EFFECTS OF INITIAL ABSTRACTION AND URBANIZATION ON ESTIMATED RUNOFF USING CN TECHNOLOGY1

ABSTRACT:

Few studies have been directioned to explore the effects of initial abstraction upon estimated direct runoff despite the widespread use of the 1 number (CN) method in many hydrologic moulds to estimate direct runoff. In this research use of a 5 percent ratio of initial abstraction (I^sub a^) to storage (S) to estimate daily direct runoff with modified CN values for a 5 percent I^sub a^/S value was investigated using the Long-Term Hydrologic Impact Assessment (L-THIA) geographic information a whole (GIS). In addition, the consequences on estimated runoff of altering the hydrologic soil assemblage due to urbanization were investigated. The L-THIA archetype was applied to the Indiana Little Eagle bight watershed with 5 percent and 20 percent I^sub a^/S values, considering hydrologic soil collection alteration due to urbanization. The rises indicate that uses of a 5 percent I^sub a^/S and modified CN values and Hydrologic Soil assemblage D for urbanized areas in original runs can improve long boundary direct runoff prediction.

(KEY TERMS: hydrology; bend number; initial abstraction; runoff; urbanization; infiltration; hydrologic soil group; antecedent moisture condition.)

INTRODUCTION

Many hydrologic originals have been developed and used; however, greatest in quantity models are limited because of their intensive input data requirements. A significant amount of time is typically required for protoplast users in preparing input data for types Calibration/ validation of the prototype requires even more efforts for accurate pattern application. Thus, there is a ne for easy-to-use hydrology prototypes with reasonable accuracies. To address this ne the L-THIA design was developed and integrated with the ArcView GIS a whole (ESRI, 2002). The L-THIA GIS combination of parts to form a whole estimates direct runoff from actual basic input data, such as drawn out term daily rainfall data, land uses, and hydrologie soil collection (Harbor, 1994; Bhaduri, 1998; Lim et al, 2001) woodland et al. (2001) applied the L-THIA GIS a whole to the Little Eagle small bay (LEG) watershed in Indiana without considering temporal changes of land use and alteration of the hydrologie soil assemblage due to possible soil compaction and disturbance that oftentimes occurs during urbanization (Ocean shire Soil Conservation District, 2001). The L-THIA type underpredicted direct runoff compared with observ U Geological view (USGS) direct runoff values from hydrograph separation. The predictions amounted to around 62 percent of observ direct runoff values. woodland et al. (2001) used the belonging to all CN method that assumes initial abstraction is 20 percent of storage. The arises indicated that a 20 percent I^sub a^/S value might not be an appropriate value. Improvements in L-THIA's predictive ability may be wait fored with a 5 percent I^sub a^/S value. Further improvements may be wait fored when land use changes are considered as well as the impacts of these changes upon hydrologie soil group.

The first objective of this investigation was to verify whether a 5 percent I^sub a^/S value with corresponding CN values improves direct runoff estimation. The next to the first objective was to assess the consequences of altering the hydrologie soil assemblage due to soil compaction associated with urbanization and the use of a 5 percent I^sub a^/S value and corresponding CN values upon the L-THIA model's prediction of direct runoff.

LITERATURE REVIEW

Curve Number (CN) [i]modus operandi[/i]

Curve Number Initial Abstraction Ratio

The relationship I^sub a^ = 02 (Equation [1]) was derived from the inquiry of many small, experimental watersheds, and details upon this relationship can be base in the National Engineering Handbook, section 4 Hydrology (USDA-SCS, 1985 1986) Since the history and documentation of this relationship are unilluminated Hawkins et al. (2002) used incident analysis and model fitting way s to determine the ratio of I^sub a^ to s with hundreds of rainfall-runoff data from numerous U watersheds.

For circumstance analysis, Hawkins et al. (2002) analyzed rainfall and runoff data for 134 USDA Agricultural Research Service (USDA-ARS) watersheds. The storm rainfall profundity when direct runoff started was considered the initial abstraction value. With the known values of circumstance rainfall, direct runoff, and initial abstraction value, s was obtained from Equation (4a). The ratio of I^sub a^ to s was obtained for each storm occurrence The median value of these ratios for all storm occurrences was used as a representative value for each watershed.

For design fitting, the ratio of I^sub a^ to s was determined by iterative least squares fitting. Rainfall and runoff data for 307 watersheds and plats were used in these original fittings (Hawkins et al., 2002) In the type fitting, the natural pairs of rainfall and direct runoff, which naturally be met with in time, and ordered pairs of rainfall and direct runoff were used. Hawkins et al. (2002) set that the ratio of I^sub a^ to s varies from storm to storm and watershed to watershed and that the assumption of I^sub a^/S = 020 is usually high. According to the fact analysis, the median I^sub a^/S ratio for each watershed varied from 00005 to 04910 and the median value was 00476 More than 90 percent of I^sub a^/S ratios were les than 02 follows from model fitting were more varied than those from occurrence analysis. I^sub a^/S ratios for natural data ranged from 0 to 0996 with a median of 0 and I^sub a^/S ratios for ordered data ranged from 0 to 09793 with a median of 00618 (Hawkins et al., 2002)