Physics and Chemistry of the Earth, Part B: Hydrology, Oceans and Atmosphere最新文献

In debris flows over erodible beds, the kinematic conditions at the bed surface, such as velocity and velocity gradient, are determined by the dynamic condition that the driving force must be equal to the yield stress at the bed surface. In debris flows over rigid beds, on the other hand, kinematic quantities depend on such conditions as bed slope, sediment discharge rate, static friction angle of sediment and friction angle of grain to the bed surface. In the present study, the differences between debris flows over erodible and rigid beds as well as the transition between the two are analyzed theoretically by solving for velocity and sediment concentration profiles. An important difference between the two lies in the shear stress distributions near the bed. The velocity gradient takes a finite value in the case of a rigid bed, and zero in the case of an erodible bed, which causes several different features in the profiles of velocity, and sediment concentration, and correspondingly in the flow resistance. The theoretical results are verified by flume data.

Retention ponds are a valuable tool for assessing sediment yield (SY) over large areas. SY is normally expressed in t ha⁻¹ a⁻¹ or t km⁻¹ a⁻¹ and therefore measured sediment volumes in ponds need to be converted to masses using representative values for the dry bulk density (dBD) and corrected for trap efficiency (TE). Mean dBD varied from 1 to 1.35 t m⁻³ for eight retention ponds in central Belgium. Within the Hammeveld retention pond, dBD ranged from 1.14 to 1.58 t m⁻³ with a mean value of 1.34 t m⁻³. Variations in dBD are mainly controlled by texture and the length of dry periods between rain events. Values of TE were calculated for this pond by comparing soil losses due to water erosion in the catchment with sedimentation rates in the pond. TE varied between 58% in spring-summer and 96% in autumn-winter because of different hydrological conditions.

A method of estimation of daily global radiation based on standard meteorological observations is presented. Global radiation may be calculated with one-dimensional regression equations, either with or without conditions (constraints). Daily duration of sunshine, mean cloudiness or temperature range can be used as predictors in either case. In general to select the proper equations with conditions, humidity was chosen as a master variable. Formulation and validation of the regression equations is done on the basis of data from 42 German radiation stations. The annual root mean square error (RMSE) between values calculated for individual days with the estimation method developed, and measured data, was about 1.6 MJ m⁻² day⁻¹ using sunshine duration as the predictor and 3.0 MJ m⁻² day⁻¹ using both temperature range and relative humidity as predictors.

This estimation method has been used for reconstruction of daily global radiation for about 400 German meteorological stations for years, and for stations without observations of global radiation. Reconstructed data will be used in agricultural and hydrological models.

A Cellular Automata model for soil erosion by water, SCAVATU, was developed. It involves a larger number of states in comparison to the previous models, including altitude, water depth, total head, vegetation density, infiltration, erosion, sediment transport and deposition.

The model was applied to the small catchment of the Fiumara Armaconi, Calabria, Southern Italy. First simulations gave encouraging results, even if field erosion data is needed for validation and future calibration and setting of the CA parameters.

The model is susceptible to improvement and could represent a valid alternative to classic physically based methods, for the description of complexity through simple local rules.

Temporal and spatial variability of water content in soil results from a complex interaction of different factors such as duration and frequency of rainfall, soil layering, vegetation, and topography. The objectives of this study were (i) to use a resistant median-polishing scheme to quantify the temporal variability of a depth and a horizontal location factor in an additive model, and (ii) to investigate the time stability of those two factors at a detailed temporal scale during different infiltration and redistributions cycles. Time series of water content were measured at 5 depths and 12 locations along a transect of 6 m using Time Domain Reflectometry (TDR). Measurements were repeated every 2-hours for 168 days under natural boundary conditions. At each time step, the mean water content of the soil profile, 5 depth factors and 12 location factors were estimated. The time series of these factors were qualitatively interpreted and related to the atmospheric and prevailing soil conditions. It was found that micro-heterogeneity plays an important role, even at this small plot-scale. The relative contributions of the factors were dependent on the antecedent soil moisture conditions. Also, the ratio of the deterministic variance, i.e., variance explained by the deterministic factors, of water content to the observed variance is variable in time.

