Current flood models do not account for cities'
impact on local rainfall patterns, an oversight that could lead to
significantly underestimating the severity and frequency of floods in
urban areas, a Purdue study finds.
Indiana's state climatologist, and collaborators at China's Tsinghua
University showed that hard, impenetrable city surfaces such as concrete
can dramatically influence the way rainfall spreads across a watershed.
Flood models that do not incorporate the ways
cities modify rainfall patterns could underestimate the magnitude of
future floods by as much as 50 percent, said Niyogi, who is also a
professor of agronomy and earth, atmospheric and planetary sciences.
"Over the last two decades, we've seen an
increase in major flooding in urban areas," he said. "Many factors are
contributing to that change, including extreme weather, climate change
and climate variability. But evidence is also emerging that cities
themselves are significantly and detectably changing the rainfall
patterns around them."
In rural areas, rainfall seeps gradually into the
soil, nourishing plants, flowing into streams and lakes and
replenishing underground aquifers. But when a region is urbanized -
topped with impervious surfaces such as asphalt - the way water moves
over land changes significantly.
Paved roads, sidewalks and roofs can hinder
rainwater's ability to filter into the ground, often creating powerful
streams by linking water flow paths and sending runoff surging into
drains and waterways along with pollutants it picks up along the way.
This dual combination of city-altered rainfall patterns and the urban
network can modify the strength, timing and duration of floods in cities
and damage local water quality.
Previous research by Long Yang, a postdoctoral
research associate at Tsinghua University and first author of the study,
suggested that covering 5 percent of a watershed with impervious
surfaces is the threshold at which local hydrology - the science of how
water moves - begins to change. These effects are magnified with
increases in impervious surfaces.
Hydrologic engineers make projections about
future floods with a liner model that assumes flood magnitude increases
at the same rate as impervious coverage and rain. But Niyogi and Yang
think this model does not tell the whole story: It treats cities as
isolated systems that respond to changes in rainfall but do not affect
the outside system, such as the atmosphere.
According to Niyogi, growing evidence shows that
cities measurably shift rainfall patterns by sending signals to the
atmosphere that can alter how rain accumulates, its intensity, the
frequency of downpours and the dynamics of how storms evolve.
By omitting this complex back-and-forth between
cities and local rain patterns, the linear model could greatly
underestimate flood changes and undermine the effectiveness of
flood-control designs, he said.
"Models need to move beyond the traditional approach to consider the feedbacks cities have on local rainfall," he said.
The researchers used a grid-based watershed model
to test how urbanized regions surrounded by rural areas, such as
grassland, crops and forests, interact with rain and affect how it is
spread across a watershed. Hydrology in these "mixed" watersheds can be
much more varied than in completely urbanized or undeveloped areas, Yang
said. To mimic real rainfall variability, they used a dataset of
30-year daily rainfall typical of an urbanized watershed.
Simulations showed that a region's hydrological
response depends on two factors - the amount of impervious coverage and
the spatial pattern of rainfall, for example, whether rain falls across
the watershed in a random way or is more centralized over impervious
The most notable changes in flood magnitude
occurred when the watershed was moderately urbanized, that is, 20-30
percent covered with impervious surfaces. The results indicate that the
risk of underestimating flood changes is larger for moderately urbanized
watersheds than completely urbanized or undeveloped areas, Yang said.
"Each city is unique," Niyogi said. "There is no
single, simple recipe for what urban areas can do to modify rainfall
potential and flood risks. Engineering will have to evolve to include
multiple factors and feedbacks. Here we've shown that cities create
their own rainfall patterns that need to be considered to mitigate flood
Fuqiang Tian of Tsinghua University also co-authored the paper.
The paper was published in Urban Climate and is available to journal subscribers and campus readers at http://www.sciencedirect.com/science/article/pii/S2212095515000073
The National Science Foundation of China and the
Ministry of Science and Technology in China and the U.S. National
Science Foundation provided funding for the study. Researchers received
support from the Foundation of State Key Laboratory of Hydro-Science and
Engineering of Tsinghua University, the NSF's Career and Computational
and Data-Enabled Science Strong City grants and the U.S. Department of
Agriculture's National Institute of Food and Agriculture Drought Trigger