Assessing Urbanization Impacts on Longterm Rainfall Trends in Houston
Parastou Hooshialsadat1, S.J. Burian2, and J.M. Shepherd3
1University
of Arkansas, 2University of Utah,
3NASA Goddard Space Flight Center
Research Objective
To determine the effect of urbanization of the Houston
metropolitan area on precipitation variability within the
city compared to regions seasonally upwind and
downwind.
Analysis Components:
1. Downscaling analysis using TRMM-PR and rain
gauge data (Shepherd and Burian, 2003)
2. Quantification of alterations to storm event
characteristics and diurnal rainfall pattern (Burian
et al., 2004a, 2004b)
3. Trend analyses of long-term rainfall records
4. Linked meteorological-hydrological modeling
Houston is the 4th largest city in
the U.S. (1.6 million) and covers an
area of 937 km2; 10th largest CMSA
(more than 4 million)
Houston sits on the 5,000 km2 Gulf
Coastal Plain with a high elevation
of 27 meters above sea level
Mean Monthly Air Temperature (C) for Houston
(Data from Bush International Airport)
35
Air Temperature (C)
30
25
20
15
10
5
0
Jan
Feb
Mar
Apr
May June July
Aug Sept
Oct
Nov
Houston’s climate is subtropical
humid with very hot and humid
summers and mild winters
Dec
Houston Urbanization
Urban growth characterized using
a combination of multi-temporal
population, multi-spectral, and
cadastral data
4.5E+06
4.0E+06
Population
3.5E+06
Average of 43% population
increase in Houston Metro
per decade since 1900
3.0E+06
2.5E+06
2.0E+06
1.5E+06
1.0E+06
5.0E+05
0.0E+00
1880
1900
1920
1940
1960
1980
2000
2020
Year
Approximately 40% increase of
urban surfaces in Houston Metro
between 1978 and 2000 (current city
limit is ~1000 km2)
Theoretical Coordinate System Defining Upwind and Downwind
Regions Based on Mean Annual 700 hPa Steering Flow from 1979 to
1998 (following Shepherd et al. 2002)
Abcissa is
aligned
along the
230º (southsouthwest)
700 hPa
mean vector
The wind rose below
indicates that the
prevailing near-surface
flow is predominately
southeasterly (e.g. sea
breeze driven),
however, for steering
flow and upwinddownwind delineation,
the 700 hPa surface is
most critical.
UCR – Upwind Control Region
Orange ellipse has a 100 km horizontal diameter and 50 km vertical
diameter and is centered on 29.77,95.38. The westernmost boundary of the
UCR is 125 km from the orange ellipse and the easternmost boundary of
the UIR is 100 km from the orange ellipse
UIR- Urban Impacted Region
Diurnal Rainfall Pattern
Annual
Warm Season
**(blue gages have both urban and pre-urban data)
 This component of the study focused on the analysis
of rain gage records with the necessary temporal
resolution (hourly or less increments) for a preurban time period (1940-1958) and an urban time
period (1984-1999)
30
35
Urban (1984-1999)
Urban (1984-1999)
30
Pre-Urban (1940-1958)
Pre-Urban (1940-1958)
Percent of Rainfall
Percent of Rainfall
25
25
20
20
15
15
10
10
5
5
0
0
0
400
800
1200
1600
Four Hours Ending (CST)
2000
2400
Average annual diurnal rainfall
distributions at gage 4311 (UA) for
the urban (1984-1999) and preurban (1940-1958) time periods
0
400
800
1200
1600
Four Hours Ending (CST)
2000
2400
Average warm season diurnal
rainfall distribution at gage 4311
for the urban (1984-1999) and preurban (1940-1958) time periods
The peak fraction of daily rainfall is more pronounced for
the 12-16 and 16-20 4-hr time increments for the urban time
period compared to the pre-urban time period; The warm
season experiences a greater diurnal modification
25
30
Percent of Rainfall
20
25
Percent of Rainfall
15
10
Urban (1984-1999)
5
Pre-Urban (1940-1958)
20
15
10
Urban (1984-1999)
0
5
0
400
800
1200
1600
2000
Four Hours Ending (CST)
Pre-Urban (1940-1958)
2400
Average annual diurnal rainfall
distribution for the average of
UCR gages 1671, 5193, 569, and
9364
0
0
400
800
1200
1600
Four Hours Ending (CST)
2000
2400
Average warm season diurnal
rainfall distribution for the
average of UCR gages 9364,
1671, and 3430
The change in diurnal rainfall distribution is visibly less in
the UCR compared to the UA; The warm season has also
experiences a greater diurnal modification
Average warm season rainfall amounts
(mm) in each time increment
UA*
UCR*
19401958
19841999
%
Change
19401958
19841999
%
Change
0-4
26
15
-42
28
29
+4
4-8
44
28
-36
54
50
-7
8-12
53
47
-11
75
54
-28
12-16
62
94
+52
50
65
+30
16-20
54
79
+46
32
35
+9
20-24
24
22
-8
28
25
-11
Total
263
285
+8
267
258
-3
* UA is the average of 4311 and 4309; UIR gages had insufficient data for warm season
analysis; UCR is the average of 1671, 3430, 9364
Storm Event Characteristics
 This component of the study focused on the analysis of
rain gage records with the necessary temporal resolution
(hourly or less increments) for a pre-urban time period
(1940-1958) and an urban time period (1984-1999)
Storm Event Characteristics
Average Warm Season Maximum 1-hr Rainfall Intensity
Data Series
Comparison
Two Sample
Test
Mean Statistically
Different ( = 0.05)?
