University of Twente Student Theses


Satellite Remote Sensing for Soil Moisture Estimation: Gumara Catchment, Ethiopia

Mekonnen, Dagnenet Fenta (2009) Satellite Remote Sensing for Soil Moisture Estimation: Gumara Catchment, Ethiopia.

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Abstract:Soil moisture is an important variable in hydrology: it affects the partitioning of rainfall into runoff and infiltration, and the partitioning of incoming solar radiation into latent and sensible heat. Soil moisture is however difficult to measure at the spatial scale of a catchment. The conventional point measurement based methods such as the neutron probe or gravimetric method are not appropriate for understanding of the spatial and temporal behaviour of soil moisture. Due to the heterogeneity of soil type, land use and topography, soil moisture may change considerably in space and time. Soil moisture can also be measured by microwave remote sensing under some topographic and vegetation cover conditions, but the spatial resolution for passive microwave and temporal resolution for active microwave commonly is low and the method cannot be applied in densely vegetated areas. An alternative approach based on optical and thermal remote sensing (the triangle method) has recently been proposed and tested to estimate soil moisture at a resolution high enough to serve applications such as semi or fully distributed hydrological modelling. This approach is based on correlating Normalized Difference Vegetation Index (NDVI) and land surface temperature (LST) retrieved from remote sensing to the ground measured soil moisture. In this study, the triangle method was tested for the Gumara catchment in northwest Ethiopia. Observed soil moisture was regressed with scaled NDVI and LST retrieved from atmospherically corrected MODIS images at 1km resolution by applying 2nd and 3rd order polynomial relations, and maps of soil moisture were produced for the catchment for 9 days in September 2008. A total of 49 sampling schemes using gravimetric and indirect soil moisture measurements by theta probe were carried out during a field campaign in September 2008. A field station provided continuous radiometric surface temperature data was installed in the study area during field campaign. MODIS Aqua and Terra images were collected for 9 days from September 18, 2008 to September 29, 2008. For LST, a comparison was made between values computed using the split window method and the MODIS LST product. The latter was used for further analysis, because it correlated well with the ground measurements. Both actual LST from the MODIS product as well as elevation corrected potential surface temperature were used. From the ground based measurements of soil moisture, 35 sampling points were used for calibration, and 14 sampling points for validation. Ground measured soil moisture compared well with simulated soil moisture R2 greater than 0.7 and RMSE of 0.045 is obtained. Applying the potential surface temperature could minimize the topographic effect that is induced from LST on the simulated soil moisture, especially on high elevated areas. Statistical spatio-temporal analysis was done to see the temporal as well as the spatial variability of soil moisture in the catchment. According to the statistical spatio-temporal analysis, the high elevated area resembles high mean soil moisture, low coefficient of variability and time stable condition. But after elevation correction the high elevated area resembles low soil moisture than low elevated area. Also, elevation is found to be the dominant terrain controlling factor for the simulated soil moisture simulated by the developed algorithm. The temporal variability of soil moisture among the pixels in a catchment is high, as compared to the spatial variability of the catchment during the study period. Key words: Triangle method, LST, NDVI, potential surface temperature, soil moisture.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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