A Modified Distributed CN-VSA Method for Mapping of the Seasonally Variable Source Areas

Pradeep Goel,Syed Ahmed,Bahram Gharabaghi,Ramesh Rudra,Kishore Panjabi

doi:10.3390/w13091270

Abstract

Many watershed models employ the Soil Conservation Service Curve Number (SCS-CN) approach for runoff simulation based on soil and land use information. These models implicitly assume that runoff is generated by the Hortonian process and; therefore, cannot correctly account for the effects of topography, variable source area (VSA) and/or soil moisture distribution in a watershed. This paper presents a new distributed CN-VSA method that is based on the SCS-CN approach to estimate runoff amount and uses the topographic wetness index (TWI) to distribute the runoff-generating areas within the watershed spatially. The size of the saturated-watershed areas and their spatial locations are simulated by assuming an average annual value of potential maximum retention. However, the literature indicates significant seasonal variation in potential maximum retention which can considerably effect water balance and amount of nonpoint source pollution. This paper focuses on developing a modified distributed CN-VSA method that accounts for the seasonal changes in the potential maximum retention. The results indicate that the modified distributed CN-VSA approach is better than distributed CN-VSA to simulate runoff amount and spatial distribution of runoff-generating areas. Overall, the study results are significant for improved understanding of hydrological response of watershed where seasonal factors describe the potential maximum retention, and, thus, saturation excess runoff generation in the watershed.

Highlights

Saturation excess is one of the dominant overland flow generation mechanisms in humid and well-vegetated regions [1,2,3]
The distributed CN–variable source areas (VSA) method was applied to the study watershed, and nine representative rainfall events out of 45 monitored events were selected for detailed simulation
The results show a better agreement of simulated runoff with observed runoff, which indicates that the modified distributed CN-VSA method is an improvement over the traditional distributed CN-VSA method

Summary

Introduction

Saturation excess is one of the dominant overland flow generation mechanisms in humid and well-vegetated regions [1,2,3]. Precipitation over saturated areas is assumed to readily generate overland flow regardless of the precipitation rate [13] This assumption is in contrast to the Hortonian theory, which suggests that runoff occurs when the precipitation rate exceeds the maximum soil infiltration capacity regardless of the soil’s degree of saturation [14]. The portion of the watershed susceptible to saturation excess runoff varies seasonally and within a storm event [15]. These areas are termed as variable source areas (VSA) or hydrologically active areas [16,17,18,19]

Methods

Results

Conclusion