Eagleson (1967) and Fogel and Duckstein (1969) in terms of mathematical equations. Hydrologists have also devised numerous techniques to obtain estimates ...
Some aspects of the effects of urban development on thunderstorms in the southeast coastal plain of Georgia, USA
A. Unal Sorman
Abstract. This research is intended to show the effect of a newly developed urban area on heavy rainfall distribution as well as to ascertain to what extent the urbanization of part of an experimental basin effects the temporal and spatial distribution of raincells. The study was carried out from the analysis of storm data collected during the summers of 1968-1970. The Little River Experimental Watershed located near Tifton, Georgia, is monitored by a dense network of raingauges covering an area of 650 km . The results obtained from graphical plots indicated that summer thunderstorms with a precipitation depth of 25 mm or more were initiated earlier on an urban area compared to the surrounding rural area. The frequency of storm occurrences from the year of urbanization and the areal depth of rainfall on the urban area increased with increase of storm depth. Finally, the transformation factor, the mean correlation coefficient, converting point rainfall into areal rainfall was determined in terms of storm area and characteristic distance parameter. The ratio d^/dQ increased with an increase of storm centre depths from 5 mm to 50 mm or more on the urban area, but it remained constant over the nonurban area. The results from the model study confirm the findings of similar studies carried out recently in this field of research.
Quelques aspects des effets du dévelopement urbain sur les averses orageuses dans les plaines côtières du sud-est de Georgia, aux Etats-Unis Résumé. On essaie de montrer l'influence d'une région récemment urbanisée sur la répartition spatiale des averses orageuses, et de vérifier les effets de l'urbanisation dans un bassin versant experimental sur la répartition temporelle et spatiale des cellules pluviales. On a utilisé les résultats d'une analyse des données pluviométriques des averses d'été pour la période 1968-1970. Le bassin versant expérimental de 'La Petite Rivière' près de Tifton, Georgia, était contrôlé par un réseau dense de pluviographes sur une surface de 650 km 2 . Les résultats montrent que sur la région urbanisée, les averses d'une hauteur de 25 mm ou plus se produisaient plus tôt qu'en région rurale environnante. La fréquence des averses et la hauteur de pluie moyenne sur la région urbanisée augmentent avec la hauteur de l'averse. Un facteur de transformation, appelé coefficient de correlation moyen, et permettant de passer d'une valeur ponctuelle à la hauteur de pluie moyenne sur le bassin, a été déterminé en fonction de la superficie de l'averse et d'un paramètre de distance caractéristique. Le rapport d^/dQ augmente avec la hauteur de pluie au centre de l'averse, de 6 mm à 50 mm ou plus, sur des régions urbaines, tandis qu'il reste constant sur des régions nonurbaines. Les résultats obtenus par le modèle utilisé dans cette étude sont en accord avec ceux des études des autres chercheurs effectuées récemment dans ce domaine.
INTRODUCTION Previous studies have indicated the importance of the effects of urbanization on the hydrological regime especially on streamflow. Only a small number of studies have been concerned with the effects of urban or suburban development on rainfall, particularly on heavy rainfall. This paper is intended to show the effect of a newly developed urban area on heavy rainfall distribution in the southeast coastal plain of Georgia, USA. The study was 3
A. Unal Sorman Legend
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FIGURE 1. Georgia.
J)
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TIFTON
/f>54
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Raingauge network on the Little River Experimental Watershed at Tifton,
carried out using storm data from the summers 1968—1970 made available by the ARS, US Department of Agriculture (Sorman and Wallace, 1972). The Little River Experimental Watershed located near Tifton, Georgia is monitored by a dense network of 55 digital type raingauges covering an area of 650 km2 (250 mi2). Figure 1 shows the basin and the location of the raingauges. The gauges are spaced at 2.5 km (1.5 mi) intervals in the upper part of the basin and in the urban area covering 20 km2. The remaining gauges in the rural area are spaced at 5 km centres. OBJECTIVES The main objective of this study is to show the effect of the newly developed urban area, Tifton, on heavy rainfall distributions as well as to ascertain to what extent the urbanization initiated on the southeast portion of the experimental basin affects raincells and their time—space distribution. This is so that urban related rain effects can be identified in order to provide valuable information for urban design, forecasting, hydrological urban design, planned weather modification and air pollution. The new raingauges (nos. 53—58) were installed in 1969 on the initiated urban area for the better understanding of the heavy rainfall distribution as pointed out by Berg (1968) 'A rural concept of soil conservation simply no longer is sufficient in a society that has become increasingly urban, a society in which the interests of the users of resources have become equal to those of the owners of resources. The demands come from the urban society . . . The quantity of resources is becoming more important, and the
Effects of urban development on thunderstorms
5
location of resource problems is shifting.' The results obtained from such studies provide valuable information for estimating precipitation patterns on similar areas as well as determining the areal mean precipitation depth by the use of storm centre depth and characteristic parameters (correlation distance, correlation coefficient etc.) defining the spatial distribution of storms.
