Booklet 3 - Discharge relationships within drainage basins

Question 1

define storm hydrograph

Answer 1

shows how the discharge of the river varies over short time periods as it responds to individual or a group of storm events. It shows the short term/sudden variation in river discharge due to recent rainfall events occurring over a few days or even hours.

Question 2

State all the key terms in a storm hydrograph

Answer 2

1. Peak rainfall - hour of greatest rainfall
2. Peak discharge - time of max discharge by river
3. Lag time - period of time between peak rainfall and discharge
4. Rising Limb - period of time of rising river discharge following rainfall
5. Normal flow - base flow of river (water flowing into the river anyway, from groundflow)
6. Falling Limb - period of time when the rivers discharge is falling

Question 3

Shapes of Hydrographs

Answer 3

flashy and subdued

Question 4

Interpreting a storm hydrograph; processes operating in a drainage basin

Answer 4

overland flow, infiltration, percolation, through flow and groundwater flow transfers water to river channel (at different speeds)

a. quick flow processes = flow of water by surface runoff + fast through flow via porous soil

b. slow flow proces = flow of water through rocks (percolation + groundwater flow) + more constant and feeds into a river to produce base flow

Question 5

Define bank full discharge

Answer 5

amount of water a river can hold

Question 6

Flood discharge

Answer 6

amount of water that will spill onto the flood plain.

Question 7

Factors influencing the shape of a storm hydrograph

Answer 7

1. Climate
   a) Precipitation intensity - if rainfall intensity exceeds infiltration capacity, then infiltration will decrease, overland flow increases = water reaches the river more quickly creating a steep rising limb. Low intensity rainfall is likely to infiltrate into the soil and percolate slowly into the rock, increasing time lag and reducing peak flow.

b) Precipitation Type - snow may lay on the ground until it melts = sudden rapid melting can cause flooding and lead to higher rates of overland flow = high peak flow + if ground is still frozen, it prevents infiltration

c) Temperature, evaporation, transpiration and evapotranspiration - higher temps = increased evaporation rates = less water reaches the river = low increase in discharge + warmer temps would indicate increased vegetation cover, more interception = increases interception loss through evapotranspiration, reducing peak discharge

d) Antecedent moisture - high antecedent moisture conditions = reduces infiltration capacity and increases overland flow, causing a steep rising limb and high peak discharge + hard, dry baked soils also encourages overland flow.

2. Drainage basin characteristics
   a) drainage basin size and shape - smaller + circular drainage basins responds faster to rainfall events because water reaches the river channel quickly over a smaller distance.

b) Drainage density - high drainage density = many streams and rivers per km^2 = water will move more quickly to the measuring point

c) Soil type - permeable soils with high sand content have higher infiltration capacity = water reaches the river channel slowly and spread out over time

d) Geology - impermeable rock such as granite restricts percolation = encourages through flow processes.

e) Slopes and relief - low gentler slops encourage infiltration and percolation = increases lag time and reduces peak discharge

f) Vegetation type - bare, low density vegetation / deciduous vegetation in the winter = low levels of interception and more rapid movement of water through the system and river

g) Land use
- urbanisation= impermeable surfaces reduce infiltration and encourage overland flow + drains encourage the quick flow process of throughfow so water reachers the river rapidly + reduction in vegetation cover reduces interception loss and increase the amount of water reaching the river.