Abstract
Abstract. Soil moisture at the plot or hill-slope scale is an important link between local vadose zone hydrology and catchment hydrology. However, so far only a few methods are on the way to close this gap between point measurements and remote sensing. One new measurement methodology that could determine integral soil moisture at this scale is the aboveground sensing of cosmic-ray neutrons, more precisely of ground albedo neutrons. The present study performed ground albedo neutron sensing (GANS) at an agricultural field in northern Germany. To test the method it was accompanied by other soil moisture measurements for a summer period with corn crops growing on the field and a later autumn-winter period without crops and a longer period of snow cover. Additionally, meteorological data and aboveground crop biomass were included in the evaluation. Hourly values of ground albedo neutron sensing showed a high statistical variability. Six-hourly values corresponded well with classical soil moisture measurements, after calibration based on one reference dry period and three wet periods of a few days each. Crop biomass seemed to influence the measurements only to minor degree, opposed to snow cover which has a more substantial impact on the measurements. The latter could be quantitatively related to a newly introduced field neutron ratio estimated from neutron counting rates of two energy ranges. Overall, our study outlines a procedure to apply the ground albedo neutron sensing method based on devices now commercially available, without the need for accompanying numerical simulations and suited for longer monitoring periods after initial calibration.
Highlights
Soil moisture plays an important role in the hydrological cycle
To test the method it was accompanied by other soil moisture measurements for a summer period with corn crops growing on the field and a later autumn-winter period without crops and a longer period of snow cover
Soil moisture is a key factor for chemistry, biology, infiltration and matter transport processes in soil, e.g. it provides the main storage of water available for vegetation (Robinson et al, 2008)
Summary
Soil moisture plays an important role in the hydrological cycle. It influences climate and weather (Wu and Dickinson, 2004), and determines surface runoff after precipitation events and controls groundwater recharge. Measurements of soil moisture at the point-scale (∼1 dm3) have advanced significantly in the last decades for a wide range of sensors. These are usually the basis for calculation of water storage and its changes at the field scale (up to 1 km). At the basin scale (2500– 25 000 km2), remote sensing technology, both active and passive, has demonstrated the potential to map and monitor surface soil moisture changes over large areas at regular intervals in time (Barrett et al, 2009). Satellites with L-band radiometers, e.g. SMOS (Kerr et al, 2001), and the planned SMAP mission (Entekhabi et al, 2010), or gravity change detection, GRACE (Tapley et al, 2004), provide a spaceborne Earth observation with opportunities to estimate soil water content at continental scale
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