CGP Variations in runoff and water cycle

• Runoff


  The water flowing on the surface of the land
• River discharge

  The volume of water in cubic metres per second flowing in a river
• High levels of runoff increase the discharge of a river because more water makes it into the river increasing its volume
• Hydrograph


  A graph of river discharge over time
• Flood hydrographs


  • Also called storm hydrographs, they show river discharge around the time of a storm event and only cover a relatively short time period (hours or days)
• Peak discharge


  The highest point on the hydrograph, when the river discharge is at its greatest
• Lag time


  The delay between peak rainfall and peak discharge, because it takes time for the rainwater to flow into the river
• Rising limb


  The part of the hydrograph up to peak discharge, where river discharge increases as rainwater flows into the river
• Falling limb


  The part of the hydrograph after peak discharge, where discharge is decreasing because less water is flowing into the river
• Bankfull discharge


  The point when the water level reaches the top of the river channel
• A basin with rapid runoff and not much storage capacity gives a hydrograph with a short lag time and high peak discharge, called a "flashy" hydrograph
• Factors affecting runoff and hydrograph shape

  • Size of drainage basin
  • Shape of drainage basin
  • Ground steepness
  • Rock and soil type
• Larger drainage basins catch more precipitation, so they have a higher peak discharge than smaller basins
• Smaller basins generally have shorter lag times because precipitation has less distance to travel, so it reaches the main channel more quickly
• Circular basins are more likely to have a flashy hydrograph than long, narrow basins because all points on the watershed are roughly the same distance from the point of discharge measurement
• Water flows more quickly downhill in steep-sided drainage basins, shortening lag time, and also means water has less time to infiltrate the soil, so runoff is higher
• Impermeable rocks and soils don't store water or let water infiltrate, increasing surface runoff and peak discharge
• Intense storms generate more precipitation and greater peak discharges than light rain showers
• Larger inputs of water can cause flows and stores to increase in size, and some flows may not be able to occur rapidly enough, increasing runoff
• In the UK, winters are normally wetter than summers
• During winter, temperatures may drop below 3°C, causing water to freeze, which can reduce the size of flows through drainage basins, while the store of frozen water grows
• When temperatures increase again, flows through drainage basins and output can be much larger as the ice melts
• Most plants show seasonal variation in evapotranspiration, usually decreasing in winter as vegetation intercepts less precipitation and slows its movement to the river channel
• The more vegetation there is in a drainage basin, the more water is lost through transpiration and evaporation directly from the vegetation before it reaches the river channel, reducing runoff and peak discharge
• Human activities affecting the water cycle

  • Farming practices
  • Water abstraction
  • Land use change
• Infiltration


  The process of water entering the soil surface
• Ploughing breaks up the surface so more water can infiltrate, reducing the amount of runoff
• Crops increase infiltration and interception compared to bare ground, reducing runoff, but also increasing evapotranspiration
• Livestock trampling and compacting the soil decreases infiltration and increases runoff
• Irrigation can increase runoff if some of the water can't infiltrate, and can also reduce groundwater or river levels if water is extracted for irrigation
• Deforestation reduces the amount of water intercepted by vegetation, increasing the amount that reaches the surface, and also reduces infiltration due to the loss of dead plant material on the forest floor
• Construction of new buildings and roads creates an impermeable layer over the land, preventing infiltration and massively increasing runoff, resulting in water passing through the system much more rapidly and making flooding more likely
• Water abstraction (taking water from stores) to meet demand in areas with high population density reduces the amount of water in stores such as lakes, rivers, reservoirs and groundwater
• During dry seasons, even more water is abstracted from stores, especially groundwater and reservoirs, for consumption and irrigation, further depleting the stores