Abstract
Abstract. In most hydrological systems, evapotranspiration (ET) and precipitation are the largest components of the water balance, which are difficult to estimate, particularly over complex terrain. In recent decades, the advent of remotely sensed data based ET algorithms and distributed hydrological models has provided improved spatially upscaled ET estimates. However, information on the performance of these methods at various spatial scales is limited. This study compares the ET from the MODIS remotely sensed ET dataset (MOD16) with the ET estimates from a SWAT hydrological model on graduated spatial scales for the complex terrain of the Sixth Creek Catchment of the Western Mount Lofty Ranges, South Australia. ET from both models was further compared with the coarser-resolution AWRA-L model at catchment scale. The SWAT model analyses are performed on daily timescales with a 6-year calibration period (2000–2005) and 7-year validation period (2007–2013). Differences in ET estimation between the SWAT and MOD16 methods of up to 31, 19, 15, 11 and 9 % were observed at respectively 1, 4, 9, 16 and 25 km2 spatial resolutions. Based on the results of the study, a spatial scale of confidence of 4 km2 for catchment-scale evapotranspiration is suggested in complex terrain. Land cover differences, HRU parameterisation in AWRA-L and catchment-scale averaging of input climate data in the SWAT semi-distributed model were identified as the principal sources of weaker correlations at higher spatial resolution.
Highlights
Evapotranspiration (ET) and precipitation are the largest components of the water balance (Nachabe et al, 2005) and yet the most difficult to estimate, over complex terrain (Wilson and Guan, 2004)
The objectives of this study are (1) to simulate and compare the results of the evapotranspiration of Soil and Water Assessment Tool (SWAT), AWRA-L and MOD16 over a complex terrain at a catchment scale in a semi-arid climate; and (2) to analyse and determine the spatial scale at which the SWAT and MOD16 ET models tend towards agreement to enhance the confidence in ET estimation in a complex terrain
The SWAT ET model is calculated at the hydrological response units (HRU) scale (Fig. 7a, b), for direct comparison with the MOD16 ET (Fig. 7c), the HRU ET results were reprocessed into 1 km2 cells using simple averaging
Summary
Evapotranspiration (ET) and precipitation are the largest components of the water balance (Nachabe et al, 2005) and yet the most difficult to estimate, over complex terrain (Wilson and Guan, 2004). ET is principally measured and estimated using ground based measurement tools and/or through various modelling techniques often involving remote sensing (Drexler et al, 2004; Tabari et al, 2013). Ground based measurement methods such as the Bowen Ratio Energy Balance (BREB), Eddy Covariance (EC), Large Aperture Scintillometers (LAS) and lysimeters have been regarded as the most accurate and reliable ET determination methods (Kim et al, 2012a; Rana and Katerji, 2000; Liu et al, 2013), but they are spatially and/or temporally limited (Wilson et al, 2001; Glenn et al, 2007). Despite the relative reliability of ground based measurement methods, there are inherent uncertainties associated with the different methods, which affect the accuracy of ET measurements
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