Deforestation and agriculture alter hydrological flows by changing vegetation cover, soil structure and water abstraction. They commonly reduce interception, infiltration and transpiration while increasing surface run-off, although the precise effects depend on land management.
This is SL and HL content in IB ESS Subtopic 4.1, Water systems. The hydrological cycle contains water storages connected by flows such as precipitation, transpiration, evaporation, infiltration, percolation, surface run-off and groundwater flow.
| Human activity | Changes to hydrological flows | Likely consequences |
|---|---|---|
| Deforestation | Removing leaves reduces interception and transpiration. Less water returns to the atmosphere, while more precipitation reaches the soil directly. | Surface run-off and river discharge often increase, raising flood and erosion risks. Root loss and soil degradation may decrease infiltration and percolation. |
| Agricultural soil compaction | Heavy machinery and livestock compress soil, reducing pore space. | Infiltration and percolation decrease, while surface run-off increases. Groundwater recharge may fall, and sediment or pollutants may enter rivers. |
| Irrigation | Water is abstracted from rivers, reservoirs or aquifers and transferred to fields. | River flow and groundwater storage may decline, while evaporation and transpiration increase. Return flows may carry fertilizers into water bodies. |
The Aral Sea demonstrates these interactions: large-scale abstraction of river water for irrigated agriculture greatly reduced inflow, causing the lake to shrink and damaging ecosystems and livelihoods. It also illustrates environmental justice, because nearby communities carried disproportionate ecological, economic and health costs.
A common misconception is that agriculture always decreases infiltration. Compaction does, but contour farming, terracing and maintaining vegetation cover can increase infiltration and reduce run-off.
Exam technique: For an “explain” question, identify a changed flow, state its direction, and give the mechanism and consequence. Use precise terms such as surface run-off, infiltration, percolation and transpiration, supported by a named example such as the Aral Sea.
