A.S.M. Gieske

Evaluation of the Surface Energy Balance System (SEBS) applied to ASTER imagery with flux-measurements at the SPARC 2004 site (Barrax, Spain)

Accurate quantification of the amount and spatial variation of evapotranspiration is important in a wide range of disciplines. Remote sensing based surface energy balance models have been developed to estimate turbulent surface energy fluxes at different scales. The objective of this study is to evaluate the Surface Energy Balance System (SEBS) model on a landscape scale, using tower-based flux measurements at different land cover units during an overpass of the ASTER sensor over the SPARC 2004 experimental site in Barrax (Spain). A sensitivity analysis has been performed in order to investigate to which variable the sensible heat flux is most sensitive. Taking into account their estimation errors, the aerodynamic parameters ( hc, z0M andd0) can cause large deviations in the modelling of sensible heat flux. The effect of replacement of empirical derivation of these aerodynamic parameters in the model by field estimates or literature values is investigated by testing two scenarios: the Empirical Scenario in which empirical equations are used to derive aerodynamic parameters and the Field Scenario in which values from field measurements or literature are used to replace the empirical calculations of the Empirical Scenario. In the case of a homogeneous land cover in the footprints of the measurements, the Field Scenario only resulted in a small improvement, compared to the Empirical Scenario. The Field Scenario can even worsen the result in the case of heterogeneous footprints, by creating sharp borders related to the land cover map. In both scenarios modelled fluxes correspond Correspondence to: J. van der Kwast (hans.vanderkwast@vito.be) better with flux measurements over uniform land cover compared to cases where different land covers are mixed in the measurement footprint. Furthermore SEBS underestimates sensible heat flux especially over dry and sparsely vegetated areas, which is common in single-source models.

Rainfall Variability over Mountainous and Adjacent Lake Areas: The Case of Lake Tana Basin at the Source of the Blue Nile River

Abstract The water resource of the Blue Nile River is of key regional importance to the northeastern African countries. However, little is known about the characteristics of the rainfall in the basin. In this paper, the authors presented the space–time variability of the rainfall in the vicinity of Lake Tana, which is the source of the Blue Nile River. The analysis was based on hourly rainfall data from a network of newly installed rain gauges, and cloud temperature indices from the Meteosat Second Generation (MSG–2) Spinning Enhanced Visible and Infrared Imager (SEVIRI) satellite sensor. The spatial and temporal patterns of rainfall were examined using not only statistical techniques such as exceedance probabilities, spatial correlation structure, harmonic analysis, and fractal analysis but also marginal statistics such as mean and standard deviation. In addition, a convective index was calculated from remote sensing images to infer the spatial and temporal patterns of rainfall. Heavy rainfall is frequen...

Determination of water and heat fluxes with MODIS imagery: Maun, Botswana

Estimation of regional evapotranspiration is of major importance in hydrological modeling, where the partitioning of available energy into sensible and latent heat fluxes is crucial. Point-based measurements are routinely obtained with micrometeorological methods through a combination of radiometers and eddy-covariance instruments. Notwithstanding closure problems, they are considered to yield reliable flux point values. However, when dealing with heterogeneous semi-arid terrain, these point estimates are not representative for regional values. In this paper the results are presented of an analysis where MODIS images are used for the mapping of energy and water balances of a heterogeneous land surface in a savannah environment on the southern fringe of the Okavango Delta (Maun, Botswana). Despite its semi-arid character, fresh floodwaters arrive through the Delta seasonally and therefore part of the area’s vegetation is always transpiring at a potential rate. The model we implemented is governed by remotely sensed values of surface temperature, reflection and vegetation density. The availability of MODIS data provided an opportunity to test the new algorithm by determining the energy balance components on a regional scale for a heterogeneous area and then comparing the results with energy flux measurements using a meteorological flux tower situated in a woodland savannah environment. The results indicate good estimates of net radiation, soil and turbulent fluxes. However, if energy closure problems are neglected, latent heat estimates show significant deviations.

