A Historical Review of Slope Based SCS Method and its Effect on CN and Runoff Potential Globally

The Soil Conservation Service Curve Number (SCS-CN) method is extensively used to calculate the runoff from rainfall over a large catchment over the world. Slope is an important criterion for runoff but a very few attempts have been made to evaluate the effect of slope on the CN with runoff potential. The objective of this paper is to summarise the historical review on the effects of slope on CN and runoff potential in various regions by the hydrologists. This paper also depicts that how the various researchers proved the importance of consideration of slope for CN and runoff estimation. In addition, paper highlights the key features of research in future like to classify the watersheds on slope based CN, accurate Antecedent Moisture Condition (AMC) and proper initial abstraction in the various regions etc. Considering these parameters an accurate runoff estimation can be predicted and managed properly in the urban watersheds.

2 %. Hence, the literature provides brief summary of the earlier research outcomes and the developments related to the slope based SCS-CN. The objective of this paper is to review the researches including the effect of slope on CN and runoff in various watersheds all over the world.

Basics of Soil Conservation Service Curve Number (SCS-CN) Equation:
The SCS CN is based on the water balance where two hypothesis (ratio of actual amount of direct runoff to the total rainfall and actual infiltration to the potential maximum retention) are equated to form the equation. The equation (1)  for P > Ia ……(1) Where, Q is surface runoff (mm), P is rainfall (mm), Ia = Initial abstraction (Ia= 0.2S) and S is the retention parameter (mm). The value of S is obtained from the given below equation (2) S= 25400 CN -254 ……. (2) Where, CN varies from 0 to 100. The CN value is determined based on land cover and hydrologic soil group using a table given by the SCS handbook [2].

The historical background of effect of slope on CN with runoff potential:
In the present study, related peer reviewed journals have been referred which take into consideration the effect of slope on CN and runoff estimation. In literature survey, it is found that very few studies are available which account the effect of slope on CN and runoff. This concept was started by the Sharpley and Williams [5], who adjusted the CN for moisture condition 2 (Average moisture condition) called as CN2 upto 5 % slope and proposed an equation (3). In the same year, Siavash et al. [9], selected the coal mining region of the southern Appalachians in U.S.A., and studied over three active surface-mine sites in East Tennessee.
Rainfall runoff was monitored during one year for 5-min intervals using a unique Pinson-type i) Slope based CN (SA-CN2) is less than the normal CN (SCS-CN) when slope is less than 5 % but it is more than SCS-CN when slope is greater than 5%. Higher the deviation from 5% slope more is the difference.
ii) Significant changes in CN values were observed in slope based forest land. Muhammad et al. [13], estimated the runoff using NRCS slope adjusted CN method in the mountainous watershed of South Korea. This study investigated two existing approaches of CN estimation and suggested a new approach. It is found that the steepness of the slope of watershed is the dominant factor to generate the runoff as compared to the plain areas or having slope less than or equal to 5%. It is also found that NRCS method using CN2 values was found 5 to be least efficient model for runoff estimation because of very small difference between the CN2 and CN2α while in Huang [4] method, improved runoff estimation was observed. The newly proposed CN2α approach is more reliable than the CN2.
Tauseef et al. [14], investigated the effect of slope on CN and runoff. In the study, it is found that weighted CN is marginally increased for all watersheds due to slope. It is also found that when slope is considered to estimate the runoff using Sharpley and William model, the runoff volumes has also increased. This study also depicts that the increase in Impervious Surface Area (ISA) has more impact on runoff volume than the slope.
Sangeeta et al. [15], analysed that when SCS-CN model is coupled with slope, increase in runoff is observed. It was also observed that the Sharpley and Williams [14] approach of slope adjustment for λ (ratio of initial abstraction to the infiltration) value equal to 0.

Conclusions:
After the literature review, it is found that instead of being a very important parameter, very few scientists have touched the slope based CN in the last 30 years. The SCS-CN (currently NRCS-CN) method has the advantages over other methods like its simplicity, predictability and used for large areas. This method is rainfall event based method rather than single annual rainfall value but still it has a more scope of research to reconsider the following parameters of the SCS-CN methods.
i) It requires more research in the steep slope (mountain) regions especially in the spatial and temporal changing pattern of land use like urban area and also in ungauged watersheds.
ii) When slope of the watersheds are more than 5% (mountains watershed) then it is very important to consider the effects of slope because it affects CN and runoff values.
iii) The modified CN model can be proposed and also the estimated and observed CN values can be validated for different urban areas.