Spatial variability of evaporation and moisture storage in the swamps of the upper Nile studied by remote sensing techniques
Introduction
The estimation of spatial variation of evaporation (E) in a catchment is fundamental to many applications in water resources and climate modeling. Evaporation, being the sum of interception, soil evaporation, open water evaporation and transpiration, is a key variable not only in water balance determinations, but also to estimate the moisture, heat and CO2 interactions between land and atmosphere (e.g. Sellers et al., 1996). Several climatic studies have indicated that atmospheric circulation and rainfall are significantly affected by the large-scale variation of soil moisture and evaporation (Savenije, 1996b; Enthekhabi et al., 1999). Three methods can be used to estimate the evaporation at a regional scale: by up-scaling point measurements, by remote sensing techniques (e.g. modeling the energy balance at the land surface) and by hydrological modeling (Savenije, 1997). Each of the three methods has its limitations, and an optimal procedure probably would be a combination of the three approaches. To maximally profit from remote sensing and hydrological modeling, data assimilating is gaining terrain in hydrological studies (Walker et al., 2001; Jhorar et al., 2002; Schuurmans et al., 2003) as well as in climate studies (Dolman et al., 2001). Extensive reviews of remote sensing flux determination methods have been presented by Choudhury (1989), Moran and Jackson (1991) and Kustas and Norman (1996).
Unlike hydrological models, remote sensing techniques compute evaporation directly from the energy balance equation without the need to consider other complex hydrological processes. As a result, the error in the quantification of other hydrological processes is not propagated into evaporation (E), and this is a strong advantage. On the other hand, a major limitation of remote sensing data is that the temporal distribution of satellite-based estimates is poor, and that interpolation techniques are necessary to define evaporation between satellite overpasses. Due to extremely scarce ground hydrometeorological data in the vast study area of the Sudd marshes, utilization of satellite imagery is an attractive study approach. Advanced remote sensing techniques have a high potential to estimate hydrological processes, and are probably far better than the scanty field data available for this large tropical wetland.
The vast wetlands in Southern Sudan are characterized by huge evaporation from the Sudd, Bahr el Ghazal and the Sobat sub-basins. The evaporation from the Sudd alone (Bahr el Jebel swamps) is estimated to be more than 50% of the Nile inflow into the Sudd near Juba, i.e. about 28 Gm3/yr out of the 49 Gm3/yr (Sutcliffe and Parks, 1999). The whole river inflow of the Bahr el Ghazal Basin (12 Gm3/yr) is evaporated before reaching the Nile. About 4 Gm3/yr is claimed to be evaporated from the Machar marches (PJTC, 1960). Therefore, the Sudd became subject of research by planners and engineers to save water and carry more water to the rapidly expanding population living in the downstream areas in North Sudan and Egypt. Their aim is to save water by reducing the evaporation (losses) from the wetlands. Numerous studies and projects were proposed to reduce these losses, of which the (uncompleted) Jonglei canal is the most famous (Jonglei investigation team, 1954; PJTC, 1960).
Most of the past studies to estimate evaporation from the Sudd wetlands rely on the computation of evaporation using meteorological ground station data under the basic assumption that the area is wet throughout the year and moisture is not limiting evaporation rates (Sutcliffe and Parks, 1999; Jonglei investigation team, 1954; Penman, 1963). Although some experiments were made to estimate evaporation from papyrus grown in water tanks (Butcher, 1938), their results were rejected in the subsequent hydrological studies as being too low (1533 mm/yr). Instead, more recent studies used open water evaporation of 2150 mm/yr estimated with the Penman formula (Chan et al., 1980; Sutcliffe and Parks, 1999). Penman (1963) assumed that transpiration from papyrus is similar to the evaporation from open lagoons. However, the computed evaporation by his well-known formula represents the potential open water evaporation, and not the actual evaporation (Ea) from the land surface with a certain heterogeneity in vegetation types and development. The major difficulty of this estimation procedure is that evaporation from point measurements is applied to the total Sudd area. One of the complexities in determining the regional scale evaporation is the variability of the boundaries of the Sudd swamps and the soil moisture behavior throughout the year.