Hydrological Balance of Lake Tana, Upper Blue Nile Basin, Ethiopia

In recent years, few studies are presented on the water balance of Lake Tana. In these studies, the water balance is closed by unknown runoff contributions from ungauged catchments. Studies relied on simple procedures of area comparison to estimate runoff from ungauged catchments. In this study, emphasis is on regionalisation approaches by the use of physical catchment characteristics and a regional model. For runoff modelling, the HVB-96 model is selected while automated calibration is applied as based on a Monte Carlo procedure. Closure of the lake water balance was established by comparing measured to estimated lake levels. Results of daily lake level simulation show a relative volume error of 2.17% and a Nash–Sutcliffe coefficient of 0.92. Results show runoff from ungauged catchments of 527 mm/year for the simulation period 1994–2003 while the closure term only is 85 mm. Compared to previous works this closure term is smallest.

Effect of spatial resolution on estimating surface albedo: A case study in Speulderbos forest in The Netherlands

Abstract Land surface albedo is one of the most important parameters accountable for the planetary radiative energy budget. It is known that albedo varies in both space and time as a result of various natural processes and human interventions. Especially in forest ecosystems these variations are much more intense due to inherent canopy structural differences and anticipated seasonal changes. In such environments, estimation of spatially distributed surface albedo poses challenges in terms of capturing the spatial variability using a remotely sensed sensor with a finite field of view. This study investigated the stand level surface albedo variability of a patchwork forest in the central part of The Netherlands. The data used for the study included airborne and satellite imageries and tower-based solar radiation measurements acquired through a dedicated field campaign. The imageries were preprocessed and atmospherically corrected to obtain top of the canopy (TOC) reflectance. The TOC reflectance bands in the visible and near-infrared domain were integrated to estimate spatially distributed surface albedo while the tower-based radiation measurements in the solar-reflective region were used to obtain the temporal variation of surface albedo over a needleleaf forest canopy. The diurnal variation of surface albedo is consistent with the previous findings for needleleaf forest canopies. The spatial mean surface albedo values estimated from remote sensing data for needleleaf (pure Douglas fir), broadleaf (pure Beech) and mixed forest classes are 0.09, 0.13 and 0.11, respectively. Both visual characteristics and descriptive statistics indicate that with increased pixel size, the spatial variability of albedo progressively decreases. The semivariogram analysis was more insightful to perceive the nature and causes of albedo spatial variability in different forest classes in relation to sensor spatial resolution.

Evapotranspiration Modeling Using Remote Sensing and Empirical Models in the Fogera Floodplain, Ethiopia

Conventional methods and remote sensing were applied for the estimation of reference evapotranspiration and actual evapotranspiration over the Fogera floodplain. Reference evapotranspiration (ET0) by Modified Makkink (MM), Priestly-Taylor (PT) and Abtew (A) simple equations was compared to the Penman-Monteith (PM) estimations, in order to decide which method for ET0 is the most suitable alternative to PM in data scarce conditions. A comparison was also made to a satellite based energy balance approach that estimated actual evapotranspiration. For the remote sensing approach, images from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor were selected. For the study, data has been used from Bahir Dar meteorological station at a distance of 50 km from the floodplain and from Woreta weather station that is located in the floodplain. The comparison of results from the conventional methods indicated that the MM method performed best over the floodplain as compared to the PM approach while the PT and Abtew (A) simple equations only produced fair results. The latter two approaches required calibration of site specific coefficients that may have affected the estimation results. Accumulated actual evapotranspiration from the satellite based approach for the year 2008 was about 1,519 mm for rice, while the reference evapotranspiration by the PM approach was 1,498 mm. A comparison of these results with literature values of the crop coefficient of rice indicated that rice transpired at a potential rate.

