Measured Water Depths Research Articles

Underwater topography detection of Taiwan Shoal with SAR images

Under suitable conditions of tidal current and wind, underwater topography can be detected by synthetic aperture radar (SAR) indirectly. Underwater topography SAR imaging includes three physical processes: radar ocean surface backscattering, the modulation of sea surface short wave spectrum by the variations in sea surface currents, and the modulation of sea surface currents by the underwater topography. The first process is described usually by Bragg scattering theory because the incident angle of SAR is always between 20°–70°. The second process is described by the action balance equation. The third process is described by an ocean hydrodynamic model. Based on the SAR imaging mechanism for underwater topography, an underwater topography SAR detection model and a simplified method for its calculation are introduced. In the detection model, a two-dimensional hydrodynamic model — the shallow water model is used to describe the motion of tidal current. Due to the difficulty of determining the expression of SAR backscattering cross section in which some terms can not be determined, the backscattering cross section of SAR image used in the underwater topography SAR detection is pro-processed by the simulated SAR image of the coarse-grid water depth to simplify the calculation. Taiwan Shoal, located at the southwest outlet of Taiwan Strait, is selected as an evaluation area for this technique due to the occurrence of hundreds of sand waves. The underwater topography of Taiwan Shoal was detected by two scenes of ERS-2 SAR images which were acquired on 9 January 2000 and 6 June 2004. The detection results are compared with in situ measured water depths for three profiles. The average absolute and relative errors of the best detection result are 2.23 m and 7.5 %, respectively. These show that the detection model and the simplified method introduced in the paper is feasible.

Errors in Surface Irrigation Evaluation from Incorrect Model Assumptions

Some two-dozen methods have been proposed in the literature for estimating an infiltration function from field measurements. These methods vary in their data requirements and analytical rigor, however most assume some functional form of the infiltration equations. In this paper, if is shown that the form of infiltration and roughness equations can cause errors in the estimation of actual conditions. For example, assumptions regarding the influence of wetted perimeter on furrow infiltration can result in inappropriate infiltration equations and parameters. Also, the Manning n has been shown to vary with time during an irrigation event as the soil is smoothed by the flowing water. Thus estimates of Manning n based on the advance curve may vary substantially from those based on measured water depths. Inappropriate selection of equations or parameter values for infiltration or roughness can lead to unrealistic parameter values for the other. The estimated parameters from evaluation of a measured irrigation event usually give reasonable estimates of actual performance. However, extrapolation to future irrigation events, particularly with a different application depth or flow rate, can lead to inappropriate recommendations.

Two Dimensional Mapping of River Bathymetry and Power Using Aerial Photography and GIS on the Brazos River, Texas

Effective management of river environments requires efficient means of gathering data on the rivers, including stream power and other hydraulic attributes. Traditional methods of data collection are spatially limited and can be restrictively expensive. This study utilizes hydraulically‐calibrated aerial photography and GIS to calculate cross‐sectional and mean stream power on a stretch of the Brazos River in central Texas. Field measured water depths are regressed against image brightness values to establish a highly accurate depth to reflectance curve. GIS‐derived water surface slopes are then combined with estimated water depths to map fully two‐dimensional hydraulic processes. This type of image‐based river monitoring provides both an advance in measurement accuracy and in temporal monitoring.

Development and testing of a GIS integrated irrigation scheduling model

A user-friendly window based irrigation scheduling model (ISM) developed by George et al. [Agric. Water Manage. 46 (2) (2000) 121] was modified by incorporating a rice irrigation scheduling model and integrated with Geographic Information System (GIS) software. The developed rice irrigation scheduling model is based on a daily water balance approach. The model was tested by comparing daily simulated and measured water depths for kharif (monsoon) 1999 and rabi (dry) 2000 seasons for outlet 5 l (12.2 ha) of the Tarafeni South Main Canal (North) in the Kangsabati irrigation project, West Bengal, India. The model testing criteria consisted of graphical comparison between simulated and measured water depths; deviation of water depth, D d (%); Nash–Sutcliff coefficient, R 2; and z-test. The model predicted daily water depth reasonably well for both single and multiple field cases and performed better during rabi than kharif season. The model application to the service area (command) of lateral (distributary) 2 l (910.4 ha) revealed that the supply was sufficient to meet about 60 and 23% of the demand, respectively for kharif 1999 and rabi 2000 seasons. Therefore, simulations were done to estimate irrigation demand for an alternative rabi crop (wheat). The comparison between supply and demand revealed that the supply was adequate to meet the irrigation demand of the distributary, and, thus wheat can be taken as an alternate crop during rabi season. Based on the results, it can be said that the ISM is a user-friendly model and can be used for irrigation management at different spatial scales.

