ABSTRACT: Few studies have been deportment ed to explore the effects of initial abstraction forward estimated direct runoff despite the widespread use of the bend number (CN) method in many hydrologic designs to estimate direct runoff.

ABSTRACT:

Few studies have been deportment ed to explore the effects of initial abstraction forward estimated direct runoff despite the widespread use of the bend number (CN) method in many hydrologic designs to estimate direct runoff. In this reflection 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 method (GIS). In addition, the issues on estimated runoff of altering the hydrologic soil arrange due to urbanization were investigated. The L-THIA pattern was applied to the Indiana Little Eagle cove watershed with 5 percent and 20 percent I^sub a^/S values, considering hydrologic soil assign places to alteration due to urbanization. The arises indicate that uses of a 5 percent I^sub a^/S and modified CN values and Hydrologic Soil dispose D for urbanized areas in pattern runs can improve long mete direct runoff prediction.

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

INTRODUCTION

Many hydrologic designs have been developed and used; however, in the greatest degree 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 originals Calibration/ validation of the pattern requires even more efforts for accurate design application. Thus, there is a ne for easy-to-use hydrology examples with reasonable accuracies. To address this ne the L-THIA mould was developed and integrated with the ArcView GIS arrangement (ESRI, 2002). The L-THIA GIS method estimates direct runoff from excessively basic input data, such as drawn out term daily rainfall data, land uses, and hydrologie soil arrange (Harbor, 1994; Bhaduri, 1998; Lim et al, 2001) wood et al. (2001) applied the L-THIA GIS connected view to the Little Eagle inlet (LEG) watershed in Indiana without considering temporal changes of land use and alteration of the hydrologie soil dispose due to possible soil compaction and disturbance that frequently occurs during urbanization (Ocean shire Soil Conservation District, 2001). The L-THIA archetype underpredicted direct runoff compared with observ U Geological inspect (USGS) direct runoff values from hydrograph separation. The predictions amounted to around 62 percent of observ direct runoff values. wood et al. (2001) used the frequent CN method that assumes initial abstraction is 20 percent of storage. The consequence s 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 awaited with a 5 percent I^sub a^/S value. Further improvements may be look fored when land use changes are considered as well as the impacts of these changes upon hydrologie soil group.

The first objective of this reflection was to verify whether a 5 percent I^sub a^/S value with corresponding CN values improves direct runoff estimation. The other objective was to assess the validitys of altering the hydrologie soil cluster 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) regularity

Curve Number Initial Abstraction Ratio

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

For termination analysis, Hawkins et al. (2002) analyzed rainfall and runoff data for 134 USDA Agricultural Research Service (USDA-ARS) watersheds. The storm rainfall stillness when direct runoff started was considered the initial abstraction value. With the known values of termination 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 affair The median value of these ratios for all storm affairs was used as a representative value for each watershed.

For original fitting, the ratio of I^sub a^ to s was determined by iterative least squares fitting. Rainfall and runoff data for 307 watersheds and combinations were used in these design fittings (Hawkins et al., 2002) In the gauge fitting, the natural pairs of rainfall and direct runoff, which naturally befall in time, and ordered pairs of rainfall and direct runoff were used. Hawkins et al. (2002) ground 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 consequence 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 ends from model fitting were more varied than those from consequence 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)