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HYDROLOGICAL PROCESSESHydrol. Process. 17, 24092422 (2003)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1250

Experimental study of water fluxes in a residential area:1. Rainfall, roof runoff and evaporation: the effect

of slope and aspect

R. Ragab,* J. Bromley, P. Rosier, J. D. Cooper and J. H. C. GashCentre for Ecology and Hydrology, Wallingford OX10 8BB, UK

Abstract:An experimental study of water fluxes from roofs in a residential area has quantified water fluxes from different typesof roof and identified the major controls on the process. Roofs with pitches of 0, 22 and 50 and orientations of 15

(from true north) (NNE) and 103 (ESE) were selected. A novel automatic system for monitoring has been developed.Noticeable differences in rainfall, runoff and evaporation were found for different roof slopes, aspects and heights.Depending on height, flat roofs collected 90 to 99% of rainfall recorded at ground level. Roofs with a 22 slope; facingsouth-south-west (i.e. facing the prevailing wind) captured most rain, whereas east-south-east facing roofs with slopesof 50 received the least. Depending on the roof slope, the average rainfall captured ranged from 62 to 93% of thatat ground level. For the same slope, the results indicated that from roofs orientated normal to the prevailing wind;(i) captured rainfall was higher, (ii) evaporation was higher and (iii) runoff was less than that from roofs having otheraspects. Monthly variations in the runoffrainfall ratio followed the rainfall distribution, being lowest in summer andhighest in winter. The highest mean ratio (0 91) was associated with the steeper roof slope; the lowest ratio (0 61)was for roofs facing the prevailing wind direction. For the same amount of rainfall, the runoff generated from asteeper roof was significantly higher than that generated by a moderate roof slope, but the lowest runoff was from

roofs facing the prevailing wind. The results have also shown that the amount of runoff collected (under UK climaticcondition) was sufficient to supply an average household in the studied area with the major part of its annual waterrequirements. The use of this water not only represents a financial gain for house owners but also will help protect theenvironment by reducing demand on water resources through the reduction of groundwater abstraction, constructionof new reservoirs, and a reduction of the flood risk as its in situ use is considered a preventive measure known as asource control. Copyright 2003 John Wiley & Sons, Ltd.

KEY WORDS Roof runoff; urban hydrology; rainfall; evaporation; slope; aspect; height; water recycle; residential areas

INTRODUCTION

Effective management of urban environments requires detailed knowledge and understanding of processes

and phenomena across a range of interlinked disciplines. Compared with rural areas, urban land use and

surface cover tends to be more diverse and hydrological processes more complex. Subsequently, processessuch as evapotranspiration, groundwater flow, soil moisture movement and contaminant transport are less

well understood.

Roof runoff, both as a drainage problem and as a potential resource, is essentially unquantified. Current

urban runoff models are based on very little roof runoff data and none contributed significantly to the

WASSP/WALLRUS/HydroWorks Models (Packman, 1992). The models assume (i) rainfall at roof level is

the same as at ground level (though roof rain is almost certainly less owing to wind turbulence and direction),

and (ii) roof runoff is the same as paved area runoff (although steeper roofs probably generate more and

faster runoff). Moreover, urban drainage models are usually verified using rain gauges sited on flat roofs (for

* Correspondence to: R. Ragab, Centre for Ecology and Hydrology, Wallingford OX10 8BB, UK. E-mail: [email protected]

Received 19 February 2001

Copyright 2003 John Wiley & Sons, Ltd. Accepted 17 October 2002

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2410 R. RAGABET AL.

security reasons). According to Hamilton (1954) and Panicucci (1986), rainfall measurements on a slope are

more accurate if the rainfall gauge is tilted by the same degree. Comparisons of roof with ground level data

are rarely made. Uncertainties also include the impact of height and exposure on storm rainfall. A study of

these would also provide information on the extent to which roof runoff can act as a domestic water source.

