Amin Zettam

The effectiveness of hydrological predictions using CFSR (Climate Forecast System Reanalysis) data varies regionally, and their suitability in North Africa's semiarid Mediterranean climate remains untested. This study represents the first of its kind in North Africa, focusing on the simulation of phosphorus, using CFSR data. We conducted a comparative analysis of hydrological simulations in the Tafna watershed using both CFSR data and real station data. Monthly and annual precipitation comparisons, as well as streamflow and phosphorus flux evaluations, were performed using the coefficient of determination ( R ² ) and the Nash–Sutcliffe efficiency (NSE). Results indicated that CFSR data provided good statistical agreement with rain gauge measurements and generated accurate streamflow simulations (NSE = 0.63, R ² = 0.64) comparable to real data (NSE = 0.50, R ² = 0.53), particularly during calibration. Phosphorus simulations, while lacking precision, yielded values consistent with observations. This study demonstrates that CFSR data can serve as a viable alternative for the SWAT (Soil and Water Assessment Tool) model when real station data are unavailable, highlighting the originality of phosphorus simulations in this region.

Dans des contextes semi-arides et fortement anthropisés la caractérisation et la quantification des transferts de polluants dans les eaux de surface et vers la mer est importante à appréhender pour aider à une meilleure gestion de la ressource en eau dans un contexte de changements environnementaux. Sur le Nord-africain notamment la difficulté d’obtenir des données en quantité et qualité suffisante est difficile. La présente étude s’intéresse aux mécanismes de transfert des nitrates et des sédiments à l’échelle du bassin versant de la Tafna (7200 km2) et des plus grands fleuves de l'Afrique du Nord par une approche couplant les mesures in situ et le modèle SWAT (Soil and Water Assessment Tool). La partie in situ a permis de quantifier la quantité de nitrates apportée par les différents exutoires des sous bassins de la Tafna. Elle a aussi mis en évidence la contribution du bassin versant à l’eutrophisation de la Méditerranée tout en mesurant les quantités de nitrates acheminées vers les eaux marines. Les résultats ont mis en évidence les facteurs qui influent sur les flux et les concentrations en nitrates et notamment l’occupation du sol et les barrages ont une grande influence sur les flux de nitrates dans la Tafna. Les stations d’étude ont été classées selon un gradient d’eutrophisation, les stations amont présentant de fortes concentrations en nitrates, elles s’opposent aux stations avals. Les résultats de simulation sur la période 2003 à 2011 ont montré qu’une grande partie de l'eau de surface de la Tafna provient du ruissellement de surface (59%) et du flux latéral (40%), alors que la contribution des eaux souterraines est insignifiante (1%). La Tafna transfère une quantité annuelle moyenne de 2942 t.an-1 de sédiment vers la mer entre 2003 et 2011. SWAT a montré aussi que les cinq barrages de la Tafna stockent une grande quantité de sédiments, plus de 27 000 t.an-1 (90% des sédiments transportés par la Tafna). Le modèle reproduit aussi le flux de nitrates dans les stations de mesure entre 2003 et 2011. La Tafna transporte une quantité moyenne annuelle de 37 à 86 tN.an-1 jusqu'à la Méditerranée. Sur la base de ces résultats, l’approche de modélisation a pemis d’effectuer un changement d’échelle vers les grands bassins du nord-africain. La simulation a montré que les barrages de la Tafna stockent une quantité importante de nitrates [80 à 486 tN.an- 1], qui représente en moyenne 62% de la quantité totale de nitrates transportée annuellement par le fleuve. L’application du modèle sur les plus grands fleuves d'Afrique du Nord, Moulouya (55 860 km2), Tafna (7 200 km2), Chélif (44 694 km2) Oued el Kbir-Rhumel (8824 km2) et Medjerda (23 213 km2) a permis de représenter les flux d'eau, de sédiments et de nitrate. Les fleuves de cette région transportent une quantité de sédiments de 706 .103tan-1 vers la Mer ou le fleuve Oued Kbir-Rhumel qui contribue à la plus grande quantité (55%).Le flux de nitrates transportés vers la mer est de 5330 tN.an-1, le fleuve Moulouya qui contribue à la plus grande quantité (32%). Les barrages de cette région stockent une grande quantité de sédiments (98%) et de nitrates (39%) transportés par les fleuves vers la Méditerranée.

