We compare methods used to measure the water content near the soil
surface. The primary objective of this project is to link remotely
sensed surface water contents to the soil water regime, in particular
to the regime of structured soils. We attempt to use the dynamics of
spatially averaged surface water contents measured with microwave
radiometry to predict preferential infiltration and drainage. Under
field conditions the so-called macropore flow plays an important role
in the infiltration and drainage behavior of a soil, as well as in the
mass transfer of all kinds of solutes to larger soil depths. These rapid processes are only detectable during the first few hours after a rainfall event, when most of the larger pores are still water filled. The main focus of our project lies in an areal integration of such processes on a field scale. For this reason, we depend on areal data with a high temporal resolution that allow to characterize the soil water dynamics.
In this study we report on a field experiment with two different
ground-based radiometers (1.4 GHz and 11.4 GHz, respectively) centered
at a 5 m × 10 m bare soil plot. The brightness temperature
measured with passive microwave sensors contains information on
surface water content that is already spatially averaged. Furthermore the water content was measured in-situ with time domain reflectometry probes (TDR) assembled at five depths. In the same depths we measured matric potential (pressure head of soil water) and soil temperature. These data were recorded every 30 min from May to July 2002. In addition, we determined the moisture profile over the top 15 cm using neutron radiography. Transmission radiographs of soil slabs vertically taken from the surface horizon allow for a high spatial resolution of the water distribution. In order to characterize the surface roughness of the soil on a mm-scale we used optical measurement techniques.
We illustrate the implications of the results from this field campaign
on the dynamics of surface water content.
Soil hydraulic properties are needed in many applications. One of the most difficult quantities to assess is the hydraulic conductivity function. One reason for this is the influence of soil structure on the infiltration capacity. In this paper we present an approach to estimate the hydraulic properties based on time-series of water contents measured in the topsoil of an experimental field plot. Based on the van Genuchten-Mualem model for the soil hydraulic functions we investigate how these properties affect the dynamics of the topsoil water content. We postulate that the water retention curve can be estimated from the range of the top soil water contents observable in the field. The experimental evidence obtained in the plot experiment supports this theoretical conjection. With regard to the hydraulic conductivity function simulations of the drying process demonstrate that there is no straightforward, linear effect of the saturated conductivity Ks on the drying rate. Depending on the initial conditions and the water retention curve drying may be faster or slower with increasing values of Ks. Despite this non-linear behavior the simulation results indicate that for certain soils the influence of soil structure on the conductivity function may be observed by monitoring diurnal cycles of water content. The lack of these cycles in the measured data points to a small Ks-value for the soil matrix of the experimental plot. This is in agreement with the infiltration patterns observed on that plot. A further way to detect bimodal pore-size distributions consists in measuring during a number of drying periods that differ substantially in the initial water distribution in the profile. Simulations show larger effects on the drying rate caused by larger Ks-values.
Various applications such as meteorology, climatology or hydrology require information about the soil hydraulic properties over large areas. Microwave radiometry is a promising approach to gather this type of information. The microwave emission from soils is strongly affected by the roughness of the soil surface. This effect has therefore to be quantified to get a reasonable estimation of the hydraulic properties. In a cooperation of the Institute of Terrestrial Ecology, Soil Physics with the Institute of Geodesy and Photogrammetry digital surface models of soils were generated to study the influence of the surface roughness on the soil measurements. Accurate Digital Surface Models (DSM) can be derived by the application of photogrammetric measurement techniques and provide the spatial basis to extract roughness information. In this paper an approach to determine the roughness of the topsoil surface is presented.
For various applications in soil science, hydrology and climatology it is important to estimate soil hydraulic properties over large areas. The methods to quantify them have been of limited success so far. One of the main difficulties is the influence of soil structure on infiltration and drainage of soils. The classical soil physical methods cannot be applied because they yield local measurements of properties, which are spatially averaged over sampling volumes of soil columns but not over plots or entire fields. One way out of this dilemma may be the use of microwave radiometry to quantify the dynamics of surface water contents and use this information as a proxy variable to estimate the hydraulic properties of the underlying soil. This study reports on a field experiment conducted from April to July 2001. Beside measuring the dynamics of the water content in the topsoil by a ground-based 11.4 GHz microwave radiometer, we carried out in-situ measurements of water content, temperature and matric potential. The infiltration pattern, visualized with a blue dye tracer added to the infiltrating solution, showed that both soils were strongly susceptible to preferential flow. The changes in topsoil water content seemed to be well comparable to the changes in the measured reflectivities.
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