METHODS OF ANALYSIS AND EVALUATION OF STORM RAINCELL DATA The study by Byers and Braham (1949) lead to the development of our knowledge on the characteristics of summer rainfalls by the analysis of surface cell patterns. The raincells are the smallest precipitation areas characterized by closed elliptical isohyets on the ground surface and have high temporal and spatial intensity gradients. The evidence for and the degree of the urban effect on raincells have been revealed recently by Huff (1973), Huff and Changnon (1973), Changnon and Semonin (1975), through comparisons of urban values with rural values. In this study, the urban effected raincells and their characteristics are compared with similar cells which developed in or passed through the rural portion of the basin. The raincell data have provided a substantial amount of information that will be useful in weather modification projects and urban storm design. In order to investigate the possible affects of urbanization on rainstorms, the areal distribution of total storm isohyets, here called surface raincells, is studied. The main advantage of this type of technique is that the total storm cells provide several cell parameters which are related to the physical processes involved rather than the integrated areal value. But on the other hand it requires considerable effort and power to reduce the data to a form which will be handy to use for analysis. The storm raincell patterns observed over urban and non-urban areas on the same days are presented in Fig. 2 for the 21 storm events during which sampled data were obtained and analysed for both urban and rural regions. There were more rural raincells than there were urban raincells because of more years of rural data and a larger rural study area over which more than one cell sample may be collected for analysis. A total of 52 storm raincells for non-urban and 21 cells for urban regions have been analysed during the summers 1968—1970. Table 1 summarizes the sampled data for the period of record and the total number of events and the number which are studied in detail. The values in Table 1 indicate that nine cells in 1969 and 12 cells in 1970 occurred in or passed over the urban area and they were all studied. Over the rural region, 14 cells selected from 1968, 24 from 1969 and 14 from 1970 were studied (52 in total) although the total number of cells which occurred or passed in the 66 events over the rural region was 119. Analyses were made between numerous raincell parameters; (1) average rainfall depth dA, (2) storm centre depth '— i
Effects of urban development on thunderstorms TABLE 1.
Non-urban area
Urban area Cells studied
Period of record
No. of storms
Total no. cells
Cells studied
9
1968 1969
12
12
1970
56 22 13 7 21
21
21
Total
30 10 9 5 12 66
14 15 9 5 9 52
No of storms
7
Distribution of rain events and raincells on urban and non-urban areas
Total no. cells
No data available 9
TABLE 2. Spatial and temporal cell characteristics on urban and non-urban areas for selected storms
Date 20/6/69 2/7/69 10/7/69 12/7/69 15/7/69 20/7/69 22/7/69 27/7/69 28/7/69 30/7/69 31/7/69 4/8/69
10/8/69 13/8/69 15/8/69 19/8/69 22/8/69 23/8/69 30/8/69 3/7/70 8/7/70
Storm Initiation centre raingauge time [type] [h, min] NU NU U NU NU NU NU NU NU U NU U NU U NU NU NU U NU NU NU NU U NU NU U NU NU U NU U NU NU U NU NU NU NU U
u
20.35 20.30 20.40 3.40 4.15 15.05 15.45 12.30 12.15 12.50 15.45 14.30 18.45 17.45 11.45 15.35 13.10 12.10 22.55 23.15 20.45 20.15 18.40 0.35 1.30 9.40 17.50 18.20 21.20 16.45 18.30 16.40 15.15 15.55 10.25 15.45 1.55 22.35 18.50
Duration dA [mm] [min]
Volume [Xl06m3]
Area [km 2 ]
_
—
__
1.78 2.33 1.17 1.50 3.05 2.91 3.80 1.80 0.68 1.02 1.56 1.18 2.58 6.38 2.97 4.75 2.68
185.5 88.8 178 205 206 194.7 219 107.5 84 168.4 171 161.4 145 228 213.5 151.3 71.2
3.00 0.84 1.15 2.53 3.50
244 97.4 149 126 230
7.06 6.20 3.70 4.96 11.30 1.07 3.02 5.93 4.25 5.7 1.32 4.61 7.05 6.98 0.106
243.5 417 261 207 446 111 200 108.8 80.3 215 147.6 453 248.8 131 26.9
d0 [in.]
dx [in.]
