Abstract
Analysis of the relationship between evapotranspiration (ET) and its natural and anthropogenic drivers is critical in water-limited basins such as the Nile. The spatiotemporal relationships of ET with rainfall and vegetation dynamics in the Nile Basin during 2002–2011 were analyzed using satellite-derived data. Non-parametric statistics were used to quantify ET-rainfall interactions and trends across land cover types and subbasins. We found that 65% of the study area (2.5 million km2) showed significant (p < 0.05) positive correlations between monthly ET and rainfall, whereas 7% showed significant negative correlations. As expected, positive ET-rainfall correlations were observed over natural vegetation, mixed croplands/natural vegetation, and croplands, with a few subbasin-specific exceptions. In particular, irrigated croplands, wetlands and some forests exhibited negative correlations. Trend tests revealed spatial clusters of statistically significant trends in ET (6% of study area was negative; 12% positive), vegetation greenness (24% negative; 12% positive) and rainfall (11% negative; 1% positive) during 2002–2011. The Nile Delta, Ethiopian highlands and central Uganda regions showed decline in ET while central parts of Sudan, South Sudan, southwestern Ethiopia and northeastern Uganda showed increases. Except for a decline in ET in central Uganda, the detected changes in ET (both positive and negative) were not associated with corresponding changes in rainfall. Detected declines in ET in the Nile delta and Ethiopian highlands were found to be attributable to anthropogenic land degradation, while the ET decline in central Uganda is likely caused by rainfall reduction.
Highlights
Terrestrial evapotranspiration (ET)—the water transferred from the soil-plant complex to the atmosphere—plays an important role in water and energy budgets [1]
Our objective was to address the following questions: (1) How are monthly ET dynamics related to rainfall in the Nile Basin? (2) How does this relationship vary with land cover? (3) What are the geographic extents and patterns of trends the Basin exhibited by ET, rainfall and vegetation dynamics? (4) Which regions exhibited change in ET
We have explored seasonal ET dynamics as well as multiyear trends, attempting to understand the relationships between ET, rainfall and land cover both in terms of monthly variability as well as the changes that occurred over the period of 2002–2011
Summary
Terrestrial evapotranspiration (ET)—the water transferred from the soil-plant complex to the atmosphere—plays an important role in water and energy budgets [1]. Accurate estimates of ET are key environmental constraints for climate and hydrological simulations [5,6,7], and for ensuring sustainable use of water resources [5,8] and the ability of biological systems to support human needs. This is especially true in the Nile Basin (Figure 1), a highly populated region in northeastern Africa, where surface water primarily originates from rainfall over an area encompassing about 25% of the Basin, with substantial parts of the Basin (about 40%) remaining water-limited and dependent on the Nile River for irrigation. The vulnerability of the Nile Basin region to climate [9] and land use [10,11] change and the responses of the hydrological cycle to these changes remain poorly understood
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