Monsoonal influences on evapotranspiration of savanna vegetation of northern Australia

Abstract

Data from savannas of northern Australia are presented for net radiation, latent and sensible heat, ecosystem surface conductance (G ) and stand water use for sites covering a latitudinal range of 5° 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 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 declined markedly during the dry season at all sites, and we conclude that the savanna water (and carbon) balance is largely determined by G and its response to atmospheric and soil water content and by seasonal adjustments to canopy leaf area.

Biotic pump of atmospheric moisture as driver of the hydrological cycle on land

“…Dry periods during the vegetative season in natural forests of higher latitudes neither bring about a decrease of transpiration (Goulden et al, 1997;Tchebakova et al, 2002). In contrast, transpiration of open ecosystems like savannas, grasslands or shrublands incapable of maintaining high soil moisture content year round, drops radically during the dry season (Hutley et al, 2001;Kurc and Small, 2004). Change of soil moisture content with time, dW/dt, is linked to precipitation P , evaporation E and runoff R via the law of matter conservation, dW/dt=P −E−R.…”

Investigation of dominant hydrological processes in a tropical catchment in a monsoonal climate via the downward approach

“…The soils are well drained with a surface texture commonly of sandy loam changing to sandy clays and clay subsoils deeper than 1-1.5 m with a massive and earthy structure (Hutley et al, 2000(Hutley et al, , 2001. The vegetation consists of open forests and wetlands.…”

Photosynthetic physiology of eucalypts along a sub-continental rainfall gradient in northern Australia

“…In savanna communities near Darwin, at the northern end of the precipitation gradient, the soil water that accumulates during the wet season is sufficient to maintain transpiration rates at constant, or even higher, rates in savanna eucalypts throughout the dry season compared to rates in the wet season Hutley et al, 2000Hutley et al, , 2001O'Grady et al, 1999). Measurements of predawn water potential suggested that these trees have access to available water even at the end of the dry season, presumably from roots that extend to several meters depth in the soil profile Hutley et al, 2000).…”