0.9 0.5 1.4 0.6 0.6 1.8 1.5 1.1 1.0 0.6 0.3 0.8 0.4 1.2 2.0 0.8 1.8 2.0 0.3 0.9 0.6 0.4 1.3 0.8
0.4 0.3 0.8 0.1 0.1 0.2 0.1 0.4 0.5 0.2 0.1 0.2 0.2 0.5 0.8 0.3 0.7 1.0
9.6 26.2 6.6 7.3 14.7 14.96 17.3 16.7 8.0 6.06 9.13 7.3 17.7 27.9 13.9 31.4 37.6
0.2 0.2 0.2 0.5 0.5
12.3 8.6 7.7 20.1 15.2
1.9 1.1 1.0 1.7 1.6 0.8 1.0 3.4 3.1 1.5 0.6 0.8 2.2 3.7 0.2
0.5 0.4 0.2 0.6 0.5 0.1 0.3 1.5 1.7 0.4 0.2 0.2 0.7 1.1 0.1
29.0 14.9 14.2 24.0 25.3 9.6 15.1 54.5 52.9 26.5 8.96 10.2 28.7 53 4
0.8/100 100 120 30 30 50 50 75 0.8/60 0.7/50 20 70 0.4/160 1.2/90 110 0.7/30 90 110 50 40 100 120 190 120 90 130 140 30 1.0/160 1.1/110 150 230 105
8
A. Unal Sorman TABLE 2 continued
Date 11/7/70 13/7/70 16/7/70 19/7/70 23/7/70 26/7/70 30/7/70 6/8/70 7/8/70 11/8/70 12/8/70 16/8/70 17/8/70 20/8/70 25/8/70
Storm centre Initiation raingauge time [type) [h, min] NU U NU NU NU NU NU NU NU U NU NU U U NU U NU U NU U U NU
u
NU+U NU
19.35 19.35 13.05 16.35 17.00 17.00 16.10 18.40 21.35 19.50 14.25 15.05 14.25 15.05 18.50 19.30 18.20 18.05 19.30 16.05 17.40 18.35 20.55 15.15 4.10
d0 dx d^ Duration [in.] [in.] [mm] [min]
Volume [Xl06m3|
Area [km 2 ]
0.5 0.4 1.1 1.0 1.0 0.9 1.0 2.3 1.6 1.3 3.3 2.4 3.1 1.2 2.0 1.7 1.3 1.8 0.8 0.9 0.6 0.6 0.7 3.3 1.2
1.43 0.97 1.65 1.40 4.37 2.46 2.96
229 179 142 100 274 207 269
2.90 2.0
163 130
6.00 3.35 0.716
137. 80 52
0.1 0.1 0.2 0.2 0.4 0.1 0.1
6.2 5.4 11.6 13.96 16.0 11.9 11.0
0.2 0.1
17.7 15.4
90 2.2/60 110 200
1.4 0.8 0.1 0.7 0.8 0.5 0.8
43.6 41.9 13.8
240 3.0/80 1.1/40
28.6 24.3 31.0 17.4 12.6 6.9 8.0 10.0 31.4 8.7
1.6/90 1.1/40 80 0.4/40 0.5/40 40 0.4/50 140 50 90
0.4 0.1 0.2 0.3 0.2 0.1
30 30 0.8/40 100
1.98 2.91 1.68 4.45 1.86 0.62 1.19 0.51 20.2 1.78
69. 119. 53. 256 147 90 149 46. 644 205
storms for urban and rural storms were plotted against frequency and the results are presented in Fig. 3. Table 3 summarizes the mean and maximum values of these parameters. DETERMINATION OF MEAN AREAL RAINFALL OF EVENTS IN GEORGIA, USA The uniformity of rainfall over an area is dependent upon the type of storm, the size and nature of the area under consideration, the storm duration and/or the total rainfall depths. The areal mean of an event has potential uses in hydrology, namely in runoff and yield studies, and in forecasting local water supplies. The spatial variability of rainfall has been derived for convective storms by Woolhiser and Schawalen (1959), Eagleson (1967) and Fogel and Duckstein (1969) in terms of mathematical equations. Hydrologists have also devised numerous techniques to obtain estimates of the mean of the rain process over a given area. Since rainfall is a random function in time and space, total rainfall depth is a random function in space. The exponential correlation function decaying as a function of distance is expressed by rid) = e ~ M where d represents the distance between any two points and h is the decaying constant called a characteristic correlation distance parameter. Recently, Rodriguez-Iturbe and Mejia (1974 a,b), Kagan (1966) and WMO (1972) used mean square errors as accuracy criteria in obtaining the areal mean precipitation and designing network densities, using random and stratified random sampling techniques. A more complete review of the literature in this field can be found in Bras and Rodriguez-Iturbe (1976a,b).
