Glaciers and ice caps melting may be due to global warming which is an indirect affect on the hydrological cycle by humans. It means that ice can hold less water, and so this excess water is no longer held as storage, but enters the cycle in oceans and lakes. This water can then evaporate and condense to become precipitation, and so rainfall may increase.
Humans may intentionally affect precipitation levels using cloud dusting. This is when planes are used to spray a chemical into the atmosphere. The chemical gives a surface for the separate water particles to attach to, and so they form a raindrop. This helps farmers in times when there is little rainfall, and it is a direct and intentional affect on the hydrological cycle by humans.
Water is taken from the water table for domestic human usage. This means that the water table drops, and so there is no base flow into rivers and rivers may dry out. It also takes the water from the water cycle. The water may then enter a human drain as it is sued which will move it quickly to a river or a lake. There is therefore less water being stored in the water table and the water is then moved to another storage or another flow to which it may add a large volume. Cities also affect the ground water because surfaces such as tarmac allow no infiltration into the soil, but instantly transport water to a river or to the sea, so instead of slowly infiltrating towards a river, the lag time is once again decreased as the discharge rapidly increases, See Figure 2.
Dams and reservoirs add extra storage and control the flow of water into rivers. They help prevent floods and regulate the discharge of a river. This would have the affect of not allowing too much water to enter a river at a time, and so a storm hydrograph would look more like Figure 1. These, along with other flood prevention methods, also affect the flow of a river, for example building of levées to contain water in a river and weirs to control the amount of water in the river at any one time. This can have various influences on a river basin.
Urbanisation leading to deforestation has the biggest affect on the flows of a hydrological cycle. Urbanisation removes vegetation, which means that stem flow, infiltration and evapotranspiration cannot occur. It also reduces infiltration and increases runoff and over ground flow. If vegetation is removed then no interception can occur, which means that no water can be stored which can later evapotranspire, and no water can be taken from the ground in stem flow. It is instead transported to a river straight away. A lot of water reaches the river in a short time so the rising limb is steep, and the lag time is shorter so peak discharge is therefore higher, see Figure 2. This increases the chance of flooding of a river. Impermeable surfaces such as concrete and tarmac cover the ground in cities so water cannot infiltrate slowly. It is run into drains and again reaches the river much more quickly than if it had infiltrated. This has the same affect on the hydrological cycle as the removal of vegetation.
With references to examples, outline the results of such modification.
I have found examples of the effects that humans have had on certain rivers in the UK. The River Ystwyth in Wales has had flood control systems installed in 1973 due to the spilling of water over the right bank, and the blockage of a lower inlet pipe makes the flows unreliable. The land usage of the surrounding area is mainly extensive hill farming which would have little effect on the river discharge levels, but there is livestock farming at lower levels which may mean that the ground has had many trees removed and the ground may have been compacted by the trampling of animals, so would mean that water would travel in overland flow because there would be no pores for the water to fill in the ground and so could not infiltrate, which would mean that it reached the river faster than usual. 18% of the area is forest, which would help in slowing down the passage of water to the river due to interception and stem flow during the periods of wet weather. The trees may also absorb through flow that would otherwise enter the river, and evapotranspiration would occur without the water ever entering the river.
The River Usk has only 3% forest, combined with dairy and livestock farming in the lower catchment areas which would mean that ground was trampled, and both of these factors would combine to help water to travel much faster by overland flow with little interception rather than by through flow, which would take much longer than overland flow. The intensive trampling of animals would crush the pores in the ground, so that little infiltration could occur. Having so little forest would mean that instead of being intercepted and water being absorbed by the roots in the ground, the water would reach the ground straight away and thus enter the river much more quickly. In the upper areas on the other hand, there are three public water supply reservoirs that have some impact on the flows of the river because they would hold much of the water that flowed from the highest part of the catchment. This would mean that the amount of water could be regulated in the upper part of the river. In contrast to this, although the River Tone has two reservoirs, Clatworthy and Luxhay, these are not large enough and do not contain enough water to have any effect on the naturally short lag time due to the speed at which water enters the river. The high hills either side of the river cause this short lag time as well as the rural land use having little effect either increasing or decreasing the river cycle as compared to an urbanised area. There are only minor water abstractions for the Public Water Supply, and so the river is relatively unaffected by human activity due to rural land uses, although the water abstractions may cause the water table to sink during certain times of the year, and so base flow into the river would decrease causing a decrease in river discharge levels.
The River Lee has had significant urbanisation in the lower part of the valley recently, which would prevent infiltration due to surfaces being covered in impermeable substances such as tarmac and concrete, and so decrease lag time and add to flood problems that may occur by putting pressure on the river due to a sudden increase in river discharge. Urbanisation also removes trees from an area, which reduces interception, and therefore water reaches the ground quickly. The excess water is run through drains and is deposited straight into the river reducing the water travel time by hours or even days. There are large amounts of ground water extraction for the Public Water Supply, which would reduce the volume of water in the river, and the pervious chalk landscape also allows water to infiltrate outside of the urbanised areas of the catchment area, and so this would help increase the lag time between peak rainfall and peak discharge, so this river, although it would be effected by humans, would not suffer and flood due to human impact.
The River Itchen has human influences such as ground water abstractions and cress farms which although originally had little effect on the river are now becoming increasing impact on the river regime as the river depends predominantly on base flow, the addition of water to a river from the water table. The extraction of water from the water table means that it becomes lower each year and so less water is added and therefore river discharge decreases. The catchment is 90% chalk with mainly arable land and a few small settlements, so these would have little effect on the river cycle, and the hugely permeable chalk would allow a lot of infiltration which would mean the water was travelling by through flow which is much slower than surface runoff, and so would increase lag time.
