Towards an Australian Root Zone Soil Moisture Product

Jeffrey P Walker

Power Point Presentation

Root zone soil moisture is a vital land surface parameter throughout a wide range of applications, from weather and climate prediction to early warning systems (eg. flood forecasting), climate-sensitive socioeconomic activities (eg. agriculture and water management) and policy planning (eg. drought relief and global warming). However, timely and accurate information on land surface soil moisture content is lacking, due to an inability to economically monitor the spatial variation in soil moisture from traditional point measurement techniques at an appropriate spatial and temporal resolution. As a result, land surface models have been relied upon to provide an estimate of the spatial and temporal variation in land surface soil moisture. Due to uncertainties in atmospheric forcing, land surface model parameters and land surface model physics, there is often a wide variation in the soil moisture forecasts of different land surface models.

Satellite remote sensing is an emerging technology that makes possible regular measurement of soil moisture content over large areas. Although current remote sensing systems only provide a measurement of the soil moisture content in a relatively thin layer of near-surface soil (approximately 1cm for C-band), there is a sizeable body of literature that has demonstrated an ability to retrieve the soil moisture content at much greater depths when this near-surface information is assimilated into a land surface model. An appropriate satellite (AMSR-E) for making these measurements (25km x 25km) was launched in May this year.

Moderate to dense vegetation, such as that along the east coast of Australia, is a major limitation on satellite measurement of near-surface soil moisture content. Moreover, accurate knowledge of soil moisture content in those areas is crucial for some applications. Assimilation of stream runoff information can potentially provide a methodology for retrieval of root zone soil moisture content in upstream forested areas, and provide further constraint to rangeland areas.

Gravity measurements provide another potential source of soil moisture information over large areas, and is unaffected by vegetation. It has been suggested that temporal variations in the earth’s gravity field, such as those currently monitored by the GRACE satellite launched in March this year, can provide information on temporal changes in terrestrial (snow, soil moisture, groundwater, dams, etc) water storage over areas of around 300km x 300km. It is proposed that this large scale change in total terrestrial water storage can be disaggregated and downscaled through the process of data assimilation. A major advantage of this type of data is the mass balance information it contains, providing yet a further constraint on soil moisture retrieval.

It is envisaged that a combination of these different data sources (near-surface soil moisture, runoff and changes in terrestrial water storage) and land surface modelling through the process of data assimilation will yield the most accurate estimates of spatial and temporal variation in soil moisture content across the Australian continent. Details of progress to date and current research plans will be discussed.