extra urban and intra urban rainfall enhancement by a medium sized city
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VOL. 17, NO. 5 WATER RESOURCES BULLETIN
AMERICAN WATER RESOURCES ASSOCIATION OCTOBER 1981
EXTRA URBAN AND INTRA URBAN RAINFALL ENHANCEMENT BY A MEDIUM SIZED CITY
Lawrence C. Nkemdirim
ABSTRACT: An experiment on urban effects of warm season rainfall of a moderately sized city and its downwind towns found no evidence of global enhancement. However, there are grounds for believing that the intra urban distribution of precipitation is influenced by urban variables among which air pollution and urban roughness are considered primary. The presence of an urban heat island did not appear to im- prove rainfall. On the contrary, it appears to be a factor in the relative aridity of the downtown sector of the city. (KEY TERMS: urban rainfa, enhancement; air pollution; heat islands.)
Urban influence on climate has been reported for over a century. Since Howard (1 833) made his study of the climate of London, several reports and monographs have been pub- lished on that subject (Peterson, 1969). If, as the literature suggests, a city can enhance precipitation within itself (Sander- son and Gorski, 1978) or downwind (Changnon, 1968) or both (Changnon, 1979), the recurrence interval for floods of all sizes in the area may be shortened. And since the con- struction of most engineering structures are based on expected flood frequencies, their capacity to fulfill design goals may be compromised by urban induced rainfall enhancement. Leo- pold (1968) discussed the consequences of urban landuse on stream flow on the premise that the change in land use is not paralleled by one in climate. But if the city does induce a change in the local rainfall regime, realistic models in urban hydrology should incorporate the impact of the change. How- ever, that effort will require the creation of a larger data base on the scope and nature of urban induced rainfall enhance- ment than what currently exists. It is with that in mind that the experiment on rainfall distribution in Calgary was ini- tiated.
A survey of warm season rainfall in Calgary (pop. ca 5 10,000) was undertaken from 1978- 1980 as part of a climato- logical study of the city and its surrounding country. Among the objectives of the study were to determine (a) the extent to which the city has affected precipitation in the city and its downwind towns, (b) if there are significant intra urban dif- ferences in rainfall which result from urban influence, and
(c) to provide base data and analysis for the assessment of the impact of future growth on the citys rainfall pattern.
RAIN GAGE NETWORK Twenty rain gages were used in the study. Of these, 15
were standard Canadian copper gages, 4 were tipping bucket recording gages, and 1 was a recording weighing type. In addi- tion to providing information on the duration and intensity of rainfall the two sets of automatic gages were calibrated against a standard gage and were consequently used as part of the standard gage network.
Decision on the location and allocation of gages was governed by the desire to achieve a uniform coverage while reflecting the topographical diversity of the city. This was done by dividing the study area into twenty 20-square kilo- meter rectangular grids. The area was then broken down into elevation districts using 38 m intervals. The percentage of land within each district was determined, and the gages were located in the grids in a manner that took account of the eleva- tion breakdown. Fortuitously a good representation of other relief features such as aspect and slope was also achieved.
The gages were sited in accordance with exposure specifica- tion of the WMO (1971). However, in a small number of cases, standard exposure was compromised by factors of safety and/ or accessibility. But such departures did not destroy the de- sign objectives of the network. Nearest neighbor statistic was calculated as a means of testing the form of the distribution. The statistic confirmed that the gages were uniformly dis- tributed over the study area.
During the study, the standard error the mean rainfall based on the network averaged less than 5 percent. For or- ganized storms such as cyclones with heavy rainfall (> 10 mm/event) the error was about 2 percent but was as high as I1 percent for convectional storms of lower intensity (< 2 mm/ event). The most widely dispersed storm occurred on July 4, 1978, when the error was about 30 percent. These figures suggest that the network was adequate for the assessment of the areal rainfall in the area. Indeed a control experiment
Paper No. 80105 of the Water Resources Bulletin. Discussions are open until June 1, 1982. Department of Geography, The University of Calgary, Calgary, Alta. T2N 1N4, Canada. 2
753 WATER RESOURCES BULLETIN
involving a similar gage network outside the University Weather Research station suggest that in the worst exposure situation the network was at least 75 percent accurate in its assessment of true rainfall.