Temperature-Emissivity separation with ASTER and LANDSAT 7 validation on the fringe of the Okavango Delta, Botswana

Land surface temperatures are important in global change studies, in estimating radiation budget, heat balance studies and as control for climate models. A new algorithm for estimating land surface temperature and emissivity spectra for multi spectral thermal infrared ranging from 8 to 12mm images has been developed recently (Schmugge et al., 2002) for use with data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on the TERRA platform. Similar methods are also used with the MODIS instrument. In this study, the method developed by Ogawa et al. (2002) was adopted to estimate the broadband emissivity from the narrow band emissivities of the five TIR channels of ASTER instrument in an area on the southern fringe of the Okavango Delta (Botswana). MODTRAN 4 was used to determine the necessary atmospheric corrections while software was developed to facilitate MODTRAN pre- and post-processing. The results were compared with field data, with a LANDSAT 7 image of the same day, and finally also with reported ASTER surface temperature and emissivities for the same image (high level ASTER product). Results indicate that the surface temperature depends rather sensitively on atmospheric transmissivity. No relation was found between broad-band emissivity and NDVI, contrary, for example, to earlier findings in Botswana by Owe and Van de Griend (1993). Using the TES method it becomes possible to obtain more reliable solutions to the energy balance and evapotranspiration problem, especially in semi-arid areas.

Multi-model Approach for Spatial Evapotranspiration Mapping: Comparison of Models Performance for Different Ecosystems

Accurate estimation of evapotranspiration (ET) is vital for water resource management. The FAO-56 Penman–Monteith (FAO-56 PM) is a standard method, but it requires numerous weather data. This challenges water resource managers to estimate ET in areas where there are no adequate meteorological data. Hence, simplified approaches that are less data intensive are the right alternatives. Here, ET was estimated using different approaches and their performances were evaluated in different ecosystems of Ethiopia. Surface Energy Balance Systems (SEBS) model was also used for spatio-temporal mapping of ET in the Fogera floodplain, Lake Tana Basin. The spatial average of actual ET (ETa) from remote-sensing (RS) data over the floodplain was less than the Penman–Monteith (PM) reference ET (ETo) in drier periods and larger in wet seasons. A sensitivity analysis of PM input variables at the Bahir Dar station showed that the incoming solar radiation and air temperature are most sensitive, and wind speed was found to be the least sensitive. The comparison of simple Enku (E) temperature method, Abtew (A) equation, modified Makkink (MM) method, and Priestley–Taylor (PT) method with the PM ETo in the different ecosystems of Ethiopia showed the MM method performed best in all the stations except Dire Dawa stations with coefficient of determination (R 2) of 0.94, Nash–Sutcliffe efficiency (NSE) of 0.88, root mean square error (RMSE) of 0.26 mm, and absolute mean error (AME) of 0.21 mm at Addis Ababa and Awassa stations. The performance of MM and PT methods in the dry and hot climate was poor. The E method performed consistently well in all the stations considered. While ET estimation from remotely sensed inputs has generally been improved, selection of the method of estimation is very important and should always be tested with observational data.

Satellite Based Cloud Detection and Rainfall Estimation in the Upper Blue Nile Basin

In this study remote sensing for rainfall estimation is evaluated. For the Lake Tana basin in Ethiopia the diurnal cycle of rainfall is assessed using satellite observations at high temporal resolution and ground based observations. Also convective activity of a cloud system on the lake has been observed through satellite imagery and shows a potential to observe characteristics of a cloud that produced extreme rainfall intensity. These characteristics include the cloud area and a volume index as well as temporal evolution of distance and direction of the centroid of a cloud mass from a rain gauge at the Gurer Island in Lake Tana. In this work it is concluded that remote sensing can be very helpful in estimating rainfall, assessing the diurnal cycle and monitoring heavy rainfall producing clouds. The high potential of remote sensing observations is mainly because the observations are consistently available with spatially continuous coverage.