Integration and application of the Rainfall Runoff Library.

The Rainfall Runoff Library (RRL) provides a convenient platform for implementing environment modelling components such as rainfall runoff models, calibration tools, and objective functions. A rainfall-runoff model widely known and used in South Korea, TANK, is added to the RRL, and used along with the models AWBM and SIMHYD to reproduce the historical time series of daily and monthly runoff at the Soyanggang Dam and Youngcheon Dam catchments located in South Korea. The features of the RRL allow for an easy comparison of different models in a standardised and common framework. Three optimisation methods (Genetic algorithm, Rosenbrock method and Shuffled Complex Evolution algorithm) were applied to calibrate the model parameters using three different objective functions. The applicability of each model to these catchments is discussed based on the resulting statistics.

Observations of evapotranspiration in a break of slope plantation susceptible to periodic drought stress.

Break of slope (BOS) plantations are advocated as a means of water table control in areas where groundwater flows through colluvial deposits overlying low permeability bedrock. It is also believed that BOS plantations can supplement their water use requirements by exploiting shallow groundwater at the breaks in topographic slope. We compared measurements of BOS plantation and pasture evapotranspiration during spring, when the weather was warm and soils moist, and late summer when drought conditions prevailed. Microlysimeters and ventilated chambers were used to determine pasture and plantation floor evaporation, and heat pulse sensors were used to determine transpiration of the plantation. In spring, pasture evapotranspiration was 65% of that of the plantation, whereas, in summer, pasture evapotranspiration was equivalent to only 35% of that of the plantation. Rainfall interception by the canopy of the plantation was twice that of the pasture, reinforcing the notion that trees can help reduce groundwater recharge and alleviate dryland salinity and waterlogging. During the summer drought period, daily plantation transpiration was only 20% of that measured during spring, suggesting that the plantation was not utilizing groundwater supplies but was instead drawing from soil water supplies. This hypothesis was supported by the comparison of relative abundances of the isotopes of water ((2)H and (18)O) in soil and wood samples. We conclude that the BOS plantation is not behaving in the manner predicted, and our findings raise doubts about the predicted advantages of establishing plantations in break of slope positions.

A review of whole-plant water use studies in tree.

Weighing lysimeters, large-tree potometers, ventilated chambers, radioisotopes, stable isotopes and an array of heat balance/heat dissipation methods have been used to provide quantitative estimates of whole-tree water use. A survey of 52 studies conducted since 1970 indicated that rates of water use ranged from 10 kg day(-1) for trees in a 32-year-old plantation of Quercus petraea L. ex Liebl. in eastern France to 1,180 kg day(-1) for an overstory Euperua purpurea Bth. tree growing in the Amazonian rainforest. The studies included in this survey reported whole-tree estimates of water use for 67 species in over 35 genera. Almost 90% of the observations indicated maximum rates of daily water use between 10 and 200 kg day(-1) for trees that averaged 21 m in height. The thermal techniques that made many of these estimates possible have gained widespread acceptance, and energy-balance, heat dissipation and heat-pulse systems are now routinely used with leaf-level measurements to investigate the relative importance of stomatal and boundary layer conductances in controlling canopy transpiration, whole-tree hydraulic conductance, coordinated control of whole-plant water transport, movement of water to and from sapwood storage, and whole-plant vulnerability of water transport to xylem cavitation. Techniques for estimating whole-tree water use complement existing approaches to calculating catchment water balance and provide the forest hydrologist with another tool for managing water resources. Energy-balance, heat dissipation and heat-pulse methods can be used to compare transpiration in different parts of a watershed or between adjacent trees, or to assess the contribution of transpiration from overstory and understory trees. Such studies often require that rates of water use be extrapolated from individual trees to that of stands and plantations. The ultimate success of this extrapolation depends in part on whether data covering short time sequences can be applied to longer periods of time. We conclude that techniques for estimating whole-tree water use have provided valuable tools for conducting basic and applied research. Future studies that emphasize the use of these techniques by both tree physiologists and forest hydrologists should be encouraged.

Estimating stand water use of large mountain ash trees and validation of the sap flow measurement technique.

