Monsoonal influences on evapotranspiration of savanna vegetation of northern Australia

Data from savannas of northern Australia are presented for net radiation, l atent and sensible heat, ecosystem surface conductance (G(s)) and stand wat er use for sites covering a latitudinal range of 5 degrees or 700 km. Measu rements were made at three locations of increasing distance from the northe rn coastline and represent high(1,750 mm), medium- (890 mm) and low- (520 m m) rainfall sites. This rainfall gradient arises from the weakened monsoona l influence with distance inland. Data were coupled to seasonal estimates o f leaf area index (LAI) for the tree and understorey strata. All parameters were measured at the seasonal extremes of late wet and dry seasons. During the wet season, daily rates of evapotranspiration were 3.1-3.6 mm day(-1) and were similar for all sites along the rainfall gradient and did not refl ect site differences in annual rainfall. During the dry season, site differ ences were very apparent with evapotranspiration 2-18 times lower than wet season rates, the seasonal differences increasing with distance from coast and reduced annual rainfall. Due to low overstorey LAI, more than 80% of wa ter vapour flux was attributed to the understorey. Seasonal differences in evapotranspiration were mostly due to reductions in understorey leaf area d uring the dry season. Water use of individual trees did not differ between the wet and dry seasons at any of the sites and stand water use was a simpl e function of tree density. G(s) declined markedly during the dry season at all sites, and we conclude that the savanna water (and carbon) balance is largely determined by G(s) and its response to atmospheric and soil water c ontent and by seasonal adjustments to canopy leaf area.