Desert City’s Green Spaces Quietly Soak Up 80 Percent of Its Rainfall

In one of the driest corners of the planet, where annual rainfall barely reaches 120 millimeters and potential evaporation tops 2,000 millimeters per year, researchers have discovered that the landscape of Sharjah in the United Arab Emirates is quietly performing an extraordinary feat of hydraulic engineering. According to a new study published in Discover Geoscience, roughly 80 percent of the precipitation that falls on the emirate each year is captured and retained by the land itself, with only about one-fifth escaping as surface runoff. The finding, derived from a spatially explicit ecosystem service model, suggests that Sharjah’s vegetated and agricultural lands are functioning as a vast, largely unrecognized sponge, one whose replacement with concrete and steel would demand a staggering capital investment.

The research, led by Rabin Chakrabortty of the American University of Sharjah together with colleagues across several departments, applied the InVEST Urban Stormwater Retention model, an open-source tool developed by the Natural Capital Project at Stanford University. Unlike conventional event-based hydrologic models such as SWMM or HEC-HMS, which simulate the response of a drainage network to a single storm, InVEST operates on an annual climatological timescale. This makes it particularly suited to strategic planning questions: how much water does a city’s landscape soak up over a full year, how much pollution does that absorption prevent from reaching the sea, and what would it cost to reproduce those services artificially?

To answer those questions, the team assembled a remarkably detailed digital portrait of the emirate. A land use and land cover map was derived from Sentinel-2 satellite imagery at 10-meter resolution and classified into seven categories, ranging from high-intensity developed zones to cultivated land, grassland, scrub and shrub, bare land, and open water. Annual rainfall came from the CHIRPS gridded climate dataset, resampled to match the satellite grid. Hydrologic soil groups were compiled from the FAO Soil Map, ISRIC SoilGrids, and the UAE Soil Survey, then reclassified into the standard USDA categories A through D according to texture and permeability. Road centerlines from OpenStreetMap were rasterized to capture the hydrologic connectivity of impervious surfaces, and all layers were clipped to the Sharjah Municipality boundary and projected into UTM Zone 40N.

The model’s core logic is elegantly simple. For every 10-meter pixel, annual precipitation is partitioned according to a runoff coefficient and a percolation ratio assigned to each land cover class. The retention ratio is the implicit complement of these fluxes, representing the fraction of rainfall captured through interception, infiltration into soil, and eventual return to the atmosphere via evapotranspiration. Crucially, the model also accounts for urban drainage connectivity: pixels flagged as directly connected impervious areas, such as high-intensity development and surfaces within 50 meters of roads, were assigned high runoff coefficients and negligible retention, because stormwater there is rapidly whisked into storm drains rather than absorbed locally. This 50-meter retention radius was chosen as representative of parcel-scale drainage distances in Sharjah’s urban fabric.

The results reveal a striking spatial divide. Across the emirate, the mean retention ratio was 0.81, translating into a total retention volume of 122.77 million cubic meters per year. Runoff, by contrast, averaged a ratio of 0.19, corresponding to 31.29 million cubic meters annually, concentrated in densely built-up districts and along major transport corridors. A separate percolation calculation estimated potential groundwater recharge at just over 5 percent of annual precipitation, or 7.97 million cubic meters per year, a modest fraction that nonetheless carries real significance in a region where aquifers are stressed by over-extraction and saline intrusion from the Arabian Gulf. The authors emphasize that this recharge represents a component of the retained water rather than an additional fraction of the water balance.

Perhaps the most consequential finding concerns which land covers deliver these services. Cultivated land contributed approximately 35 percent of total citywide retention, grassland about 25 percent, and scrub and shrub a further 20 percent, meaning that vegetated and semi-natural classes together supplied roughly 80 percent of the retained volume, far out of proportion to their share of the map. Bare land accounted for around 10 percent, while high-intensity developed areas contributed a mere 5 percent, and open water was essentially negligible. In other words, the emirate’s remaining green and agricultural patches are doing the overwhelming majority of the hydrologic work, a pattern consistent with prior studies in Doha, Qatar, where researchers documented a 422 percent increase in stormwater runoff between 1984 and 2020 as urban expansion replaced natural land with impervious surfaces.

