Hydrol. Earth Syst. Sci. Discuss., 11, 5515-5558, 2014
© Author(s) 2014. This work is distributed
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This discussion paper is under review for the journal Hydrology and Earth System Sciences (HESS).
Assimilation of near-surface cosmic-ray neutrons improves summertime soil moisture profile estimates at three distinct biomes in the USA
R. Rosolem1, T. Hoar2, A. Arellano3, J. L. Anderson2, W. J. Shuttleworth4, X. Zeng3, and T. E. Franz5
1Queens School of Engineering, University of Bristol, Bristol, UK
2NCAR Data Assimilation Research Section, Boulder, USA
3Department of Atmospheric Sciences, University of Arizona, Tucson, USA
4Department of Hydrology and Water Resources, University of Arizona, Tucson, USA
5School of Natural Resources, University of Nebraska-Lincoln, Lincoln, USA

Abstract. Aboveground cosmic-ray neutron measurements provide an opportunity to infer soil moisture at the sub-kilometer scale. Initial efforts to assimilate those measurements have shown promise. This study expands such analysis by investigating (1) how the information from aboveground cosmic-ray neutrons can constrain the soil moisture at distinct depths simulated by a land surface model, and (2) how changes in data availability (in terms of retrieval frequency) impact the dynamics of simulated soil moisture profiles. We employ ensemble data assimilation techniques in a "nearly-identical twin" experiment applied at semi-arid shrubland, rainfed agricultural field, and mixed forest biomes in the USA The performance of the Noah land surface model is compared without and with assimilation of observations at hourly intervals and every 2 days Synthetic observations of aboveground cosmic-ray neutrons better constrain the soil moisture simulated by Noah in root zone soil layers (0–100 cm) despite the limited measurement depth of the sensor (estimated to be 12–20 cm). The ability of Noah to reproduce a "true" soil moisture profile is remarkably good regardless of the frequency of observations at the semi-arid site. However, soil moisture profiles are better constrained when assimilating synthetic cosmic-ray neutrons observations hourly rather than every 2 days at the cropland and mixed forest sites. This indicates potential benefits for hydrometeorological modeling when soil moisture measurements are available at relatively high frequency. Moreover, differences in summertime meteorological forcing between the semi-arid site and the other two sites may indicate a possible controlling factor to soil moisture dynamics in addition to differences in soil and vegetation properties.

Citation: Rosolem, R., Hoar, T., Arellano, A., Anderson, J. L., Shuttleworth, W. J., Zeng, X., and Franz, T. E.: Assimilation of near-surface cosmic-ray neutrons improves summertime soil moisture profile estimates at three distinct biomes in the USA, Hydrol. Earth Syst. Sci. Discuss., 11, 5515-5558, doi:10.5194/hessd-11-5515-2014, 2014.
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