Water balance of tropical rainforest canopies in north Queensland,Australia

The water balance of four different rainforest types in the Wet Tropics region of north Queensland is inferred from measurements of canopy hydrological components undertaken for periods between 391 to 657 days. These measurements of rainfall, cloud interception, stem-flow, throughfall, canopy interception and transpiration have revealed considerable differences in the canopy water balance of different locations as a result of forest structural differences, altitude, exposure and climate. Cloud interception is a significant extra input of water to forests at high altitude sites (>1000 m) and varies between 7 and 29% of the total water input. At coastal and lower montane rainforests annual total evaporation is consistently around 50% of the total water input, but in upper montane cloud forest this drops dramatically to only 13% of the water input. At all sites actual evaporation is greater than potential evaporation for most of the year and on an annual basis exceeds potential by between 2 and 53%. The source of this additional energy is uncertain, but is likely to come from advection. Annual interception at all the rainforest sites was greater than annual transpiration, with transpiration dominating in the dry season and interception dominating in the wet season. All of the rainforests have a large annual net water balance to sustain runoff and recharge. Towards the end of the dry season runoff and recharge can cease in coastal lowland and lower mountain forests and they may have to draw on soil moisture and/or ground water at this time. In contrast, upper montane cloud forests have a positive net water balance throughout the year and are therefore an important source of dry season river flows. Furthermore, their exceptionally large annual runoff (∼6500 mm year⁻¹) is a major source of downstream water. Copyright © 2007 John Wiley & Sons, Ltd.     

Evapotranspiration and water source of a tropical rainforest in peninsular Malaysia

To evaluate water use and the supporting water source of a tropical rainforest, a 4‐year assessment of evapotranspiration (ET) was conducted in Pasoh Forest Reserve, a lowland dipterocarp forest in Peninsular Malaysia. The eddy covariance method and isotope signals of rain, plant, soil, and stream waters were used to determine forest water sources under different moisture conditions. Four sampling events were conducted to collect soil and plant twig samples in wet, moderate, dry, and very dry conditions for the identification of isotopic signals. Annual ET from 2012 to 2015 was quite stable with an average of 1,182 ± 26 mm, and a substantial daily ET was observed even during drought periods, although some decline was observed, corresponding with volumetric soil water content. During the wet period, water for ET was supplied from the surface soil layer between 0 and 0.5 m, whereas in the dry period, approximately 50% to 90% was supplied from the deeper soil layer below 0.5‐m depth, originating from water precipitated several months previously at this forest. Isotope signatures demonstrated that the water sources of the plants, soil, and stream were all different. Water in plants was often different from soil water, probably because plant water came from a different source than water that was strongly bound to the soil particles. Plants showed no preference for soil depth with their size, whereas the existence of storage water in the xylem was suggested. The evapotranspiration at this forest is balanced and maintained using most of the available water sources except for a proportion of rapid response run‐off.     

Water table and transpiration dynamics in a seasonally inundated Melaleuca quinquenervia forest,north Queensland,Australia

Water table fluctuations and transpiration were monitored in a seasonally inundated Melaleuca quinquenervia floodplain forest at Cowley Beach, north Queensland, Australia. Techniques were developed to reconstruct inundation duration and seasonal and inter‐annual variability at this site using long‐term stream flow data. It was estimated that the median duration of inundation in any year was 75 days with maximum and minimum durations of 167 days and 8 days, respectively. Measurements of individual tree transpiration using heat‐pulse techniques showed a strong relationship between tree size and tree water use, which was used for scaling to stand transpiration. Stand transpiration rates were found to be closely tied to atmospheric drivers of evaporation, and transpiration of M. quinquenervia was found to be unaffected by inundation. This ability to transpire during inundation may be due to physiological adaptations of this species. These adaptations are believed to include dynamic root systems that can quickly respond to rising and falling water tables and dense networks of fine apogeotropic roots, which grow on and within the papery bark. Rates of stand transpiration remained low throughout the study (0·46 mm d^?1, 164 mm y^?1) despite the fact that transpiration was not limited by solar energy inputs or soil moisture deficit. Low stand transpiration was attributed to the low density, stunted nature and small sapwood area of trees at this site. Copyright © 2007 John Wiley & Sons, Ltd.     

Modelling wetland connectivity during overbank flooding in a tropical floodplain in north Queensland,Australia

