5.3 The water balance and river regimes

Question 1

What is a water budget?

Answer 1

In the short term, water availability varies from day to day, month to month and year to year. Over a longer period, there is usually a natural rhythm that balances out times of surplus and times of deficit. This balance is called the water budget and reflects the differences between inputs and outputs in any given area

Question 1

How else can the water budget be expressed?

Answer 1

as the water balance equation:
Precipitation (P)
runoff/river discharge (Q)
Potential evapotranspiration (E)
soil moisture and groundwater storage (S)
Each year, whatever falls as precipitation should be balanced out by the other components

P = Q + E +/- S

Question 2

What are the inputs of a water budget?

Answer 2

-precipitation
-Water diversion into the area
-Groundwater flow into the area
-Surface water flow into the area
-Surface runoff into the area

Question 3

What are the outputs of a water budget?

Answer 3

Evapotranspiration
Water diversion, groundwater flow, surface water flow/runoff out of the area
Industrial or residential uses within the area

Question 4

What is the field capacity?

Answer 4

at this point, the soil is full of water and cannot hold anymore

Question 5

Look at the components of a water budget source… what does part A show?

Answer 5

Precipitation exceeds potential evapotranspiration. Soil water is full with a surplus for plants, runoff and groundwater recharge

Question 6

Look at the components of a water budget source… what does part B show?

Answer 6

Potential evapotranspiration exceeds precipitation. Soil moisture is used up by plants, or used by evaporation (called soil moisture utilisation)

Question 7

Look at the components of a water budget source… what does part C show?

Answer 7

Soil moisture store is now used up. Precipitation is likely to be absorbed by the soil, rather than produce runoff. River levels will fall or dry up completely

Question 8

Look at the components of a water budget source… what does part D show?

Answer 8

Deficiency of soil moisture, as the store is used up. PE exceeds precipitation. Plants must adapt to survive and crops need irrigation

Question 9

Look at the components of a water budget source… what does part E show?

Answer 9

Precipitation exceeds PE. Soil moisture is recharged

Question 10

Look at the components of a water budget source… what does part F show?

Answer 10

Soil moisture is now full and field capacity is reached. Additional rainfall will percolate to the water table, and groundwater stores will be recharged

Question 11

What does the amount of water reaching a river depend on?

Answer 11

both local and drainage basin wide factors, as well as climatic control

Question 12

What is a river regime?

Answer 12

The annual pattern of a river’s flow (or discharge) which reflects local, drainage basin wide and climatic factors

Question 13

What is a tropical river’s regime like?

Answer 13

Generally have fairly uniform flow patterns, especially in areas where rain falls every month.

Question 14

What are worldwide river regimes like?

Answer 14

They tend to have a seasonal pattern whereby they directly respond to the amount of precipitation

Question 15

What are the two types of river regimes?

Answer 15

Simple and complex

Question 16

What are simple river regimes?

Answer 16

Where the river experiences a period of seasonally high discharge , followed by low discharge

Question 17

What are examples of where you would find a simple river regime

Answer 17

• They are typical of rivers where the inputs depend on glacial meltwater, snowmelt or seasonal storms (monsoon).
  -Rivers with temperate climates, which rise in mountainous regions where summer snowmelt occurs, tends to be like this (e.g. the Rhone)

Question 18

What is a complex river regime?

Answer 18

Where larger rivers cross several different relief and climatic zones, and therefore experience the effects of different seasonal and climatic events

Question 19

What are two rivers that have complex river regimes?

Answer 19

Mississippi
Ganges

Question 20

What else causes rivers to have complex regimes?

Answer 20

-human factors, such as damming rivers for energy or irrigation
-The longer the river, the more complex the variables (land use, precipitation, evapotranspiration) - making it more complex

Question 21

What are flow patterns like in the Yukon river in Alaska, US?

Answer 21

high flow in spring and summer, caused by snowmelt
low flow in winter when precipitation is frozen
Large seasonable variability
Relatively few human influences apart from some HEP use for mining industries

Question 22

What are major influences on the Yukon river?

Answer 22

Tundra, taiga and mountain climates

Question 23

What are flow patterns like in the Amazon, Brazil?

Answer 23

High flow in wetter season
Low flow in drier season
Moderate seasonable variability- fed by Andean rivers outside rainforest region
Human influences are increasing as large dams are used by Brazil’s cities for irrigation and HEP

Question 24

What are the major influences of the Amazon?

Answer 24

Rainforest climate, precipitation and evapotranspiration rates are high

Question 25

What are some of the main factors that influence a river regime?

Answer 25

-size of river - the longer it is, the more complex it is
- amount, pattern and intensity of rainfall
-temperature - influencing evaporation
-geology and soils
- amount and type of regulation
-human activities

Question 26

What is a hydrograph?

Answer 26

a graph showing the discharge of a river at a given point over a period of time

Question 27

What shows discharge is increasing on a hydrograph?

Answer 27

The rising limb

Question 28

Why does it take time for discharge to increase?

Answer 28

When rain falls, only a fraction of it will fall directly into the river, so discharge doesn’t increase immediately. Most rain falls on valley sides and takes time to reach the river

Question 29

What is the gap between peak rainfall and peak discharge?

Answer 29

lag time

Question 30

What are some of the physical factors that affect hydrographs?

Answer 30

-rock type
-tidal conditions
-slope
-shape
-area
-soil
-precipitation and temperature
-land use
-drainage density

Question 31

How does rock type affect hydrograph shape?

Answer 31

Permeable rock means rapid infiltration and little overland flow, therefore it has a shallow rising limb

Question 32

How does tidal conditions affect hydrograph shape?

Answer 32

high spring tides can block the normal exit for the water, therefore extending the length of time the river basin takes to return to base flow

Question 33

How does shape affect hydrograph shape?

Answer 33

Elongated basins will produce a lower peak flow and longer lag time than a circular one of the shape

Question 34

How does slope affect hydrograph shape?

Answer 34

Channel flow can be faster down a steep slope making a steeper rising limb and a shorter lag time

Question 35

How does area affect hydrograph shape?

Answer 35

Large basins receive more precipitation than small and therefore have a larger runoff
Larger size means longer lag time as water has a longer distance to travel to reach the trunk of river

Question 36

How does soil affect hydrograph shape?

Answer 36

Infiltration is generally greater on thick soil, although less permeable soils e.g. clay act as impermeable layers
The more infiltration that occurs, the longer the lag time and shallower the rising limb

Question 37

How does precipitation and temperature affect hydrograph shape?

Answer 37

• short intense rainstorms can produce rapid overland flow and a steep rising limb
• if there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface runoff
• snow on the ground can act as a store, producing a long lag time and a shallow rising limb. Once a thaw sets in, the rising limb will become steep

Question 38

How does land use affect hydrograph shape?

Answer 38

-urbanisation- impermeable surfaces create a steep rising limb and shorter lag time
-afforestation- intercepts the precipitation, creating a shallow rising limb and lengthening lag time

Question 39

How does drainage density affect hydrograph shape?

Answer 39

a higher drainage density will allow rapid overland flow

Question 40

How does land use influence the decisions made by planners?

Answer 40

Planners must giver considerable weighting to environmental considerations as well as the need for development. Developers must show that land use changes will not increase runoff beyond that estimated for a ‘greenfield site’

Question 41

What are some examples of sustainable drainage systems?

Answer 41

-green roofs- vegetation cover planted over a water proof membrane
-permeable pavements
- detention basins
- rainwater harvesting- collecting rainwater from roofs to be recycled