Measured data on the water budget of urban areas are scarce and carry considerable uncertainty in many

of the components of the budget (Owen, 1995; Van den Ven, 1990; Lerner, 1997; Grimmond and Oke, 1991;

Whitlowet al., 1992; Stephenson, 1994; Makin and Kidd, 1997). However, hydrological and hydrodynamic

models need to be calibrated using accurate measurements before they are used for designing urban systems.

Clearly, there is a need to collect real data that can reliably be used for water management purposes as

well as for the calibration of hydrological and hydrodynamic models for the design of urban systems (Owen,

1995). For the above-mentioned reasons the Centre for Ecology and Hydrology, Wallingford set up a pilot

project to identify and quantify water fluxes related to roof runoff, road runoff, evaporation and infiltration

for different surfaces.

The main objectives of Part 1 of this paper were to study the effect of slope and aspect on roof rainfall,roof runoff and evaporation, to identify the processes and quantify fluxes; Part 2 is focused on the effect of

different road surfaces on infiltration and road runoff to the River Thames.

Why roof runoff?

Rainfall runoff from roofs provides a potential alternative source of water for (non-potable) domestic use

(toilets, washing machine, car washing, garden irrigating, etc.). However, the volume of roof runoff available

as a water resource is currently not known. Quantifying this resource will contribute to the management of

urban areas. One of the long-term objectives of this project is to couple the results of physical monitoring and

modelling to a socio-economic component, with a view to exploring the policy implications in relation to the

Sustainable Cities agenda. Stored recycled water has many economic benefits, one of the more significant of

which might be to help prevent building subsidence during extended periods of drought, providing a significant

value to both house owners and insurers alike. In addition, at a macroscale, it is likely that water savingsmade through urban water runoff management, could have implications for overall urban water demand, and

thus reduce the need for new reservoir construction. Moreover it could lead to a reduction in groundwater

abstraction, which in some cases has a negative impact on river flows, wetlands, ecosystem and biodiversity

(the effect of abstraction at Compton, Berkshire is an example of such negative impact on the Pang River

flow). With the present increased rate of house building and potential climate change, water saving and

recycling could be an important policy issue in the future and could demonstrate the benefits of a change in

the way we view water in an urban environment. Some of these issues are discussed by Maksimovic (1996).

MATERIALS AND METHODS

Site

The study was conducted in a small housing estate in Crowmarsh Gifford near Wallingford, Oxfordshire,

UK. The estate was built in the late 1960s over an area of 005 km2. It comprises 121 houses, 61 with roofs

sloping at 50, and 60 with roofs sloping at 22; some houses have flat-roofed garages or extensions. Five

houses with the same roof material were selected on the estate; a sixth site was set up at the CEH offices, a

few hundred metres to the north. Details of house orientations, roof slopes, position of rain gauges and roof

areas are given in Table I. Thus, for example, house JB has an orientation of 15 (approximately NS), a

roof slope of 22; the rain gauge is located on the east-facing roof, which has a surface area of 34 27 m2 and

a horizontally projected area of 3177 m2. The horizontally projected area was obtained by multiplying the

actual (sloping area) by the cosine of the angle of the roof slope. The roofs at house HA and the CEH site

were flat. In addition, rainfall was recorded from the ground-level gauge (as described by Rodda et al., 1986)

Copyright 2003 John Wiley & Sons, Ltd. Hydrol. Process.17, 24092422 (2003)

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WATER FLUXES IN RESIDENTIAL AREA 1 2411

Table I. Slopes, orientations and rain gauge positions, and the surface area of the houses studied in Crowmarsh Gifford,

Wallingford, UK during 20002001

House JB GP AJ IT HA CEH

Slope () 22 22 22 50 0 0House orientation NS E W E W NS Flat FlatDegrees from true north 15 103 103 15Rain-gauge position East side North side South side East side

Surface area (m2) 3427 3745 5668 4361 2793 7373

Plan area (m2) 3177 3472 5255 2803 N/A N/A

North

Raingauge

AJ: Slope = 22 GP: Slope = 22

HA: Flat roof CEH: Flat roof

Prevailing wind direction

IT: Slope = 50 JB: Slope = 22

Figure 1. Layout of the roofs studied with respect to orientation, slope and prevailing wind direction

at the CEH meteorological site. A plan view of the five estate houses selected and the CEH site together with

the prevailing wind direction and the roof slopes are shown in Figure 1.

