attributes depends on the rainfall regime. In regions with unimodal and highly seasonal rainfall regimes, we found that cereal production is mostly correlated with the onset (median ρ=-0.39) and duration (ρ=0.24) of the rainy season, whereas in regions with semi-arid bimodal rainfall regime, cereal production is more correlated with the rainfall seasonality (ρ=0.21). The median cereal production losses due to late-onset and shorter duration over the RFA areas are estimated to be ∼1.5% and 1.1% per 5-day period respectively, but much larger losses of up to 12% can be experienced in some regions.

How to cite: Wakjira, M. T., Peleg, N., Anghileri, D., Molnar, D., Alamirew, T., Six, J., and Molnar, P.: The connections between rainfall regimes and crop production in semi-arid and sub-humid climates: an example from Ethiopia, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-244, https://doi.org/10.5194/iahs2022-244, 2022.

Recent floods have highlighted the high vulnerability of Haitian territories to hydro-meteorological hazards. For example, the coastal city of Cayes (southern Haiti) has been heavily impacted by the torrential rainfall of October 5 to 6, 2018, with numerous damaged buildings. In Haiti, the only time period of continuous observations of both rainfall and streamflow lasts from 1920 to 1940. The concomitance of rainfall and streamflow observations at daily timestep enable the characterization of Haitian catchment hydrological response over that 20-year period. However, Haiti has experienced massive deforestation over the last century, and the Caribbean is a hotspot of climate change impacts such as warmer air and sea surface temperatures, leading to stronger hurricanes. Hence, the relevance of the characteristics estimated over the 1920-1940 period is questionable for present-time studies.

The objectives of this work are firstly to estimate flow characteristics of gauged Haitian catchments considering 1920-1940 temporal series, and secondly to use this historical information to estimate present-time hydrological characteristics. After digitalization and quality control tests of daily rainfall and streamflow temporal series, two types of hydrological characteristics were estimated on each gauged Haitian catchment. First, the catchment annual water balance was described by calculating the ratio between mean annual precipitation, evapotranspiration and runoff (using Turc-Budyko representation). Then, daily flood characteristics were estimated with a rainfall-runoff event-based study, focused on rainfall event duration and amount, peak streamflows and runoff ratio. Finally, present-time flow characteristics were estimated using a global rainfall-runoff model (GR4J) calibrated on the digitalized series, and then fed by the MSWEP dataset. This global database is available at the daily time step from 1979 to 2017, and merges gauge, satellite, and reanalysis data. The uncertainties related to the temporal stability of the rainfall-runoff model parameters and the potential impact of the deforestation on flow characteristics were then discussed.

How to cite: Bathelemy, R., Brigode, P., Boisson, D., Tric, E., and Andréassian, V.: Climate change and deforestation in Haiti: are historical hydro-meteorological observations relevant to describe present catchment hydrological characteristics? , IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-475, https://doi.org/10.5194/iahs2022-475, 2022.

Abstract:

Runoff simulation in highly anthropized watersheds is complex, but essential for water management, especially in poorly gauged and data-scarce hydrosystems of the West African Sahel. In this study, the physically-based and semi-distributed hydrological model SWAT (Soil and Water Assessment Tool) was applied to simulate daily runoff in the Nakanbé watershed at Wayen (21,178 km²) in Burkina Faso over the period 2006-2012. The data used concerned 9 water reservoirs, 5 hydrometric stations, 15 rain gauges, 3 synoptic stations, a digital elevation model (spatial resolution: 30m), a soil map (scale: 1:5,000,000), a land use and land cover map (scale: 1:200,000). The 5 hydrometric stations (Tougou, Dombré, Rambo, Ramsa and Wayen) gauge 5 nested watersheds ranging in size from 38 to 21,178 Km². The added value of nested watersheds was assessed through the following three calibration/validation (C/V) schemes: one-site C/V (OS) at the watershed outlet (Wayen); multi-sites C/V considering nested sub-basins (MS1) and multi-sites C/V without considering nested sub-basins (MS2). The results at the watershed outlet (Wayen) indicated that the three C/V schemes (OS, MS1, MS2) are overall satisfactory (KGE above 0.7). However, MS2 scheme (KGE = 0.75) performed better as compared to the traditional one-site calibration scheme (OS: KGE = 0.73). On the other hand, the MS1 scheme (KGE =0.71) performed worse than the OS scheme. The water balance analysis at Wayen showed that 60 to 90% of the annual rainfall is lost as evapotranspiration whereas 5 to 8% contributes to surface runoff, groundwater recharge remaining very low. The comparison of the three calibration/validation schemes provides evidence that accounting for nested sub-basins does not necessarily improve the quality of rainfall-runoff simulations. Yet, multi-sites modeling remains the best scheme when the watersheds used are not nested. The availability of higher resolution data might allow a better understanding of spatial scale effects.

Keywords:

Nested watersheds, SWAT model, Nakanbé, West Africa Sahel, Burkina Faso.

How to cite: Gbohoui, Y. P., Yonaba, R., Fowé, T., Paturel, J.-E., Karambiri, H., and Yacouba, H.: Comparison of One-Site vs Multi-Sites calibration/validation schemes for hydrological modelling of nested watersheds in the West African Sahel, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-69, https://doi.org/10.5194/iahs2022-69, 2022.

A good understanding of the hydrological cycle, and discharge in particular, in tropical tidal rivers enables reliable forecasting and decision making by researchers and policy makers.

The study proposes a low-cost technique to estimate tidally-influenced river discharge based on high frequency in-situ measurements. This technique pursues the work of Camenen et al. 2021 and estimates discharge based on a stage-fall-discharge rating curve, which requires the estimation of the river’s slope.

