**4. Discussion**

Rainfall erosivity factor (R) plays a vital role in soil loss. The catchment under study has R-value of between 723.7 and 902.1 (**Table 1** and **Figure 3**). High R-factor is usually associated with soil loss due to high kinetic energy of rain drops that dislodges *Spatial Soil Loss Assessment Using USLE in Lake Ol Bolossat Catchment DOI: http://dx.doi.org/10.5772/intechopen.112129*

and disintegrates soil particles that are easily carried away by surface runoff [3, 36, 37]. However, other factors such as vegetation cover and erosion management factors also determine the rate of soil erosion [22, 36]. According to the study areas with high R-values and high vegetation cover such as western strip of study area have little soil loss (less than 5 tons/year). In comparison, areas with moderate R-value (about 723.7) and little or no vegetation cover experience high soil loss indicating that C-factor plays a major role in controlling soil erosion. In the view of this, it can be concluded that there is no soil erosion factor acts in isolation but all USLE factors are interrelated.

Vegetation cover is an important determinant of soil erosion. Vegetation intercepts the rain drops reducing their impact on soil particles. In addition, vegetation cover slows down the surface runoff allowing for more for water to infiltrate and consequently reducing the scouring ability of the runoff [29, 36, 37].

The type of soil in an area determines the soil erodibility value. The study area has four dominant soil types i.e. nitisol, phaeozem, luvisol and solonchack which have different erodibility values (**Table 4**). The results of the study show that nitisol is erodible even though they have a high erodibility value (0.25) because these soils are deep and supports dense vegetation. Solonchacks are highly erodible (K-value of 0.35) and areas dominated by this type of soil have high erosion rate. Most of these areas are around the Lake and have a soil loss of over ten tons per hectare per year which is above tolerable values of 9 tons per hectare per year [35, 37]. Luvisols and phaeozems are relatively erodible and areas dominated by these soils have high rate of soil erosion. In addition to soil types, overgrazing also contributes to high soil erosion rate


#### **Table 4.**

*k-values.*

especially around the Lake. The same observation was made by [35] when they were assessing erosion hazard in Upper Ewaso Ng'iro Basin of Kenya.

Slope length and slope angle (LS factors) also influence the rate of soil erosion. Steep slope especially Satima escarpment experience high rate of soil erosion. These sloppy areas have relatively high vegetation cover and erosion management practices as shown in **Figures 5** and **6** respectively, however the erosion rate is high possibly due to high erosivity factor.

The results of this study are based on use of empirical model, USLE. It is therefore recommended that other methods such as experimental plots be used so as to compare the results. The results of prediction models can be improved by carrying out semidetailed or detailed soil surveys so as to enable a better way of computing erodibility factor. In addition, field assessment of soil and water conservation measures would help to accurately determine erosion management practices (P-factor). In addition, further research is recommended on assessment of soil erosion hot spots so as to formulate immediate soil conservation measures.

The study revealed that the spatial rate of soil loss in Lake Ol Bolossat catchment is between zero (0) and 22,575.5 tons per year. The rate of soil loss in most of the parts of the study area is alarming and requires mitigation measures for it is above the tolerable levels of 9 tons per hectare. The study also revealed that rainfall intensity, soil erodibility, slope length, slope angle, vegetation cover and erosion management are factors that contribute to soil erosion in Lake Ol Bolossat catchment. The spatial locations of high erosion rates are agricultural areas (croplands) and grazing areas around the Lake which are dominated by barelands and sparse vegetation.
