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1. Effects of Future Land Use Variability on Nutrient Loads in a Fast‐Urbanizing Landscape

   https://doi.org/10.1111/1752-1688.70009  

   Santos, Andres Lora; Tarabih, Osama M; Arias, Mauricio E; Rains, Mark C; Zhang, Qiong (April 2025, JAWRA Journal of the American Water Resources Association)

   ABSTRACT Urbanization, driven by population growth, alters watershed hydrology and nutrient runoff. However, the complex interplay between urbanization and nutrients in regional watersheds remains an open question. This study assessed how urbanization affects streamflow, total nitrogen (TN), and total phosphorus (TP) loads in six diverse Florida watersheds covering an area of 10,600 km². This was carried out by introducing 2070 land use/land cover (LULC) projections to a watershed hydrology/water quality model. We investigated how different levels of urban density, as a proxy for urbanization patterns, affect streamflow and nutrient variability. Results indicate that urban land could increase from 14% to 27% in 2070. This expansion could lead to monthly streamflow increases of 0%–36%, based on watershed and urbanization patterns. Future TP loads could change by −8% to +140%, with decreases attributed to LULC transitions from high‐use fertilizer agriculture to low/medium density residential classes. Projected TN loads are more consistent, with simulated changes of −1% to +26%. Among LULC transitions, the largest increases in TP and TN are caused by potential urbanization of freshwater wetlands. This study provides knowledge relevant to regions undergoing similar urbanization trends, enabling managers to make better land development plans with water quality considerations. It also contributes a detailed modeling framework that can be adopted even with the use of different LULC datasets and software. 

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2. Long-term effects of land-use change on water resources in urbanizing watersheds

   https://doi.org/10.1371/journal.pwat.0000083  

   Talib, Ammara; Randhir, Timothy O. (April 2023, PLOS Water)

   Cheema, MJM (Ed.)

   The changes in energy balance resulting from land-use change may significantly affect the amount and timing of water loss to the atmosphere as evapotranspiration (ET). Also, these will impact water fluxes in the watershed system, influencing runoff rate, flow volume, intensity, and frequency of floods. During the past century, land-use change in the SuAsCo (Sudbury-Assabet and Concord) watershed has altered basin hydrology, sediment, and nutrient load that is detrimental to water resources in SuAsCo. This study uses an integrated physically-based model Hydrological Simulation Program-FORTRAN (HSPF), along with Land Transformation Model (LTM), to assess predicted temporal and spatial changes in water, nutrient, and sediment yields for future land-use scenarios of 2035, 2065, and 2100. Results showed that a 75% increase in effective impervious area and a 50% decrease in forest area in 2100 (from 2005 baseline levels) are projected to cause a 3% increase in annual streamflow and a 69% increase in total yearly mean surface runoff. The average annual total suspended solid (TSS) yield at the watershed outlet is estimated to increase by 54% in 2100. An increase of 12% and 13% concentrations of average annual total phosphorus (TP) and total nitrogen (TN) are predicted by 2100 due to urban expansion and increased runoff volume. This integrated modeling approach will inform watershed managers and landowners about critical areas of the SuAsCo watershed to apply best management practices (BMPs) to mitigate the effects of land-use land cover (LULC) change. 

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3. Trade-off of ecosystem productivity and water use related to afforestation in southcentral USA under climate change

   https://doi.org/10.1016/j.scitotenv.2024.170255  

   Zhang, Tian; Yang, Jia; Winrich, Abigail; Will, Rodney E.; Zou, Chris B. (March 2024, Science of The Total Environment)

   The increase of tree canopy cover due to woody plant encroachment and tree plantations modifies both carbon and water dynamics. The tradeoffs between ecosystem net primary productivity (NPP) and water use with increasing tree cover in different climate conditions, particularly under future climate scenarios, are not well understood. Within the climate transition zone of the southern Great Plains, USA, we used the Soil and Water Assessment Tool+ (SWAT+) to investigate the combined impacts of increasing tree cover and climate change on carbon and water dynamics in three watersheds representing semiarid, subhumid, and humid climates. Model simulations incorporated two land use modifications (Baseline: existing tree cover; Forest +: increasing evergreen tree cover), in conjunction with two climate change projections (the RCP45 and the RCP85), spanning two time periods (historic: 1991-2020; future: 2070-2099). With climate change, the subhumid and humid watersheds exhibited a greater increase in evapotranspiration (ET) and a corresponding reduction in runoff compared to the semi-arid watershed, while the semi-arid and subhumid watersheds encountered pronounced losses in water availability for streams (>200 mm/year) due to increasing tree cover and climate change. With every 1 % increase in tree cover, both NPP and water use efficiency were projected to increase in all three watersheds under both climate change scenarios, with the subhumid watershed demonstrating the largest increases (>0.16 Mg/ha/year and 170 %, respectively). Increasing tree cover within grasslands, either through woody plant expansion or afforestation, boosts ecosystem NPP, particularly in subhumid regions. Nevertheless, this comes with a notable decrease in water resources, a concern made worse by future climate change. While afforestation offers the potential for greater NPP, it also brings heightened water scarcity concerns, highlighting the importance of tailoring carbon sequestration strategies within specific regions to mitigate unintended repercussions on water availability. 

