Monsoonal influences on evapotranspiration of savanna vegetation of northern Australia

Data from savannas of northern Australia are presented for net radiation, latent and sensible heat, ecosystem surface conductance (G(s)) and stand water use for sites covering a latitudinal range of 5 degrees or 700 km. Measurements were made at three locations of increasing distance from the northern coastline and represent high(1,750 mm), medium- (890 mm) and low- (520 mm) rainfall sites. This rainfall gradient arises from the weakened monsoonal influence with distance inland. Data were coupled to seasonal estimates of 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 reflect site differences in annual rainfall. During the dry season, site differences 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 water vapour flux was attributed to the understorey. Seasonal differences in evapotranspiration were mostly due to reductions in understorey leaf area during 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 simple 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 content and by seasonal adjustments to canopy leaf area.