Pre-Urban
UAR > UCR
t Test
No
Wilcoxon
No
t Test
Yes
Wilcoxon
Yes
t Test
Yes
Wilcoxon
Yes
t Test
No
Wilcoxon
No
Post-Urban
UAR > UCR
UAR Post-Urban
> Pre-Urban
UCR Post-Urban
> Pre-Urban
Average maximum 1-hr rainfall intensity during the
warm season has increased by 16% in the UAR
compared to 4% in UCR
Storm Event Characteristics
Average Warm Season Number of Heavy Rainfall
Events (> 25 mm)
Data Series
Comparison
Two Sample
Test
Mean Statistically
Different ( = 0.05)?
Pre-Urban
UAR > UCR
t Test
No
Wilcoxon
No
t Test
Yes
Wilcoxon
Yes
t Test
Yes
Wilcoxon
Yes
t Test
No
Wilcoxon
No
Post-Urban
UAR > UCR
UAR Post-Urban
> Pre-Urban
UCR Post-Urban
> Pre-Urban
Average number of “heavy” rainstorms (> 25mm)
during the warm season increased by 35% in the UAR
compared to a 3% decrease in the UCR
Trend Analysis
The trend analysis used 10 rain gauges from the UA, and 20
each from the UIR and UCR. The gauges selected had the
longest record lengths and the highest data coverage for the
50-year study period (1950-2000)
Annual
Warm Season
UA
UIR
UCR
UA
UIR
UCR
Mean (mm)
1201
1210
993
326
289
232
St. Dev. (mm)
249
212
218
124
95
95
Cv
0.21
0.18
0.22
0.38
0.33
0.41
Skew
0.09
0.25
-0.01
0.82
0.40
0.92
Kurtosis
-0.64
-0.23
-0.45
-0.40
-0.70
0.71
 Average annual rainfall amount is greater in the UA
and UIR than the UCR at the 0.95 confidence level
 Average warm season rainfall amount is greater in the
UA than the UCR and UIR at the 0.95 confidence level
 There is no statistical difference between average
annual rainfall in UA and UIR at the 0.95 level
 Average warm season rainfall amount is greater in the
UIR than the UCR at the 0.95 confidence level
Average annual trends
1800
Rainfall (mm)
Linear: no trend exhibited (slope
not significantly different from 0)
for UCR; increasing trend for UA
and UIR at 0.95 level
Mann-Kendall: Annual rainfall is
significantly increasing with time
(90% confident) in each region.
For UA and UIR, results are
significant even for =0.05.
1400
1200
1000
Urban Area (UA)
800
mean = 1205 mm
600
400
1950
1960
1970
1980
1990
2000
Year
1800
1800
y = 4.5627x - 7801.4
R2 = 0.1019
1600
1600
y = 3.5935x - 6104.2
2
R = 0.0601
1400
Rainfall (mm)
1400
Rainfall (mm)
y = 5.4931x - 9647.3
2
R = 0.107
1600
1200
1000
1200
1000
800
800
Urban Impacted Region (UIR)
mean = 1210 mm
600
Upwind Control Region (UCR)
600
mean = 993 mm
400
1950
1960
1970
1980
Year
1990
2000
400
1950
1960
1970
1980
Year
1990
2000
350
Avg warm season trends
250
Rainfall (mm)
Linear: no trend exhibited (slope
not significantly different from 0) at
0.95 level
Mann-Kendall: there is no evidence
to conclude that the amount of
warm season rainfall is increasing
with time.
