Research

WP10 - Remote sensing of hydrological parameters
What?

The main objective is to adapt the WetSpa model (used in WP11) for the use of remote sensing derived information. On the one hand, remote sensing derived parameters are used as a direct input for the rainfall-runoff model enabling a better estimation of the rainfall-runoff processes. The focus lies on sealed surfaces and vegetation dynamics. On the other hand, sensing of energy balance terms is used to create measured evapotranspiration maps, enabling an improved model calibration.


Why?

Impervious land-cover or so-called sealed surfaces play a key factor for hydrological changes due to urbanization. However, currently most physically based hydrological models use a land-use class based approach, often disregarding the spatial variability of imperviousness in urban areas. Chormanski et al. (2008) have shown that recently developed remote sensing image processing techniques, similar to those used in WP2, allow a better estimation of the spatially distributed sub-pixel imperviousness. Such measurements could considerably improve assessments of urbanization on the hydrological processes. Integration of vegetation dynamics allows to also account for the impacts on the recharge areas, which are mainly covered with pastures and crops. The use of a timeseries of evapotranspiration, one of the terms in the water balance, in a data assimilation approach is expected to improve model calibration.


How?

The WetSpa model was adapted for the use of the remote sensing derived information. The new script for runoff and depression storage calculation used the sub-pixel maps (output from WP2). A runoff coefficient of 1 is assumed for the impervious pixel fraction and a literature based value for the non-impervious part of the pixel. The vegetation dynamics in non-urban pixels are parametrized using the leaf area index (LAI). Based on established relations in literature between the LAI and a number of land-use parameters (such as root depth, interception capacity, etc.) spatially distributed input maps are produced. Seasonal variations are considered using a sinusoidal function between the minimum and maximum values. A surface energy balance model is used to estimate a timeseries of evapotranspiration (ET) maps. These models use the thermal infrared band of MR imagery. To improve the spatial resolution different thermal sharpening techniques are tested, especially with respect to urban applications.


Results

Using the sub-pixel maps with sealed proportions for the urban pixels (from WP2 ) spatially distributed and time-varying runoff coefficients for the Tolka catchment in Dublin were calculated with the WetSpa model. The figure on the left shows the runoff coefficient maps for the years 1988 and 2001. Note the steady increase of surface with high runoff coefficients (orange and red color) reflecting the urbanisation during the past decades. Also the spatial variability within the urban range – city centre of Dublin is located in the east of the Tolka catchment – is clearly visible. The integration of the vegetation index parametrization is still tested.




 
Webmaster
Last modification date = 30-04-2009