Land use change

Question 1

What are the main types of land use change and their impacts on the hydrological cycle?

Answer 1

Urbanisation and Agricultural development

Question 2

Effect of urbanisation on the hydrological cycle (processes and impacts)

Answer 2

PROCESSES

1) Water supply and wastewater systems
- These are usually contained within pipe systems so they have relatively little impact on the sizes of floods and droughts (except WWTP discharges)

2) Extra impervious areas (e.g. buildings, paths, roads, carparks)
- change the relative proportions of infiltration, evaporation, runoff

IMPACTS

1) Surface runoff is larger
- peak discharges, often fed by stormwater drains, are larger
- lag time is shorter

2) Infiltration is lower
- low flows are lower

Question 3

Effect of rural land development on the hydrological cycle

Answer 3

1) Mechanised intensive farming
- compaction of soil - Increased surface runoff and soil erosion

2) Agricultural drainage
• tend to modify the timing of runoff and the peak flow rather than the volume of runoff
• peak flows tend to be reduced in clay catchments and increased in more permeable soils
• effects on local streams, only limited evidence at larger scales

3) Afforestation/deforestation

Question 4

What are the possible impacts of forestation/afforestation?

Answer 4

The ‘sponge’ model:
forests soak up water during rainy spells and release it during dry periods
- they increase low flows and reduce floods

The ‘pumps’ model:
forests remove water via transpiration during dry periods
- they reduce water yield, whilst having little influence on floods

Question 5

How can we quantify the effects of urbanisation of flood risk?

Answer 5

Hydrograph parameters (e.g. peak discharge, time to peak, lag time)
can be related to the surface area (or proportion) of impervious
surfaces.

And equations from the FEH using Urban Adjustment Factors for ungauged sites

Question 6

Give an example of Sustainable (urban) Drainage Systems (SuDS)

Answer 6

Green roofs
Wet ponds
Pervious surfaces

Question 7

Estimating the impacts of rural land development (2)

Answer 7

1) Generalise results from field studies on paired catchments
2) Use a catchment rainfall-runoff model to estimate the change in simulated river flows under different land use conditions (encapsulated in the model parameters for land cover and soil characteristics)

Question 8

How to mitigate the impacts of rural land development (3)

Answer 8

vegetated riparian buffer strips to intercept sediment runoff

• partial blocking of drains to provide storage for flood runoff
• constructed wetlands to store flood runoff and intercept pollutants

Changes at localised spots have little effect on downstream river flows: an integrated, catchment-scale approach is needed!

Question 9

Floods and low flows are affected by climate and catchment characteristics, what are the 4 main catchment characteristics?

Answer 9

Topography
Geology
Soils
Vegetation

Question 10

Characteristics and benefits of SuDS?

Answer 10

Cost effective solutions -
easy to manage, require little energy input, resilient to use

Low environmental impact -
reduced runoff rates (reduce DS flood risk), increased infiltration (increase GW recharge), improved water quality, providing habitat within the urban catchment

Aesthetically attractive