(a) Describe the methods of irrigation agriculture in either the Nile Valley or the Niger Valley (b) Give reasons for the irrigation agriculture in any one ...

Irrigation agriculture in the Nile Valley (Egypt/Sudan)

(a) Methods of irrigation

• Basin irrigation: low-lying fields are enclosed by earthen banks (bunds); flood water from the annual rise of the Nile is trapped in these basins, allowed to soak the soil and deposit silt, then the surplus is drained off before planting. This is the traditional method.
• Perennial (canal) irrigation: a network of main canals, branch canals and field ditches distributes water throughout the year. Regulators and barrages (e.g. at the Aswan High Dam) store flood water and release it as needed, allowing two or three crops per year.
• Lift irrigation using traditional devices: the shaduf (a counter-weighted bucket on a pole), the sakia (an animal-turned water wheel) and the Archimedean screw (tambour) raise water from the river or canal onto higher fields.
• Modern mechanical pumps driven by diesel or electricity lift large volumes of water from the river and canals to the fields.

(b) Reasons for irrigation agriculture in the Nile Valley

• The region has a hot desert climate with very low, unreliable rainfall, so crops cannot grow on rainfall alone.
• The Nile provides a permanent, reliable supply of water flowing across the desert.
• The annual flood deposits fertile silt (alluvium), producing rich soils that support intensive cultivation.
• High temperatures and abundant sunshine throughout the year allow all-year cropping once water is supplied.
• A large, dense population needs food and cash crops such as cotton, hence the demand for continuous production.

(c) Problems associated with irrigation agriculture in the Nile Valley

• Salinisation: continuous irrigation with poor drainage causes salts to accumulate in the soil, lowering fertility.
• Waterlogging: over-watering raises the water table and floods the root zone, damaging crops.
• Loss of silt: since the Aswan High Dam traps silt in Lake Nasser, downstream fields no longer receive natural fertiliser and farmers must buy expensive chemical fertilisers.
• Spread of water-borne diseases such as bilharzia (schistosomiasis) and malaria, which breed in the standing canal water.
• Silting of canals and reservoirs reduces storage capacity and raises the cost of maintenance and dredging.
• High evaporation losses from canals and reservoirs in the hot desert climate waste water.
• Pests, weeds and disputes over water sharing between countries and farmers also occur.