Hydrological connectivity between floodplain wetlands and rivers is one of the principal driving mechanisms for the diversity, productivity and interactions of the major biota in river–floodplain systems. This article describes a method of quantifying flood‐induced overbank connectivity using a hydrodynamic model (MIKE 21) to calculate the timing, the duration and the spatial extent of the connections between several floodplain wetlands and rivers in the Tully–Murray catchment, north Queensland, Australia. Areal photogrammetry and field surveyed stream cross data were used to reproduce floodplain topography and rivers in the model. Laser altimetry (LiDAR)–derived fine resolution elevation data, for the central floodplain, were added to the topography model to improve the resolution of key features including wetlands, flow pathways and natural and artificial flow barriers. The hydrodynamic model was calibrated using a combination of in‐stream and floodplain gauge records. A range of off‐stream wetlands including natural and artificial, small and large were investigated for their connectivity with two main rivers (Tully and Murray) flowing over the floodplain for flood events of 1‐, 20‐ and 50‐year recurrence intervals. The duration of the connection of individual wetlands varied from 1 to 12 days, depending on flood magnitude and location in the floodplain, with some wetlands only connected during large floods. All of the wetlands studied were connected to the Tully River for shorter periods than they were to the Murray River because of the higher bank heights and levees on the Tully River and wetland proximity to the Murray River. Other than hydrology, land relief, riverbank elevation and levee banks along the river were found key factors controlling the degree of connectivity. These variations in wetland connectivity could have important implications for aquatic biota that move between rivers and off‐stream habitats during floods. Copyright © 2011 John Wiley & Sons, Ltd.     

Contrasting recovery of shallow and deep water seagrass communities following climate associated losses in tropical north Queensland,Australia

Tropical seagrass decline and recovery from severe storm impacts was assessed via quarterly measurements of seagrass biomass, species composition and experimental investigations of recovery in north Queensland. Shallow and deep seagrass meadows suffered major declines. Significant recovery in the two years following loss only occurred at deeper sites. Halophila spp. in deep water areas had a high capacity for recovery through the availability of seed banks. In contrast, the shallow species did not recover quickly from experimental disturbance, had poor seed reserves and relied on asexual propagation. The potential for shallow species to recover rapidly from widespread losses was limited as seed banks were limited or non-existent. Understanding inter- and intra-specific differences in seagrass recovery and how this interacts with location is critical to predict the consequences of climate events to tropical seagrasses. This is especially important as more frequent severe storms are predicted as a consequence of climate change.     

A simple method using climatic variables to estimate canopy temperature,sensible and latent heat fluxes in a winter wheat field on the North China Plain

Estimation of evapotranspiration from a crop field is of great importance for detecting crop water status and proper irrigation scheduling. The Penman–Monteith equation is widely viewed as the best method to estimate evapotranspiration but it requires canopy resistance, which is very difficult to determine in practice. This paper presents a simple method simplified from the Penman–Monteith equation for estimating canopy temperature (T_c). The proposed method is a biophysically‐sound extended version of that proposed by Todorovic. The estimated canopy temperature is used to calculate sensible heat flux, and then latent heat flux is calculated as the residual of the surface energy balance. An eddy covariance (EC) system and an infrared thermometer (IRT) were installed in an irrigated winter wheat field on the North China Plain in 2004 and 2005, to measure T_c, and sensible and latent heat fluxes were used to test the modified Todorovic model (MTD). The results indicate that the original Todorovic model (TD) severely underestimates T_c and sensible heat flux, and hence severely overestimates the latent heat flux. However, the MTD model has good capability for estimating T_c, and gives acceptable results for latent heat flux at both half‐hourly and daily scales. The MTD model results also agreed well with the evapotranspiration calculated from the measured T_c. Copyright © 2008 John Wiley & Sons, Ltd.     

Precipitation interception in Australian tropical rainforests: II. Altitudinal gradients of cloud interception,stemflow, throughfall and interception

This article presents a comprehensive study of canopy interception in six rainforests in Australia's Wet Tropics for periods ranging between 2 and 3·5 years. Measurements of rainfall, throughfall, stemflow and cloud interception were made at sites characterized by different forest types, canopy structure, altitude, rainfall and exposure to prevailing winds. Throughfall at these sites ranged between 64 and 83% of total precipitation inputs, while stemflow ranged between 2 and 11%. At sites higher than 1000 m, cloud interception was found to contribute up to 66% of the monthly water input to the forest, more than twice the rainfall at these times. Over the entire study period, cloud interception accounted for between 4 and 30% of total precipitation inputs, and was related more to the exposure of sites to prevailing winds than to altitudinal differences alone. Over the duration of the study period, interception losses ranged between 22 and 29% of total water input (rainfall and cloud interception) at all sites except the highest altitude site on Bellenden Ker, where interception was 6% of total water input. This smaller interception loss was the result of extremely high rainfall, prolonged immersion in cloud and a sparser canopy. On a monthly basis, interception losses from the six sites varied between 10 and 88% of rainfall. All sites had much higher interception losses during the dry season than in the wet season because of the differences in storm size and rainfall intensity. The link between rainfall conditions and interception losses has important implications for how evaporative losses from forests may respond to altered rainfall regimes under climate change and/or large‐scale atmospheric circulation variations such as El Niño. Copyright © 2007 John Wiley & Sons, Ltd.     

