Chapter-6
Effect of projected urban settlement on soil loss from the
management factor; P is the support and conservation practices factor. L, S, C and P factors are dimensionless. Determination of all of these factors are stated below-
(a) Rainfall erosivity factor 'R': Rainfall erosivity factor 'R' stands for the eroding capability of rainfall (Renard et al. 1997). Both due to the impact of raindrops and heavy storm runoff resulting from intense rainfall, soil particles are dislodged which lead to the occurrence of sheet or rill erosion (Biswas and Pani 2015). R factor can be calculated by the equation given by Renard and Freimund (1994). This revised relation was derived based on more data.
R= j (6.2)
where, E= =Total storm kinetic energy in MJ/ha I30 = Maximum 30 minutes rainfall intensity
j = Index for the number of years used to compute the average k = Index of the number of storms in each year
n = Number of years to obtain average m = Number of storms in each year
er = 0.29 [1- 0.72 exp (-0.05) ir] = Rainfall energy per unit depth of rainfall per unit area for the rth increment of m numbers of divisions of the storm hydrograph in MJ ha-1mm-
1.
ir= = Rainfall intensity for the rth increment in mm h-1. ΔVr= Depth of rain falling in the rth increment.
Δtr = Duration of increment in h (hour).
Sarma et al. (2005) obtained the R factor value for Guwahati city as 9259 MJ.mm ha-1 h-1year-1. The highest value of R factor in the US is 10000 MJ mm ha-1 h-1 year-1 (Foster et al. 1981). From that point of view, R factor value equal to 9259 MJ mm ha-1 h-1 year-1 is an acceptable value since Guwahati is a place of heavy rainfall (Sarma et al. 2005).
In this study, soil loss calculation has been done by taking R= 9259 MJ.mm ha-1 h-
1year-1 (Sarma et al. 2005; Sarma et al. 2013; Sarma et al. 2015).
(b) Soil erodibility factor, K: Soil erodibility factor K indicates the resistance of soil particles against detachment and movement caused by the impact of rainfall and runoff (Renard et al. 1997). It can be defined as the amount of soil loss per unit rainfall erosivity factor from a plot of clean fallow land with a uniform slope of 9% and a slope length of 22.1 m (Weesies 1998, Brady and Weil 2012). K factor can be determined by using soil-erodibility nomograph based on soil texture, composition, organic content and permeability (Wischmeier et al. 1971). In this study, K factor values are taken depending on soil texture class and organic content (2%) as given in Stewart et al.
(1975). Necessary unit conversion of K factor has been carried out as per Foster et al.
(1981). Soil texture classes for hills of Guwahati city are taken from Das (1992) and also from soil maps of Guwahati city collected from Assam Remote Sensing Application Centre (ARSAC). These soil maps were prepared by using satellite data of years 1993 and 1996; and the SOI (survey of India) map of the year 1995. K factor values for all the hills of Guwahati city are displayed in Table 6.1.
Table 6.1: K factor values for hills of Guwahati city
Hill ID Name of hills Soil Type
K factor (t h MJ-1mm-1) (Stewart et al. (1975)
1 University Sandy loam 0.031608
2 Fatasil Clay loam 0.032925
3 Kalapahar Sandy loam 0.031608
4 Sonaighuli Sandy loam 0.031608
5 Sarania Clay loam 0.032925
6 Kharguli Clay loam 0.032925
7 Japorigog Clay loam 0.032925
8 Burhagosain Silty clay 0.030291
9 Khanapara Clay loam 0.032925
10 Garbhanga Clay loam 0.032925
11 Kamakhya Sandy loam 0.031608
12 Kahilipara Silty clay 0.030291
13 Betkuchi Sandy loam 0.031608
14 Chunsali Silty clay 0.030291
15 Koinadhara Clay loam 0.032925
(c) Slope length and slope steepness factor, LS: This factor was introduced to account the effect of topography on soil loss (Renard et al. 1997). Slope length is the distance from the starting point of overland flow to the point where deposition starts or runoff goes into a defined channel due to the sufficient decrease of slope and slope steepness means slope gradient (Wishmeier and Smith 1978). Both the quantity and
velocity of runoff increase with the increment of slope length and steepness. As a result, soil loss increases. Though originally USLE suggests consideration of an average straight slope, it was found to underestimate the LS factors as it neglects three- dimensional complex form of the topography (Foster and Wishmeier 1974). Use of remote sensing and GIS in the determination of LS factors make possible to consider both the upslope contributing area and the complex three-dimensional form of the topography (Gelagay and Minale 2016). In this study, Eq. (6.3) (Moore and Burch 1986, Desmet and Govers 1996a) has been applied to calculate LS factors in ArcGIS (software) platform. In many studies, this equation has been applied (Moore and Wilson 1992, Mitasova et al. 1999, Gelagay and Minale 2016).
(6.3) where, Ac and θare the specific catchment area (m2) and natural slope angle (degree), respectively. In "Map Algebra" tool of ArcGIS, Eq. (6.3) should be input as-
LS=Power ("Flow accumulation"*cell size/22.13, 0.6)*Power
(Sin("Slope"*0.01745)/0.0896, 1.3) (6.4)
Slope and flow accumulation maps (raster) for the watersheds of 15 hills of Guwahati city have been derived from an SRTM (Shuttle Radar Topography Mission) DEM (Digital Elevation Model) of 1 arc-second resolution.
(d) Cover management factor, C and support practice factor, P: Cover management factor is the ratio of soil loss from a plot of land having a particular land cover condition (natural or artificial) to the soil loss in the condition of continuous tilled fallow on the same soil (Renard et al. 1997). The highest value of cover management factor is '1' assigned to completely bare land and the lowest value '0' is assigned to a very well protected soil. It is the most dynamic factor in RUSLE (Gelagay and Minale 2016). On the other hand, support practice factor (P) is the ratio of soil loss from a plot of land having a specific soil conservation practice to that from straight row cultivation with up and down slope (Pandey et al. 2007). For hills of Guwahati city, there are no support practices. Hence, in this study, P factor is taken as 1. Again, C factor values have been assigned based on land cover types as given in LULC maps (Fig. 4.3). These are shown in Table 6.2. For urban settlement in general located in plain area, C-factor
value (Cg) has been assigned by considering 60% of the settlement area being impervious and the remaining 40% being in bare condition [According to Guwahati Metropolitan Development Authority [GMDA] (2006), maximum coverage area of building in a plot of land can be 60% of plot area]. C-factor value for the impervious area is taken as '0' (Sarma 2011).
Table 6.2: Cover management factor (C) values for LULC of hills of Guwahati city
LULC type C factor
Bare soil 1 (Sarma et al. 2005, Sarma 2011)
Forest 0.01 (Sarma et al. 2005, Gelagay and
Minale 2016)
Scrub land 0.014 (Wishmeier and Smith 1978,
Gelagay and Minale 2016)
Marshy land 0 (Sarma 2011)
Water bodies 0 (Erdogan et al. 2007, Gelagay and Minale 2016)
Urban settlement in plain area, Cg (60%
impervious, 40% bare)
0.4 (=0x0.60+1x0.4)