Feedback in water cycle

• Rising temperatures affect the water cycle at the global scale
• In a warmer world, evaporation increases and the atmosphere holds more vapour. The result is greater cloud cover and more precipitation. These changes create a positive feedback effect
• Because water vapour is a greenhouse gas, more vapour in the atmosphere increases absorption of long-wae radiation from the Earth, causing further rises in temperature
• Alternatively, more atmospheric vapour can induce negative feedback. This works as follows: more vapour creates greater cloud cover which reflects more solar radiation back into space. And as smaller amounts of solar radiation are absorbed by the atmosphere, oceans and land, average global temperatures fall
• In drainage basins in the longer term, inputs and outputs of water are in equilibrium. The main input, precipitation, is balanced by outputs of evapotranspiration and run-off. However, this balance varies from year to year. The system responds to above average precipitation by increasing river flow and evaporation; and excess water recharge aquifers, increasing water storage in permeable rocks
• During droughts the system adjusts to lower precipitation by reducing run-off and evapotranspiration. Meanwhile, as the water table falls, springs and seepages dry up, helping to conserve groundwater stores
• Feedback in the water cycle also takes place at the smallest scale. In most years precipitation is sufficient to satisfy an individual tree's demand for water. However, in drought years, shallow-rooted trees like silver birch become stressed: water lost in transpiration is not replaced by a similar uptake of water from the soil. The tree responds, reducing transpiration losses by shedding some or all of its leaves. This negative feedback loop restores the water balance and ensures the tree's survival