Regional evaporation estimates from flux tower and MODIS satellite data
Introduction
The ability to monitor evaporation1 from land surfaces is important for applications requiring spatially-resolved estimates of moisture availability over large areas continuously at weekly to monthly timescales. Examples of such applications include irrigation scheduling (e.g. Dodds et al., 2005), managing carbon, water and land resources (e.g. Meyer, 1999, Raupach, 2001), and risk assessments for bushfires, dust storms and flooding. Evaporation is a large component of the terrestrial water balance, so improving the accuracy of evaporation estimates will significantly reduce uncertainties in terrestrial water balance modelling and improve the quality of information used in these applications.
Hydrometeorologists have striven for decades to use the global coverage of satellite-based remote sensing to provide accurate estimates of evaporation at daily to weekly time scales and at fine spatial scales (100 to 103 m). These efforts have been hindered by two problems: firstly, that the quantities of interest, such as carbon and water fluxes and their associated stores, must be estimated indirectly using algorithms that relate measured radiances to, for example, leaf area index (Myneni et al., 2002), gross and net primary productivity (Running et al., 2004), vegetation indices (Huete et al., 2002) and land surface temperature (Wan et al., 2002). Secondly, one of the biggest impediments to global, multi-temporal satellite-based monitoring is the conflicting requirement for algorithms that are biophysically realistic yet simple enough for global parameterisation and implementation. Zhao et al. (2005) demonstrates this for the MODIS primary productivity products, while a global MODIS evaporation product remains elusive because no algorithm has yet been found that achieves the right balance between accuracy and simplicity.
We present a new approach to building a global land surface evaporation algorithm using optical/thermal satellite data. Our goal is to develop an observation model appropriate for global implementation and routine monitoring of landscape-scale evaporation at weekly to monthly timescales. In this paper we use data products from the MODIS Moderate Resolution Imaging Spectroradiometer on the polar-orbiting Terra satellite, which has a daily overpass at around 10:30 h local time. With these constraints, the following model attributes are required:
- i)
Model inputs and parameters must be routinely available at daily time and local space scales, for large regions such as continental Australia, and globally.
- ii)
The model needs to be robust, i.e. evaporation estimates are constrained by energy and mass conservation and have relatively low sensitivity to the input data and parameters.
- iii)
The model needs to be insensitive to constraints imposed by the once-daily overpass of the polar orbiting satellite and the necessary cloud screening and compositing procedures.
- iv)
The model needs to be validated using comparable evaporation measurements from a diverse range of bioclimates.
These objectives are consistent with the ultimate goal for Fluxnet (Baldocchi et al., 2001), which seeks to integrate flux and concentration measurements, remote sensing and land-surface modelling to yield a comprehensive global biosphere monitoring network (Running et al., 1999, Zhao et al., 2005). Fluxnet encompasses over 400 towers distributed across the globe, providing hourly measurements of carbon, water and sensible heat fluxes across a diverse range of ecosystems and climates for multiple years (Baldocchi et al., 2001; http://www.daac.ornl.gov/FLUXNET/fluxnet.html). The insights and constraints provided by the simultaneous measurement of these fluxes and their corresponding scalar fields ensures that Fluxnet provides an excellent data set for land surface model development and testing. Data from two Australian flux stations (Ozflux: http://www.dar.csiro.au/lai/ozflux/) are used in this paper to test two evaporation models: i) an aerodynamic resistance–surface energy balance model and ii) the Penman–Monteith (P–M) equation, where the required surface conductance is estimated from remotely-sensed vegetation indices (leaf area index and NDVI).
The plan of the paper is as follows: Section 2 presents the fundamental energy balance and evaporation equations that underpin the evaporation modelling approaches that are tested; Section 3 describes the micrometeorological flux measurements used to develop and test the models; and Section 4 evaluates the two modelling approaches. 5 Implementing P–M at regional scales, 6 Concluding comments then implement the successful model to determine monthly evaporation fluxes at 1-km resolution for the Australian continent. This demonstrates the potential to monitor monthly land surface evaporation at the regional-scale by combining surface-based meteorological measurements with MODIS remote sensing.