A recently developed method for event-based daily bias-correction of synoptic precipitation observations regarding systematic measuring errors was transfered at the Global Precipitation Climatology Centre (GPCC) from regional to global applications. Using the reported present weather, an analysis based on more than 600 000 global synoptic data from 16 winter months was done, which made it possible to relate air temperature and dew point temperature to the probable distribution of liquid, solid and mixed precipitation phase. Based on this information, synoptic precipitation observations can be corrected regarding systematic measuring errors on a daily resolution, which makes the estimation of extreme precipitation events more reliable.

We present numerical simulations of incompressible, two-dimensional, unsteady flow over a backward facing step. The velocity field is made time-dependent through a vortical perturbation of given amplitude and frequency imposed upstream of the step. On being convected downstream by the flow, the perturbation interacts with the detached shear layer and causes the recirculating flow to periodically shed large-scale vortices. By means of Lagrangian tracers we study the fluid exchange between the recirculation region and the incoming flow. The numerical experiments have application to processes of enviromental relevance, i.e. the trapping of a pollutant within a recirculating structure and its release, which may be found in the lee side of a sharp crest, such as a row of hills laid across the wind.

As there is currently no remote-sensing method that can supply high-quality information on all meteorological parameters from one system alone, integrated stations for vertical profiling are needed. This paper defines a classification about integrated remote-sensing stations for wind, temperature and humidity profiling and analyzes their performance. Based on a SODAR/RASS, two different windprofiler/RASS devices and a microwave radiometer profiler, it is shown that reliable profiling of wind, temperature and humidity is possible in the troposphere and lower stratosphere with sufficient vertical resolutions in relation to the typical length scales of the boundary layer as well as the free atmosphere. Finally, the synergy of an integrated profiling station is demonstrated by an example of the accurate determination of the PBL depth.

Since 1997, IWMI has been developing models to investigate future food and water requirements. Since the initial results were published (Seckler et al., 1998), the data and methodologies were refined substantially, evolving into the PODIUM model. The model estimates projected increases in water demand in 2025 resulting from the expected population growth and changes in consumption pattern, for individual nations. The PODIUM model provides a user-friendly means to analyze alternative future scenarios and conduct sensitivity analysis.

As part of the World Water Vision 2025 exercise, the PODIUM model was used to test a range of scenarios related to food and water demand. In the IWMI base scenario, 33 percent of the population of the studied countries will face absolute water scarcity. These countries will not have sufficient water resources to meet water needs.

Another 45 percent of the population live in countries that will face economic scarcity. Countries in this category may not have the capacity or financial resources to develop sufficient water resources. Globally, water diversions to agriculture will grow by 17 percent. Fifteen countries, mainly in the Middle East and Africa, will rely on cereal imports for more than 25 percent of their grain consumption (Seckler et al., 2000).

This paper presents the modeling strategies adopted in the PODIUM model and the results obtained during the development of the World Water Vision (Rijsberman and Cosgrove, 2000). These results indicate the need for substantial investment in water resources development, improving agricultural water use and expansion of both irrigated and rain-fed agriculture.

In the multiple-aquifer system in the sediments above the Gorleben salt dome, an upper fresh-water body is underlain by saline water. The salinity of the water generally increases with depth. The salt dome is crossed by a subglacial erosion channel, in which the lowermost aquifer is locally in contact with the caprock or the salt itself. Saturated brines are found in these areas. Information on the flow system can be derived from the observed density distribution. Two-dimensional numerical studies have been conducted to determine the transient density distribution and the associated flow field. They have demonstrated the sensitivity of the system to changes in the hydrogeological structure, initial density distribution, and modeled time period (palaeohydrogeological effects). A few of the calculated present density distributions compare well with measured field data.