2 Fig. 2.1 shows a cross-section of a hillside where gully erosion has occurred. (a) Describe three features visible in a gully that distinguish it from ril...

Labelled answer diagram:

(a) Three features that distinguish a gully from rill erosion [3]:

1. A gully is too deep to be crossed by normal tillage equipment (typically deeper than 30 cm), whereas rills can be ploughed over and temporarily smoothed out. [1]
2. Gully sides are vertical or near-vertical, creating a steep-walled channel rather than the shallow groove of a rill. [1]
3. The gully may have a waterfall or head-cut at the upper end, where flowing water drops over an exposed face and undercuts the soil, causing the gully to extend upslope. [1]

Also acceptable: width greater than 30 cm; carries water only during heavy rain events.

(b) How deforestation on upper slopes led to gully formation lower down [4]:

1. Trees intercept rainfall with their canopy, reducing the force with which raindrops strike the soil. Without this canopy, rain hits exposed ground at full energy, breaking up soil aggregates. [1]
2. Tree roots bind the soil and increase infiltration by maintaining soil structure and creating channels for water to percolate downward. [1]
3. Without trees, surface runoff increases dramatically in both volume and speed. Water that would have soaked in now flows overland. [1]
4. This concentrated, fast-moving flow cuts into the unprotected soil lower down the slope and progressively deepens into a gully as each storm event removes more material. [1]

(c) Two methods to rehabilitate the gully (with explanations) [4]:

1. Build check dams (gabions or stone barriers) across the gully floor. These structures slow the velocity of water flowing through the gully and cause sediment to be deposited behind them. Over successive rainy seasons, trapped sediment gradually fills the gully and reduces its depth. [2]
2. Plant deep-rooted vegetation (e.g. vetiver grass) on the gully sides and edges. The dense root systems stabilise the soil and bind it together, preventing further widening and collapse of the gully walls. Above-ground growth slows surface runoff entering the gully from the sides. [2]

Also acceptable: divert water away from the gully head using a diversion ditch, which reduces the volume of water entering and deepening the gully.

(d) Two effects of gully erosion on farmland productivity [2]:

1. Loss of fertile topsoil directly reduces crop yields because the nutrient-rich layer and organic matter have been physically removed. [1]
2. Land area is reduced as the gully widens, and fields may be divided, making cultivation with machinery difficult or impossible across the gully. [1]