Key idea: The hydrological cycle influences water budgets and river systems at a local scale.

Lesson Objectives:

To understand how the water budget shows the annual balance between inputs and outputs. To understand how different climates can influence the water budget. River regimes are an important annual indicator in water availability within a drainage basin.

Water Budgets= the annual balance between inputs into a drainage basin and outputs. It is important for understanding the processes operating in a drainage basin and water balances throughout the year. The water budget can be used to help people manage water supply and to predict whether or not there will be a water surplus or deficit.

The water balance can be shown using the formula:

Precipitation (P) = discharge (streamflow) (Q) + evapotranspiration (E) +/- changes in storage (S)

P=Q+E +/- S

The diagram below (figure 1) illustrates the main features of the water budget:

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Figure 2: Provides a clear breakdown of what the water budget shows.

Key points about water budgets: You can look at the water balance at a global level (hydrological cycle), at a local level (drainage basin cycle)

or even just a field. At a global level oceans tend to experience greater outputs (evaporation) than inputs (precipitation). This is

because oceans are large unshaded bodies of water that have regular winds blowing saturated air on land, allowing greater evaporation. In addition oceans don't tend to suffer from the same amount of relief and convectional rainfall as land does. On land, inputs (precipitation) tends to be greater than outputs (evaporation). This is because lands suffers from larger amounts of frontal, relief and convectional rainfall, as well as much of the lands water being protected underground or in shaded areas reducing evaporation. At a global level there obviously has to be an equilibrium between inputs and outputs. The excess precipitation on land is returned to the oceans by channel flow, surface run-off and to a lesser extent groundwater flow. The excess of evaporation is returned to the land from the sea by winds blowing saturated air on land.

In contrast, at a local level there tends to be an imbalance in inputs and outputs within drainage basins due to them being an open system.

Limitations on a local level (drainage basins): they look at yearly averages, so don't take into account months and years of above average or below average temperatures and rainfall.

In the UK (a temperate climate): The general water balance in the UK shows seasonal patterns. In wet seasons (winter) precipitation is greater than evapotranspiration which creates a water surplus. Ground stores fill with water which results in increased surface runoff, higher discharge and higher river levels. This means there is a positive water balance. In drier seasons evapotranspiration exceeds precipitation. As plants absorb water ground stores are depleted. There is a water deficit at the end of a dry season (summer). SEE FIGURE 2.

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Key words:

Surplus= If precipitation exceeds evapotranspiration and the excess is not been used by plants. A water surplus can result in wet soils, high river levels and run-off.

Deficit= Evapotranspiration exceeds precipitation and there tends to be water shortages. A deficit leads to dry soil, falling river levels and possibly drought.

Recharge= Replacement of water lost during drier periods.

Field capacity= The maximum amount of water soil can hold.

Potential evapotranspiration (PE) = An estimate of the amount of water lost through evaporation and transpiration in any given period, depending on temperature and air humidity.

Soil moisture excess= When soil moisture and groundwater is replenished. The excess may lead to saturation and increased surface run-off.

Soil moisture utilisation= When precipitation reduces and soil moisture stores begin to be used

Soil moisture deficit= When there is a shortage of soil moisture stores and deeper groundwater reserves and surface reservoirs need to be used.

Soil moisture recharge= When precipitation increases and soil moisture stores fill, high infiltration and little surface run-off. Recharge occurs when precipitation exceeds potential evapotranspiration.

Task 1: Answer the below questions using the information above as well as the water budget graph below:

1. In which months is there a water surplus?

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2. Why is there soil moisture recharge in October?

3. When is field capacity attained?

4. Make a copy of the water balance equation

5. Define the terms precipitation and potential evapotranspiration

6. Explain how these two terms are used to work out the water budget?

Task 2: Use the two water balance graphs below to describe and explain the water balance for both the Atacama Desert Peru AND Bolivia ( Tropical climate )

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Task 3: Add the statements below onto the water budget graph

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Task 4: Using the water budget graph for Birmingham, England (a temperate climate) answer the following questions

Q1. Name the months when there is a soil moisture surplus.

Q2. State one of the months when potential evapotranspiration exceeds precipitation the most and by what amount?

Q3. Give the meaning of soil moisture recharge.

Mark scheme:

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River Regimes=The annual variation in discharge or flow of a river at a particular point or gauging station, usually measured in cumecs. The regime of a river is expected to have a seasonal pattern of discharge during the year. This is due to factors such as: climate, local geology and human interaction etc., Tropical rivers generally have fairly uniform flow patterns, especially in areas where rain falls every month. However, worldwide, river regimes tend to have a seasonal pattern whereby they respond directly to the amount of precipitation e.g. the UK.

The character of a regime of the resulting stream or river is influenced by several variable factors:

The size of the river and where measurements are taken in the basin: many large rivers have very complex regimes resulting from varied catchments.

The amount, pattern and intensity of the precipitation: regimes often reflect rainfall seasonal maxima or when the snow fields or glaciers melt (for snow the peak period is in spring and glaciers in summer).

The temperatures experienced: evaporation will be marked in summer as the temperatures are warmer. The geology and overlying soils, especially their permeability and porosity: water is stored as groundwater

in permeable rocks and is gradually released into the river as base flow, which tends to regulate the flow during dry periods.

The amount and type of vegetation cover: wetlands can hold the water and release it very slowly into the system.

Human activities, such as dam building, which can regulate the flow. Overall the most important factor determining stream flow is climate, as shown below.

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Different types of regimes:

Simple regimes= These show times of high water levels followed by lower levels. They exist as a result of a glacier melt, Snowmelt, or seasonal rainfalls such as monsoons.

Complex regimes= If a river has more than one period of high water levels and/or low water levels, a more complex regime results. It is more common on large rivers that flow through a variety of relief and receive their water supply from large tributaries, for example, The Ganges.

Contrasting River Regimes EXAMPLES:

1) The Amazon: is 6,308km long and has a drainage basin of nearly 6million km², it is in a humid, tropical climate and flows over an ancient shield area of rock. Major influences= rainforest climate, seasonal precipitation and evapotranspiration levels are very high. Its peak discharge is in April- May and lowest discharge is in September which is linked to wet and dry seasons and snowmelt from the Andes. Human influences= large dams used by Brazil’s major cities and for HEP. Moderate variability.

2) The Yukon (Alaska): is 3,540km long and has a drainage basin of about 850,000 km², it is in a Tundra climatic area and flows through a mountain range. Its peak discharge is in May-June with a dramatic increase due to melting of snow and ice; it is low from December to May due to frozen precipitation. Very large seasonal variability. Human influences= relatively few- most of its landscape is wilderness. Some HEP.

3) The river regime of the Nile was changed significantly by the construction of the Aswan Dam in 1970. The flow of river below the dam was reduced by about 65% and became regulated between the seasons so that the flood peaks in September were greatly reduced. Overall, the dam is regarded to have had a severe impact on the river regime (as seen below).

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Homework: Using the data below for South Hampshire (UK), draw a water budget graph and complete it so it resembles the ones above.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Rainfall (mm) 78 59 48 49 54 48 58 60 63 70 75 77

Potential evapotranspiration (mm)

5 11 31 50 82 95 97 88 60 33 12 5

Answer the following questions: A) Does South Hampshire have an annual water surplus or deficit? B) Using the data, outline the duration and characteristics of the periods of: recharge, surplus, utilisation and deficit in South Hampshire on your graph. Hints: Make sure you use the definitions and the water budget graphs to help you. Also the x axis will be the months, the left hand y axis rainfall (mm) and the right hand y axis potential evapotranspiration (mm).

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