The application to the Saigon river (Vietnam) shows that its hydrological dynamics are ruled by a complex combination of atmospheric, continental and oceanic factors and pointed out unexpected hydrological behaviour: the 2018 rainy season was marked by heavy precipitations during four months, especially in November (typhoon USAGI). These high precipitations were associated with an increased water level; but surprisingly, the net water discharge decreased substantially during the same period (figure 9a, 9c). 

We propose an in-depth study of in-situ water level time series at several locations including in the Mekong delta, in the Saigon river and in the surrounding coastal area. The tidal signal obtained via harmonic analysis is extracted from the water level time series and the residual signal is then filtered using a low-pass filter as proposed in Trinh et al. 2020. This discriminates non-tidal fluctuations of water level and thus, permits the analysis of the impact of non-tidal signal such as storm surge or precipitation.

Our analysis supports that the coastal water level dynamics actually control the slope of the Saigon river and thus, its net discharge. Additionally, the coastal water level might be itself regulated by the seasonal flood of the Mekong delta, so that the Mekong river discharge management would indirectly influence the Saigon river dynamics, even if these two basins are hydrologically disconnected. 

The technique developed will help in better understanding the hydrological pattern of complex tropical estuarine rivers and disentangle the factors that influence their discharges as well as provide insights on tidal river interactions with coastal waters.

How to cite: Amaral, F., Gratiot, N., and Pellarin, T.: The Saigon's river response to coastal dynamics, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-496, https://doi.org/10.5194/iahs2022-496, 2022.

The aim of this study is to assess the impacts of climate change and forestation on the hydrology of a tropical catchment in Java, Indonesia. The study area is an exemplary case in Indonesia where a forestation programme has been implemented at the catchment scale. The hydrological effects of this forestation programme, in combination with climate change effects, have not been assessed. The individual and combined effects of climate change and forestation on the hydrology are assessed by employing a calibrated Soil and Water Assessment Tool (SWAT) model. Different water balance components are considered including streamflow, surface runoff, lateral flow, base flow and evapotranspiration. The results show that the forest area has increased from 2.7 to 12.8% of the catchment area from 2006 to 2019. In the same period the mean annual and seasonal temperature, rainfall and streamflow have increased as well. The SWAT simulations show that the effects of forestation on the annual and seasonal water balance components were relatively small, while the effects of climate change on these components were more pronounced. The combined effects of climate change and forestation show the dominant influence of climate change and both opposite effects of climate change and forestation on water balance components (e.g. for streamflow) and similar effects (e.g. for evapotranspiration). The outcomes of this study indicate that the forestation programme has small impacts on water balance components in the study catchment on an annual scale compared to the impacts of climate change. However, seasonally, forestation has resulted in a decrease in streamflow and surface runoff during the wet season which may reduce the flood risk. At the same time, the effects of forestation on severe drought events in the dry season should be carefully monitored. In conclusion, this study showed the relative importance of climate change and forestation impacts on water balance components and the potential of forestation programmes to flatten extreme events on a seasonal scale.

How to cite: Marhaento, H., Booij, M. J., and Rahardjo, N.: Impacts of climate change and forestation on the hydrology of a tropical catchment in Indonesia, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-532, https://doi.org/10.5194/iahs2022-532, 2022.

Le Fouta Djallon est considéré comme le « château d’eau » de l’Afrique de l’Ouest. La plupart des grands cours d’eau d’Afrique de l’Ouest y prennent leur source. Les eaux provenant de ce massif constituent l’essentiel du débit des grands cours d’eau apportant de l’eau douce à la bande sahélo-soudanienne, en particulier, d’ouest en est, la Gambie, le Sénégal et le Niger.

Ce massif est caractérisé par une forte hydraulicité. Ceci est lié avant tout à une forte pluviométrie, à des pentes appréciables et à une lithologie peu perméable. Les roches sont en effet peu poreuses, ce qui s’oppose grandement à la nation de « château d’eau ». de fait, les réserves sont faibles, et essentiellement liées à la présence de 8000 petites zones humides qui stockent des volumes d’eau permettant de garantir des écoulements faibles en fin de saison sèche, mais pérennes sur de nombreux cours d’eau.

La couverture végétale du Fouta Djalon n’est pas très dense, du fait d’une lithologie peu propice. Mais elle ne montre pas, ni sur le court ni sur le long terme de signes de dégradation pouvant menacer sa résilience.

Après la période hyper humide 1950-1967 et la longue sécheresse de 1968 à 1993, la pluviométrie annuelle est en croissance au point de dépasser, dans de nombreuses régions d’Afrique de l’Ouest, la moyenne de long terme. Mais les débits mettent du temps à rejoindre ceux de la période antérieure à la phase hyper humide, du fait de la reconstitution des stocks d’eau suite à leur vidange progressive durant la phase sèche où elles ont longtemps soutenu les étiages.

Cette recherche vise à déterminer les conditions de la résilience hydrologique du massif du Fouta Djalon, source majeure d’alimentation en eau douce de la bande sahélo-soudanienne.

How to cite: Descroix, L., Manga, S. P., Dièye, A., Diedhiou, A. B., Diallo, K., Sakho, Y., Fall, A., Andrieu, J., Ogilvie, A., Marega, O., Diouf, O., and Faty, B.: Le château d’eau de l’Afrique de l’Ouest est-il hydrologiquement résilient ?, IAHS-AISH Scientific Assembly 2022, Montpellier, France, 29 May–3 Jun 2022, IAHS2022-593, https://doi.org/10.5194/iahs2022-593, 2022.