A physically meaningful technique to determine the effective depth to the water table, as a measure of the initial storage capacity of a basin is developed. The estimation of the initial storage capacity prior to a given flood event is essential to obtain useful results from storm runoff prediction models based on saturation excess overland flow. It is shown how this effective depth to the water table can be related to streamflow measurements at the outlet of the basin. The analysis is based on Boussinesq's standard hydraulic groundwater theory. The main feature of the present formulation is that it allows the estimation of catchmerit-scale parameters, namely the aquifer hydraulic conductivity and the average depth to the impervious layer. The estimation of these parameters is based on a drought flow analysis which is consistent with the hydraulic groundwater theory used to develop the described technique. This hydraulic theory is found to be applicable for a catchmerit under humid temperate climatic conditions, namely the Zwalm catchmerit situated in East-Flanders, Belgium. The results of the proposed analysis are used to estimate the initial conditions in a partial area runoff generation model. It is shown that accurate estimates of total runoff volume are obtained without further calibration of the model. a physically-based flood forecasting model (Topmodel) for three U.K. catchments, J. Hydrol., 69, 119-143, 1984. Boussinesq, J., Recherches th•oriques sur l'•coulement des nappes d'eau infiltr•es dans le sol et sur le d•bit des sources, J. Math. Pures Appl., 5th Ser., 10, 5-78, 1904. Brutsaert, W., and J. L. Nieber, Regionalized drought flow hydro. graphs from a mature glaciated plateau, Water Resour. Res., 13(3), 637-643, 1977. Castany, G., Traitd Pratique des Eaux Souterraines, 657 pp., Dunod, Pads, 1963. Dunne, T., Field studies of hillslope flow processes, in Hillslope Hydrology, edited by M. J. Kirkby, pp. 227-293, John Wiley, New York, 1978. Dunne, T., T. R. Moore, and C. H. Taylor, Recognition and prediction of runoff-producing zones in humid regions, Hydrol. Sci. Bull., 20(3), 305-327, 1975. Famiglietti, J. S., E. F. Wood, Comparison of passive microwave and model derived estimates for soil moisture fields, paper pre. sented at 5th International Colloquium, Physical Measurements and Signatures in Remote Sensing, Courchevel, France, 1991. Lebbe, L., Uitvoering van pompproeven en interpretatie door middel van een invers model, thesis ("Aggregaatsproefschrift"), 563 pp., Fac. of Sci., Univ. of Ghent, Ghent, Belgium, 1988. Polubarinova-Kochina, P. Ya., Theory of Groundwater Movement, translated from Russian by R. J. M. DeWiest, 613 pp., Princeton University Press, Princeton, N.J., 1962.
Abstract:In this paper, we investigate the possibility to improve discharge predictions from a lumped hydrological model through assimilation of remotely sensed soil moisture values. Therefore, an algorithm to estimate surface soil moisture values through active microwave remote sensing is developed, bypassing the need to collect in situ ground parameters. The algorithm to estimate soil moisture by use of radar data combines a physically based and an empirical backscatter model. This method estimates effective soil roughness parameters, and good estimates of surface soil moisture are provided for bare soils. These remotely sensed soil moisture values over bare soils are then assimilated into a hydrological model using the statistical correction method. The results suggest that it is possible to determine soil moisture values over bare soils from remote sensing observations without the need to collect ground truth data, and that there is potential to improve model-based discharge predictions through assimilation of these remotely sensed soil moisture values.
The advent of digital elevation models (DEMs) has made it possible to objectively extract, calculate and store geomorphological parameters for hydrological modelling at several scales. For a grid-based DEM, the threshold area used to extract the channel network is analogous to the scale of the map produced. In addition to the map scale, the effects of the vertical resolution of the DEM on some frequently used geomorphological parameters in hydrology are examined using high-resolution DEMs of two natural and two artificial catchments. The vertical resolution was varied between 1 cm and 1 m, the most common vertical resolution of DEMs. At a fixed map scale, the mean absolute percentage error in the geomorphological parameters caused by a decrease in vertical resolution is within the range 0-5% for the medium-sized catchments and 0-10% for the small catchments studied. Although it is true that a change in vertical resolution may cause a change in the individual pixel slope, area and topographic index (area/slope), particularly in low relief terrain, their cumulative distributions do not show any significant change with the vertical resolution. The shape of the normalized width function is not very sensitive to the vertical resolution and the map scale. For small catchments order change may occur at different map scales for the different vertical resolution DEMs of the same catchment, causing a significant change in order-related parameters such as Horton ratios. It is suggested that the vertical resolution of the DEM of a catchment be considered satisfactory for most hydrological applications if the ratio of the average drop per pixel and vertical resolution is greater than unity. This ratio criterion could be used to define the minimum pixel area for reliable channel network definition for any given vertical resolution. The minimum pixel area places a lower bound on the horizontal resolution with which a channel network can be extracted from a DEM. These results could potentially be used to assess the adequacy for hydrological purposes of existing and proposed digital elevation databases.
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