ISSN:0262-6667    

DOI:10.1080/02626669809492146

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0262-6667    

DOI:10.1080/02626669809492147

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Surface soil moisture, defined here as the volumetric soil water content in the top 10 cm of the soil, shows a great deal of variability. The variance observed within a square metre can be as large as for a whole field. For that reason alone, point measurements in low quantities cannot be representative for the average soil moisture of a whole field. Soil moisture estimated by remote sensing techniques, especially by microwave sensors, might give adequate information on areally-averaged soil moisture. If these techniques are to be worthwhile in hydrological modelling, it will be necessary to link the various spatial resolutions of the different instruments to each other and to point-scale measurements. Despite differences in instrument configuration it is shown that it is possible, under certain circumstances, to downscale high resolution areal soil moisture estimates to low resolution soil moisture estimates using data from HAPEX-Sahel'92 and Washita'94.

ISSN:0262-6667    

DOI:10.1080/02626669809492148

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The emissivity of soil surfaces at microwave frequencies depends strongly on the near-surface water content. However this relationship varies between soils. Use is made of a model (MICRO-SWEAT) which couples water, heat and microwave radiation transfers through soils to show that the main cause of soil variation in the relationship between apparent emissivity at 1.4 GHz (eapp) and the average water content of the upper 2 cm of the profile (θ0-2) are differences in the dielectric properties. A simple semi-empirical model is proposed, based on the effect of soil particle size distribution on bound/free water partitioning, which provides a calibration between eappand θ0-2. Although, in principle, the detailed shape of the water content distribution close to the surface will influence the eapp/θ0-2relationship, such effects are only significant during short periods (< 1 h) following the onset of rainfall which produces steep wetting fronts close to the soil surface.

ISSN:0262-6667    

DOI:10.1080/02626669809492149

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0262-6667    

DOI:10.1080/02626669809492150

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The basic principles are derived of steady momentum dissipation during laminar boundary-layer flow in macroporous soils. The principles lead to a flux law and to geometrical measures of the flow paths. The theory of kinematic waves evolves, and the shock fronts are routed with the method of characteristics. The approach is applied to TDR soil moisture readings at depths 0.15, 0.35 and 0.55 m in a soil under meadow during an infiltration experiment (rate and duration of sprinkling were 2.78 x 10−5m s−1and 4500 s, respectively). Momentum dissipation during flow within the soil profile is independent of momentum added to the soil surface by sprinkling. Specific momentum dissipation per unit depth in the soil profile was about 0.02 kg m−2s−1, with a tendency to increase with depth.

ISSN:0262-6667    

DOI:10.1080/02626669809492151

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Knowing the root distribution in soil is essential for estimating water uptake by plant roots. It is difficult, however, to characterize and model undisturbed root systems. Root development in a two-dimensional potential field is simulated with the dielectric breakdown model (DBM), which implies a similarity between electric discharge and root distribution. A weighted potential gradient with an exponent η was used to create probability distributions of root growth. Each root was generated step-by-step by selecting random numbers from probability distributions. Simulations were performed for point sources and horizontal line sources. In both cases, as η increased, the root system changed from a cluster and dense type to a slender and sparse type and the fractal dimension decreased. For a point source of water, there was a definite pattern of root development toward the source. It is concluded that the DBM effectively describes root development under various water conditions.

ISSN:0262-6667    

DOI:10.1080/02626669809492152

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Infiltration experiments with intensities varying from 4.1 x 10−6to 2.8 x 10−5m s−1were conducted on a column of undisturbed soil. Flow was investigated using kinematic wave theory. The approach was first applied to the drainage curve of outflow. In this case, preferential flow occurred during intermediate infiltration rates. The approach was further applied to soil moisture variations at three depths of the column. The wetting front dispersion increased with depth according to the analysis of mobile soil moisture contributing to total flow, whereas the analysis of mobile soil moisture contributing to rapid flow indicated flow through macropores. The kinematic wave model simulates reasonably well both measured drainage of outflow and of soil moisture. A simple method was used to separate mobile soil moisture contributing to rapid flow and that contributing to diffusive flow. The validity of the model was evaluated by water balance comparison, with particular attention given to the distinction between the two types of soil moisture.

ISSN:0262-6667    

DOI:10.1080/02626669809492153

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The seasonal movement of soil water through a deep vadose zone of a sandy hill was studied in the central part of The Netherlands. The behaviour of soil water was tracked by monitoring soil water fluctuations and changes in the environmental tracer18O in vertical profiles at test sites with grassland, heathland and forest. In the temperate climate of the study area a soil moisture deficit develops during summer and a precipitation surplus prevails during winter. The18O content in precipitation exhibits a distinct seasonal cycle with the highest (relatively enriched) values of c. −6%vsV-SMOW occurring in summer and the lowest (relatively depleted) values of c. −9%vsV-SMOW in winter. The influence of this cycle can be traced back in the subsurface to a depth of sometimes more than 6 m. This seasonal fluctuation moves around the average annual18O content in rainfall, tending closer to the average as depth increases. In the saturated zone the same average value is observed. This suggests that, although the precipitation surplus is limited to the winter period, part of the summer precipitation also infiltrates to below the root zone and contributes to the groundwater recharge. The results of this study indicate that preferential flow dominates in the root zone. Below the root zone of low homogeneous vegetation covers, the soil water movement is dominated by diffuse flow. In forests, a more heterogeneous root pattern causes a more preferential type of flow, also at greater depths below the root zone.

ISSN:0262-6667    

DOI:10.1080/02626669809492154

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0262-6667    

DOI:10.1080/02626669809492155

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor