Role of a cascade of reservoirs in regulating downstream transport of sediment, carbon and nutrients: Case study of tropical arid climate Tana River Basin

摘要

Mass balances for dissolved and particulate organic C (DOC and POC), particulate nitrogen (PN) and dissolved inorganic nitrogen (DIN), dissolved PO~(3−)_4 and dissolved Si (DSi) were estimated by considering the annual fluxes in and out of three cascading reservoirs (Masinga, Kamburu and Gitaru) on the Tana River, Kenya. Flux data were obtained through biweekly sampling of the main inflows and outflows of the three reservoirs between May 2011 and May 2013. The degree of retention of particulate and dissolved materials generally was higher in Masinga Reservoir, compared to Kamburu and Gitaru reservoirs, which could be attributable to the much higher water retention time (i.e., 163, 19 and 3 days for Masinga, Kamburu and Gitaru, respectively, in 2012; 136, 14 and 2 days for Masinga, Kamburu and Gitaru reservoirs, respectively, in 2013). It was also observed the two smaller reservoirs (Kamburu and Gitaru) did not always act as net sinks of materials, but occasionally were a net source of materials transported downstream. The role of reservoirs in material retention was also found to exhibit interannual variability, likely controlled by the total annual water discharge into the reservoirs.