Measurement and utilization of on-site soil moisture data

Programs for the on-site measurement of soil moisture in the USA are reviewed. These are regional and national measurement programs that may be useful for the verification of remotely sensed soil moisture estimates and for hydroclimatic studies. Location and type of measurement are described. A technique for the utilization of on-site data of soil moisture and discharge together with remotely sensed data of the surface soil moisture is proposed for the estimation of soil water content aggregated over large areas. The technique is based on large-scale conceptual hydrologic models and on state estimation techniques that allow explicit account to be taken of measurement uncertainty. The proposed approach is explored in an example formulation applied to a 40 year record of monthly data from a 4672 km(2) natural catchment in Illinois. This study shows the ability of simple conceptual hydrologic models to simulate well both flow and soil water in humid areas and with monthly data. It is further shown that, even when the remotely sensed measurements of the surface soil moisture carry substantial measurement errors, inference of lower soil water and of total soil water is possible with an expected error that is smaller than that achieved without the use of the remotely sensed data.