**4. Estimation of runoff**

There are different methods used for determination of runoff in a catchment. In the present study some tables and empirical formulae are used depending on availability of data to estimate the quantity of rainwater available for runoff in Gosaba block.

*Binnie's coefficient method:* According to this method, runoff and rainfall are correlated as *R* = *KP*, where P is precipitation in cm, R is runoff in cm and K is the runoff coefficient that changes with the landcover type (Usual values of K are given in **Table 1**).

*Ingles and De Souza's formulae:* According to Ingels and De Souza, runoff calculation based on rainfall can be done in two separate equations one for plains and the other for hills. The equations are –

For plains, *R* = (*P*-17.8) *P*/254; for hilly areas, *R* = 0.85*P*-30.5. The calculations are shown in **Table 2**.

*Barlow's Table*: T.G. Barlow divided the catchment into 5 different types named as A-E after many studies to estimate the percentage of runoff generated by them. The present study area comes under category A (Flat, cultivated, absorbent soil) and the coefficient of this type is 25 percent of the total rainfall. Thus, the equation stands *R* = 0.25*P*, and the calculation of availability of runoff according to this equation is given in **Table 3**. The seasonal variation of rainfall in Gosaba block of South 24 Parganas is shown in **Figure 3** (Source: IMD Kolkata [15]), that depicts that the area


#### **Table 1.**

*Average availability of rainwater for runoff (mm) according to Binnie Values of runoff coefficient K (source: Ministry of Water Resources, GoI [13]); Rainfall Data (source: Meteorological Department, GoI [14]).*

*Analysis of Rainwater Harvesting Method for Supply of Potable Water: A Case Study of Gosaba… DOI: http://dx.doi.org/10.5772/intechopen.106537*


**Table 2.**

*Average availability of rainwater for runoff (mm) according to Ingels and De Souza.*


#### **Table 3.**

*Average availability of rainwater for runoff (mm) according to Barlow.*

#### **Figure 3.**

*Monthly variation of rainfall in Gosaba block of S 24 Parganas, WB.*

receives maximum rainfall in the month of August and minimum rainfall in the month of January.

From the calculations, it is found that the area receives good quantity of runoff that can be harvested for future use. The runoff needs to be collected, stored and utilized to reduce water scarcity and reduce the overuse of groundwater. During the rainy season excess runoff should be stored for use in dry season. Different amount of runoff is generated by different types of terrain (hilly, plains) and different areas (forest, residential, farms etc.). So, design of RWH plant should be done based on the type of land. The present study area is in plain land, so the calculations and design are done with a normal rainfall of 196 cm.