A comparison of the morphology, gas space anatomy and potential for internal aeration in Phragmites australis under variable and static water regimes

To examine the effect of water regime on the potential for internal aeration in Phragmites australis, the morphology and gas space anatomy of plants grown in ponds under constant water levels were compared with those of plants growing in the field where water levels varied by 1.24 m over the growing season. Pond plants were grown at depths ranging from 5 to 65 cm depth whilst the field survey examined plants growing at instantaneously measured water depths ranging from 7 to 63 cm. Pond- and field-grown plants responded differently across the instantaneously measured water depth. Resistance to convective flow did not change in the culms of pond-grown plants whereas there was an increase in resistance in plants from the field with increasing instantaneously measured water depth. On the other hand, diffusive resistance remained constant in the developing rhizomes of field-grown plants but decreased with increasing water depth in pond-grown plants. Further, diffusive resistances in the rhizomes of field-grown plants were approximately twice those found in pond-grown plants. The rhizome length of pond plants was similar at all depths whereas field-grown plants showed a decrease in rhizome length with increasing instantaneously measured water depth, possibly reflecting the decrease in potential for aeration in the above-ground parts of the plant. The results suggest that a variable water regime may affect the performance of P. australis by constraining oxygen supply to the below-ground parts of the plant.

Evolutionary processes a focus of decade‐long ecosystem study of Antarctic's Lake Vostok

As scientists probe for life in new habitats and try to understand the processes that triggered the origin and guided the evolution of life on Earth, environments beneath large ice sheets are beginning to emerge as key ecosystems. Modern subglacial environments are analogues both for the icy moons of Jupiter and the environmental stresses that led to widespread evolutionary radiation following the Neoproterozoic “snowball” Earth.The largest modern analogue to these distant systems is Lake Vostok, a great Antarctic subglacial lake, and the international science community is developing a plan to systematically survey and explore this complex system over the next decade. Approximately the size of Lake Ontario, Lake Vostok lies beneath the 4 km thick East Antarctic ice sheet (Figure 1). The lake is much deeper than Lake Ontario—remotely measured water depths reach 670 m.

Coastal Bathymetry from the Wave Spectrum of Spot Images

Two SPOT images were used to derive the surface wave spectra for mapping water depths in the coast zone. By the general dispersion relation of water wave, the surface wave changes its wavelength over varying water depth. The estimated depths from this wave spectrum bathymetric (WSB) method were further corrected to water depths relative to the mean lower low tide. The average error of this image-derived water depth is about 10% from the measured water depths. This WSB method takes advantages of the remote sensing technology to obtain the synoptical bathymetry of coastal waters safely, economically and quickly. It is especially suitable for the coastal zone west of Taiwan where the bathymetry changes yearly near some river mouths, the bottom slope is gentle enough, satisfying the requirement of WSB method, and the non-uniformity of water quality incapacitate the application of optical bathymetry method.

Magnetotelluric imaging of the Society Islands hotspot

In April‐June 1989, seafloor magnetotelluric data across and along the leading edge of the Tahiti hotspot were obtained. The magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects, and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced. The corrected response functions are consistent with a two‐dimensional structure. Inversion of the data shows a slightly higher conductivity (relative to a reference site located away from the hotspot) down to 130 km depth beneath the active area southeast of Tahiti underlain by a more resistive structure. There is a suggestion for a change in conductivity in the 400–450 km depth range, which is consistent with elevated temperatures. This result is consistent with a mantle plume of limited extent (less than 150 km radius) located near the leading edge of the Tahiti hotspot. The magnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area, as would be required by a superswell model.

Quantification of channel bed morphology in gravel-bed rivers using airborne multispectral imagery and aerial photography

The potential for mapping in-channel morphology within shallow gravel-bed rivers using airborne multispectral imagery and aerial photography is illustrated using a case study from the River Tummel, Scotland. The technique described relies on a good correlation between observed light reflectance levels from a water body and water depth. Measured water depths are regressed against reflectance levels derived from airborne multispectral imagery and black-and-white aerial photographs, to obtain equations that can be used for mapping channel bathymetry. The technique has a great deal of potential for wide-ranging applications, including detailed morphological surveys, assessing in-channel changes and mapping riverine habitats.© 1997 John Wiley & Sons, Ltd.