SPECIFIC DEGRADATION OF WATERSHEDS

An extensive database of reservoir sedimentation surveys throughout continental United States is compiled and analyzed to determine specific degradation SD relationships as function of mean annual rainfall R, drainage area A, and watershed slope S. The database contains 1463 field measurements and specific degradation relationships are defined as function of A, R and S. Weak trends and significant variability in the data are noticeable. Specific degradation measurements are log normally distributed with respect to R, A, and S and 95% confidence intervals are determined accordingly. The accuracy of the predictions does not significantly increase as more independent variables are added to the regression analyses.     

Mali’s many shields of its past

There is a civilization at risk. In the middle of West Africa, just south of the Sahara, where the Niger River spills into a vast floodplain (the Middle Niger), an indigenous urban civilization emerged around two thousand years ago. The story of that civilization is just beginning to be uncovered from the thousands of tells (ancient settlement mounds) that litter the Middle Niger, but archaeologists have until recently been loosing the race with the looters who plunder these sites for ancient statues and bronze art. The nation of Mali, and the local community of Jenne at the epicenter of this looting recently invented effective shields for the remains of this civilization, protections against local looters in the employ of a syndicate of wealthy European antiquities dealers. In the person of the new, democratically-elected archaeologist-President, the nation has articulated a vision of Mali’s future linked to the preservation of its past. The government has promulgated effective regulations and education programs, and initiated with the United States the world’s first bilateral import ban between a major “source” and a major “market” nation. This 1993 bilateral accord has become a prototype for many nations. In that same year, Mali established a Cultural Mission at Jenne to reinforce local pride in a local past. This local pride, in only a short period, has turned into a total protection of some 200 sites within a ten-kilometer radius of Jenne. These indigenous actions have, in less than a decade, transformed the nonrenewable archaeological resources of the Jenne region from surely one of the world’s most endangered, to a case study in local and national protection     

Analyses de grilles pluviométriques et principaux traits des changements survenus au 20ème siècle en Afrique de l'Ouest et Centrale

Résumé

A partir des grilles mensuelles de pluie que met à disposition l'unité de recherche climatologique (CRU) de l'Université d' East Anglia, Royaume Uni, nous avons entrepris d'analyser les grands traits des changements pluviométriques qu'elles mettent en évidence en Afrique de l'Ouest et Centrale au cours du 20ème siècle. D'après ces grilles et en s'appuyant sur des tests statistiques robustes, il apparaît que l'Afrique de l'Ouest et Centrale a connu, au cours du siècle dernier, des alternances de saisons sèches et humides qui ont eu une ampleur spatiale et temporelle très variable. Au nord de l'Equateur, les changements climatiques les plus nets sont des sécheresses alors qu'au sud de l'Equateur, ce sont des épisodes plus humides. Au cours de la première moitié du siècle, les modifications ont eu tendance à correspondre à une augmentation de la pluviométrie annuelle alors que la tendance s'est inversée après, avec en point d'orgue la fin des années 1960, le début des années 1970. Cette dernière période marque un phénomène de sécheresse climatique, hydrologique et agricole qui est, de mémoire d'homme et statistiquement, le plus important en intensité et en étendue géographique du 20ème siècle. De nombreux travaux menés à des échelles plus petites confirment cette analyse globale, même si parfois, ils mettent en avant certains particularismes locaux, voire régionaux, que les données utilisées par la CRU et la résolution spatiale de leurs grilles ne peuvent pas toujours révéler. Ces grilles ont cependant leur utilité en hydrologie régionale.

Citation Paturel, J. E., Boubacar, I., L'Aour, A. & Mahé, G. (2010) Analyses de grilles pluviométriques et principaux traits des changements survenus au 20ème siècle en Afrique de l'Ouest et Centrale. Hydrol. Sci. J. 55(8), 1281–1288.     

Changes in rainfall regime over Burkina Faso under the climate change conditions simulated by 5 regional climate models

Sahelian rainfall has recorded a high variability during the last century with a significant decrease (more than 20 %) in the annual rainfall amount since 1970. Using a linear regression model, the fluctuations of the annual rainfall from the observations over Burkina Faso during 1961–2009 period are described through the changes in the characteristics of the rainy season. The methodology is then applied to simulated rainfall data produced by five regional climate models under A1B scenario over two periods: 1971–2000 as reference period and 2021–2050 as projection period. As found with other climate models, the projected change in annual rainfall for West Africa is very uncertain. However, the present study shows that some features of the impact of climate change on rainfall regime in the region are robust. The number of the low rainfall events (0.1–5 mm/d) is projected to decrease by 3 % and the number of strong rainfall events (>50 mm/d) is expected to increase by 15 % on average. In addition, the rainy season onset is projected by all models to be delayed by one week on average and a consensus exists on the lengthening of the dry spells at about 20 %. Furthermore, the simulated relationship between changed annual rainfall amounts and the number of rain days or their intensity varies strongly from one model to another and some changes do not correspond to what is observed for the rainfall variability over the last 50 years.