Dynamics of the Zambezi river plume

Abstract

Freshwater discharges from river runoff into the ocean have an important influence on the dynamics of many coastal regions. The plumes resulting from the encounter between fresh and saline waters have a variety of impacts, including changes in the availability of biological resources. Although there have been several general studies on the plume dynamics (and their relation to the rainfall), no quantitative analysis of the relationship between river plumes and its main controlling factor (i.e. the river discharge) has yet been reported. In order to establish a relationship between the river discharge and the plume size, both two-dimensional particle-tracking two-layer model and satellite ocean-colour observations were used for the Zambezi River plumes in Mozambique. The model included a random walk component, and it was run under realistic windstress and river discharge conditions. Discrete particles of freshwater released every 30 minutes, the counting of which allowed the computation of the plume size, were used to parameterize the river discharge. On the other hand, the observed plume size was estimated using the red to blue ratio of the normalized water-leaving radiation from 73 MODIS-Aqua images that spanned 5 years. The plume size obtained by these two methods was correlated with the river discharge measured at the Tete gauge station. Both model and imagery have revealed that the Coriolis deflection plays a predominant role in the dispersion patterns of the Zambezi plumes. The time lag between the river discharge and the maximum plume size was 17 days for near field plumes and 7 days for the far-field plumes. Ninety percent (90%) of the freshwater volume discharged per day was found to be diluted or dispersed in the same day. The area-runoff relation is dependent on the time of the year, and for the plumes observed in the far field during the period January-March (i.e. when the discharges are at their maximum), this relation is linear with a slope of 1.74 x10-³ s m-¹ . For the same period, the model has reproduced a similar relation with a slope of 1.73 x10-³ s m-¹