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4. Positive unintended consequences of urbanization for climate-resilience of stream ecosystems

   https://doi.org/10.1038/s42949-024-00144-1  

   Banner, Jay L; Black, Bryan A; Tremaine, Darrel M (December 2024, npj Urban Sustainability)

   Abstract Developing sustainable urban systems is a fundamental societal challenge for the 21st century, and central Texas faces particularly synergistic challenges of a rapidly growing urban population and a projected increasingly drought-prone climate. To assess the history of urbanization impacts on watersheds here, we analyzed 51 cores from bald cypress trees in paired urban and rural watersheds in Austin, Texas. We find a significant contrast between rural and urbanized watersheds. In the rural watershed, tree-ring-width growth histories (“chronologies”) from 1844–2018 significantly and positively correlate (p < 0.01) with (1) one another, and (2) regional instrumental and proxy records of drought. In the urbanized watershed, by contrast, chronologies weakly correlate with one another, with instrumental records of drought, and with the rural chronologies and regional records. Relatively weak drought limitations to urban tree growth are consistent with the significant present-day transfer of municipal water from urban infrastructure by leakage and irrigation to the natural hydrologic system. We infer a significant, long-term contribution from infrastructure to baseflow in urbanized watersheds. In contrast to the common negative impacts of ‘urban stream syndrome’, such sustained baseflow in watersheds with impaired or failing infrastructure may be an unintended positive consequence for stream ecosystems, as a mitigation against projected extended 21st-century droughts. Additionally, riparian trees may serve as a proxy for past impacts of urbanization on natural streams, which may inform sustainable urban development. 

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5. Impacts of Land Cover Change on the Spatial Distribution of Nonpoint Source Pollution Based on SWAT Model

   https://doi.org/10.3390/w15061174  

   Zhang, Zeshu; Montas, Hubert; Shirmohammadi, Adel; Leisnham, Paul T.; Negahban-Azar, Masoud (March 2023, Water)

   Nonpoint source (NPS) pollution is a pressing issue worldwide, especially in the Chesapeake Bay, where sediment, nitrogen (N), and phosphorus (P) are the most critical water quality concerns. Despite significant efforts by federal, state, and local governments, the improvement in water quality has been limited. Investigating the spatial distribution of NPS hotspots can help understand NPS pollutant output and guide control measures. We hypothesize that as land cover changes from natural (e.g., forestland) and agricultural to suburban and ultra-urban, the distribution of NPS pollution source areas becomes increasingly spatially uniform. To test this hypothesis, we analyzed three real watersheds with varying land cover (Greensboro watershed for agriculture, Watts Branch watershed for suburban, and Watershed 263 for ultra-urban) and three synthetic watersheds developed based on the Watts Branch watershed, which ranged from forested and agricultural to ultra-urban but had the same soil, slope, and weather conditions. The Soil and Water Assessment Tool (SWAT) was selected as a phenomenological model for the analysis, and SWAT-CUP was used for model calibration and validation. The hydrologic responses of the three real and synthetic watersheds were simulated over ten years (1993–2002 or 2002–2011), and calibration and validation results indicated that SWAT could properly predict the export of runoff and three target NPS pollution constituents (sediment, total nitrogen, and total phosphorus). The results showed that the distribution of NPS pollutant outputs becomes increasingly uniform as land cover changes from agriculture to ultra-urban across watersheds. This research suggests that the spatial distribution of NPS pollution source areas is a function of the major land cover category of study watersheds, and control strategies should be adapted accordingly. If NPS pollution is distributed unevenly across a watershed, hotspot areas output a disproportionate amount of pollution and require more targeted and intensive control measures. Conversely, if the distribution of NPS pollution is more uniform across a watershed, the control strategies need to be more widespread and encompass a larger area. 

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