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Author Metzen, Daniel

Title Unravelling spatio-temporal water balance patterns in topographically complex landscapes

Published 2017


Location Call No. Status
Physical description 1 online resource
Thesis notes Thesis (PhD thesis)-- School of Ecosystem and Forest Sciences 2017
Summary The Budyko framework for understanding how precipitation (P) is partitioned into evapotranspiration (ET) and streamflow has been shown to be remarkably robust at large spatial scales. The Budyko model simply uses P and potential ET as variables. However, at smaller spatial scales additional predictor variables are required to partition precipitation. In steep uplands, topography appears to exert strong control on the water balance at the hillslope scale. Organization of vegetation suggest heterogeneity in the water balance at these scales. This topographic control though is poorly characterized in most environments and therefore not well represented in models. The aim of this thesis is to quantify the effect of local topography (aspect and drainage position) on forest water balance as a first step towards a down-scaling of the Budyko model in steep upland terrain. Six intensively instrumented sited were established on three drainage positions and two aspects in mixed species eucalypt forests (MSEF), with all other variables remaining constant. Continuously monitored water balances were extrapolated across a ̃70 ha catchment using a Random Forest model and LiDAR characterization of stand density and structure. The study demonstrated that spatial vegetation patterns emerged in response to topographic control on water-availability via soil depth, water redistribution and sub-canopy radiation loads. Moreover, short-term variations of overstory transpiration (To) were driven by atmospheric forcing, whereas seasonal and annual To patterns were explained by sapwood area index (As, R̂2:0.89). Understory and forest floor evapotranspiration (ETu) was controlled by sub-canopy short-wave radiation. Further, the combined effect of aspect and drainage position on water balance partitioning markedly diverged along the south and north-facing transect. All plots on northern aspects had a positive water balance (P> ET), whereas only the ridge plot on the south-facing slope had a positive water balance, while the lower hillslope had higher ET rates than rainfall inputs.ET measurements from the distributed plots could be up-scaled using terrain and vegetation information derived from LiDAR and unveiled strong spatial variability of To (4.5-fold), ETu (3.5-fold) and total ET rates (2-fold) over as little as 200 m distance. The observed ET range corresponds to eucalypt forests typically located >100s of kilometers apart, with the lower end similar to arid open woodlands in Western Australia and the upper end to tall mountain ash forests in the Victorian highlands. Predicting ET using the Budyko framework revealed strongly biased ET estimates in relation to landscape position, where ̃18% of the catchment area plotted above the theoretical water limit, confirming the importance of topographic water redistribution. Further, model residuals were explained well by As and terrain patterns. My thesis presented strong links between vegetation patterns, topography, soil depth and energy and water fluxes in upland MSEF. Ultimately, the study demonstrated the potential of remotely sensed vegetation and terrain patterns to infer and scale water-balance patterns in heterogeneous upland forests.
Subject topography forest water balance evapotranspiration vegetation patterns sap flow scaling LiDAR