There was considerable areal variability as indicated by the number of days when rain, though occurring at several stations within the city, did not occur in at least one site. The mini- mum number of days in which such an event was experienced was nine (1978) at Station 7. The maximum was 21 (1980) at Station 15.
Figure 1. Calgary: Topography, Land Use, and Rain Gage Stations.
STUDY PERIOD AND DATA SUMMARY
The study was conducted over three years from 1978-1 980. It includes all rainfall events between May and September. In 1978 and 1980, the mean warm season rainfall was above aver- age for the time of the year; but 1979 was relatively dry (Table 1). Taken together, the mean of the three years is very close to the normal rainfall expected at that time of the year.
TABLE 1. Summary of Rainfall Data. ~~ ~ ~~
1978 1979 1980 Mean
Total Rainfall (mm) 330 164 320 271 Percent of Mean for May-September 125 62 121 103 Number of Rain Days 74 59 75 69 Number of Events* 51 43 57 50.3
*The number of events is less than the number of days of rainfall be- cause some events extended beyond one day.
Potential Inmence of Calgary on Downwind Rainfall Since the Changnons (1968) La Porte, Indiana, study we
have become aware of the potential of atmospheric pollutants from urban sources to significantly enhance rainfall and related weather downwind. Although not generally a serious environ- mental problem in Calgary, there has been anoticeable increase in the level of air pollution in the past few years (Nkemdirim, et al., 1975) and under certain types of weather, pollution has reached levels that can be considered hazardous (Nkemdirim and Leggat, 1978).
Winds prevail from the northwest in Calgary (Brinkman, 1969), but during storms they are equally likely to come from the southeast (Audrey, 1979). During the study period 61 percent of the storms were accompanied by northwesterlies or southeasterlies demonstrating the dominance of these directions in the rainfall process. Consequently, if there is a downwind effect, it should be experienced in Gleichen and High River to the south and east and at Cochrane and Crossfield to the north- west (Figure 2). Two tests were conducted to verify possible enhancement. In the first test, trend line were fitted to May- September data (1962-1980) for Calgary airport and for down- wind sites. The rainfall regimes in these places (Figure 3) was not different from those for other areas in the Prairies (Long- ley, 1972) nor was the declining trends for each of the stations statistically significant (0.05 level). Moreover, there was no statistically significant difference in the slope of the trend line among the stations. In the second test, downwind sites were paired against nearby nondownwind stations, and the ratio of the annual warm season rainfall (1962-1980) between the pairs computed. The pairs used were Crossfield against Madden, Gleichen against strathmore, Cochrane against Madden, and High River against Blackie. There was neither a net increase in the size of the ratios over the period in question nor were any trends observed in the time series. In addition, Calgary airport whose rainfall is lower than the regional average did not ex- perience any net gain in relation to Gleichen, High River, Crossfield, and Madden over that period. These results suggest that there has not been any rainfall enhancement downwind of Calgary as yet and that Calgary itself has not received more warm season rainfall as a result of urbanization. The results support Changnon (1 976) who found no eivdence of anomalies in smaller cities. Perhaps they suggest that the city has not grown either in size or in pollution status to influence the climate of the area around it. However, the results do not ex- clude the possibility of an internal distribution of rainfall in- side the city which may be traced to urban influence.
WATER RESOURCES BULLETIN
Extra Urban and Intra Urban Rainfall Enhancement by a Medium Sized City
- \ MacLeod
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Figure 2. Distribution of Mean Warm Season Rainfall in Calgary and Surrounding Towns (adjusted for elevation
difference) May-September 197 8-1 980.
CALGARY , GLEICHEN
*- HIGH RIVER
1962 1978 1962 1978
Figure 3. Trends in Warm Season Rainfallin Calgary and Three Downwind Towns.
Intra Urban Rainfall Dism'bution The data from all 20 stations were tabulated according to
stations and date (event). A two-way ANOVA test showed that the differences in stations and the distribution of storm
rainfall were statistically significant at 0.05 level. The con- clusion was therefore drawn that the variable spatial distribu- tion of the warm season rainfall (Figure 4)