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1
Estimating the Impact of Floods on House Prices: An Application to the 2005 Carlisle Flood
Paper presented at ERES 2009 Stockholm
Gwilym Pryce, Yu Chen, Danny MackayUniversity of Glasgow
Structure of Presentation
1. Introduction
2. Theoretical framework
3. Proposed econometric model
4. Background to 2005 Carlisle Flood
5. Data
6. Estimation
7. Future work
1. Introduction: the project
Downscaled Climate Change & Flood Risk Estimates
EWESEM Model
(Based on impact of
past floods)
Simulate Socio-Economic
Impacts for Case Study Area
Digimap terrain, SWERVE 2008
Stakeholder Engagement
(PP2)
+
Web Interface
(WISP)
From SWERVE
Introduction
• Aims of this paper: – To estimate the impacts of historical flood events
on house prices– To capture spatial spill-over effects of floods
• Why house prices?– If heterogeneities in the type of dwelling can be
controlled for, • variation in house price across space offers a way
of placing a monetary value on the variation in the desirability (and hence quality of life) of a location,
• and of the willingness to pay for avoiding flood risk.
– house price is potentially a powerful measure of the impact on wellbeing of extreme weather
2. Economic Theory
• Q1/ Why should house prices change at all in the event of a flood?– If markets are efficient, prices should already be
fully risk adjusted.– Areas with higher perceived flood risk will have
lower house prices, all else equal.– Yet, previous studies do indeed find:
• Temporary fall in house prices after a flood
• Followed by a gradual bounce back
• A1/ The most plausible explanation is market amnesia– Market prices drift away from the risk adjusted
level the longer the time lapse since last flood.– People actively cover up evidence of flood risk– Framing & herd behaviour (Zeckhauser 1996):
• tendency to underestimate risks that appear distant or global, or which others seem to accept without concern
• JARring Actions: Jeopardize Assets that are Remote (Zeckhauser 2006)
• Q2/ Why do house prices bounce back?• A2/ This is what you’d expect if the amnesia
hypothesis is valid– Flood event is a reminder of the true risk– The more frequent the reminder, the less prices
will diverge from the risk adjusted price• So prices will not fall so much, and the bounce back
effect will be correspondingly smaller.
– Crucially, house prices observed in the aftermath of the flood reveal the true risk adjusted house price.
• Whether floods are frequent or rare in an area, the price observed in the aftermath of a flood should be a good estimate of the risk adjusted price.
• This is important, because climate change will lead to more frequent flooding, and so prices in areas worst affected will eventually converge to their risk adjusted price as floods become more frequent.
• This allows us to estimate future house price impacts of flood risk.
3. Proposed econometric model
ln(pricei) = f(Si , Z, CBD, Green, Dep, year),where,
pricei = selling price of dwelling iSi exp(ӨDijHi)
= distance decay flood event variable captures the spill-over effect
Dij = distance from dwelling i to nearest flooded postcode unit j
Hi = elevationZ = vector of dwelling characteristicsCBD = distance to central business districtGreen = distance to woodland Dep = index of deprivation year = year dwelling sold
• Distance decay methods
• Explicit spatial econometrics models
4. Background to Carlisle 2005 Floods
• Located in northwest England, capital of Cumbria
• A long historical record of flooding. – Over 50 flood events occurred from 1800 to 1979, with
severe flooding every 11.4 years and major floods every 42.7 years
• January 2005: 15% of average annual rain fell in 36 hrs, once in 150 years
• Flood Defences were overwhelmed by the extreme flows. – 1,925 properties were flooded up to two metres. – 3 people died – Over 3000 people were made homeless for up to 12
months– Infrastructure was destroyed– An estimate of losses exceeded 450 million pounds
Source: EA 2005
5. Data
• Housing transaction data – House prices, property attributes– Nationwide building society 2006-07
• Location and accessibility measures– Elevation, distance to CBD, woodland. – Ordinance Survey
• Neighbourhood variable– Index of multiple deprivation
• Flood: – Flood outline overlayed with postcode boundaries in
GIS
• Distance between each postcode and its nearest flooded postcode
6. Estimation: functional form
• We incorporated distance to the nearest flooded postcode and height above sea level into the functional form of the flood variable:
Si = exp(ӨDijHi)
• A Maximum likelikood grid search procedure on the following model,
LnP = a0 + a1Bathroom + a2Bedroom + a3lnfloorsize + a4Centralheating + a5Newproperty + a6bungalow + a7lnCBD +a8lnwoodland +a9 imd + a10Si + a11year2007
0 10 20 30 40mean of ll
D_H_p09
D_H_p08
D_H_p07
D_H_p06
D_H_p055
D_H_p05
D_H_p045
D_H_p04
D_H_p035
D_H_p03
D_H_p025
D_H_p02
D_H_p015
D_H_p01
D_H_p005
D_H_p001
D_H_p0009
D_H_p0008
D_H_p0007
D_H_p0006
D_H_p0005
D_H_p0004
D_H_p0003
D_H_p0002
D_H_p0001
D_H_1p1
D_H_1p0
0 1 2 3 4mean of t_abs
D_H_p09
D_H_p08
D_H_p07
D_H_p06
D_H_p055
D_H_p05
D_H_p045
D_H_p04
D_H_p035
D_H_p03
D_H_p025
D_H_p02
D_H_p015
D_H_p01
D_H_p005
D_H_p001
D_H_p0009
D_H_p0008
D_H_p0007
D_H_p0006
D_H_p0005
D_H_p0004
D_H_p0003
D_H_p0002
D_H_p0001
D_H_1p1
D_H_1p0
Log likelihood values for different values of Ө
T-values (based on White’s Standard Errors) for different values of Ө
It reveals that the most appropriate value for Ө to be -0.005.