Measurements

The rain-gauge funnels were mounted on the roofs such that the mouth of the funnel was at the same

angle as the slope of the roof. The funnel is connected to a tipping bucket rain-gauge system situated

at ground level. A new automatic system for monitoring has been developed. Each house has a separate


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system. The system consists of an in line funnel rain gauge (contrary to the usual horizontal roof

rain gauges) positioned 1 m upslope from the gutter, a water butt with a 12 V bilge pump capable of

draining the water within 2 min, and a data logger system to record the rainfall and water level in the

butt every 2 min. The roof runoff system is shown in Figure 2, and a schematic representation given

in Figure 3. Wind direction data presented in Figure 4 have been obtained from the CEH, Wallingford

meteorological site. The runoff volume was calculated using the water depth in the butt and the cross-

sectional area. The volumetric runoff was converted into millimetres of water by dividing the runoff

volume by the horizontally projected area; the latter was also used to convert rainfall in millimetres into

a volume.

Measurements were made at 2-min intervals and later stored in a Microsoft Access database. Analysis was

made on a rainfall-event basis with the rainfall aggregated over the total duration of the event; calculation of

Figure 2. Photograph of the apparatus configuration


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I

Upper float switch -turns pump on

IInput from house roofvia down pipe

2000 l hr-112 volts dc operatedboat bilge pump outputs waterto gravel soak away

Lower float switch -turns pump off

Mounting pole forfloat switches andbilge pump

Pressure transducer tomonitor water level

Mounting pole forpressure transducer

House

12 voltbattery

Logger

Figure 3. Schematic diagram to show the system developed for emptying the butt and recording water level, rainfall and runoff

runoff for the same event was extended to 6 h after the cessation of rain to allow for any delayed drainage.

Both rainfall and runoff were also aggregated and analysed on a monthly basis.Evaporation is calculated as the difference between rainfall and runoff. This difference is assumed to

represent the amount of water soaked up by the roof surfaces and eventually released back to the atmosphere

by evaporation. Measurements were expressed as a volume in litres and as a depth in millimetres by dividing

the volume of water by the horizontally projected roof area.

RESULTS AND DISCUSSIONS

Total and cumulative rainfall, runoff and evaporation

Table II shows the annual measured rainfall and runoff as well as the calculated evaporation for each

site. The measured rainfall ranges from 851 mm at site AJ to 509 mm at IT. At house AJ the roof faces the

dominant wind direction and helps explain the high figure; the low rainfall at IT is partly due to the position of

the gauge with respect the prevailing wind (Figure 4) and the steep slope of the roof. During the same period,

the runoff ranged from a high of 624 mm at GP to a minimum of 456 mm at CEH; maximum evaporation of

286 mm was at CEH, the minimum of 75 mm at IT. The differences between sites are attributed to differences

in slope and aspect and are discussed further in the following sections. An example of the time-series results

is graphically illustrated in Figure 5 for runoff.

The effect of aspect on rainfall, runoff and evaporation

To investigate the effect of aspect on rainfall interception and runoff, three houses of the same slope (22)

but with different aspect were selected. Table III shows the cumulative rainfall, runoff and evaporation for

houses AJ (south-facing), JB (east-facing) and GP (north-facing). The results show that the south-facing


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2414 R. RAGABET AL.

20

40

60

0

20

40

60

020

40

60

80

100

120

140

160180

200

220

240

260

280

300

320

340

Degrees

Frequency

Wind Direction: Wallingford Jun 2000 to Jun 2001

Figure 4. Wind direction rose for Wallingford site

Table II. Measured annual runoff, rainfall and evaporation over houses studied in Crowmarsh Gifford, Wallingford, UK 29June 2000 to 30 June 2001

JB GP AJ IT HA CEH Meteorologicalsite

Rainfall (l) 256342 244544 446824 142676 226125 546888 N/ARunoff (l) 193177 216588 297733 143924 159393 336263 N/AEvaporation (l) 63320 27956 149091 21023 66732 210940 N/ARainfall (mm) 8069 7043 8510 5091 8096 7417 8181Runoff (mm) 6081 6238 5665 5135 5707 4561 N/AEvaporation (mm) 1993 957 2845 2861 750 2389 5406

roof (AJ) has the highest cumulative rainfall and evaporation, but the lowest recorded runoff (Figure 6).

