Samenvatting
Samenvatting Water 1 (HWM 10303)
- Instelling
- Wageningen University (WUR)
Dit is een samenvatting van het bachelor vak Water 1. Het omvat alle stof die je moet weten voor het tentamen (behalve oefenmateriaal).
[Meer zien]Voorbeeld 4 van de 35 pagina's
Voorbeeld 4 van de 35 pagina's
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Voorbeeld van de inhoud
Chapter 1 – Catchments [take another look at the Catchments are of-figures!] ten characterised by
4 thing: the size, to-
1.1 Catchment characteristics pography, land-
The catchment (= basin, drainage basin, watershed) of a river is the area from which cover and geology.
excess water flows towards that river.
o It is the area draining towards the river’s outlet (or debouchment), which is at sea or
when the river joins a larger river.
Determining the catchment boundary (or water divide) is called catchment delineation.
o The surface water divide is found by following the river channels upstream until
their sources and drawing the divide between the channels leading to the outlet and
the channels leading to another river.
Mountainous catchments -> water divide is often
highest point in landscape. This is the topographical
water divide, which can deviate from the actual water
divide, because infiltrating water may hit an imper-
meable layer in the ground and be led in another direc-
tion underground (fig. 1.2 p. 3) -> the groundwater di-
vide is different from the surface water divide.
Difference between topographical and actual water divide often occurs in
catchments with a high contribution of groundwater flow.
In many places, people have changed the course of groundwater and surface water flow,
changing the catchment boundary.
o Catchment boundaries in polders are often dikes: they divide the areas draining to-
wards a certain pumping station.
We can subdivide the catchment area of a river, from source to outlet, in 3 zones:
o Zone 1: upper course/upstream area, which consists of various sub-catchments with
a dense dendritic network of channels. Also called production zone, because this is
the region where the surface water and sediments are formed.
o Zone 2: middle course, which consists of a distinguished main channel which often
meanders through a landscape. Only has a few branches. Also called transport zone.
o Zone 3: lower course/downstream area, which is the delta where the river divides
into multiple branches again as a result of the flat landscape and the large amount of
water and sediments present in the river. Also called deposition zone (low flow velo-
city).
The topography (variation in elevation) plays an important role in the formation and trans-
port of surface water. It partly determines which areas within a catchment are relatively wet
and vulnerable to flooding.
Draining area of a certain point in the landscape is the surface area uphill (above that point
and up to the water divide): water from that area all flows towards that point.
o Large draining area -> large amount of water has to pass by this point -> increased
wetness and flood risk.
If the local slope of the land surface at that point is high, the water flows quickly away from
that point, reducing the wetness.
draining area
Topographic wetness index tells you something about flood risk: .
local slope
1
, o At locations with high topographic wetness index, ponds form, and channels begin.
Valleys are often wet (large draining area, small local slope); closer to the top of a
mountain it is usually dryer (draining area is small and local slope steeper).
Hillslope shapes:
o Converging hillslope = hillslope shaped like a funnel. All water is led to 1 point ->
bottom of converging hillslope has a high topographic wetness index.
o On diverging hillslopes, the water from the draining area is spread out more -> less
wet.
o Parallel hillslope = neither converging nor diverging.
Hillslope slopes:
o Straight hillslope -> slope is the same everywhere.
o Concave hillslope (hollow) -> slopes are higher near the top and
lower near the bottom, which increases wetness and flood risk.
o Convex hillslope (rounded) -> slopes are lower near the top and
higher near the bottom, decreasing the wetness at the bottom.
Topographic wetness index and hillslope can also be applied to the mi-
crotopography of a garden or agricultural field.
The presence and type of vegetation impacts many hydrological processes. Leaves intercept
rainwater, roots extract water from ground, water seeps into ground more easily near stems.
Subsurface plays a major role in storing and transporting water in catchments. For soil hydro-
logy, the major soil properties are:
o Porosity: the fraction of open spaces between the soil particles. It determines how
much water can be stored in the ground.
o Conductivity/permeability: measure of how easy it is for water to flow between soil
particles. It determines how quickly water is transported.
Porosity and conductivity are determined by size, shape and origin of soil particles. They are
often related:
o Gravel (very large particles) has a very high porosity and very high conductivity.
o Sand (large particles) has a high porosity and a high conductivity.
o Clay (small particles) has a low porosity and a low conductivity.
o But: peat has a very high porosity but low conductivity because it consists of partly
composed plant material which fixates water.
Aquifer = layer of permeable soil. It can store and transport water well.
Under the layer(s) of soil (loose material) is the bedrock (solid rocks). In mountainous areas,
the soil can be just a thin layer or completely absent -> porosity and conductivity of rocks are
often low -> important factor when determining a catchment’s response to rainfall.
Every catchment can be subdivided into sub-catchments. This way, hydrologists can gain in-
sight into hydrological processes which are important there.
A digital elevation model (DEM) gives the elevation for each pixel (often 30×30m) on a map.
It is easy to distinguish valleys (with possibly streams) from hills. The topographic wetness in-
dex can be determined automatically from a DEM. Wet areas are located in the valleys.
Geographical Information Systems (GIS) is a generic name for spatial datasets like land use,
soil type, precipitation, etc. They can be used as input for computer models simulating the
hydrological processes in different parts of the catchment and predicting river discharge.
1.2 Channels
The topology (channel networks) and geomorphology (origin of the landscape and channels)
reveal much about the past and current climate, geology and land use.