Some of the earlier studies intended to define the extent of the Sudd wetlands from satellite imagery. Travaglia et al. (1995) applied a methodology based on the thermal inertia difference between dry and wetlands from the National Oceanic Atmospheric Administration-Advanced Very High Resolution Radiometer (NOAA-AVHRR). NOAA-AVHRR thermal data were complemented with maps of the Normalized Difference Vegetation Index (NDVI). They defined the seasonal and inter-annual variation of the Sudd area between 28.0 and 48.0 Gm2 during 1991–1993. Mason et al. (1992) used the thermal channel of Meteosat to estimate the Sudd area during 1985–1990 between 8.0 Gm2 at its minimum and 40.0 Gm2 at its maximum. These methods showed a successful application of NOAA-AVHRR and Meteosat thermal data to define the area of wetlands, but such methods fail in the rainy season (cloudy conditions) and do not determine the evaporation.
The approach adopted in this study is based on the derivation of the actual evaporation using the SEBAL techniques (Bastiaanssen et al., 1998a). All components of evaporation (evaporation from open water, from vegetative surface and from bare soil) are calculated based on the surface reflectance and emittance in different parts of the spectrum. This forms basic data that can be used in hydrological and meteorological studies. SEBAL was selected because it does not require any input data, except routine weather station data.
Moisture evaporated into the atmosphere may again precipitate at a different location, but in the same basin (Savenije, 1995, Savenije, 1996a; Trenberth, 1999). If this holds true, the need for reclamation efforts loose ground. Estimation of evaporation by remote sensing over the Sudd is a first step in a larger research project to model moisture recycling in the Nile basin. The derived evaporation results from remote sensing will be used subsequently to verify moisture exchanges between land surface and atmosphere in a regional climate model.
Section snippets
The study area: Sudd, Bahr el Ghazal and Sobat basins
NOAA-AVHRR images have been selected to cover three sub-basins: the Sudd, Bahr el Ghazal and the Sobat basins. These basins stretch from the outfalls on the White Nile to the upstream side near the discharge measuring stations. The names of the most important places are provided in Fig. 1. Fig. 1 shows the natural drainage network within the image boundary, superimposed over the digital elevation model.
The largest wetlands of the Sudd are located at the center of the image, extending from
Major SEBAL model principles
The Surface Energy Balance Algorithm for Land (SEBAL) is an energy partitioning algorithm over the land surface, which has been developed to estimate actual evaporation from satellite images (Bastiaanssen et al., 1998a, Bastiaanssen, 2000). The scheme has found applications in different basins of the world, e.g. Snake River basin in Idaho, USA (Allen et al., 2002), the lake Naivasha drainage basin in Kenya (Farah, 2001), all river basins in Sri Lanka (Bastiaanssen and Chandrapala, 2003) and the
SEBAL results
Eq. (1) has been applied to compute the energy balance components and hence to derive the latent heat flux on a pixel-by-pixel basis over an area of 1000 km×1000 km, covering the Sudd and the neighboring swamps of the Ghazal and the Sobat basins. SEBAL has been applied for 37 images acquired during individual days of the year 2000, Table 1 for the date of acquisition. The daily integrated Ea values have been obtained through the evaporative fraction as defined in Eq. (2) and applied in Eq. (4).
Monthly water balances
A monthly water balance has been computed for the three sub-basins to verify the plausibility of the new evaporation results: Sudd, Ghazal and Sobat. These sub-basins have different land use patterns, precipitation rates and accuracy of available hydrological data. Table 2 shows that the Sudd basin has ponding water at the surface throughout most of the area, and that apart from some higher located spots, the entire sub-basin is flooded in the wet season, or has very shallow groundwater table.
Conclusions
Monthly actual evaporation and soil moisture maps during the year 2000 have been computed for an area of 1000 km×1000 km of the Nile basin covering the wetlands of the Sudd, the Bahr Ghazal, and the Sobat. The SEBAL algorithm has been used to derive the energy balance components at the land surface from the NOAA-AVHRR images because only routine weather data need to be known. The evaporation estimates were verified against water balance results, and close resemblance was derived for two of the
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