In contrast the house with the north-facing roof (GP) has the lowest rainfall and evaporation but has the

highest runoff.

The effect of slope on rainfall, runoff and evaporation

To investigate the effects of slope, two houses with the same aspect but different slopes were compared.

Table IV shows that the shallower sloping roof (22) collected more rainfall, generated more runoff (Figure 7)

and has higher evaporation than the steeper roof (50).


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01/06/00 01/10/00 01/02/01 01/06/01 01/10/01

Cumulativerunoff(mm)

0

100

200

300

400

500

600

700JBGPAJTYITHA

Figure 5. Cumulative runoff collected from the houses studied

Table III. Effect of aspect for houses with 22 slope

Measurement (mm) South-facing (AJ)

East-facing (JB)

North-facing (GP)

Rainfall 851 807 704Runoff 567 608 624Evaporation 285 199 96

Italics minimum

22slope

01/06/00 01/10/00 01/02/01 01/06/01 01/10/01

Cumulativerunoff(mm)

0

100

200

300

400

500

600

700

JB, roof orientation: South - East

GP, roof orientation: North - East

AJ, roof orientation: South - West

Figure 6. Effect of aspect on runoff collected from houses with the same roof slope of 22


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2416 R. RAGABET AL.

Table IV. Effects of slope for east-facing and flat roofs

Measurement(mm)

22

Slope (JB)50

Slope (IT)Flat

(HA)Flat

(CEH)Meteorological

site

Rainfall 807 509 810 742 818Runoff 608 514 571 456Evaporation 199 75 239 286

01/06/00 01/10/00 01/02/01 01/06/01 01/10/01

Cumu

lativeruno

ff(mm

)

0

100

200

300

400

500

600

700JB, South - EastIT, South - East 22

50

Figure 7. Effect of slope on the runoff collected from houses with the same aspect but with different roof slopes of 22 and 50

01/06/00 01/10/00 01/02/01 01/06/01 01/10/01

Cumu

lativera

infall(mm

)

0

200

400

600

800

1000TYHAMet site

Figure 8. Comparison between rainfall collected at two different flat surfaces with different heights and at ground level at CEH, Wallingford

The two sites with near zero slopes (flat roofs) were also compared and included in Table IV, along with

the rainfall recorded from the ground level gauge at the CEH meteorological site. The rainfall measured at

the meteorological site (818 mm) was slightly higher than HA (810 mm) and both were slightly higher than

CEH (742 mm), as shown in Figure 8. The runoff from the flat surface HA was greater than the CEH site

because evaporation was higher from the CEH site.


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Rainfall measurements at different heights

The results in Table IV show that the 818 mm of rainfall recorded at ground level at the CEH meteorological

site was higher than both the flat roof sites. Rainfall at the CEH flat roof site, approximately 4 m above ground

level, is 9% lower; but at the HA site, which is 25 m high, the difference reduces to only 1%. The implication

is that recorded rainfall tends to reduce at increasing height.

Seasonal variations in rainfall, runoff and runoff/rainfall ratio

Seasonal variation in runoff with respect to rainfall was investigated on a monthly basis. Figure 9 shows the

monthly runoff from the six sites together with the monthly rainfall (measured at ground level). Generally, the

runoff follows the rainfall distribution, being lowest in the summer then rising towards a peak in the winter

from October to November when it falls back to the summer levels. The ratio of runoff to rainfall measured

at each site on a monthly basis reveals the same pattern, with higher ratios in the winter falling to a minimum

in summer (Figure 10).The highest runoffrainfall ratios are at site IT followed by GP then JB, with the remaining sites plotting

closely below. The average runoff over the year, as given in Table V, ranges from 58% to 91% of rainfall.

