The water budget, or water balance, of a drainage basin shows the relationship between precipitation, evapotranspiration, surface runoff, and changes in water storage. It is often expressed as an equation where precipitation equals surface runoff plus evapotranspiration plus or minus changes in storage. The water budget is useful for hydrologists to understand water surplus and deficit, and to plan for potential water shortages or recharge after deficit. A water budget graph illustrates surplus when precipitation exceeds evapotranspiration and deficit when the reverse is true.

1. The Water Budget (Balance) What is it? The balance between inputs and outputs in a drainage basin, usually shown on a graph. It is useful to hydrologists who can plan and manage water supply e.g. Possible water shortages. It is often expressed as an equation: P = Q + E +/- changes in storage P = precipitation, E = evapotranspiration, Q = runoff

2. The Water Budget Learning Objectives; To understand how the balance of inputs and outputs of water in a drainage basin can be shown in a calculation of ‘the water budget’ To suggest how the water budget is useful to hydrologists managing water surplus and deficit.

3. The balance between the input and output of a drainage basin may be shown as a WATER BUDGET GRAPH When precipitation is higher than Evapotranspiration there will be a WATER SURPLUS 2. When Evapotranspiration exceeds precipitation there will be SOIL MOISTURE UTILISATION occurring, this leads to WATER DEFICIT (soil moisture utilisation means water is being lost from the soil, through interception, evaporation mostly) 3. When the soil is recovering from deficit we term it SOIL MOISTURE RECHARGE 4. We then Consider over a whole year whether a place has a ………… positive or negative WATER BUDGET

4. The Water Budget

5. The Water Budget Potential evapotranspiration (PET): the amount of water that could be evaporated and transpired if there was sufficient water available. Soil Water Deficit means that the soil is dry, or could hold more water. Soil Water Recharge means the soil is re-absorbing moisture after a shortage Soil water utilisation is when the water contained in the soil is being lost again Field Capacity is the amount of water that the soil type can hold after excess has run off and downward movement into ground water sources has slowed to a minimum.

6. One pupil to come and explain what is happening here- use as many key terms as possible

7. Discuss in pairs the water budget for Khartoum.

8. Pupils to come up and explain what happens in each diagram. Imagine this was an exam question (GAS) Do we think that the UK and Greece have an overall positive or negative Water Budget?

9. Reminder.. General Anomaly Specific GAS….

10. The Water Budget: Tasks Make a large copy of a water budget graph of a drainage basin and label it. (Fig 1.4) “The UK usually has a positive water budget” is this statement true or false and why? Describe how the water budget in a drainage basin may change over the course of a year in the UK. Explain how the water budget is a useful calculation for hydrologists and environmental scientists.

11. The Water Budget (RECAP) Learning Objectives; To understand how the balance of inputs and outputs of water in a drainage basin can be shown in a calculation of ‘the water budget’ To suggest how the water budget is useful to hydrologists managing water surplus and deficit.

12. Independent Learning this week: Annual cycle of soil moisture sheet. Interception and infiltration sheet. Key Words, just the ones we have done. Presentation on your river to present to the group (2 mins) 2 WEEKS TIME COVER WILL BE SET FOR NEXT WEEK IN ADDITION TO THIS!