Satellites Track Changing Water Story Across Egypt’s Dry Coast

A 25-year satellite study has revealed striking differences in how Egypt's Mediterranean coastal ecosystems turn scarce water into plant growth, while exposing the influence of agricultural reclamation, urban expansion and landscape degradation. Published in the journal npj Scientific Reports, the research paper titled 'Assessment of water use efficiency in arid ecosystems of the coastal land of Egypt using MODIS dataset' was written by Amira M. Hotaiba, Boshra B. Salem and Marwa Waseem A. Halmy of Alexandria University.

Their findings offer the first remote-sensing assessment focused on water-use efficiency across Egypt's northern coastal drylands, a region where limited rainfall, high temperatures and growing pressure on land make every unit of water increasingly valuable.

Measuring how much plant growth is produced from every unit of water

Water-use efficiency, known as WUE, describes the balance between carbon absorbed through plant growth and water lost through evapotranspiration, which combines evaporation from soil and water surfaces with moisture released by plants. A landscape with high WUE produces more organic material for the water it uses, making the measure valuable for understanding crop performance, ecosystem health, carbon storage and resilience to drought.

The researchers examined more than 1,200 kilometres of Mediterranean coastline, covering the Nile Delta, newly reclaimed farmland west of the delta, coastal wetlands, salt marshes, dunes, desert plains, rocky surfaces and the northern Sinai Peninsula. Much of this landscape receives little dependable rainfall and contains sparse vegetation, allowing direct evaporation to account for a large share of total water loss.

Satellite records from NASA's Moderate Resolution Imaging Spectroradiometer, or MODIS, provided gross primary productivity and evapotranspiration data from 2000 to 2024 at a spatial resolution of 500 metres. The team calculated annual WUE by dividing plant carbon uptake by water loss, then examined its pattern across ten land-cover classes.

Land-cover maps were produced for 2000 and 2024 using a Random Forest machine-learning model supported by vegetation, built-up, elevation, slope, soil-carbon and proximity-to-water information. The maps achieved overall accuracies of 82% and 81%, with Kappa scores of 0.80 and 0.78, showing considerable agreement with independently interpreted reference points.

Reclaimed farmland and reed beds recorded the strongest efficiency

Reclaimed agricultural land recorded the highest values, ranging from 2.7 × 10⁻³ to 2.9 × 10⁻³ grams of carbon per millimetre of water, while reed vegetation remained close to 2.6 × 10⁻³. Established agricultural areas averaged around 2.5 × 10⁻³ and showed a mild downward trend after 2010.

Wetlands produced lower and declining values, while salt marshes fluctuated sharply without a clear direction. Gravel and sand flats remained in the middle range, and vegetated sand plains and bare rock recorded the lowest efficiency. The highest overall WUE occurred in 2010, possibly because lower temperatures or drought-related conditions reduced evapotranspiration. The authors caution that extreme drought can lower efficiency by disrupting photosynthesis and increasing evaporation from exposed soil.

Trend testing showed that most natural and semi-natural land covers experienced small but statistically significant declines. Water bodies recorded the steepest downward trend, followed by gravel and sand flats, wetlands, sand plains, reed vegetation and reclaimed land. Agriculture, salt marshes and bare rock showed no significant long-term direction, while urban areas registered a slight but significant increase. The size of these changes was generally small, meaning they represent gradual shifts rather than sudden ecological collapse.

WUE patterns moved together most strongly in agriculture and reclaimed land, reclaimed land and reed vegetation, and urban areas and reclaimed land. These correlations suggest that the categories may be responding to shared climate conditions, irrigation practices or development pressures; they do not prove that change in one land cover directly caused change in another.

Agricultural reclamation expanded as coastal landscapes were transformed

Egypt's reclaimed land expanded from 6,843 square kilometres in 2000 to 10,490 square kilometres in 2024, an increase of more than 53%. Agricultural land grew by about 19.4%, rising from 4,177 to 4,987 square kilometres, while wetlands expanded by 5.7%. Sand plains gained more than 4,157 square kilometres, representing the largest increase in absolute area.

Reclaimed farms performed surprisingly well despite the western desert's high temperatures, dry winds, low humidity and intense evaporation. The researchers link this result to centre-pivot and drip-irrigation systems supplied largely by groundwater, which can reduce surface water losses and deliver moisture closer to crops. Local water-management practices may have a stronger influence on WUE than the surrounding desert climate alone would suggest.

Turning native desert into managed farmland can increase plant productivity and improve the carbon-to-water ratio while still causing soil salinisation, depletion of fossil groundwater and displacement of natural habitats. A rising WUE value should not be treated automatically as evidence of ecological improvement.

Wetlands, open water and salt marshes displayed more complicated behaviour because they are influenced by natural hydrology and nearby development. Conversion to aquaculture, discharge of untreated agricultural or municipal waste, irrigation construction and the spread of reeds can all change their structure and water balance. Tourism, road construction, limestone extraction and urban or industrial growth are also reshaping dunes, rocky areas and desert flats along the western coast.

Greater water efficiency does not always mean a healthier landscape

The team used an Index of Landscape Conservation State to separate strongly degraded, moderately degraded, stable and improving areas. Most of the study region remained stable, particularly the broad sandy flats of the Western Desert and Sinai, while degradation clustered around urban expansion, agricultural reclamation and coastal construction east and west of the Nile Delta. Only 1.75% of the region showed landscape recovery.

Areas classified as strongly degraded recorded a 22.5% rise in WUE, largely because reclaimed farms replaced low-productivity desert and introduced efficient irrigation. Stable areas, including the traditional agricultural heart of the delta, retained the highest absolute WUE but improved by only 1.07%, possibly because crop productivity is already near its practical ceiling and flood irrigation keeps water losses high. Moderately and strongly improving landscapes recorded WUE gains of 10.7% and 11.4%, though their absolute values remained lower than those of cultivated land.

These results reveal why water efficiency and conservation must be assessed together: commercial agriculture can produce more biomass per unit of water while weakening natural ecosystems, whereas wetlands and native vegetation may show lower WUE because they lose more water naturally and lack the fertilisation-driven productivity of farmland.

The authors acknowledge that MODIS pixels can contain several land-cover types, creating uncertainty in fragmented areas, particularly for reeds, wetlands and urban land. Egypt also lacks enough ground-based carbon and water monitoring stations to fully validate the satellite estimates. Future work using higher-resolution imagery and eddy-covariance towers could reveal finer ecological changes and separate the effects of climate, hydrology and land management.