200
150
100
Urban Area (UA)
50
mean = 163 mm
0
1950
1960
1980
1990
2000
300
250
y = 0.4351x - 714.73
R2 = 0.0184
250
Rainfall (mm)
y = 0.2726x - 422.33
R2 = 0.0073
200
150
100
50
200
150
100
50
Urban Impacted Region (UIR)
Upwind Control Region (UCR)
mean = 116 mm
0
1950
1970
Year
300
Rainfall (mm)
y = 0.4657x - 756.62
R2 = 0.0124
300
1960
mean = 145 mm
0
1970
1980
Year
1990
2000
1950
1960
1970
1980
Year
1990
2000
Trend Analysis (cont’d)...
 The same battery of trend assessment
tests were conducted for a difference
statistic that represents the difference in
average rainfall amount in a given year or
warm season between the UA and UCR
(R UA-UCR), UIR and UCR (R UIR-UCR), and
the UA and UIR (R UA-UIR)
 Objective: Isolate the trend of differences
between the three regions
500
Annual
400
300
y = 0.0748x - 139.49
 R (mm)
200
y = 0.0382x - 66.821
100
y = 0.0366x - 72.672
0
1940
-100
-200
1950
1960
1970
1980
1990
2000
2010
UA-UCR (mm)
UIR-UCR (mm)
UA-UIR (mm)
-300
Year
Linear: increasing trend (slope > 0 at the 0.95 level) for UAUCR only
Mann-Kendall: no significant trends found down to the 0.90
confidence level for all combinations; UA-UIR and UA-UCR
differences increasing at =0.20
100
UA-UCR (mm)
Warm
Season
UIR-UCR (mm)
75
UA-UIR (mm)
 R (mm)
50
25
Linear (UA-UCR
(mm))
Linear (UIR-UCR
(mm))
Linear (UA-UIR
(mm))
y = 0.1287x - 222.86
y = 0.1083x - 194.93
y = 0.0204x - 27.929
0
1940
1950
1960
1970
1980
1990
2000
2010
-25
-50
Year
Linear: increasing trend (slope > 0 at the 0.95 level) for UAUCR only
Mann-Kendall: no significant trends found down to the 0.80
confidence level for all combinations
Conclusions
• Comparison of pre-urban and urban time
periods suggests the diurnal rainfall
distribution has been modified in urban areas
beyond that responsible from natural
background climate variability
• Urbanization in Houston may be responsible
for increased rainfall amounts during the midafternoon to late evening time periods in the
urban area
Conclusions
• For recent period: annual and warm season
diurnal rainfall patterns in the Houston UA
and UIR display greater late afternoon and
early evening rainfall amounts and
occurrences compared to the UCR
• This corroborates findings by Balling and
Brazel (1987) for Phoenix and Huff and Vogel
(1978) for St. Louis
Conclusions
 Statistical comparison of average storm event
characteristics from a pre-urban period and
an urban time period indicates:
1. Average maximum 1-hr rainfall intensity
during the warm season has increased
in the UAR, but not in the UCR
2. Average number of “heavy” rainstorms
(> 25mm) during the warm season has
increased in the UAR, but decreased in
the UCR
Conclusions
 Annual rainfall amounts have had a
strong increasing trend from 1950-2000
in the UA and UIR; and a weak trend in
the UCR
 Warm season rainfall amounts have
had very weak increasing trends from
1950-2000
Conclusions
 An increasing trend of R UA-UCR versus
time and population is observed for
annual and warm season rainfall in
Houston
 No trend is observed for R UIR-UCR and
R UA-UIR versus time and population
Acknowledgements
• This work has been supported by a
NASA/ASEE Summer Faculty Fellowship
(Burian), a NASA New Investigator Program
(NIP) Grant (Shepherd), and a NASA
Precipitation Measurement Mission award
(PMM-0022-0069) (Shepherd, Menglin, and
Burian)
Questions???
Steve Burian
burian@eng.utah.edu