Investigation of the rainfall variability in central Tunisia / Investigations sur la variabilité pluviométrique en Tunisie centrale

Abstract

The decrease in runoff of the Merguellil basin, Tunisia recorded during the decade 1989–1998 led to the study of several time series derived from daily rainfall. It was found that variability characteristics related to daily rainfall greater than 30 mm are significantly different among successive time periods. In addition, the probability distributions of these rains, recorded at different raingauge sites, are significantly different between the period 1976–1989 and the prior and posterior observation periods. Furthermore, the surface covered by daily rains greater than 30 mm decreased between 1976 and 1989. It was also noted that the probability distributions of the surfaces covered changed significantly after 1976. The combination of human action and pluviometric variability (rainfall increase in the period 1989–1998, notably the increase in the number of days of rainfall greater than 30 mm) may explain the decrease in runoff of Merguellil Wadi.     

Avenir des ressources en eau glaciaire de la Cordillère Blanche / On the future of the water resources from glacier melting in the Cordillera Blanca,Peru

Abstract

The hydrological data (since 1953) of the Llanganuco basin (87.0 km², 39% glacierized) show an increase of the glacial melting during the last quarter of the 20th century. These results were supplemented (since the end of 2000) by the data of the small basin of Artesoncocha (8.4 km², 79% glacierized). The basin runoff is well correlated to the atmospheric temperature derived from the NOAA-NCEP re-analysis above the Cordillera Blanca. At the monthly time scale, the temperature is a good proxy of the glacier melting. The retreat of several glaciers in the Cordillera Blanca has been well documented for 50 years, highlighting an acceleration of the deglaciation in the mid-1970s. The use of these data of various origins permits one to model the behaviour of glaciers, especially the meltwater production, and then to predict their future evolution. The model was calibrated over the 1950–2000 period, thus providing a possible optimistic evolution range (underestimation if the climate change becomes more intense). The forcing of the model by forecasts of the future temperature evolution above the Cordillera Blanca, derived from the regionalization of global climatic models, allows improvement of the estimations only based on past glacial behaviour.     

Some effects of sampling design on water quality estimation in streams / Quelques effets de la stratégie d’échantillonnage sur l’estimation de la qualité de l’eau des rivières

Abstract

Abstract Stream sampling programmes for water quality estimation constitute a statistical survey of a correlated population. The properties of parameter and other estimates made from sample values from such programmes are set in the context of statistical sampling theory. It is shown that a model-based rather than a design-based approach to statistical analysis is usually appropriate. The influence of model structure and sampling design on the robustness and suitability of estimation procedures is investigated, and relationships with kriging are demonstrated. Methodology is discussed with reference to data from a UK sampling programme     

Le coefficient de compacité de Gravelius: analyse critique d'un indice de forme des bassins versants

Résumé

Les classiques de l'hydrologie proposent, pour caractériser la géométrie d'un bassin versant, différents indices de forme destinés à comparer les bassins voire à estimer certaines de leurs caractéristiques hydrologiques. L'indice auquel il est le plus communément fait référence est l'indice de compacité, dit de Gravelius. Il est défini comme le rapport du périmètre du bassin étudié à celui d'un cercle de même surface. L'approche par la géométrie fractale amène à s'interroger sur la validité de cet indice. En effet, dans la définition précédente, seule la surface du bassin est une grandeur bien définie. Sa mesure est bien sûr entachée d'une incertitude mais il est possible d'en déterminer par exemple un minorant et un majorant. Le périmètre est par contre totalement dépendant de sa jauge de mesure et n'a donc aucun caractère objectif, ce qui retire toute signification à l'indice ainsi construit.     

Uncertainty and sensitivity analysis of runoff and sediment yield in a small agricultural watershed with KINEROS2 / Analyse d'incertitude et de sensibilité des simulations d'écoulement et de transport solide de KINEROS2 dans un petit bassin versant agricole

Abstract

Using the Monte Carlo (MC) method, this paper derives arithmetic and geometric means and associated variances of the net capillary drive parameter, G, that appears in the Parlange infiltration model, as a function of soil texture and antecedent soil moisture content. Approximate expressions for the arithmetic and geometric statistics of G are also obtained, which compare favourably with MC generated ones. This paper also applies the MC method to evaluate parameter sensitivity and predictive uncertainty of the distributed runoff and erosion model KINEROS2 in a small experimental watershed. The MC simulations of flow and sediment related variables show that those parameters which impart the greatest uncertainty to KINEROS2 model outputs are not necessarily the most sensitive ones. Soil hydraulic conductivity and wetting front net capillary drive, followed by initial effective relative saturation, dominated uncertainties of flow and sediment discharge model outputs at the watershed outlet. Model predictive uncertainty measured by the coefficient of variation decreased with rainfall intensity, thus implying improved model reliability for larger rainfall events. The antecedent relative saturation was the most sensitive parameter in all but the peak arrival times, followed by the overland plane roughness coefficient. Among the sediment related parameters, the median particle size and hydraulic erosion parameters dominated sediment model output uncertainty and sensitivity. Effect of rain splash erosion coefficient was negligible. Comparison of medians from MC simulations and simulations by direct substitution of average parameters with observed flow rates and sediment discharges indicates that KINEROS2 can be applied to ungauged watersheds and still produce runoff and sediment yield predictions within order of magnitude of accuracy.     

