Soil Moisture Monitoring: Whither Shall We Measure?
Jeffrey Walker and Andrew Western
In spite of its importance, there is no operational system for reliably estimating the global soil moisture status. The reasons for this are that (i) models yield poor estimates as a result of poor parameters, atmospheric forcing and process understanding, (ii) satellite near-surface soil moisture products are not yet mature, are limited to regions of low-to-moderate vegetation, and need to be value added to yield spatial and temporally continuous root zone soil moisture estimates, and (iii) in-situ soil moisture sensors can only monitor a limited area at an appropriate spatial resolution. Nevertheless, it is imperative that long-term in-situ soil moisture monitoring sites be established under a range of climatic, soil, land cover, land use, topographic, etc conditions in order to (i) achieve improvements in process understanding and models, (ii) mature current and planned satellite near-surface soil moisture products, and (iii) validate derived root zone soil moisture. However, if such data is to be useful, careful consideration must be given to network design. The most important considerations include (i) spatial configuration – broadly distributed individual sites across large areas and/or more closely spaced measurements across a focussed area, such as a satellite footprint, (ii) vertical resolution and extent – matching both remote sensing observation depth and root zone extent, (iii) sensor type, installation and calibration and (iv) the minimum set of complimentary measurements required to ensure appropriate value can be extracted from the soil moisture measurements. This talk will discuss such issues with reference to the authors experience within the Australian Monitoring Network (www.oznet.unimelb.edu.au).