Examples of How Land Use Changes Can Affect the Water Cycle

Deforestation

South America generates over a quarter of the world’s river discharge, but over the last 50 years, the area has undergone major deforestation and explosive development due to increased demand for cattle feed, beef and sugar cane for ethanol.
Of all the rainforest in the area, about 10% of it has been developed to cattle pasture and agricultural land.
Deforestation/forest degradation can majorly change streams of many sizes. New vegetation to replace that which has been lost is younger so has shallower roots and fewer leaves.
This means that less water evaporates from the land surface, more water runs off the land so stream flow increases.
The extent of change depends on amount of rainfall, how much of the watershed is cut down, topography, soils and land use after deforestation.
Studies suggest that if less than 20% of a basin is cut down, there is very little effect, in contrast with 50-100%, when changes are major.
If deforestation does not result in a decrease in rainfall via atmospheric feedback, river discharge will tend to be greatly increased.
If deforestation does result in a decrease in rainfall via atmospheric feedback, the resulting decrease in river discharge could be greater than changes without feedbacks.
Changes in water resources due to atmospheric feedbacks are not limited to catchment areas where deforestation has been carried out, but it will be spread unevenly throughout the whole basin by atmospheric circulation.

Soil Drainage

Subsurface drainage removes excess water from the soil profile. It is carried out through a network of perforated tubes (tiles) between 60 and 120 cm below the surface.
Perforations allow water to enter the plastic tubing, only when the soil’s water table is higher than the tiles.
The process lowers the water level to be in line with the tiles over a period of several days. The tiles allow excess water to leave fields.
Agricultural drainage is good for moderately/poorly drained soils, as it increases the productivity of the field and improves efficiency of growers.
There are lots of advantages for drainage of marginal farmland. An improved soil structure develops so the land becomes easier to work.
Greater roots penetration can be achieved so roots can travel further and faster. There is also improved aeration, so microorganisms can thrive, which means organic matter is broken down into hummus more quickly.
Improved aeration also means soil is warmed more easily, which means farmers can sow seeds earlier, increasing the likelihood of successful germination.
Heavy machinery works on the land without risk of compaction. Plenty of animals can graze on the land without risk of compaction.
There are also many disadvantages of agricultural drainage. Application of drains into the soil increases throughflow speed in the soil.
This increases likelihood of flooding. Another disadvantage is that if the dry top soil is not protected well, it can be prone to wind erosion.
The average rate of wind erosion is of the order of 0.1-2 tonnes/hectare/year. A final disadvantage of agricultural drainage is that the water drains, containing nitrates, and finds its way to local watercourses. The ponds are enriched with nitrogen or phosphorus, so eutrophication can occur.
A solution to the problems surrounding agricultural drainage is to use controlled drainage, which keeps the water table high when crops are not growing.
This increases rate of denitrification the process of converting nitrate to harmless nitrogen gas as soon as saturated soil warms up in spring, which means the loss of nitrates to the environment is reduced.

Water Abstraction

There can be problems in a basin when demand for water exceeds the available amount of water during a certain period. It often happens in areas with low rainfall but high population density as well as in areas with intensive agricultural/industrial activity.
In much of Europe, groundwater is the main source of freshwater, so it is pumped from the ground faster than it is replenished by rainfall.
As a result, the water tables sink, wells are empty, pumping costs increase and in coastal areas, saltwater from the sea intrudes and degrades the fresh groundwater.
Saline intrusion is a major issue along Mediterranean coastlines including Spain, Italy and Turkey, as tourist resorts increase the demand for water.
Malta now has the issue of being able to use very little of its groundwater for domestic consumption/irrigation so has expensive desalinisation plants.
Sinking water tables reduces the reliability of rivers, as springs tend to dry up during the dry season because the water table falls so significantly.
Surface reservoirs like lakes and wetlands are very productive as ecosystems and for leisure activities but are threatened by over-abstraction.
In agricultural areas, the main cause of exploitation of groundwater is irrigation. For example, in Italy, over-exploitation of the Po River in the Milan aquifer region has resulted in a 25-40m decrease in groundwater levels throughout the past 80 years.
Water abstraction from the chalk of southern England
The North and South Downs and the Chilterns are chalk hills, on which rain falls and replenishes the water in the chalk aquifer of southern England.
Recharge tends to take place in winter months, as potential evapotranspiration is low and soil moisture deficits are negligible.
Groundwater amounts vary seasonally, rising in the autumn through spring.
In summer, potential evapotranspiration generally exceeds precipitation, so there is soil moisture deficit and very little percolation takes place.
During the summer, water leaves chalk from springs and by abstraction from boreholes. Rainfall varies in time and location, so the pattern is not constant.
Rivers that are fed by groundwater from chalk aquifers sometimes have intermittent sections which are referred to as ‘bournes’ and are common in chalk downlands.
If there is one or more dry winter, when rainfall for recharge is low then the rivers can dry up totally.
Some of the most severe problems with over-abstraction are in chalk stream systems, where up to 95% of the flow is derived from underground aquifers.
The chalk stream catchments give underground reservoirs which are usually quite high-quality groundwater, so is abstracted for public supply and industry.
Abstraction has greatly reduced the flow in lots of chalk streams and can also completely dry up certain sections of the rivers, especially in summer, as this is when public demand is greatest.
It affects local communities economically and socially, as they cannot fish, and the river views are ruined.

Water Abstraction in the London Basin

The diagram is the subsurface geology of the London Basin. Chalk layers form a syncline below London, with the Chilterns to the North West and the North Downs to the South East.
On the exposed chalk hills, precipitation soaks into the porous rock, is stored and then released naturally at springs.

Water has been abstracted from London from wells and boreholes penetrating the chalk. In the 19^th and early 20^th century, chalk-basal sands aquifer was increasingly exploited due to industrialisation.
The 1960s was the height of abstraction, at groundwater levels were at 88m below sea level in London at this point as there was a large depression in the water table.
Since the mid-1960s, economic activity in London has changed to service industry/commerce instead of heavy industry.
This means that less abstraction has been taking place, so groundwater levels recovered by up to 3m/year by the 1990s, so the water table rose.
However, this resulted in threats to structures in the Basin, like the London Underground, as well as the building foundations.
Therefore, the General Aquifer Research, Development and Investigation Team (GARDIT) was introduced to control water levels.
Abstraction and artificial recharge were carefully managed by the GARDIT, so aims had been mostly achieved by the year 2000.

Differences in Groundwater Levels Between January 2000 and January 2014

Levels have risen in West London due to limited abstraction in the area at about 4-8m since 2000, levelling off in recent years
Levels have fallen in Central and East London in the order of 5-7m since 2000 as result of increased abstraction.
Levels fallen over 2m across much of South London around many large public water supply abstractions, levels have fallen up to 12m.
Risk of saline intrusion in east London where there are chalk outcrops happens when groundwater levels near the river are lower than water level of the River Thames where saline river water may enter the chalk aquifer.