Section snippets
Modelling land surface evaporation
Energy partitioning at the surface of the earth is governed by the following three coupled equations:where H, λE and A are the fluxes of sensible heat, latent heat and available energy, Rn is net radiation, G is soil heat flux; ΔS is the heat storage flux; Ts, Ta are the aerodynamic surface and air temperatures; es, ea are the water vapour pressure at the evaporating surface and in the air; Ra is the aerodynamic resistance, Rs is the surface
Flux measurement sites
Fluxes of sensible and latent heat used in the model evaluation were measured over two strongly contrasting ecosystems. The first is a wet/dry tropical savanna located in northern Queensland (Virginia Park, 19°53′00″S, 146°33′14″E, elevation of 200 m ASL), the other is a cool temperate, broadleaved forest in south east New South Wales (Tumbarumba, 35°39′20.6″S, 148°09′07.5″E, elevation of 1200 m ASL). Complete site descriptions, including the soil, rainfall climate and flux station
Climatology and surface properties
Time series of TsR and ND for the 7 × 7 km2 MODIS cutouts for Tumbarumba and Virginia Park are shown in Fig. 1. Gaps in the original record due to presence of persistent cloud cover during the wet season have been filled using linear interpolation, which may limit the validity of TsR for the Virginia Park site in the first wet season (Dec. 2001–Feb. 2002). The ND for Tumbarumba had a mean value of 0.80 for 2001–2003, with a fairly small annual amplitude ranging from a maximum of 0.89 (late
Using non-local meteorological forcing
The above results suggest that the P–M model is the most appropriate model but these results were achieved using locally measured meteorological forcing. The next step is to investigate model performance when these local measurements are replaced by inputs derived from calibrated functions, broader scale meteorological data and look-up tables for parameters that vary with vegetation type. Daily values of incoming solar radiation, temperature and humidity are available from a variety of sources,
Concluding comments
Our goal was to develop a global model for monitoring land surface evaporation at weekly to monthly timescales and at 1-km to continental spatial scales using surface meteorology and MODIS remote sensing. We compared the remote sensing approach that uses radiative surface temperature to estimate evaporation with the P–M model. The resistance–surface energy balance approach yielded implausible and unrealistic estimates of evaporation. This approach failed because of the difficulties in using 8-
Acknowledgements
The authors gratefully acknowledge the technical support given by Steve Zegelin, Dale Hughes and Mark Kitchen in the design, construction and maintenance of the flux stations, and to Christine Pierret for establishing the database used in processing the flux station results. We are also grateful to Faith Ann Heinsch for extracting the DAO meteorological fields for the Ozflux sites and the comments of Dr Damian Barrett (CSIRO Land and Water) on an earlier draft. This work was supported in part
References (60)
- et al.
A remote sensing surface energy balance algorithm for Land (SEBAL): Part 1. Formulation
Journal of Hydrology
(1998) - et al.
The surface energy balance algorithm for land (SEBAL): Part 2. Validation
Journal of Hydrology
(1998) - et al.
Modelling sensible heat fluxes from a wheat canopy: An evaluation of the resistance energy balance method
Journal of Hydrology
(1995) - et al.
Measuring and modelling soil evaporation in wheat crops
Physical Chemistry of the Earth
(1996) Satellite-derived estimates of evapotranspiration in the Gediz basin
Journal of Hydrology
(2000)- et al.
Overview of the radiometric and biophysical performance of the MODIS vegetation indices
Remote Sensing of Environment
(2002) - et al.
Using spatial interpolation to construct a comprehensive archive of Australian climate data
Environmental Modelling and Software
(2001) - et al.
Estimating evaporation from pasture using infrared thermometry: Evaluation of a one-layer resistance model
Agricultural and Forest Meteorology
(1990) - et al.
Maximum conductances for evaporation from global vegetation types
Agricultural and Forest Meteorology
(1995) - et al.
Evaluation of soil and vegetation heat flux predictions using a simple two-source model with radiometric temperatures for partial canopy cover
Agricultural and Forest Meteorology
(1999)