5.3

• The Soil Water Budget

  Precipitation (P): All forms of moisture reaching the Earth’s surface from the atmosphere.

  Potential Evapotranspiration (PE): The possible amount of water that may be evaporated from the soil or from plants given the average temperatures.

  
• Soil Water Surplus: When P continues to be higher than PE so that the soil is saturated. Continued precipitation will become overland flow.

  Soil Moisture Utilisation: When PE rises above P the moisture in the soil will begin to fall due to evaporation and transpiration by plants not being replaced by rainfall.

  Soil Moisture Recharge: P rises above PE and water in the soil begins to refill.

  Field Capacity: The total amount of water that a soil can collect in its pores before it becomes saturated.
• A river regime describes the annual variation in the discharge of a river and is influenced by the factors that affect the drainage basin system and water budgets. This includes climate, geology and soils.
• Baseflow - The starting and finishing level of discharge. This water is produced by Groundwater. It will not change a great deal, however permeable geology will have a greater impact on Baseflow.

  Rising Limb - As storm water enters the system the discharge rises

  Peak discharge - The highest flow after a precipitation event.

  Lag Time - time interval between peak rainfall and peak discharge

  Recession/Falling Limb - The receding water level after the peak discharge.

  Bankfull Discharge - when the water level reaches the top of the channel.
• Steepness of the Rising Limb - if it is steep it indicates a Flash Flood. If it is gentle then it is Subdued.

  Length of the Lag Time - if short then this indicates a Flash Flood. If long then it indicates a Subdued hydrograph.

  Steepness of the Falling Limb - if steep and short then it will reflect the Flashy nature of the event. If long and gentle it will reflect the Subdued nature of the flood
• factors affecting the shape of the Hydrograph


  Intensity and duration of the storm

  Antecedent Moisture

  Impermeable rock types

  Size + shape of drainage basin

  Temperature

  Vegetation

  Land use

  Deforestation

  Urbanisation - drains and sewers