Hydrology and modelisation a quick outlook. Etienne Leblois Cemagref Lyon. