Impacts of Land Use Change on a Watershed

A watershed is an area of land that collects precipitation, such as rainfall and snow, and directs it into a common water body—whether a stream, river, or, in the case of Prince William County, the Occoquan Reservoir. Land use changes can dramatically alter the natural functioning of watersheds, with impacts beginning almost immediately after land is cleared, leveled, or developed with roads and impervious surfaces. These changes result in increased volume and speed of stormwater runoff. Over the course of 20 to 50 years, initial disturbances lead to lasting ecological and physical transformations. When 35–50% of a forested watershed is replaced by impervious surfaces—well above the 10% threshold where significant degradation typically starts—the movement and fate of water within the landscape are permanently altered.

When Impacts Happen

Watersheds respond quickly to shifts in land use. The removal of vegetation and the installation of pavement cause stormwater runoff to become nearly instantaneous during rainfall events, with surface runoff reaching up to 16 times that of undisturbed areas. This dramatic increase highlights the immediate impact of development. To address these concerns, stormwater management measures are often integrated into development plans, aiming to slow, capture, and treat runoff before it enters local water bodies.

• Short-Term (1–10 Years): Substantial increases in streamflow and sediment loads occur soon after clear-cutting and construction activities.
• Long-Term (20–50 Years): Over decades, cumulative effects emerge, such as persistent deficits in soil moisture and changes in water flow pathways due to road networks and infrastructure.

What are the Impacts to the Watershed

When impervious surfaces account for 35–50% of a watershed, notable hydrological and ecological changes take place. Pavement and buildings prevent precipitation from soaking into the soil. Under natural conditions, much of the rainfall infiltrates the ground, helping to replenish aquifers. However, when surfaces are sealed, infiltration declines sharply, which in turn reduces groundwater recharge.

During dry periods, streams depend on groundwater—known as baseflow—to maintain their flow. A high percentage of impervious cover reduces this recharge, lowering groundwater levels and separating streams from their underlying water sources. This results in reduced streamflow during dry spells and can transform perennial streams into intermittent ones.

Hydrological Changes

Flashier Streams: Stormwater runoff reaches streams more rapidly and in larger volumes, causing higher peak flows and more frequent flooding.

Reduced Groundwater Recharge: Impervious surfaces prevent water from infiltrating the soil, leading to lower water tables and streams that may dry up during summer months, ultimately causing streams to become intermittent.

Physical and Water Quality Changes

Increased speed and volume of stormwater runoff erode stream banks, cause incision (downcutting), and result in "blowouts" that destroy aquatic habitats. Stormwater runoff collects contaminants such as oils, heavy metals, road salts, and nutrients (nitrates and phosphates) from paved surfaces, and carries them directly into waterways without the natural filtering effects of forests. Rainwater heats up as it travels over sun-exposed pavement, raising stream temperatures and stressing or killing sensitive aquatic organisms. The reduced groundwater availability limits natural cooling, resulting in higher temperatures in urban and suburban areas.

Ecological Decline

At high levels of development, sensitive species such as trout and salamanders disappear, leaving only more pollution-tolerant organisms. The removal of old-growth trees eliminates deep roots and canopies that previously provided carbon storage, pollutant trapping, and soil stabilization, leading to further ecological imbalance.

Role and Limitations of Green Infrastructure & LID Features

Green Infrastructure (GI) and Low Impact Development (LID) are increasingly promoted as solutions to watershed challenges. These approaches focus on natural processes—such as infiltration, evaporation, and transpiration—to manage water where it falls. However, their effectiveness is generally limited to handling the first inch of rainfall, making them an incomplete solution for all impacts of land use change.

Unlike forests, which are self-sustaining, GI requires ongoing maintenance. For example, permeable pavements must be vacuumed to prevent clogging, and bioswales need sediment removal to maintain infiltration rates. Research suggests that Green Infrastructure should be designed for future climate scenarios, including increased storm intensity, to ensure sustainability over a 20-year lifecycle.

While GI and LID strategies can reduce the impacts of 35–50% development, they typically result in a "managed" watershed that is more costly to operate and more vulnerable to extreme weather events than a natural system. In watersheds with 35–50% impervious cover, Green Infrastructure is moderately effective at improving groundwater recharge, but its success depends on implementation density, storm event scale, and system maintenance. Unlike forests, GI systems require routine human intervention, such as vacuuming permeable pavements and maintaining bioswales, which are ineffective as roadways and in regions with frequent freeze-thaw cycles.

Green Infrastructure aims to slow down water movement and promote infiltration. It is most effective for frequent, low-intensity rainfall events (typically less than 0.8 to 1.0 inch). In these cases, dense GI installations can capture and infiltrate enough water to closely mimic natural forested conditions. Studies of urban catchments with approximately 35% impervious cover have shown that retrofitting with GI elements like rain gardens and porous pavements can increase infiltration and reduce total surface runoff.

In highly urbanized areas with 64% impervious cover, only about 2.4% of rainfall naturally infiltrates the ground. Implementing infiltration practices in zones with high impervious surfaces can raise this rate to roughly 5.2%, doubling recharge. This is still far below the recharge in a natural environment which is about 50%.

 

from U.S. EPA

 References:

Pegah Jalali, Sergey Rabotyagov, Quantifying cumulative effectiveness of green stormwater infrastructure in improving water quality, Science of The Total Environment, Volume 731, 2020, 138953, ISSN 0048-9697, https://doi.org/10.1016/j.scitotenv.2020.138953.

 https://www.mdpi.com/2073-4441/11/10/1992

Alam, T.; Mahmoud, A.; Jones, K.D.; Bezares-Cruz, J.C.; Guerrero, J. A Comparison of Three Types of Permeable Pavements for Urban Runoff Mitigation in the Semi-Arid South Texas, U.S.A. Water 2019, 11, 1992. https://doi.org/10.3390/w11101992

 Zuo, Shangjun. (2025). Green Infrastructure / LID for Urban Stormwater Management. Science and Technology of Engineering, Chemistry and Environmental Protection. 1. 10.61173/pz840329.