The water quality implications follow directly from the hydrology. Because no local stormwater monitoring data were available, the team adopted screening-level event mean concentrations from peer-reviewed arid-region studies: 2.0 milligrams per liter for total nitrogen and 0.4 milligrams per liter for total phosphorus. Applying these values to the modeled volumes, the researchers estimated that annual runoff exports roughly 62,580 kilograms of nitrogen and 12,516 kilograms of phosphorus toward receiving waters, while the retention process avoids nearly four times as much, approximately 245,536 kilograms of nitrogen and 49,107 kilograms of phosphorus per year. In a coastal emirate where stormwater is often discharged untreated into the Arabian Gulf, these avoided loads represent a meaningful, if screening-level, indication of how green infrastructure could reduce nutrient enrichment and eutrophication risk in sensitive marine waters.

To translate these biophysical services into economic terms, the study applied a replacement cost method, multiplying the modeled retention volume by an assumed benchmark of 1,800 UAE dirhams per cubic meter of engineered stormwater storage. The authors are careful to stress that the resulting figure is an illustrative, order-of-magnitude capital-equivalent benchmark rather than an annual benefit estimate or a formal benefit-cost analysis, and that it is sensitive to the assumed unit cost. Even so, the calculation makes the central point vivid: reproducing the retention capacity that Sharjah’s landscape currently provides for free, using detention basins, underground storage tanks, and pumping facilities alone, would require capital investment on a scale that dwarfs typical municipal stormwater budgets. The implicit message for planners is that every hectare of farmland or green space paved over is infrastructure quietly decommissioned.

The study is candid about its limitations. The annual time step of the InVEST model cannot capture the flash floods and first-flush pollutant pulses that dominate flood risk in hyper-arid climates, where rainfall arrives in short, violent winter storms between December and March; complementary event-based modeling would be needed for detailed flood design. The nutrient estimates rest on assumed uniform concentrations rather than locally calibrated measurements, and the soil group reclassification reflects generalized infiltration potential rather than site-specific hydraulic testing. The economic valuation, moreover, was aggregated rather than spatially explicit, limiting its use in pinpointing conservation hotspots. The authors call for local soil infiltration measurements, stormwater quality monitoring campaigns, sensitivity analyses, and scenario-based studies of land-use change and climate variability to sharpen the picture.

Even with those caveats, the work stands as one of the first integrated, city-scale assessments of stormwater retention, nutrient load reduction, and economic valuation in a hyper-arid Gulf city, and its framework is deliberately replicable. Built entirely on openly available datasets, Sentinel-2 imagery, CHIRPS rainfall, OpenStreetMap roads, and public soil maps, and executed with a standardized open-source modeling platform, the approach could be transferred to other rapidly growing dryland cities across the Gulf, North Africa, and beyond. As climate change renders extreme precipitation more intense and erratic even in arid zones, and as recent flooding episodes in Sharjah and Dubai have exposed the limits of engineered drainage, the study offers a quantitative argument that the cheapest and most versatile stormwater infrastructure a desert city possesses may already be growing out of the ground. Protecting it, the authors conclude, is not merely an environmental nicety but an economically justified pillar of water security and climate resilience.

Subject of Research: Nature-based stormwater retention and water quality assessment using the InVEST model in the arid city of Sharjah, UAE

Article Title: Nature based stormwater management for annual retention and water quality in Sharjah, United Arab Emirates: an InVEST assessment

Article References: Chakrabortty, R., Ali, T., Abouleish, M., Atabay, S., Ahmad, N., Abu-Rukba, R., & Meraj, G. (2026). Nature based stormwater management for annual retention and water quality in Sharjah, United Arab Emirates: an InVEST assessment. Discover Geoscience, 4(1), Article 323. https://doi.org/10.1007/s44288-026-00620-4

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00620-4

Keywords: stormwater retention, nature-based solutions, InVEST model, ecosystem services, Sharjah, United Arab Emirates, water quality, groundwater recharge, urban hydrology, green infrastructure, arid cities, Sentinel-2