Effects of urban development on thunderstorms
t(d)
0.0
2.0 2.5 3.0 3.5(inch) 0.5 (12.7) (25.4) (38.1 I (50.8) (76.2) (mm) STORM CENTER DEPTH ( d . )
"0
50
100
150
200
250
STORM CELL AREA
300
350
(km2)
(A)
(c)
6 8 10 WATER VOLUME ( V w )
FIGURE 3.
l2X!06m3
20
60
100
140
180
220
Storm raincell characteristics.
Problem formulation A stochastic event which is continuous in space must be estimated by discrete observations derived from N sample points. The criteria function measuring the performance of the network with N raingauges can be characterized by the mean rainfall variance a% (Rodriguez-Iturbe and Mejia, 1974a, b) either in terms of point rainfall variance 0^ and the mean value of the correlation coefficient between two randomly chosen points [T(xl - x~2)lA] or by mean random error variance plus spatial variation in the precipitation field (Kagan, 1966; WMO, 1972). If one is interested in the correction or transforming factor relating mean rainfall depths over the area, then one should fit the correlation structure function by preserving the correlation coefficient at the characteristic correlation distance for a specified type of storm and number of stations. In order to determine the range of variation in the parameters of the spatial
10
A. Unal Sorman TABLE 3. Comparison of mean and maximum raincell parameters for urban effect and non-urban effect Mean
Maximum
Parameters
NU
U
NU
U
d/i [mm] A [km 2 ] Kw[106m3] d0 [mm]
18.3 198 3.24 30.7
21.7 103.6 2.28 36.0
52.2 644 9.66 86.4
53 243 7.05 94.0
TABLE 4.
Spatial characteristics of raincells for convective storms
d0 (storm centre depth in mm)
5-20
20-35
35-50
Non-urban A (storm area in km 2 ) h (correlation parameter in 1/km) Ah2 (dimensionless number)
164 0.127 2.64
168 0.125 2.63
208 0.113 2.66
Urban A h Ah2
100 0.186 3.50
120 0.148 2.60
140 0.088 1.08
correlation function for urban and non-urban areas of the little River Experimental Watershed, typical average distances called the characteristic correlation distances were determined in the present research and then the changes in the ratio of areal mean to point rainfall values were followed for the storm centre depths of 5—50 mm. These distances varied from 6 to 7 km for the non urban area and from 5 to 6 km for the urban region. The exponential correlation structure functions were then fitted for the estimated correlation coefficients determined at the correlation distances for the three storm groups selected. The decay parameters are produced to compare the results derived for urban and non-urban areas. Table 4 summarizes the values derived from the above procedure so that the changes in the correlation parameter (h), average storm size (J4) and a dimensionless number (Ah2) can be easily observed.
