How human activities cause changes to the water and carbon cycles and how are they linked?

What is the local scale impact of human activities on the water cycle?

• Rising demand for water for irrigation, agriculture and public supply especially in arid or semi-arid landscapes.

• The quality of freshwater resources has declined. Over pumping of aquifers in the coastal regions of Bangladesh has led to inclusions of salt water - unfit for agriculture or consuming

What is an example of rising water demand and the impact on the water cycle?

The Colorado Basin in the south west of the US, surface supplies have reduced as more water is abstracted from rivers and huge amounts are evaporated from reservoirs like Lake Mead and Lake Powell

What is the impact of deforestation on the water cycle?

• Reduces evaporation and therefore precipitation; increase surface run-off; decrease throughflow; lower water table

• In Amazonia, forest trees are a key component of the water cycle, transferring water to the atmosphere by evapotranspiration which is returned through precipitation. Extensive deforestation has broken this cycle, causing climates to dry out and preventing regeneration of the forest.

How are the two cycles linked: Photosynthesis

• CO2 is a greenhouse gas that is a key reactant in photosynthesis needed in plants, which are significant carbon stores, extract water from the soil and transpiration needed in plants/vegetation and phytoplankton.

• Plants, which are significant carbon stores, extract water from the soil and transpire it as a part of the water cycle.

• Water is then evaporated from the oceans to the atmosphere, and CO2 is exchanged between the two stores.

How are the two cycles linked: Oceans

• Ocean acidity increases when exchanges of CO2 aren’t in balance (when input exceeds outputs). The solubility of CO2 in the ocean increases with lower Sea Surface Temperatures.

• Atmosphere CO2 levels influence - SSTs and the thermal expansion of water, air temperatures, melting of ice sheets and glaciers, seal level

How are the two cycles linked: Vegetation and soil

Water availability influences the rate, NPP, inputs of organic content. Temperatures and rainfall aspect decomposition rates and the release of CO2 to the atmosphere.

How are the two cycles linked: Cryosphere

• CO2 levels in the atmosphere determine the intensity of the greenhouse effect and melting of ice sheets, glaciers, sea ice and permafrost.

• Melting exposes land and sea surfaces which absorb more solar insolation and raises temps further.

• Permafrost melting exposes organic material to oxidation and decomposition which releases CO2 and CH4.

• Run-off, river flow evaporation respond to temperature change.

What are the impacts of land-use changes on the water cycle?

• Urbanisation - converting farm and woodland into housing which is done through cleaning the land and replacing it with impermeable surfaces for roads. These surfaces don’t allow much infiltration and provide minimal water storage capacities to buffer run-off.

• Urban areas also have drainage systems designed to remove surface water rapidly (e.g: pitched roofs, gutters, sewage systems). As a result high proportion of water from precipitation flows quickly into streams and rivers leafing to rapid rise in water level

• Urbanisation encroaches on floodplains, which are natural storage areas for water. Urban development on flood plains reduce water storage capacity in drainage basins, increasing river flow and flood risks.

What are the impacts of land-use changes on the carbon cycle?

• It also impacts the carbon cycle as it removes vegetation and its replacement with urban surfaces results in a reduction in the amount of organic carbon. The development of factories and homes, along with increased vehicle usage, resulting in more CO2 in the atmosphere.

What are the impacts of farming on the carbon cycle?

• The clearance of forest for farming reduces carbon storage above and below biomass.

• Soil carbon storage is reduced through ploughing and the exposure of organic matter to oxidation.

• Harvesting crops also returns small amounts of carbon back into the soil

• Aeoline forces erode and lifts crops leading the lifting of crops and the soil having no protective cover.

• NPP is more productive but photosynthesis exchanges are lower than in natural ecosystems are lower than in natural ecosystem (due to a lack of biodiversity). In part this is because crops from soil are reduced to four five months of growth

What are the impacts of farming on the water cycle?

• Crop irrigation diverts surface water from rivers and groundwater to cultivated land. Most of the water is lost to evaporation and soil drainage.

• Interception of rainfall by annual crops is less than in forest and grassland ecosystems. + Evaporation and transpiration

• Ploughing increases evaporation and soil moisture loss. The furrows ploughed downslope act as drainage channels, accelerating run-off and soil erosion. Infiltration due to ploughing is usually greater in farming systems while artificial underdrainage increases the rate of water transfer to streams and rivers. Surface run-off increases where heavy machinery compacts soils. This peak flows on streams draining farmland are generally higher than in natural ecosystems.

What are the impacts of forestry on the carbon cycle?

• Local impact: clearing felling to harvest timber creates sudden but temporary changes to the local H2O cycle, increasing run-off, stream discharge and reducing evapotranspiration.

• Changing farmland to forestry increases CO2 stores. In a typical plantation, mature forest trees contain on average 170-200 tonnes. This is x10 higher than grassland and x20 higher than heathland.

• They extract and sequester CO2 for hundreds of years as its stored in the wood. However trees only absorb carbon after 100 years after planting.

• The amount of CO2 absorbed levels off and is balanced by the inputs of litter to the soil, the release of CO2 in respiration and by activities of soil decomposes. Forestry plantations usually have a rotation of 80-100 years

What are the impacts of forestry on the water cycle?

• High rates of rainfall interception - in Eastern England, interception rates for Sitka spruce are as high as 60%. The needle-like structure of conifer leaves contribute to high rates of interceptions.

• Increased evaporation - a large proportions of intercepted rainfall is stored of leaves surfaces and is evaporated directly to the atmosphere.

• Reduced run-off and stream discharge - with high interception and evaporation trees roots drainage basin hydrology is altered. Streams drainage plantations typically have relatively long lag times, low peak flows and total discharge. The effect of conifer plantations in upland catchments is often to reduce water yield for public supply

What is transpiration rates of Sitka Spruce in the Pennines?

350 mm/year

How much carbon does forest soil store?

500 tonnes C/ha

What is the example of water extraction and it’s impact on the water cycle?

• The River Kennet in the south of England drains an area of around 1200 km2 in Wiltshire and Berkshire.

• The upper catchment consists of highly permeable chalk and thus groundwater contributed most of the flow.

• The water is filtered through chalk and fauna such as brown trout.

• Several habitats and urban areas rely on the water in the basin such as Swindon with a population of 220,000 is the largest area.

• It supplies water for industries, agriculture and public water. Thames Water abstracts groundwater from the upper catchment from boreholes, which isn’t returned to the river as waste water

• Rates of extraction have exceeded rates of recharge, and the falling water table has reduced flows in the river Kennet by 10-14%.

• During the 2003 drought, flow levels fell by 205.

• Lower groundwater levels have caused springs and seepages to dry up and reduce the incidence of saturated overland flow on the chalk hills of Marlborough