Site IT recorded the highest and CEH the lowest. For houses with the same 22 slope the mean ratio for site

GP (86%) was higher than JB (71%) and AJ (61%). These variations reflect the effect of the aspect on the

runoffrainfall relationship. For flat roofs the CEH mean ratio was 58% and for HA it was 67%.

Jun-0

0

Ju

l-00

Aug-0

0

Sep

t-00

Oc

t-00

Nov-0

0

Dec-0

0

Jan-0

1

Feb-0

1

Ma

r-01

Ap

r-01

May-0

1

Jun-0

1

Ju

l-01

Rainfall(mm)

0

50

100

150

200

250

300

Runoff(mm)

0

20

40

60

80

100

120 RainfallJBGPAJTY

ITHA

Figure 9. Seasonal and monthly variation of rainfall and runoff collected from the houses studied

Table V. Monthly runoff/rainfall ratios 29 June 2000 to 20 July 2001

JB GP AJ IT HA CEH

Maximum 0847 1036 0861 1214 0816 0710Minimum 0457 0705 0383 0494 0482 0456Mean 0711 0856 0610 0905 0667 0581


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2418 R. RAGABET AL.

Rainfall(mm)

0

50

100

150

200

250

300

Ratio

ofRunoff/Rainfall

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

RainfallJBGPAJTYITHA

Jun-0

0

Ju

l00

Aug-0

0

Sep

t00

Oc

t-00

Nov

00

Dec-0

0

Jan

01

Fe

b-0

1

Mar

01

Apr-

01

May

01

Jun-0

1

Ju

l01

Figure 10. Seasonal and monthly variation of rainfall and runoff/rainfall ratio of the houses studied

The highest monthly maximum runoffrainfall ratio was recorded at site IT (12). The remaining houses

were either close to 1 such as GP, or lower. The lowest minimum monthly ratio was recorded at site AJ(038) and the highest minimum at GP (071).

Values above 1, observed at sites IT (50) and GP (22), were in most cases associated with rainfall events

greater than 4 mm and on a few occasions with minor events of 05 mm. This apparent anomaly could be

attributed to a combination of a number of factors:

1. the effect of air turbulence giving rise to an underestimate of rainfall;

2. splash out (water splashes out of the rainfall funnel) effects (Maksimovic, 1996);

3. the inclination of the rainfall;

4. insensitivity of the rain gauge for small eventswe have a tip only every 05 mm;

5. at site IT the location of the gauge, the steep slope and the angle of the wind to the roof has a large

impactthe rain gauge may be in a mini-rainshadow.

In this paper all calculations assume rain falls vertically and the receiving area is the projected flat area with

a zero slope. In the event of rain falling at an angle less than or greater than 90 , the receiving area will be

different from the assumed horizontal area and that could contribute to inaccuracy of rainfall measurements

and hence the runoff/rainfall ratio (Panicucci, 1986).

The results obtained by Hollis and Ovenden (1988) showed that runoff from roofs averaged 569% but

when events above 5 mm only were considered the average went up to 904%. Their results also indicated

that pitched tile roofs had a higher runoff percentage at 54% for all storms and 81% for storms above 5 mm.

Similar values higher than one (12 or 120%) were reported by Davies and Hollis (1981) for events greater

than 41 mm and Hollis and Ovenden (1988) reported monthly ratios of 126% and 155% for events greater

than 5 mm.


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The threshold rainfall value for runoff generation

To derive the value of rainfall above which runoff is generated, a correlation between rainfall and runoff

was conducted. Figure 11 indicates the strong linearity between the two variables. However, the relationship

does not start at the origin (0,0); for a given value of runoff on the yaxis, one can see that the rainfall amount

required to produce such a runoff is less for a house with 50 slope than for one with a 22 slope (given the

same aspect) and is less for a northern and eastern facing roofs than for a south-facing roof for the same roof

slope. The zero slope roofs are similar to those facing east. For the same amount of rainfall, AJ generates

75% of the runoff from GP, and JB produces 97% of GP. Having the same aspect, but different slope, JB

generates 70% of the runoff from IT. The minimum rainfall required to generate runoff is 07 mm for JB and

HA, 06 mm for IT and AJ, 05 mm for CEH and 04 mm for GP. These are taken as mean values over the

whole period.