Mountain ash (Eucalyptus regnans F.J. Muell.) forest catchments exhibit a strong relationship between stand age and runoff, attributed inter alia to differences in tree water use. However, the tree water use component of the mountain ash forest water balance is poorly quantified. We have used the sap flow technique to obtain estimates of daily water use in large mountain ash trees. First, the sap flow technique was validated by means of an in situ cut tree experiment. Close agreement was obtained between the sap flow estimate of water use and the actual uptake of water by the tree from a reservoir. Second, we compared the variability in sap velocity between a symmetric and an asymmetric tree by using multiple sap flow loggers. In the symmetric tree, velocity was fairly uniform throughout the xylem during the day, indicating that accurate sap flow estimates can be obtained with a minimal number of sampling points. However, large variations in sap velocity were observed in the asymmetric tree, indicating that much larger sampling sizes are required in asymmetric stems for an accurate determination of mean sap velocity. Finally, we compared two procedures for scaling individual tree sap flow estimates to the stand level based on stem diameter and leaf area index measurements. The first procedure was based on a regression between stem diameter and tree water use, developed on a small sample of trees and applied to a stand-level census of stem diameter values. Inputs to the second procedure were tree water use and leaf area of a single tree and the leaf area index of the stand. The two procedures yielded similar results; however, the first procedure was more robust but it required more sampling effort than the second procedure.

Long-term growth and water balance predictions for a mountain ash (Eucalyptus regnans) forest catchment subject to clear-felling and regeneration.

We used a physically based ecohydrological model to predict the water balance and growth responses of a mountain ash (Eucalyptus regnans F. Muell.) forest catchment to clear-felling and regeneration. The model, Topog-IRM, was applied to a 0.53 km(2) catchment for a 3-year pretreatment period, and a 20-year period following clear-felling and reseeding of 78% of the catchment area. Simulations were evaluated by comparing observed and predicted streamflows, rainfall interception and soil water values. The model faithfully simulated observed temporal patterns of overstory live stem carbon gain and produced a leaf area trajectory consistent with field observations. Cumulative throughfall was predicted within 1% of observations over an 18-year period. Over a 4-year period, predicted soil water storage in the upper 1.5 m of soil agreed well with field observations. There was fair correspondence between observed and predicted daily streamflows, and the model explained 76% of the variation in monthly flows. Over the 23-year simulation period, the model overpredicted cumulative streamflow by 6%. We argue that there is a useful role for physically based ecohydrological models in the management of mountain ash forest catchments that cannot be satisfied by simple empirical approaches.

Relationships between stem diameter, sapwood area, leaf area and transpiration in a young mountain ash forest.

We examined relationships between stem diameter, sapwood area, leaf area and transpiration in a 15-year-old mountain ash (Eucalyptus regnans F. Muell.) forest containing silver wattle (Acacia dealbata Link.) as a suppressed overstory species and mountain hickory (Acacia frigescens J.H. Willis) as an understory species. Stem diameter explained 93% of the variation in leaf area, 96% of the variation in sapwood area and 88% of the variation in mean daily spring transpiration in 19 mountain ash trees. In seven silver wattle trees, stem diameter explained 87% of the variation in sapwood area but was a poor predictor of the other variables. When transpiration measurements from individual trees were scaled up to a plot basis, using stem diameter values for 164 mountain ash trees and 124 silver wattle trees, mean daily spring transpiration rates of the two species were 2.3 and 0.6 mm day(-1), respectively. The leaf area index of the plot was estimated directly by destructive sampling, and indirectly with an LAI-2000 plant canopy analyzer and by hemispherical canopy photography. All three methods gave similar results.

Integration of sapflow velocity to estimate plant water use.

The heat pulse technique provides an estimate of sapflow velocity at one position within the xylem of an individual plant. Previous experience has shown that the velocity profile across the conducting area cannot be assumed to be constant, necessitating several such point estimates for a reasonable characterization of the velocity profile with depth. These point estimates must be integrated over the sapwood conducting area to derive flux. The most common method is to fit a one degree of freedom least-squares polynomial to the point estimates, and then to integrate this equation across the sapwood profile and around the bole. An alternative method is presented based on a weighted average approach. The latter is shown to be a more robust estimator of flux when velocity profiles exhibit large curvatures.