The effect of three fire regimes on stream water quality, water yield and export coefficients in a tropical savanna (northern Australia)

Abstract

The effects of three fire regimes-(1) burning early in the dry season (June), (2) burning late in the dry season (September) and (3) not burning (protected from wildfires)-on the water quality, water yield and export coefficients of three intermittent streams, which flow between December and June, have been examined in a tropical savanna in northern Australia. The study was conducted over a three year period in Kakadu National Park, and employed a comparative catchment approach though without any pre-treatment data. The canopy cover, density of riparian vegetation, litter- and ground-cover of the catchment burnt early in the dry season (catchment E, stream E) and the unburnt catchment (catchment U, stream U) were similar. Fires lit late in the dry season (catchment L, stream L) however resulted in tree mortalities, and a lower canopy cover (50% less), riparian tree density (80% less) and litter cover, and increased amounts of bare ground; thereby increasing catchment L's susceptibility to erosion. This resulted in episodic runoff events from catchment L in November and December, before continuous wet season flow. These events, absent in catchments E and U, featured high concentrations of total suspended sediment (TSS), volatile suspended sediment (VSS), N, P, Fe and Mn up to 10 times those measured later in the wet season. During continuous wet season flow between December and June, baseflow water quality of the three streams were similar. Storm runoff concentrations for N and P were also similar, however stream L storm runoff concentrations of TSS, VSS, Fe and Mn were 2-5 times higher than those measured in streams E and U. Despite this, only the export coefficients for TSS from catchment L (average 61 kg ha-1) were significantly higher (average 2.4 times) than catchment E and U coefficients. This was attributed to the overwhelming influence of stream volume, relative to concentration, in determining stream load and hence catchment export coefficients (load/catchment area). The apparently negligible impact of the fire regimes on VSS, N, P, Fe and Mn export coefficients, and also the overall low sediment export coefficients for the three catchments which were up to 100 times less than that reported for other tropical environments, were ascribed to the low catchment slopes (average 0.5%), low soil fertility, maintenance of a protective surface gravel lag, the negligible impact of the fire regimes on water yield, and the sometimes lengthy (maximum 6 months) period between burning and runoff. (C) 2000 Elsevier Science B.V.