Water and carbon cycles 4

Deforestation - before:

• The world's largest continuous tropical forest and savannah ecosystems are contained in the tropics of South America.
• This region is environmentally important not only because of traditional ecological measures, such as its high biodiversity, but also because it generates more than a quarter of the world’s river discharge.
• 10% of the rainforest has been converted into cattle pasture and agriculture.
• Deforestation and forest degradation result in a complex set of changes to streams of all sizes.
• The result is that less water evaporates from the land surface to be returned to the atmosphere; more water runs off of the land and stream flow is increased.
• The amount of change that occurs depends on local conditions including the amount of rainfall, how much of a watershed is deforested, topography, soils and the land use after deforestation. 
• Studies have shown that there is little effect with less than 20% of a basin deforested but a large increase with 50 to 100% of a basin deforested. These changes occur at the local scale, but rivers of all sizes are affected when deforestation is extensive.

Much of the high precipitation is returned to the atmosphere by evapotranspiration before deforestation. Overland flow is minimal. Most of the water that reaches the forest floor, inflitrates into the soil and travels slowly to the river by throughflow, maintaining a steady flow in the river.

After deforestation, although the precipitation stays the same, evapotranspiration is lower due to the replacement vegetation has smaller leaves and roots is less dense. Overland flow and throughflow occur because of the lack of vegetation. This leads to increased discharge and flashiness. This can cause localised flooding; suggesting that several important points about the climate, land surface and water cycle:

• If deforestation does not cause decreased rainfall, discharge will likely be significantly increased throughout the entire southern Amazon.
• If rainfall does decrease, the resulting decrease in river discharge may be greater than the changes without feedbacks.
• Changes in water resources caused by atmospheric feedbacks will not be limited to those catchment areas where deforestation has occurred, but will be spread unevenly throughout the whole Amzon basin by atmospheric circulation.

Soil drainage:

Subsurface drainage removes excess water form the soil profile. It is carried out usually through a network of perforated tubes installed 60-120cm below the surface of the soil. These tubes are called 'tiles' because they were originally made from short lengths of clay pipes. Water would…