Unit 3.1 Water

Question 0

Identify difficulties in the collection and processing of rainfall data and identify ways of overcoming these difficulties.

Answer 0

Buildings etc. nearby can block rain or create wind turbulence. Gauges should be separated from any obstacle by a lateral distance of at least four times the height of the obstacle.

Leaves and other detritus falling into the gauge or blocking the funnel entrance. The mouth of the gauge is set 12 inches above ground level.

Gauges do not tell us the intensity of rainfall, only the total amount over a given period. To measure intensity, automated recording gauges have been developed.

Automated rain gauges can malfunction.

Question 1

Calculate mean rainfall over an area from data from irregularly distributed raingauges.

Answer 1

Thiessen polygons, and weight each polygon by its proportion of total area.

Question 3

Convert between mass and molar concentration data for ions in water.

Answer 3

1 mol of any substance is its total atomic weight in kg
e.g. 1 mol of S (atomic weight 32.1) is 32.1kg

To convert a measurement in kg to moles, divide the recorded kg by the weight of a single mol. For example, 7kg of S is equivalent to 7 / 32.1 = 0.22 mol.

To convert to moles of charge, multiply this by the charge.

Question 4

Give the routes through which precipitation may leave a catchment.

Answer 4

Water falling directly into watercourses: channel precipitation (Qp)

Water flowing overland into watercourse: overland flow (Qo)

Water penetrating the ground and percolating into the water table: groundwater flow (Qg)

Water penetrating the ground and flowing through the upper unsaturated layers: throughflow (Qt)

Question 5

Account for the origins of dissolved ions in rainwater and cite evidence in support.

Answer 5

• sulphate (SO4^2-): burning fossil fuels and volcanic sources.
• nitrate (NO3-): vehicle emissions
• hydrogen carbonate (HCO3-): reaction of atmospheric CO2 with H2O
• Na and Cl from seawater: higher recorded levels in coastal areas
• Ca from weathering of rocks
• NH4+ from fertilisers

Hubbard Brook Experimental Forest have been taking readings for the last 30 years.

Question 6

Outline methods of estimating throughflow.

Answer 6

Throughflow penetrates the ground surface, and flows through the upper unsaturated layers. The speed of flow is dependant on the material of these layers, and so a distinction is made between Quick Throughflow (Qqt) and Slow Throughflow (Qst).

Speed of throughflow is given by Darcy’s law.

Question 7

Outline methods of estimating stemflow.

Answer 7

In forests, about 10-15% of precipitation does not reach the ground, as it is intercepted by trees.

Interception loss = precipitation - throughfall - stemflow

Throughfall is water which drips through the leaf cover.
Stemflow is the water which runs down the stem.

Stemflow can be measured directly using a collection device which attaches as a ring around the stem.

Question 8

Indicate the factors that affect the relative proportions of each of the routes of runoff.

Answer 8

The relative proportion of channel precipitation (Qp) depends on the surface area of water channels. This route is significant in catchment areas with large lakes, or when large areas are flooded.

The relative proportion of overland flow (Qo) is higher where water has less chance to infiltrate the ground, such as steep slopes or impervious material such as concrete or Tarmac.

The relative proportion of throughflow (Qt) and groundwater flow (Qg) are determined by ground conditions, such as soil or waster table.

Question 9

Account for the changes in ion concentrations when rain passes through a forest canopy.

Answer 9

As it falls through the canopy of a tree, water comes into contact with leaves. Leaves intercept aerosols, adsorb gases and are coated with dust. Rain falling onto a leaf will dissolve these materials, and carry them onward. As a result, the concentration of ions increases as rain falls through the canopy.

The exception to this is hydrogen ions (H+), which are absorbed by the surface of the leaf in exchange for sodium or magnesium ions.

Question 10

Estimate the energy required for the vaporisation of water.

Answer 10

Latent heat of vaporisation of water

• units of joules per kilogram
• varies with temperature.

Multiply this by the weight of water to be vaporised.

Question 11

Outline the major factors in evapotranspiration in the field and link them to the main ideas of the Penman-Monteith model.

Answer 11

Evapotranspiration is influenced by:

• soil moisture content (and so water potential)
• atmospheric conditions (and so water potential)
• vegetation type, which may have increased resistance (stomatal and aerodynamic)
• solar radiation, which provides energy for evaporation

The Penman-Monteith formula connects evapotranspiration with humidity, available radiant energy, and vegetative resistance. It does not take into account advection: the horizontal movement of air through the atmosphere.