GENERAL DISCUSSION

The results obtained so far show the average annual rainfall captured by houses with roof slopes of 22 is

925% of the CEH meteorological site (ground level), whereas that intercepted by houses with a 50 slope is

only 622%. The total runoff generated from houses with a 50 roof slope is only 86% of those with a roof

slope of 22.

The results also illustrate that the rainfall amount received over the different roofs having the same 22

slope is higher for roofs that face south into the prevailing wind, than roofs that face east and north. Rainfall

received by south-facing roofs is between 5 and 17% higher. Amounts are also higher for 22 than 50 slopes.

Runoff, on the other hand, shows the opposite relationship; it is the south-facing roof that generates the

least amount of runoff given the same 22 slope. The south-facing roof generates between 6 and 9% less. For

Rainfall (mm)

0 5 10 15 20 25 30 35

Runoff(mm)

0

10

20

30

40

JB, y = 0.8863x - 0.5930, r2 =0.9795, South - East

GP, y =0.9808x - 0.3535, r2 =0.9811, North - East

AJ, y =0.7560x - 0.4725, r2 =0.9816, South - West

TY, y =0.6781x - 0.3064, r2 =0.9589

IT, y =1.2468x - 0.7057, r2 =0.9389, South - East

HA, y =0.8186x - 0.5359, r2 =0.9748

50

22

22

220

0

Figure 11. The rainfall/runoff regression slopes for the houses studied


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2420 R. RAGABET AL.

Table VI. Evaporation/rainfall ratio

Site Roof aspect Roof slope

Evaporation/rainfall ratio

CEH Flat: 4 m high 0 038AJ South-facing 22 033HA Flat: 25 m high 0 029JB East-facing 22 025IT East-facing 50 014GP North-facing 22 014

the same aspect, rainfall, runoff and evaporation were higher for houses with a lower slope. For example, IT

(east-facing; 50) received 46% less rainfall than site JB (east-facing; 22), 16% less run-off and lost 62%

less in evaporation. Evaporation here is calculated as a difference between rainfall and runoff. Evaporationlosses as a ratio of rainfall are given in Table VI. For the same slope 22, the house with the south-facing roof

(the same direction as the prevailing wind) had more losses (334%) by evaporation when compared with the

north-east or south-east facing roofs.

The high ratio recorded at the CEH flat roof site is attributed firstly to the ponding of water, which is then

able to evaporate, and secondly to the nature of its black rubber surface that absorbs and stores heat and

subsequently enhances evaporation. The nature of this surface could explain why the ratio is much higher

than the 029 figure recorded for the HA flat roof site, which is covered in bitumen with felt and chippings.

For the same aspect, but different slopes, the ratio at IT was 10% lower than site JB.

The ratio between runoff and rainfall follows the runoff amount; north and east-facing roofs and the roof

with a steeper slope have higher runoff ratios than that facing south. The lowest ratio was 0 58 for the CEH

site. For the same slope, the ratio was lowest for the south-facing site (AJ, 0 61), but higher for the north-

facing roof (GP, 0

86). For the same amount of rainfall, the runoff generated from the 50

roof represented143% of that generated by 22 roofs. For the same slope the runoff generated from north and east-facing

roofs for the same amount of rainfall represented 133% of that generated by south-facing roofs.

