1.5 water balance

1. Describe in your own words the ‘water balance’, include seasonal variations?

2. Water balance is the split between where water is stored and how it is travelling throughout the system and is used to help hydrologists plan for the future for the water supply and for flood control for that particular drainage basin. It is calculated by using the equation: P = Q + E +/- S or precipitation = total runoff + evapotranspiration +/- storage. This varies during different seasons, for example there may be more precipitation in the winter and more storage in the summer

3. Write down the Water balance equation in both letters and words?

4. P = Q + E +/- S or precipitation = total runoff + evapotranspiration +/- storage

5. Identify 5 factors which affect surface runoff? Include man made situations.

6. An increase in carbon in the atmosphere can increase levels of precipitation which can increase surface runoff

7. Urbanisation can decrease how absorbent the soil is which can increase surface runoff

8. Deforestation can decrease the amount of water that gets absorbed from the soil by trees and can decrease evapotranspiration which can increase surface runoff

9. The geology of the area, as a base rock like chalk is very permeable and water can run through it but a bedrock like granite is impermeable which can increase surface runoff

10. Afforestation can cause greater absorption of water from the soil and increases transpiration which means that there is less water in the soil, decreasing surface runoff

11. Read about the Wye Valley Drainage basin and evaluate how ‘humans’ have affected the water balance? Find enough notes to support a 20 mark question

Human activity has significantly disrupted the Wye Valley's water balance through increased agricultural pollution, particularly phosphorus from fertilisers and livestock, leading to eutrophication and altered water quality. Urbanisation, with its sealed surfaces and combined sewer overflow (CSO) events, accelerates runoff and discharges pollutants directly into the river. In addition, river channel modifications, such as dredging and bank reinforcement, alter natural flow and floodplain connectivity, while irrigation practices can deplete local water resources. 

The Wye catchment suffers from a substantial phosphorus surplus, accumulated over decades from excessive use of fertilisers and livestock manure.

Soils in the Wye catchment are less effective at retaining phosphorus, leading to a significant loss of these nutrients into the river system, causing water quality issues like eutrophication. 

Increased arable farming for animal feed has led to greater water usage for irrigation, potentially depleting local water resources and contributing to water stress. 

Over 75% of sediment in the River Wye originates from agricultural land and riverbank erosion, which increases due to farming practices. The growth of urban areas with impermeable surfaces (e.g., concrete, tarmac) reduces infiltration and encourages rapid surface runoff, increasing flood risk and delivering pollutants to the river more quickly. Frequent overflow incidents from Combined Sewer Systems (CSOs) during high rainfall release untreated sewage into the river, further degrading water quality and impacting aquatic life. The construction of flood defences, embankments, and channel dredging separates the river from its natural floodplain and modifies natural flow, reducing the river's ability to absorb water and slowing the natural flow of water through the catchment. While not as prominently featured in the specific Wye context as in general water cycle discussions, the removal of trees for agriculture or other land uses reduces interception and transpiration, altering the amount of water that returns to the atmosphere. Human activities have shifted the components of the water balance equation, primarily affecting output (Q, streamflow) and evapotranspiration (E), and consequently increasing the risk of deficits or surpluses in the basin. Urbanisation and channel modification lead to faster, more direct runoff, increasing river flow and flood risk. Agricultural and urban runoff introduce pollutants and sediment, affecting water quality and ecosystem health. Over-abstraction for irrigation can lead to local water deficits, particularly in groundwater resources. Modifications to the river and floodplain can reduce natural water storage, while increased pollution concentrates in the aquatic environment. 

1. Complete the data in the table from page