Question 12

Interpret hydrographs linking rainstorm and river flow rate with time.

Answer 12

• plots discharge against time
• base flow is discharge before the rising limb and after the recession limb
• may also plot rainfall against time (bar chart)
• total precipitation is the total depth of rainfall multiplied by the surface area of the catchment
• may plot multiple precipitation events
• time between peak rainfall and peak discharge is the lag time

Question 13

Estimate flood frequencies from discharge and stage data.

Answer 13

Data records are used to produce a frequency curve, which plots discharge against the average time interval before a similar discharge occurs again (the return period).

River height (stage) is empirically related to discharge by a rating curve.

Taken together, these can tell us the frequency at which the river will reach a certain height, e.g. the height at which it will breach its banks.

Question 14

Outline briefly and identify differences between the Horton and Hewlett hypotheses.

Answer 14

The Horton hypothesis calculates the amount of time at which a rate of precipitation will no longer be able to be infiltrated by the surface, and will become excess precipitation, which flows overland (Qo).

The Hewlett hypothesis suggests that, as rainwater is absorbed by the ground, the water table rises. Once the water table has reached the surface, no more water can be absorbed, and so becomes overland flow (Qo).

Question 15

Describe and account for the annual variation in baseflow on a hydrograph.

Answer 15

Baseflow is the contribution to river flow by groundwater. This is at its highest at the end of winter, and lowest during summer into autumn.

Baseflow is connected to groundwater levels. The high in late winter occurs as groundwater levels are high due to seasonal precipitation, and low in summer as the rate of evapotranspiration is greater than precipitation at this time.

Question 16

Differentiate between porosity and permeability.

Answer 16

Porosity is the amount of water which can be held. Total porosity is the percentage of total volume that is void space.

Permeability is the ease at which water can flow through a material. Larger pore sizes give greater permeability than small pores.

e.g. Clay has many pores, and so is very porous. However, these pores are very small and so it has poor permeability.

Question 17

Outline the different types of aquifer.

Answer 17

Unconfined aquifer: underlaid by impermeable rock, but the rock above is permeable, and so the aquifer is recharged by rainwater.

Confined aquifer: underlaid and overlaid by impermeable rock. - Water is trapped, and at a higher-that-atmospheric pressure. Water is referred to as artesian water, and wells into confined aquifers artesian wells. At these wells, water will rise to the potentiometric surface.

Question 18

Apply Darcy’s law to groundwater flow.

Calculate transmissivity.

Answer 18

Flow = conductivity x gradient x cross-sectional area

The product of the hydraulic conductivity (K) and thickness (height: b) of the aquifer gives the transmissivity (T). This is usually expressed in m^2 d^-1.

Question 19

Point out the limitations of Darcy’s law.

Answer 19

• assumes that rock is homogeneous, and that therefore flow is equal throughout, which is rarely the case
• takes no account of tortuosity: that water must flow around grains, rather than taking a direct route
• applies only to slow-moving water with a luminal flow, and not to faster water which may be turbulent

Question 20

Give examples of methods used to calculate the age of groundwater.

Answer 20

CFC concentrations, where water has entered an aquifer over the past 50 years

Radiocarbon dating, looking at the ratio of parent and daughter isotopes of 14C, where:
D / P = 2^n - 1
with n the number of half-lives (lambda)

Question 21

Outline and account for variations in the chemical composition of groundwater with rock type.

Answer 21

Chemicals dissolved in groundwater are derived mainly from the dissolution of minerals in the soil and the rocks with which it has been in contact.

Some minerals are more soluble than others, and it is the quantities of the most soluble, including halite, gypsum and calcite, which will define water chemistry.

The solubility of a mineral is given in its associated ionic product (K).

Example: calcite in water from the Chalk aquifers.

Question 22

Outline and account for variations in the chemical composition of groundwater with age.

Answer 22

For unconfined aquifers: Chebotarev sequence. Over time the water picks up ions from the surrounding rock:

• concentrations of hydrogen carbonate ions increase
• sulfide becomes the major anion
• calcium and magnesium replaced with sodium and potassium

Due to low levels of oxygen, reduction can occur:

• sulfide is reduced to hydrogen sulfide
• carbon dioxide is reduce to methane

Question 23

Plot the results of chemical analysis of a groundwater sample into a Piper diagram.

Answer 23

Concentrations must be expressed as percentages of moles of charge per litre.