The objective of this study is to propose a modelling approach to quantify the contribution of mains coastal catchments of North Africa in sediment and nitrate fluxes to the Mediterranean Sea. We use the Soil and Water Assessment Tool (SWAT) on the biggest coastal rivers, Moulouya (55 860 km²), Tafna (7200 km²), Chelif (44 694 km²) Oued el Kebir (8 824 km²) and Medjerda (23 213 km²). Simulation was performed monthly from 2003 to 2013. The model represents correctly the dynamics of water, sediment and nitrate flux. Rivers carry between 22 .10³ to 390 10³t yr^-1 of sediments and Oued el Kebir contributes with 55% of the total amount, while nitrate fluxes vary from 182 to 1715 t yr^-1 and Moulouya contributes with 32% of the total quantity. The simulations show us that the North African dams stocked a large amount of sediment (98%) and nitrates (39%) transported by rivers.

A major problem facing the water sector in Algeria is deteriorating water quality, which is strongly influenced by nutrient inputs to agricultural watersheds. To mitigate the effects of agricultural activities and enhance water quality, this study considered the impact of different best management practices (BMPs) in agriculture. The objective of the research was to evaluate the impacts of various agricultural BMPs on nitrate losses in the Sebdou catchment in north‐west Africa using the Soil and Water Assessment Tool (SWAT) model in order to identify and recommend the most appropriate BMP. The study was carried out in two parts: first, the SWAT model was calibrated and validated to simulate discharge and nitrate losses at the outlet of the catchment, and second, the calibrated model was used to assess the effects of BMPs on nitrate losses. The hydrological modelling fitted well with the observed data for the calibration and validation periods (NSE between 0.63 and 0.65 and R² between 0.80 and 0.92). Nitrate simulation was also satisfactory (NSE = 0.58 and R² = 0.58), with the average nitrate exports at the outlet varying between 136 and 149 t N year⁻¹. Agricultural tillage practices were the main N input and plant uptake was the main N output. The simulation results of BMP scenarios (contour farming, strip cropping and terracing) showed a significant decrease in nitrate quantity in soil, nitrate yield in surface run‐off and nitrate exports. The maximum reduction was observed from the use of parallel terraces, which reduced nitrate in soil by 17% (41 kg ha⁻¹) and decreased nitrate flux exportation to below 133 t N year⁻¹ (a reduction of about 11%). This study provided useful information for implementation of the most effective agricultural BMPs in the catchment for stakeholders who are interested in soil and water management.

An elevated nitrogen concentration in water is one of the main problems affecting water quality in Mediterranean rivers. The objectives of this study were (1) to evaluate the contribution of the Tafna catchment to the nitrate load entering the Mediterranean Sea, (2) to quantify the impact of agriculture on the nitrate concentration in water bodies, (3) to evaluate nitrate loads entering groundwater, and (4) to quantify the role of reservoirs in nitrate retention. A SWAT model was applied during the period 2003 to 2011. The discharge calibration was based on a previous study by Zettam et al. (2017). NSE efficiencies ranged from 0.421 to 0.75, R² ranged from 0.25 to 0.84, and PBIAS ranged from 3.68 to 39.42. The simulations of monthly nitrate loads were satisfactory in the upstream sampling stations, with NSE between 0.48 and 0.65 and R² between 0.63 and 0.68. The PBIAS was satisfactory in all the sampling stations (− 36.30 to 10.42). In the downstream sampling stations, the calibration of the monthly nitrate loads was unsatisfactory (NSE ranged from − 0.26 to 0.21 and R² ranged from 0.02 to 0.25). Fertilisation was the main N input in the catchment, while the main N output was plant uptake. The Tafna River carried an annual average of 37 to 85.5 t N year⁻¹ into the Mediterranean Sea. The simulation also showed that reservoirs in the Tafna basin contain a large quantity of nitrates, i.e. 62% on average of the total amount of nitrates transported annually by the Tafna River.

Sediment deposits in North African catchments contribute to around 2%–5% of the yearly loss in the water storage capacity of dams. Despite its semi-arid climate, the Tafna River plays an important role in Algeria’s water self-sufficiency. There is continuous pressure on the Tafna’s dams to respond to the demand for water. The Soil and Water Assessment Tool (SWAT) was used to evaluate the contribution of different compartments in the basin to surface water and the dams’ impact on water and sediment storage and its flux to the sea in order to develop reservoir management. The hydrological modelling fitted well with the observed data (Nash varying between 0.42 and 0.75 and R² varying between 0.25 and 0.84). A large proportion of the surface water came from surface runoff (59%) and lateral flow (40%), while the contribution of groundwater was insignificant (1%). SWAT was used to predict sediments in all the gauging stations. Tafna River carries an average annual quantity of 2942 t·yr⁻¹ to the Mediterranean Sea. A large amount of water was stored in reservoirs (49%), which affected the irrigated agricultural zone downstream of the basin. As the dams contain a large amount of sediment, in excess of 27,000 t·yr⁻¹ (90% of the sediment transported by Tafna), storage of sediment reduces the lifetime of reservoirs.