Precipitation interception in Australian tropical rainforests: I. Measurement of stemflow,throughfall and cloud interception

Methods for measuring throughfall, stemflow and, hence, interception in the tropical rainforests of the Wet Tropics region of North Queensland, Australia, were tested at three sites for between 581 and 787 days. The throughfall system design was based on long troughs mounted beneath the canopy and worked successfully under a range of rainfall conditions. Comparison of replicated systems demonstrated that the methodology is capable of capturing the variability in throughfall exhibited beneath our tropical rainforest canopies. Similarly, the stemflow system design which used spiral collars attached to sample trees worked well under a range of rainfall conditions and also produced similar estimates of stemflow in replicated systems. Higher altitude rainforests (>1000 m) in North Queensland can receive significant extra inputs of water as the canopy intercepts passing cloud droplets. This additional source of water is referred to as ‘cloud interception’ and an instrument for detecting this is described. The results obtained from this gauge are compared with cloud interception estimates made using a canopy water balance method. This method is based on stemflow and throughfall measurements and provides an alternative means to fog or cloud interception gauge calibration techniques used in the literature. Copyright © 2007 John Wiley & Sons, Ltd.     

Hydrological characteristics of groundwater in a subtropical coastal plain with large variations in salinity: Pimpama,Queensland, Australia

Abstract

The Pimpama coastal plain is situated in southern Moreton Bay, in subtropical eastern Australia. The plain is low lying and tidal and is situated behind a large sand barrier island. Largely due to recent (30 years) drainage networks within the flood plain, surface water quality has declined. Groundwater hydrographs have enabled the determination of different flow systems: a deeper system responding to seasonal weather patterns and a shallower flow system more responsive to individual rainfall events. Elevated potentiometric heads in semi-confined aquifers reflect upward movement of saline to hypersaline groundwaters. However, interaction of this deeper groundwater with shallower groundwater and the surface drains is yet to be determined. Recharge to the shallower system is by direct infiltration while recharge to the deeper system includes a component from landward ranges or bedrock outcrops within the plain. Discrimination between groundwater bodies is possible using salinity, ionic ratios and stable isotopes. Features of groundwater hydrology, the distribution of salinity and variations in water chemistry all suggest that under current conditions infiltration has increased, plus there is a greater landward migration of groundwaters of marine origin.     

Comparison of interception,forest floor evaporation and transpiration in Pinus radiata and Eucalyptus globulus plantations

Studies of evapotranspiration (ET) processes in forests often only measure one component of total ET, most commonly interception. This study examined all three components of annual ET (interception, evaporation from the forest floor and transpiration) and the correlations between them at 18 plantation forest sites in two species. All plantations had closed canopies, and sparse or no understorey. Single‐sided leaf area index averaged 3.5 (standard deviation ±0.5) in Eucalyptus globulus Labill. and 6.1 (±0.8) in Pinus radiata D.Don. Measurements included annual totals of rainfall in the open and under the canopy, stem flow (four sites only), evaporation from the forest floor and transpiration by the overstorey. Interception (I) averaged 19% (±4.9) of annual rainfall in E. globulus compared with 31% (±11.1) in P. radiata. However, higher annual interception in P. radiata did not result in higher total ET because annual evaporation from the forest floor (E) averaged 29% (±4.9) of rainfall in E. globulus but only 15% (±3.5) in P. radiata. Hence, the relative contribution of annual I plus E to ET did not differ significantly between the two species, averaging 48% (±7.3) of annual rainfall in E. globulus compared with 46% (±11.8) in P. radiata. As reported previously, transpiration did not differ significantly between the two species either, but was strongly related to depth‐to‐groundwater. In closed canopy plantations, mean annual ET did not differ between the two species. We conclude that when grown in plantations under similar soil and climatic conditions, conifer and broad‐leaved tree species can have similar annual ET, once the canopy of the plantation has closed. Lower average annual interception in broad‐leaved trees was offset by higher soil evaporation. These results highlight the importance of measuring all components of ET in studies of vegetation water use. Copyright © 2014 John Wiley & Sons, Ltd.     

Assessing stand water use in four coastal wetland forests using sapflow techniques: annual estimates,errors and associated uncertainties

Forests comprise approximately 37% of the terrestrial land surface and influence global water cycling. However, very little attention has been directed towards understanding environmental impacts on stand water use (S) or in identifying rates of S from specific forested wetlands. Here, we use sapflow techniques to address two separate but linked objectives: (1) determine S in four, hydrologically distinctive South Carolina (USA) wetland forests from 2009–2010 and (2) describe potential error, uncertainty and stand‐level variation associated with these assessments. Sapflow measurements were made from a number of tree species for approximately 2–8 months over 2 years to initiate the model, which was applied to canopy trees (DBH > 10–20 cm). We determined that S in three healthy forested wetlands varied from 1.97–3.97 mm day^?1 or 355–687 mm year^?1 when scaled. In contrast, saltwater intrusion impacted individual tree physiology and size class distributions on a fourth site, which decreased S to 0.61–1.13 mm day^?1 or 110–196 mm year^?1. The primary sources of error in estimations using sapflow probes would relate to calibration of probes and standardization relative to no flow periods and accounting for accurate sapflow attenuation with radial depth into the sapwood by species and site. Such inherent variation in water use among wetland forest stands makes small differences in S (<200 mm year^?1) difficult to detect statistically through modelling, even though small differences may be important to local water cycling. These data also represent some of the first assessments of S from temperate, coastal forested wetlands along the Atlantic coast of the USA. Copyright © 2014 John Wiley & Sons, Ltd.