Application of the Water Budget to the Urban Hydrology of Knoxville, Tennessee

Southeastern Geographer Vol. 27, No. 1, May 1987, pp. 38-47 APPLICATION OF THE WATER BUDGET TO THE URBAN HYDROLOGY OF KNOXVILLE, TENNESSEE* Hsiang-te Kung and Gregory J. McCabe, Jr. Water budget analysis is a useful method for the study of water surplus and runoff in a drainage basin. (J) This study endeavors to use the Thornthwaite water budget method to calculate water surplus and runoff in Knoxville, Tennessee. The Knoxville area watersheds offer interesting problems as the various drainage basins are underlain by carbonate rocks where several streams disappear and become part of the groundwater system. (2) The increase of urbanization in Knoxville adds to the complex hydrology of this area. The purpose of this paper is to evaluate and apply the Thornthwaite water budget method to studying the possible effects of urban land use (impervious surfaces) and geologic conditions upon total amount of surface runoff in the Knoxville area. STUDY AREA. The study area consists of most of the City of Knoxville and small portions of Knox County outside the city (Fig. 1). Knoxville is the third largest city in Tennessee, and urbanization is occurring rapidly , especially in the western Knox County corridor (along Kingston Pike and 1-40). The complexity and variation of the physical environmental characteristics, such as drainage density, geology, slope, elevation , soil, and urban land uses can be expected to produce significant intrametropolitan differences in runoff. Five drainage basins in Knoxville/Knox County (First Creek, Second Creek, Third Creek, Fourth Creek, and Ten Mile Creek) were chosen for analysis because of their variation of environmental characteristics and land use (Table 1). Limited data on stream runoff and precipitation are available from the Division ofWater System Management of the Tennessee Valley Authority. Several years of data were available * The authors gratefully acknowledge the helpful comments and encouragements from Professors Edwin H. Hammond and W Theodore Mealor, Jr. Appreciation is also extended to Ms. Melanie Wilson for typing the manuscript . Drs. Hsiang-te Kung and Gregory J. McCabe, Jr. are Assistant Professors of Geography and Planning at Memphis State University in Memphis, TN 38152. Vol. XXVII, No. 1 39 Fig. 1. Drainage basins included in the Knoxville, Tennessee study area. for the First Creek drainage basin. However, other nearby basins provided more limited data (Third, Fourth, and Ten Mile Creeks), but were included in this study as support to the findings of the First Creek data. No runoff data were available for the Second Creek basin. These hydrologie data make possible the comparison of simulated runoff from the water budget method with measured total stream runoff. These comparisons can be used to examine the relationship of runoff to the environmental characteristics of each watershed. METHODS AND PROCEDURES. The Thornthwaite water budget, as a hydrologie model, employs an accounting procedure in analyzing the allocation of water at the earth's surface. (3) It involves three independent variables: precipitation, potential évapotranspiration (PE), and soil moisture storage capacity (ST). The remaining variables in the budget (change in soil moisture storage, AST; actual évapotranspiration, AE; water deficit, D; water surplus, S; and runoff, RO) are dependent upon the interaction of the three independent variables. The quantities of 40 Southeastern Geographer TABLE 1 ENVIRONMENTAL CHARACTERISTICS OF KNOXVILLE BASINS Basin Characteristics Drainage Basins FirstSecondThirdFourthTen Mile CreekCreekCreekCreekCreek Basin Size (km2) Areas Drained into Sinkholes (km2) Drainage Density (km/km2) Local Relief (m) Basin Shape Impervious Surfaces (%) 57.08 9.79 18.39 1.30 14.499.66 182.9179.8 Irregular Elongated 17.5 26.0 44.03 3.19 16.10 161.5 Circular 24.0 24.99 2.59 16.10 125.0 Circular 24.0 41.70 41.70 16.10 152.4 Circular 15.2 Source: Authors' calculations. water involved in the dependent variables are determined by the definitions of the hydrologie processes incorporated in the water budget method. With information available for the independent variables under naturally vegetated (non-urban) conditions, it is possible to determine the quantity of water which would have been available for runoff in the absence of impervious surfaces (urban development). The estimated runoff would then reflect the natural variations in these independent variables over time. Values of the three independent variables can...