Virtual Joint Field Campaign: a framework of synthetic landscapes to assess multiscale measurement methods of water storage

Abstract. Various methods are available to measure water storage across different landscape compartments, e.g. cosmic-ray neutron sensing, remote sensing, or hydrogravimetry. All these methods provide integral estimates that are representative of their corresponding measurement volume. However, we typically do not know the true value of water storage in these measurement volumes, which makes it difficult to assess the accuracy of such methods. In this study we suggest a synthetic virtual landscape that allows for an exact definition of all variables of interest and, consequently, constitutes the so-called “virtual truth” free of knowledge gaps. Such a landscape can be explored in various “virtual field campaigns” using “virtual sensors” that mimic the response and characteristics of actual devices. We use dedicated physically based models to simulate the signal a sensor would receive. These model outputs, termed “virtual observations”, can be explored and also allow for the reconstruction of water storage, which can then readily be compared to the virtual truth. Insights from this comparison could help to better understand real measurements and their uncertainties and to challenge accepted knowledge about signal processing and data interpretation. The Virtual Joint Field Campaign is an open collaborative framework for constructing such landscapes. It comprises data and methods to create and combine different compartments of the landscape (e.g. atmosphere, soil, vegetation). The present study demonstrates virtual observations of water storage with cosmic-ray neutron sensing, hydrogravimetry, and remote sensing in three exemplary landscapes. It enables unprecedented opportunities for the systematic assessment of the sensor's strengths and weaknesses and even their combined use.