Unlocking the Power of Moisture Recycling for Sustainable Development
Effective irrigation planning is crucial for sustainable agricultural development and ecosystem restoration projects in arid regions. With respect to ambitious greening initiatives, Saudi Arabia is establishing a national strategy toward a more sustainable and eco-friendly future not only for itself but also for the broader Middle East region. Thus, comprehensively understanding the water cycle in the region is essential to identify the most suitable target locations for afforestation and reforestation while considering the potential role of irrigation.
Herein, in addition to traditional pedoclimatic factors, we introduce a complementary consideration—”irrigation recycling.” Building on the well-established concept of atmospheric moisture recycling and taking advantage from an atmospheric trajectory dataset, we track the path of evaporated water from current or potential irrigated sites to the location where the evaporated water eventually falls as precipitation. Our analysis offers two key benefits. First, it helps pinpoint the regions in which and the periods during which water recycling is maximum within the country, aiding more precise calculations of the investment return value for irrigation infrastructures. Second, it helps identify the land-use change patterns that contribute to international efforts such as drought mitigation in East Africa as an example.
Quantifying the Origins and Destinations of Atmospheric Moisture
We found that one-third of the actual precipitation in the current Saudi irrigated sites originated from evapotranspiration over land, mainly from Saudi Arabia and surrounding countries. Interestingly, most of the evapotranspiration from these irrigated sites will eventually fall somewhere over land (primarily in Iran). Controlling the seasonality and spatial distribution of the future irrigation expansion will allow controlling the atmospheric moisture recirculation in Saudi Arabia and nearby drought-prone regions such as Eastern Africa.
Origin of precipitation over the main agricultural sites in Saudi Arabia (backward footprint)
The spatial distribution of the backward footprint of precipitation averaged over the agricultural sites in Saudi Arabia exhibits marked seasonal variations. On average, evapotranspiration from global land areas contributes to one-third of the precipitation over the Saudi sites. The majority (two-thirds) of the backward footprint is attributed to evaporation from seas and oceans.
In all seasons, the Red Sea plays a dominant role in moisture transport to the Arabian Peninsula via the Red Sea Convergence Zone (RSCZ) and the Tokar wind. The contribution from the Eastern Mediterranean is more prominent in summer, while evaporation from the Arabian Gulf gains importance in autumn. The total contribution from the seas is larger in winter (~ 70%) and smaller in summer (~ 52%).
The primary contributor to the backward footprint over the five agricultural sites is Saudi Arabia itself, accounting for about a quarter of the total land contributions. In summer, the relative contribution from nearby countries increases, and that from Turkey becomes equal to the contribution of Saudi Arabia. Egypt remains a prominent contributor in most of the seasons, making up 7.5–10% of the total contribution from land. In spring and summer, the role of distant contributors from Eurasia, such as Greece, Italy, and Russia, emerges.
Destination of evapotranspiration from the main agricultural sites in Saudi Arabia (forward footprint)
According to the mean atmospheric moisture trajectory for the period 2008-2017, nearly all water used in the selected agricultural sites of Saudi Arabia will eventually reprecipitate over local or remote lands (94% on average), especially in spring and autumn. The average forward footprint from the five agricultural sites in Saudi Arabia differs considerably depending on the seasons.
On average, most of the water is transported to Iran (about 17%); this finding is consistent with the climatological seasonal atmospheric circulation. Iran is the main beneficiary of Saudi irrigation in all seasons except summer. Saudi Arabia is the second beneficiary only in spring, when ~ 7% of water is recirculated locally. In summer, most of the water is transported to Ethiopia (27%). A considerable fraction of Saudi irrigation also benefits India (23%) and Yemen (12%). In summer, only 6% of the water from the irrigated sites recirculates internally in Saudi Arabia.
Optimizing Internal and External Moisture Recycling
Internal recycling ratio: recycling within Saudi Arabia
The internal recycling ratio represents, for each potentially irrigated location in Saudi Arabia, the resulting portion of evapotranspiration that will eventually rain again somewhere within the country. This portion can contribute to agricultural development, natural vegetation restoration, or groundwater recharge depending on where it falls.
The recycling ratio is 4% on average in winter and reaches 11% at the southeastern boundary of the country. In spring, the recycling ratio is larger (7% on average and reaches 19% in the southeast). In summer, the recycling ratio homogeneously peaks to about 8% over a wide region in the central–western part of the country, while it is only ~ 5% on average for the country as a whole. In autumn, the recycling ratio is homogeneously distributed throughout the country, but it shows a tendency toward higher values in the southwest (7% on average, 14% at maximum).
These results highlight the larger irrigation recycling potential of the southwestern part of the country over most of the year.
External recycling ratio: recycling to Eastern Africa
The portion of rain potentially generated in each Saudi location that may fall over the drought-prone regions in East Africa (Sudan, Ethiopia, and Kenya) shows considerable seasonal variations and different spatial patterns.
On average, the external recycling ratio to East Africa is ~ 2% in winter, 3% in spring, 8% in summer, and 7% in autumn. At maximum, it reaches 13% and 14% in the central/southeastern and southwestern parts of Saudi Arabia in summer and autumn, respectively.
Therefore, the southwestern part of Saudi Arabia is clearly emerging as a reasonable area where irrigation can be expanded to maximize both the internal irrigation recycling to Saudi Arabia and the external irrigation recycling to East Africa.
Atmospheric Circulation Patterns Supporting Moisture Dynamics
The analysis of the low-level circulation and vertically integrated moisture transport is consistent with the observed patterns of moisture recycling. In winter, a prominent feature over the Arabian Peninsula region is the Arabian anticyclone, whose western flanks interact with another seasonal high-pressure system located over northeast Africa. In spring, the Arabian anticyclone weakens and retreats to the east, generating southerly winds from the Indian Ocean to the Arabian Peninsula.
In summer, the circulation changes dramatically, with a low-level system connected to the Indian monsoon traversing through the southern part of the Arabian Peninsula. Northerly winds blow over the Red Sea and the western side of the Arabian Peninsula, feeding the northern branch of the Intertropical Convergence Zone in the Sahel region. In autumn, the Arabian anticyclone is reestablished.
These atmospheric circulation patterns, together with the regional orography, play a crucial role in shaping the moisture recycling processes in the region, with implications for irrigation planning and afforestation efforts.
Conclusion: Towards Sustainable Water Management through Moisture Recycling
Our study provides valuable insights into the origin and destination of atmospheric moisture in Saudi Arabia and neighboring regions, highlighting the importance of considering moisture recycling in land-use planning and water resources management. By identifying the regions with the highest potential for internal and external moisture recycling, this analysis can support decision-making for the Saudi and Middle East Green Initiatives, as well as similar large-scale afforestation and irrigation projects worldwide.
Controlling the seasonality and spatial distribution of future irrigation expansion can help optimize the atmospheric moisture recirculation, benefiting not only Saudi Arabia but also drought-prone regions like Eastern Africa. The methodology presented in this study offers a pragmatic framework that can be applied to other regions, making it a valuable approach for global sustainability programs.
Ultimately, the insights gained from this research underscore the need for a holistic, transboundary approach to water resources management, recognizing the interconnectedness of land-use changes and their atmospheric impacts. By leveraging the power of moisture recycling, Saudi Arabia and its neighbors can enhance regional water security, boost ecosystem resilience, and contribute to global sustainability efforts.
