A land surface temperature model-data differencing approach to quantifying subsurface water use by vegetation: application in the Condamine region, south-eastern Queensland

Gow, Laura (2017). A land surface temperature model-data differencing approach to quantifying subsurface water use by vegetation: application in the Condamine region, south-eastern Queensland PhD Thesis, School of Geography, Planning and Environmental Management, The University of Queensland. doi:10.14264/uql.2017.789

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Author Gow, Laura
Thesis Title A land surface temperature model-data differencing approach to quantifying subsurface water use by vegetation: application in the Condamine region, south-eastern Queensland
School, Centre or Institute School of Geography, Planning and Environmental Management
Institution The University of Queensland
DOI 10.14264/uql.2017.789
Publication date 2017-06-02
Thesis type PhD Thesis
Supervisor Stuart R. Phinn
Damian J. Barrett
Luigi J. Renzullo
Anthony P. O'Grady
Total pages 145
Language eng
Subjects 0909 Geomatic Engineering
0102 Applied Mathematics
0406 Physical Geography and Environmental Geoscience
Formatted abstract
Water plays a crucial role in plant community and landscape function; plants use stomata to regulate water loss via transpiration as the ‘cost’ of carbon assimilation, which in turn regulates surface temperature of leaves. Presently there are a number of aspects of vegetation water-use dynamics that are poorly understood, including the degree of dependence on water sources at different depths in the soil profile; timing, frequency, duration and magnitude of water use; and ecosystem strategies to acquiring water from different sources as water availability varies.

The research presented in this thesis aimed to: (1) quantify timing and frequency of the use of subsurface water by woody vegetation, (2) quantify the confidence of these predictions, and (3) identify subsurface water-use strategies and physiological responses employed by woody vegetation with variation in climatic and water-deficit conditions. These aims were achieved through development, application and assessment of a novel land surface temperature (LST) model-data differencing approach applied to a subtropical woodland environment in eastern Australia. The approach detects subsurface water use by vegetation in space and time within 95% confidence intervals, through differences in modelled LST (Ts.mod) and satellite observations of LST (Ts.obs) after accounting for random and systematic error in the model and data.

Modelled LST was derived using a two-layer surface energy balance (SEB) model and captures water use from surface water and soil water in the top 30 cm of the soil profile (i.e. shallow soil water). Satellite observations of LST were obtained from Terra-MODIS thermal infrared imagery and captures water use from all available sources. When compared, temperature differences between independently-derived Ts.mod and Ts.obs reveal subsurface water use (i.e. below 30 cm depth) plus systematic and random error. Systematic error or bias, was estimated from temperature differences of grassland vegetation based on an assumption that these vegetation communities do not access subsurface water under certain climatic conditions. Therefore Ts.mod should equal Ts.obs. Random error in Ts.obs was estimated from the literature while sensitivity and uncertainty analyses were used to quantify Ts.mod random error arising from five SEB model resistance terms and their associated input parameters. A Student’s t-test was then used to account for systematic and random error in the detection of subsurface water use by vegetation within 95% confidence intervals.

The LST model-data differencing approach was applied in a 3200 km2 region of the subtropical Condamine River Catchment, south-eastern Queensland Australia. The study area contains a mixture of native remnant vegetation, agriculturally-intensive areas, and coal seam gas development. Extensive eucalypt woodlands dominate low-lying hill slopes, while rainfed pastoral grasses and irrigated crops are common throughout the alluvial floodplain and topographic lows adjacent to hill slopes. Timing and frequency of subsurface water use by woodland vegetation in the study area was quantified over a 13-year period (2000–2012). Subsurface water use and LST time-series were compared against local meteorological and vegetation data to resolve physiological response and subsurface water-use strategies of woodland vegetation at multiple temporal scales under different climatic and water-deficit conditions.

Error analysis showed that the two-layer SEB model performed poorly where vegetation height was less than 0.5 m and leaf area index was less than 0.5 m2 m-2, such that the approach was considered unsuitable for vegetation with these characteristics. Low wind speed (<1.5 m s-1) also introduced considerable error to Ts.mod such that small-magnitude subsurface water use may not be detectable due to low signal-to-error ratio. Total frequency of subsurface water use by eucalypt woodland vegetation in the study area varied spatially from 100–937 days which is equivalent to 5–50% of time period of investigation. Availability of shallow soil water was an important determinant of subsurface water-use strategies of vegetation and vegetation physiological response at intra- and inter-annual scale.

Frequency of subsurface water use by vegetation was lowest at the start of the wet season when shallow soil water was abundant as a consequence of more frequent, intense rainfall events. Other times of the year, frequency of subsurface water use was higher, commensurate with decreasing shallow soil water availability and/or increasing evaporative demand. Over multi-year dry periods, when shallow soil water availability was depleted, the vegetation increased its frequency of subsurface water use and reduced leaf area to balance latent heat loss. Leaf area reduction was greatest when dry conditions persisted and frequency of rainfall was reduced. During wet or ‘recovery’ years, the vegetation increased its leaf area by the same amount irrespective of the severity of previous dry period or present wet conditions. These results suggest the recovery response of woodland vegetation in the study area was more sensitive to onset of increased water availability rather than volume of water availability and that physiological responses (namely adjustments to leaf area) and water-use strategies employed by the vegetation maintained evapotranspiration at relatively constant rates irrespective of prevailing inter-annual conditions.
Keyword Groundwater-dependent vegetation
Land surface temperature
Remote-sensing
Surface energy balance modelling
Sensitivity and uncertainty analysis
Timeseries analysis

Document type: Thesis
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Created: Mon, 22 May 2017, 22:00:48 EST by Laura Gow on behalf of Learning and Research Services (UQ Library)