Causes and Management of Salinity in the Breede River Valley, South Africa

The water-distribution system of the Breede River Irrigation Scheme, South Africa, consists of the river itself, a series of canals, and various pumping schemes. However, the river not only conveys water to the various farms, but it also acts a a huge drainage ditch that receives all the drainage water from the irrigated areas. Since the 1960's, an awareness of salinity levels in the Breede River during the summer months has grown considerably. The perception of increasing salinity over time gave rise to concerns about the sustainability of using the water for irrigation of high-value, salt-sensitive crops. Various studies were undertaken to determine the origin of the river salinisation and how the situation could be managed. Isotopical an geohydrological investigations largely ruled out natural groundwater discharge as the major cause of river salinity; rather, irrigation return flow emanating from excessive leaching under irrigation is probably the major contributor. This contribution is aggravated by the mobilisation of salt in saline sedimentary deposits when these are disrupted during the preparation of new irrigation lands. River salinity is presently being managed by releases of freshening water. Complementary management options have been investigated or are under consideration, including construction of a return-flow intercepting drain or a high-level irrigation canal, and a reduction in irrigation leaching.Original Abstract: Le reseau d'alimentation du plan d'irrigation de la rivieere Breede (Afrique du Sud) est constitue par la riviere elle-meme, par un ensemble de canaux et par des stations de pompage. Cependand, la rivieere ne sert pas seulement a l'alimentation, mais c'est aussi un immense fosse de drainage qui recoit toutes les eaux drainees venant des zones irriguees. Depuis les annees soixante, on se preocupe de plus en plus de la salinisation des eaux de la riviere Breede pendant les mois d'ete. L'accroissement constate de la salinite au cours du temps a fait prendre conscience de la possibilite de maintenir l'usage de l'eau pour l'irrigation de cultures sensibles aux sels et de haute valeur ajoutee. Differentes recherches ont ete entreprises dans le but de determiner l'origine de la salinisation de la riviere et de definir la facon de gerer cette situation. Des etudes isotopiques et hydrogeologiques ont clairement mis hors de cause l'ecoulement souterrain naturel de l'essentiel de la salinite de la riviere; en revanche, l'apport principa