**3. Principle of soil erosion processes and development around the Mubi region**

The underlying principle of such as gully erosion is governed by flow conditions on watersheds. Gullying occurs whenever the water flow rate (runoff) on a slopping landscape exceeds the threshold limit or resistance of soil, then erosion is initiated, followed by downward incision [33] and upstream head-cut migration [34]. Likewise, whenever the flow rate drops below the erosion potential, then the erosion process ceases [35]. Gully erosion processes are active on a sloppy or rolling *Evaluation of Soil Erosion and Its Prediction Protocols around the Hilly Areas of Mubi Region… DOI: http://dx.doi.org/10.5772/intechopen.100477*

topography that increases soil particle detachments on usually two intersecting planes and/or watershed areas due to applied runoff force that voids the soil surfaces such as around the Mubi region. The soil detachment continues in time steps, except otherwise, limited by the effect of slope and/or vegetation roughness. Since the flow rate is unsteady and spatially varied, the head-cut migration rate, rate of sediment entrainment, transport, channel width, and deposition will all vary accordingly in time and space [34, 36].

**Figure 2.** *(a–f) Showing some channelized erosion features in the Mubi region.*

#### **Figure 3.**

*Schematic diagrams of EG erosion showing, (a) EG erosion channel formed on a sloping intersectional watershed areas, and, (b) erosion processes describing a developing EG channel with an actively migrating head-cut in the upstream direction. Source: adapted from [32, 34, 40].*

The periodic erosion processes, therefore, yields both head-ward migration in an upstream direction and soil sediments transportation at the gully outlets as deposited materials. The flow rate is proportional to the upstream drainage area that supplies runoff for transporting detached particles downslopes. The distance between the head-cut and the gully outlet defines the actual concentrated flow length. Depending on additional runoff, the head-cut first incises down to the tillage layer (lower boundary), before it starts migrating backward at a rate proportional to the flow rate [37]. As the erosion progresses, the head-cut continues to migrate upstream (**Figure 2**), and the contributing drainage area decreases, so that discharge at the head of the EG also decreases until it attains a maximum EG length for a given watershed area.

#### **3.1 Conceptual framework of soil erosion processes**

The concept of soil erosion formation begins with the understanding of the actual erosion process that is often caused by rainfall impacts, soil factors, and topographic variables that initiate soil erosion, then followed by subsequent channel morphological stages of development, if left unobliterated [13], as illustrated in **Figure 2**. Soil erosion is a natural phenomenon that is as old as the earth itself, and whose effects are targeted at a man and his ecosystem [38].

The soil erosion process starts with the gradual wash of soil surfaces by either water, wind, or human activities [39]. Generally, the soil erosion management principle is centered on prevention, rather than ignoring it to degenerate before controls, which often comes at very prohibitive costs. As has been the case around the neighboring parts of Adamawa State, Nigeria, and in most other parts of the world, the impacts of soil erosion such as sheet, rill, and gully erosion activities are widely spread across the regional landscape of the Mubi and her environs (**Figure 3**).

### **4. Soil erosion predictions around the Mubi region**

In the past, erosion assessment tools were used to determine surface and channel erosion development, soil losses, and their morphological processes around the *Evaluation of Soil Erosion and Its Prediction Protocols around the Hilly Areas of Mubi Region… DOI: http://dx.doi.org/10.5772/intechopen.100477*

Mubi region using field measurements (estimations) of such as sheet, rill, and gully erosion features [10, 11]. In addition, the use of empirical models for predicting area, volume, and weights of soil loss was developed and tested by [23]. Other linear models such as the universal soil loss equation were tested by [11]. Trials of sophisticated prediction models such as the ephemeral gully erosion model (EGEM), and its adapted versions, and the water erosion prediction project-WEPP model were respectively tested by [24, 32], while the RUSLE-2 and ArcGIS software 10.3 were also tested by [11, 12]. Even though, few erosion prediction technologies were tried around the Mubi region, yet, several other researchers are still only concerned about the channel morphological properties. Future studies are expected to be more involved in predictive, rather than limiting efforts to document channel properties without including soil losses and their accompanying economic implications in the region (**Table 4**).

#### **4.1 Field studies of channelized erosion features around the Mubi region**

#### *4.1.1 Empirically predicted soil losses*

Earlier, [9] reported that gullying activities are widely spread in areas along the foothills of the Mandara mountain ranges in the Mubi region. Researches have been documented on the scale and intensity of such channelized erosion processes in the region by a handful of earth scientists in recent times. **Table 5** presents the yearly soil loss reported at some gully erosion sites in the Mubi area during 2003/2004 and 2008/2009 respectively.

The erosion indices reported in **Table 5** shows an erosion trend from 2003 to 2004, and from 2008 to 2009. The reports clearly suggest a relative decrease in soil loss rates at the same erosion sites over the observation time intervals. These reductions were largely influenced by the conservation measures adapted at the erosion sites in order to curtail erosion progress at the same sites during the 6 years period.