2
, Ltot
The drainage density Dd quantifies the abundance of channels: D d = [km-1].
A
o Ltot [m] = the total length of all channels in a certain area A [m 2].
o High drainage density indicates that a certain area has many channels (per km 2) ->
average distance from a location on land to the nearest channel is small. Water only
has to travel small distance over & through the ground to the surface water network.
o In temperate climates, Dd is ~1.5 km-1 -> 1.5 km of channel is necessary to drain a
catchment with an area of 1 km2. In wet climates, more water has to be drained ->
more channels and high Dd.
Drainage densities are high in areas with low soil conductivity: if it is difficult for water to
flow through the ground, it has to flow over the ground. Agricultural areas also have a high
(man-made) drainage density.
Natural channel networks are organized efficiently: nature strives for minimal energy use and
an equal energy distribution over the entire system. Most common river network is the
dendritic pattern (tree shaped).
Drainage patterns (fig. 1.9 p. 13):
o Dendritic pattern: present in landscapes with homogeneous geological formations.
This river network has many tributaries (branches).
o Radial pattern: streams start from one central point and flow into all directions (vol-
canoes and mountains).
o Trellis pattern: often found in areas with folds in the earth crust. There is a relatively
large number of channels that have the same direction on each side of the river.
o Parallel pattern: occurs on steep slopes, where the water flows downhill fast. The
channels are very straight lines.
Channels that are located in the headwater (upstream part) of a catchment behave differ-
ently from channels further downstream -> channel network can be divided into links (chan-
nel segments) and junctions (locations where channels merge). A source is a location up-
stream where channel starts. Links between source and first junction are exterior links (nr 1);
links between two junctions are interior links.
o Strahler order numbering:
All exterior links get the order 1.
When links of the same order merge, add one for the link downstream of the
junction (2 second order streams merge -> downstream link is 3 rd order).
When links with different order merge, the link downstream of the junction
will have the same order as the highest order of the merging links.
The highest order is always found at the outlet. This is the Strahler order of
the catchment.
1.3 Climate
Rivers are fed by precipitation (P). Some of that water is used for evapotranspiration (ET)
and the rest is effective precipitation: the water that will eventually be discharged by rivers.
Precipitation, evapotranspiration, discharge are the most important fluxes in a catchment.
Precipitation can occur as rain, snow, hail, fog or dew. Two types of precipitation:
o Stratiform precipitation (frontal precipitation) often occurs in winters in temperate
climates and is characterized by prolonged periods of extensive areas with a low
precipitation intensity. Stratiform clouds are wide, thin blankets with homogeneous
grey or white colours.
3
, o Convective precipitation often occurs in tropical climates and summers in temper-
ate climates and is characterized by short-term, intensive showers on a local scale.
The clouds are small and towering with vertical structures.
Spatial variability in precipitation is caused by several factors:
o Hourly, daily scale: size and shape of clouds. Stratiform precipitation has a low spa-
tial variability and convective precipitation has a high spatial variability.
o Monthly, yearly scale: precipitation is high downwind of areas where much water
evaporates (oceans, large lakes) and on the windward side of mountains because
clouds are forced upwards, where the air is colder and water vapor turns into water
droplets (condensation) or ice particles (sublimation). The smaller amount of pre-
cipitation on the downwind side of the mountains (leeward side) is called rain
shadow. Also more precipitation in and downwind of large cities (hot air in cities).
Precipitation happens during precipitation events (showers or storms), which last from
minutes to hours or days, with long or short dry spells in between.
Precipitation often varies seasonally. In some countries, most rain falls during several
months in the monsoon season. In the Netherlands, there is hardly any variation in the pre-
cipitation sums between winter and summer, but precipitation type and intensity does vary.
Climate change affects both the average annual precipitation sum and the type of precipita-
tion. In many areas, more severe storms and longer dry spells are expected.
Rain is often measured with rain gauges; strongly influenced by wind: rain droplets are
blown over the gauge, and rainfall is underestimated.
o The most common gauges collect water in a reservoir, its volume is measured and
divided by the surface area of the gauge to obtain a rainfall sum in mm.
o A tipping-bucket gauge leads the rain to a small reservoir, which tips when it holds a
certain amount of water -> tipping moments are registered -> precipitation amounts
and intensities can be calculated.
o Heated gauges are used in cold areas to melt snow and measure the liquid water.
Rain gauges measure at 1 location, but this may not be representative for a whole catch-
ment. A weather radar can sense the location and severity of a precipitation further away.
Radio waves are sent into the atmosphere and reflected by water droplets and ice particles -
> reflected percentage of radio waves can be converted to precipitation intensity. Disad-
vantage: they measure precipitation high above land surface -> radar beams can shoot over
cloud -> precipitation can be less than on the ground.
Evaporation is the transition from liquid water to water vapor and requires much energy:
evaporation of 1 kg of water with T=295K requires 2.45*10 6J (= heat of vaporization) of en-
ergy.
o Sun provides the energy by short-wave radiation, which is
partly reflected by the land surface. The albedo (= fraction of
sunlight that is reflected) depends on the land cover: 0.8-0.95
for snow, 0.2-0.3 for agricultural crops, 0.2-0.25 for deciduous
forests (broad-leaf), 0.1-0.15 for coniferous forests (needle-
leaf) and 0.06 for water bodies.
The earth emits long-wave radiation, which is partly reflected back by
clouds.
Difference between incoming and outgoing radiation at the earth surface is the net radi-
ation [W/m2].
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