PhD defence: Mie Andreasen

Mie Andreasen defends her thesis,

Hydrology and Cosmic radiation
Neutron intensity measurements and modeling at specific field locations

Supervisors:
Professor Karsten Høgh Jensen, IGN
Associate Professor Majken C. L. Zibar, IGN
Dr. Heye Bogena, Forschungszentrum Jülich IBG-3 - Germany

Assessment Committee:
Professor Harry Vereecken, Forschungszentrum Jülich IBG-3 - Germany
Assistant Professor Trenton Franz, University of Nebraska - USA
Professor Lars Nielsen (chair), IGN

Summary:
Soil moisture is a key variable for hydrological studies as processes like infiltration and evapotranspiration are closely linked to the wetness of the soil. Soil moisture is temporally and spatially highly variable. Hydrological modeling at catchment scale is often used for weather and climate prediction, and soil moisture measurements at a comparable scale to the discretization of these models are of great importance for calibration and validation. However, traditionally soil moisture measurements are provided on either point or kilometer scale. The cosmic-ray neutron intensity (eV range) just above the ground surface is inversely correlated to all hydrogen present in the first few hectometers in the air above the land surface and the upper few decimeters of the subsurface. Neutron intensity measurements are appropriate for soil moisture detection since it for most time forms the major dynamic pool of hydrogen. The scale of neutron intensity measurement compares well with the discretization of many catchment scale models. However, the neutron intensity may also be affected by other hydrogen pools including atmospheric water vapor, biomass, litter, soil organic matter, snow and canopy interception. The uncertainty of soil moisture estimation is larger at field sites featured by more and sizeable pools of hydrogen (e.g. forest). The neutron intensity method may therefore be improved if the signals of the other featuring components are identified and adjusted for. Identifying the different signals may furthermore be used to extent the application of neutron intensity detection.
The neutron transport at the lower part of the atmosphere is examined using measured and modeled thermal and epithermal neutron intensity at field sites of varying environmental settings. Initially, a method enabling comparability of measured and modeled neutron intensity is developed. Following, site-specific models were used to test the sensitivity of soil moisture and other pools of hydrogen on neutron intensity. The sensitivity of soil moisture on neutron intensity was found to be dependent on neutron energy and land cover. The signal of biomass on neutron intensities was found to be significant for the ground level thermal-to-epithermal neutron ratio. The ratio increases considerably with increasing amounts of biomass and satisfactory agreement of measurements and modeling is obtained at the field sites of varying amounts of vegetation. A minor increase in ground level thermal neutron intensity was provided with canopy interception.
The change is within the measurement uncertainty when hourly time scales are regarded.

The thesis is available from the PhD administration at office 04.1.417.