6. Estimation: spatial econometrics
• Spatial Auto-Regressive Model SAR: – y = ρWy + Xβ + e– Correction for house price in place i depending on the weighted
average of house prices nearby
• Spatial Error Model SEM:– y = Xβ + u; u = λWu + e– To adjust errors caused by omitted variables which vary
spatially
• General Spatial Model GSM:– y = ρWy + Xβ + u; u = λWu + e– Both a spatially lag variable and a spatially weighted error term
• Estimation methods:– Maximum Likelihood: typically used but with problems, e.g.
assuming normality– Generalised Moment Method as an alternative
A spatial weight matrix
• A square matrix measuring closeness in space
• Spatial contiguity matrix dij =1/0– where 1 denotes locations sharing the same boundary– only allow contiguous neighbours to affect each other
• K nearest neighbours: – where 1 denotes locations being one of the k nearest neighbours
• Defining a neighbour using a distance threshold
• Ways of calculating distance:– straight line distance– great circle distance– travel time– economic distance – trade costs, market access
• Row standardised
Selecting a spatial weight matrix
330 340 350 360 370 380 390
1
5
9
13
17
21
25
29
K n
eare
st n
eig
hb
ou
rs
Log-likelihood
Log-likelihood of SEM models
0 100 200 300 400 500 600 700
0
100
200
300
400
500
600
700
nz = 11981
Plot of W21 (783*783)
SEM model using a spatial weight matrix with 21 nearest neighbours has the highest log-likelihood.
Contiguity
List of models using spatial weight matrix with 21 nearest neighbours
Variable OLS SAR_ML SEM_ML GSM_ML SEM_GMM
constant 10.278587*** 10.262238*** 9.504726*** 9.380543*** 9.606181***
bathroom 0.107283*** 0.107987*** 0.065869*** 0.061220*** 0.068351***
bedroom 0.055401*** 0.056796*** 0.070982*** 0.072790*** 0.070008***
lnfloorsize 0.593950*** 0.586525*** 0.573139*** 0.569168*** 0.575033***
centralheating 0.098258** 0.099148** 0.124554*** 0.126222*** 0.123285***
newproperty 0.194032*** 0.197093*** 0.195552*** 0.194077*** 0.194811***
bungalow 0.250270*** 0.246316*** 0.249200*** 0.248236*** 0.249605***
lnCBD -0.083740*** -0.082627*** -0.032512** -0.025310* -0.039321*
lnwoodland -0.061884*** -0.060543*** -0.032726** -0.026478 -0.036334**
imd -0.014007*** -0.014018*** -0.011483*** -0.011129*** -0.011690***
flood -0.310461*** -0.307233*** -0.151275** -0.126336** -0.172821**
year2007 0.067920*** 0.068077*** 0.085071*** 0.087032*** 0.084149***
rho 0.002657 -0.016973
lamdba 0.883962*** 1.123361*** 0.796783***
Rbar-squared 0.724700 0.724900 0.782200 0.786200 0.772400
log-likelihood 311.112610 385.526510 384.383000
N 783 783 783 783 783
Normality of error term in SEM was rejected. SEM_GMM is more appropriate.
OLS: Ordinary Least Squares
SAR: Spatial Autoregressive Model
SEM: Spatial Error Model
GSM: General Spatial Model
ML: Maximum Likelihood
GMM: Generalised Moments Method
7. Future work
• Estimate the location value impact of the flood: PA t-1 - PA t
– Predict the CQP surface before the flood– Subtract the CQP surface after the flood
• Do the CIs overlap?
– How will size of impact vary across space?
• Simulate house price impact of a hypothetical flood event due to climate change
Thank you!