Usefulness of roof runoff water

According to the Environment Agency, UK (2001), the average UK outdoor water use, which includes

garden watering and car washing accounts for only 6% of annual domestic water consumption. However, on

hot dry summer evenings when supplies are most stressed, 50% or more of the water supply may be used

for garden watering. Using rainwater for garden watering, toilet flushing and washing machines can save

up to 50% of household water use. Commercial rainwater recycling systems are currently being installed in

Germany at a rate of 50 000 year1. There are two main systems; one pumps water direct to the appliance and

the other pumps rainwater to a header tank in the loft that feeds into the appliances via pipes. Currently, there

are no UK regulations relating to the required quality for WC and washing machine use. However, studiesin Germany concluded that correctly collected rainwater is suitable for such use without disinfection. Some

countries or regions within countries do require that new buildings must include a rainwater system. An added

value of rainwater is that it is soft and thus ideal for clothes washing (after passing through a filter) and

toilet flushing. Washing machines fed with rainwater have an extended life compared with those fed with hard

water. Typically urinals account of about 20% of office water use, although this figure can vary a good deal.

The water supply (water fittings) regulations of 1999 require urinals to use no more than 7 5 l per bowl per

hour. Monitoring at a high school showed urinals to be responsible for over 40% of total water consumption.

It is also known that typical household WC flushing uses 50 l day1 (18 250 l year1), accounting for 30% to

40% of household water use. The use of roof rainwater for WC flushing using a collection system has been

discussed by Fewkes (1999).


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Table II shows the volume of runoff in litres collected over almost 1 year from half the roof surface of

all houses apart from the flat roof garage of HA and the CEH site. The annual total volume collected was

more than 18 250 l, with HA and IT being a little less. However the HA roof is for a small garage. If the

total area is considered, all the houses studied should be easily able to meet the 18 250 l value required for

WC flushing, as well as washing machines, outdoor garden watering and car washing. More efficient systems

such as water saving WCs, water use efficient washing machines, growing more drought tolerant plants in the

garden and the use of a trickle system will all help make a good and efficient use of the rainwater. However,

a simple butt is unlikely to be sufficient and a proper commercial system with a large tank will be needed to

make full use of rainfall throughout the year.

CONCLUSIONS

The present study has quantified the impact of aspect, slope and height of roofs on rainfall capture, runoff

and evaporation in an urban environment. The following general conclusions can be made.

1. Rainfall received by sloping roofs is less than that measured at ground level; the steeper the slope the less

rainfall is received.

2. Lower slopes experience higher rainfall, runoff and evaporation given the same aspect.

3. Roofs facing into the prevailing wind (south-facing) receive higher rainfall but generate less runoff.

4. South-facing roofs experience more evaporation than those facing east and north.

5. Monthly rainfallrunoff ratios follow the same distribution as rainfall, being highest in winter and lowest

in summer. North and east-facing roofs have higher ratios (061) than south-facing slopes (086).

6. Runoff from a 50 slope is 40% higher than from a 22 slope; runoff from north and east-facing roofs are

30% higher than south-facing roofs, given the same slope.

7. The amount of runoff that can be collected could vary under different climatic conditions. However, under

the UK condition especially where the study was carried out, the amount collected was sufficient to supplyan average household with its annual indoor and outdoor water requirements (i.e. WC flushing, urinals (for

schools, organizations, etc.), washing machines, car washing and watering gardens). The use of this water

not only represents a financial gain for house owners but also will help protect our environment through

reduced demand on water resources (i.e. over abstraction of groundwater) and the need for new or large

supply reservoirs as well as reducing the flood risk as its in situ use is considered a preventive measure

known as a source control.

ACKNOWLEDGEMENTS

We are grateful to the director of Centre for Ecology and Hydrology (CEH), Wallingford, Professor Jim

Wallace for supporting this project. The help of Martin Lees, Terry Marsh, John Packman and Graham Leeks

of CEH, Wallingford is sincerely appreciated. Thanks are also due to the Workshop staff for their help indeveloping the system and to Ivor Standbridge for system installation. The authors are very grateful to the

homeowners who are taking part in this project. Their support and co-operation was essential to carry out

this study. Special thanks to Mr Geoff Pearce, Mr Illtyd Thomas, Mr Nigel Aplin, Ms Helen Aplin and Dr

Andrew Johnson for offering their homes to be part of this project.

REFERENCES

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