Hydrochemical facies of groundwater in the Gaza Strip,Palestine / Faciès hydrochimiques de l’eau souterraine dans la Bande de Gaza,Palestine

Abstract

Abstract Groundwater in the Gaza Strip is the only source of water for domestic, agricultural and industrial uses. Extensive pumping has caused serious quantitative and qualitative problems in the aquifer. The hydrochemical facies are evaluated using the trilinear diagram for 200 water samples. Groundwater in the north and west is mostly characterized by Ca-Mg-HCO₃ facies (alkaline water), and in the south and east by Na-Cl-SO₄ facies (saline water). Sand dunes and rainfall are the major factors controlling the distribution of hydrochemical facies. The eastern edge of the sand dune belt is considered the barrier that separates the two major facies. Brackish water flowing from the east is mixed with rainwater which infiltrates through the sand dunes to the aquifer. Other factors, e.g. seawater intrusion and extensive pumping, play a minor role in the distribution of the hydrochemical facies.     

New data for water losses from mature Sitka spruce plantations in temperate upland catchments / Nouvelles données pour les pertes d’eau de plantations adultes de sapins de Sitka en bassins versants tempérés d’altitude

Abstract

Abstract Accurate estimates of water losses from mature Sitka spruce (Picea sitchensis) plantations in the UK uplands are required to assess the sustainability of water supply in the event of land-use change. Many investigations have demonstrated that afforestation increases water losses from temperate upland catchments, to up to 40% of annual site rainfall. In a 0.86 km² upland water supply catchment in southwest Scotland, interception loss in a Sitka spruce-dominated 37-year old plantation, was 52% of annual precipitation (2912 mm), considerably higher than reported in previous studies of similar catchments. From direct measurements of rainfall, cloudwater, discharge and soil evaporation, the catchment water balance was 96–117% complete, within the limits of measurement error. The most probable explanation for the higher forest interception loss reported here is the inclusion of cloudwater measurements.     

Identification of rainfall–runoff models for operational applications / Identification de modèles pluie–débit pour des applications opérationnelles

Abstract

The problem of selecting appropriate objective functions for the identification of a lumped conceptual rainfall–runoff model is investigated, focusing on the value of the model in an operational setting. A probability-distributed soil moisture model is coupled with a linear parallel routing scheme, and conditioned on rainfall–runoff observations from three catchments in the southeast of England. Using an abstraction control problem, which requires accurate simulation of the intermediate flow range, it is shown that using the traditional RMSE fit criterion, produces operationally sub-optimal predictions. This is true in the identification period, when applied to a testing period, and to proxy catchment data. Using a second case study of the Leaf River in Mississippi (USA), where the focus changes to predicting flood peaks over a specified threshold, also suggests that the relevant flood threshold should govern the objective function choice. It is concluded that, due to limitations in the structure of the employed model, it would be counter-productive to try to achieve a good all-round representation of the rainfall–runoff processes, and that a more empirical approach to identification may be preferred for specific forecasting problems. This leaves us with the question of how far hydrological realism should be sacrificed in favour of purpose-driven objective functions.     

Stream depletion by a pumping well including the effects of nonlinear variation of captured evaporation from a phreatic aquifer / Vidange en cours d’eau par puits de pompage incluant les effets de la variation non-linéaire de l’évaporation capturée d’une nappe à surface libre

Abstract

Abstract A study was made to develop a model that can be used to predict the steady-state stream depletion rates caused by a continuous pumping well located in a water table aquifer. The effects of nonlinear variation of evaporation with the depth to water table on steady-state stream depletion rate were investigated using model results. Dimensional analysis was used to determine the relationship between the scaled steady-state stream depletion, the scaled pumping distance, the scaled hydraulic conductivity, and the scaled initial depth to the water table. A dimensionless graph was developed for a wide range of these parameters. Analysis of this graph showed that the steady-state stream depletion rate decreases as the pumping distance between the well and the stream increases. The dimensionless graph also showed that steady-state stream depletion rates strongly depended on the initial position of the water table. Analysis indicated that, as the saturated conductivity increased, the effect of the initial position of the water table on the magnitude of stream depletion rate was more influential. Analysis also showed that, as the value of saturated conductivity decreased, the relative error produced by the assumption that at steady state all the pumped water is captured from the evaporation, also decreased.     

Transfert d'énergie thermique vers le fond d'un réservoir lacustre / Thermal energy transfer towards the bottom of water reservoirs (lakes)

Abstract

In water reservoirs (lakes) in the summer period, three layers can be observed: the epilimnion, metalimnion and hypolimnion. In summer an increase in the thickness of the epilimnion layer is observed together with a movement of the metalimnion towards the bottom and a significant increase in the temperature of the two layers. An increase in hypolimnion temperature is observed in autumn during the cooling of the surface layers of the water. The increase in temperature of the epilimnion and metalimnion in summer and the downward movement of the metalimnion has not been clearly explained. The movement of the metalimnion is connected with thermal energy transfer, but previous theories do not take into account the phenomenon observed. In this paper, a new theory of thermal energy transfer in water reservoirs is elaborated and subsequently checked by laboratory experiments. A physical model of the water reservoir was designed and constructed. During the investigations it was noted that thermal energy can be transferred towards the bottom of the reservoir due to the existence of anti-convectional currents, which appear with the water temperature inversion in the upper part of the water column. The results obtained in the laboratory were confirmed by measurements in the Solina Reservoir in Poland.