DISCUSSION OF RESULTS By comparing the histograms presented in Fig. 3 and the values given in Table 4 for urban and non-urban areas the following major results and conclusions can be derived: (1) Average areal rainfall depth is greater on the urban area when its value becomes >25 mm [Fig.3(a)] and the frequency gets smaller when the depth value becomes 25 mm is initiated more frequently in an earlier time period on the urban area compared to the surrounding non-urban region. (2) The earlier occurrence of the first storm of the day and the intense storm (do > 25 mm) on the urban area is due to high surface heating and produces greater average areal rainfall when storm centre values vary between 25 and 50 mm [Fig. 3(b)]. (3) The mean water volume for the urban area is greater when its value varies
Effects of urban development on thunderstorms 6
11
3
between ~0.0 and 4 X 10 m and then the volume becomes smaller due to the smaller storm area covered in the urban region than the non-urban one [Fig. 3(c) and Table 3]. (4) The occurrence of rainfall events was also studied to obtain information on whether the potential urban effects tended to be more pronounced during certain periods of the day. The 1968—1970 summer thunderstorms were divided into 3-h periods. On the urban area, a late afternoon maximization in the high rainfall centres occurred in the 1800—2100 h period. The time of thunderstorm activity on the urban region may be related to a combination of heating and urban thermal outputs and/or conditions favouring development of convective activity over the urban region. This supports the findings in h as being urban effects on thunderstorm frequencies [Fig.3(d)]. (5) The urbanized storms cover a smaller area due to the smaller number of raingauges spaced more densely on the urban area covering 104 km2. Although the surface raincell storms with a mean area of 198 km 2 cover a larger number of raingauge stations over the larger network area [Fig 3(e)]. (6) No significant differences were recognized between the storm duration parameters on urban and non-urban areas. The most frequent storm duration varied between 60 and 100 min [Fig. 3(f)]. (7) The areal depth of rainfall occurring on the urban area increased with an increase of storm centre depth of 25 mm or more but the relationship was the reverse for storm centre depths either smaller than 25 mm or larger than 75 mm. (8) There is an indication that the storm occurrences on summer thunder-days increased in absolute frequency over the urban region from the year of urbanization initiated in 1969. (9) Finally, the comparison of results presented in Table 4 indicates that the rate ofd^/do increases with an increase of storm centre depth varying from 5 mm (less intense) to 50 mm (intense storm) in urban effected storms, but it remains unchanged over the non-urban area. REFERENCES Berg, Norman A. (1968) Communities of tomorrow. Soil Conservation SCS, USDA. 33, no.7. Bras, R. L. and Rodriguz-Iturbe, I. (1976a) Evaluation of mean square error involved in approximating the areal average of the rainfall event by a discrete summation. Wat. Resour. Res. 12, no. 2, 1 8 1 - 1 8 4 . Bras, R. L. and Rodriguez-Iturbe, I. (1976b) Network design for the estimation of areal mean of rainfall events. Wat. Resour. Res. 12, no. 6, 1 1 8 5 - 1 1 9 6 . Byers, H. R. and Braham, R. R. (1949) The Thunderstorm: US Weather Bureau, Government Printing Office, Washington, DC, 287 pp. Changnon, S. A. and Semonin, R. G. (1975) Studies of selected precipitation cases from Metromex. Illinois State Water Survey Urbana, Report, no. 81. Eagleson, P. S. (1967) Optimum density of rainfall network. Wat. Resour. Res. 4, no. 3, 1021-1033. Eagleson, P. S. (1970) Dynamic Hydrology, chapter 11, pp. 1 5 9 - 2 1 0 : McGraw-Hill. Fogel, M. M. and Duckstein, L. (1969) Point rainfall frequences in convective storms. Wat. Resour. Res. 5,no.6, 1229-1237. Huff, F. A. (1973) Summary report of Metromex studies 1971 — 1972. Illinois State Water Survey Urbana, Report, no. 74. Huff, F. A. and Changnon, S. A. (1973) Precipitation modification by major urban areas. Bull. Amer. Met. Soc. 54, no. 12, 1220-1232. Kagan, R. L. (1966) An evaluation of the representativeness of precipitation data. Works of the Main Geophysical Observatory, USSR 191. Rodriguez-Iturbe, I. and Mejia, J. M. (1974a) The design of rainfall networks in time and space. Wat. Resour. Res. 10, no. 4, 7 1 3 - 7 2 8 . Rodriguez-Iturbe, I. and Mejia, J. M. (1974b) On the transformation of point rainfall to areal rainfall. Wat. Resour. Res. 10, no. 4, 7 2 9 - 7 3 5 . Sorman, U. A. and Wallace, J. R. (1972) Digital simulation of thunderstorm rainfall. OWRR Project Report, ERC-0972, Georgia Institute of Technology, 111 p. Woolhiser, D. A. and Schawalen, H. C. (1959) Area-frequency relations for thunderstorm rainfall in southern Arizona. Tech. paper 527, University of Arizona Agric. Exp. Station, Tucson. WMO (1972) Casebook on Hydrological Network Design Practice: WMO Publ. no. 324.