Question 24

Outline methods for estimating the surface storage capacity in vegetated areas.

Answer 24

In forested areas, the interception loss formula may also be of assistance. Interception can be equated with storage capacity.

Question 25

Use a topological map to plot a longitudinal profile of a river.

Answer 25

Plot elevation (from contours) against distance from source.

Result: long profile.

Question 26

Identify conditions when laminar or turbulent flow is likely to occur.

Answer 26

Reynolds number tells us whether to expect flow to be laminar or turbulent.

Re = ( speed of water x depth of water ) / viscosity of water

A value below 500 indicates laminar flow. Laminar flow only occurs with low speed, low depth or high viscosity.

Question 27

Construct isovel lines over the cross-section of a river using positional water speed.

Answer 27

Isovel lines are the equivalent to isobars: lines which connect similar values, in this case speeds.

Typically, the fastest speeds will be in the middle of the river; water along the sides or floor are slowed by friction.

Question 28

Calculate values for the hydraulic radius of a river channel.

Answer 28

Hydraulic radius (m) = cross-sectional area (m2) / wetted perimeter (m)

It is sometimes best to simplify the shape of the channel into, e.g. a semicircle or a rectangle.

Channels with a low hydraulic radius are more efficient at transporting water.

Question 29

Apply the Manning equation to estimate speed of flow in a river.

Answer 29

The Manning equation combines:

• hydraulic radius (R): metres
• gradient of the water surface (s)
• Manning roughness coefficient (n), which is dependant on the size and shape of the channel, and the surrounding vegetation

Question 30

Comment on and suggest reasons for the differences in dissolved ion concentrations in rainwater and stream water.

Answer 30

Precipitation reaches stream channels by falling directly into the channels, by surface runoff, or by shallow or deep thoroughflow.

Interception of precipitation by vegetation can significantly change the dissolved material in water: various ions in water are exchanged for H+ in the leaf.

The chemical composition of water is also changed by interaction with the soils and rocks through which it flows. Na/+, K/+, Ca/2+, and Mg/2+ are the major cations; HCO3/-, SO4/2- and NO3/- are the major anions.

Question 31

Account for the seasonal variations in dissolved ion concentrations in rainwater and streamwater.

Answer 31

Stream water contains throughfall, which has picked up ions from the ground, and direct precipitation, which has not and so is poor in ions. Seasonal variations in streamwater stem from the different proportions of these sources.

In periods of high rainfall or snowmelt, the ion-poor source is dominant, diluting the water and leading to lower overall ion content.

Ammonia (nitrogen), potassium, phosphate and nitrate are absorbed by plants during the growing season (April to July).

Question 32

Estimate the relative proportions of dissolved carbon-containing species in water of different pH values.

Answer 32

Stonefly will be present in water throughout the pH spectrum.

Mayfly will be present in waters with a pH above approximately 6.

Water snails and other invertebrates with calcium shells will only be present in waters with pH above 7.

< this may be referring to carbon minerals >

Question 33

Suggest reasons for changes in dissolved oxygen in a river and the potential effect on aquatic life.

Answer 33

Increased organic matter in water boosts populations of respiring microorganisms, leading to an increased use of O2, which leads to reduced levels.

Oxygen may also be limited in murky rivers, where low light levels limit photosynthesis.

Oxygen is also less abundant at night, while photosynthesis is not taking place, than urging the day time.

Question 34

Use the HERMES model to estimate the downstream profiles of point chemical discharges into a river.

Question 35

Explain the hydrological cycle in terms of reservoirs of water, and movement of water between them.

Answer 35

The majority of water is held in the OCEANS.

This may be evaporated into the ATMOSPHERE. Water in the atmosphere may fall over and return to the ocean, or it may fall on land:

• GLACIERS and ice caps hold water for moderate periods
• LAKES and inland seas
• RIVERS
• soil and GROUNDWATER; the latter may hold for long periods

Water in rivers will run into the sea, but water in all continental reservoirs are also affected by evaporation, returning water to the atmosphere.

Question 36

Calculate the residence time of water within a reservoir.

Answer 36

Residence time = mass in reservoir / flux into or out of reservoir

Question 37

Demonstrate how to set up a water balance equation in terms of outputs, inputs and storages of water.

Answer 37

Inflow = outflow + change in storage

The size of the system is dependant on the investigation: it may be a single stream, catchment, or the entire world.

Question 38

Use a simple hydrological model to both simulate and forecast runoff.

Answer 38

< activity 5.1 >