Remote Sensing: Technical Cooperation in Water Sector (Maghreb)
Coopération Régionale pour une Gestion Durable des Ressources en Eau du Maghreb - CREM
Report of the project:
The regional cooperation in water sector in Maghreb aims at supporting regional authorities in obtaining qualitative and quantitative information about the use of water for agricultural purpose, with the sake of a sustainable regional ground water management. Within this project, the role of the BGR is to assist the regional organizations for collecting and interchanging relevant hydrogeological information, and to provide modern and efficient methods for estimating and monitoring ground water condition. An important aspect is also the training of the regional administrations into this new knowledge, which should ensure a sustainable ground water management. The remote sensing department of the BGR intercedes in this project with the utilization of multispectral and RADAR remote sensing data in order to estimate and monitor the water needs mainly for agricultural purpose. In particular, land cover classification for parameter retrieval to calculate water requirements and land subsidence due to ground water discharge are evaluated.
Example of the Nebhana Region in Tunisia
Through the use of the RADAR technique of Persistent Scatterer Interferometry, subsidence in the city of Sousse, Tunesia, can be assessed. A point scatterer situated in the region A shows a velocity rate of about 2 cm over one year. Futher in the interior of the country, a more stable behavior is detected (B)
Source: BGR
The region situated between the Nebhana dam in the west and the city of Sousse in the east is an important agricultural region for Tunisia. The water needs for irrigation are partly covered by the dam water and precipitation. Further sources are boreholes in phreatic aquifer. The lack of a sustainable ground water management and control of intensive water withdrawal induces problems due to medium term ground water overexploitation, such as water pollution, water salination, bad harvest, habitat damage, relocation of the population towards the cities, and shortage of drinking water supply.
The remote sensing department of the BGR supports the local authorities in estimating their water consumption with two complementary methods:
- Land cover classification using both multispectral and RADAR data
- Estimation of land subsidence due to ground water discharge, using RADAR data
Land cover classification
Depending of the type of crop cultivated, a varying amount of water, over a varying period of time, is needed. The mapping and regular monitoring of crop types and changes will allow the local authorities to better estimate the water needs and sensitize and encourage farmers to exercise a water-saving farming, adapted to the crop needs. Furthermore, such determination of the cultivated area allows to detect which areas belong to the official irrigated sites (from the dam water or official wells) and which ones are not yet registered.
For this purpose, both multispectral and RADAR data are investigated. The multispectral data come from the SPOT, Rapid Eye, Sentinel 2 and Landsat 8 satellite missions, and the RADAR data come from the Sentinel 1 and TerraSAR-X (SM) missions. Figure 1 shows an overview of both data types above the region of Nebhana, Tunisia.
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Estimation of land subsidence
The irrigation conducted in the Nebhana region induces the intensive use of every possible water supply. As an example, the water level of the Nebhana dam drastically dropped in the past years (Figure 2). This, in addition to dryer rain seasons and redirection of the water to the larger cities for the needs of tourism, causes irreversible depletion of the water quantity and quality in the region. The shortcoming of water from the dam is compensated by water from the aquifers, drawn from boreholes.
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| Figure 2: Evolution of the water level in the Nebhana dam since 2012 | ||
This triggers an overexploitation of the aquifer system that cannot entirely naturally be counterbalanced by the precipitations. Depending on the soil nature, this ground water withdrawal results in land subsidence due to underground compaction, without compensating renewal of the underground water body. Such ground movements are detected using the RADAR techniques of Persistent Scatterers Interferometry (PSI) and Small-BAseline Subset (SBAS). Those techniques provide information about the relative ground movements on local and regional scale (Ferretti et al., 2014; Bernardino et al., 2002).
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Using multiple dataset acquired over a long period of time, they allow to evaluate long term trends such as land subsidence caused by irreversible water discharge, but also shorter term trends due to seasonal effects, where local uplift can occur due to partly recharge of ground water during the rainy season. Such information helps the local authorities to sensitize farmer to a water-saving agriculture but also help them monitor the available water quantity for a better management. Furthermore, intensively used boreholes affect also locally the soil structure: more important subsidence patterns are expected around such well. These techniques can therefore help the authorities to detect not registered boreholes.
| To this goal, data from the spaceborne sensors TerraSAR-X (SM) and Sentinel 1A (IW) are investigated. Figure 3 shows the results of a PSI analysis over the Nebhana region. Larger subsidence is observed in parts of the agricultural areas, and in the city of Sousse. The associated graphs represent the movement of selected PS over the defined period of time. A visual interpretation allows to delineate regions dominated by uplifts, stable, or dominated by subsidences (Figure 4). |
Partner: Sahara and Sahel Observatory (OSS) (Lead Partner), GIZ
Literature:
- Stoffner.F. and Dubois. C.(2021): Gestion durable des ressources an eauxs souterraines au Maghreb - Estimation de l’utilisation des eaux souterraines à usage agricole sur la base de données de télédétection
- Stoffner, F. and Dubois, C. (2021): Sustainable management of groundwater resources in the Maghreb - Estimation of groundwater use for agricultural purpose based on remote sensing data.
- Ferretti, A., Prati, C., and Rocca, F.: Nonlinear subsidence rate estimation using permanent scatterers in differential SAR interferometry, IEEE Transactions on Geoscience and Remote Sensing, vol.47, pp. 2202–2212, 2000.
- Berardino, P.; Fornaro, G.; Lanari, R.; Sanssti, E.: A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms, IEEE Transactions on Geoscience and Remote Sensing, vol.40, no.11, pp. 2375- 2383, 2002.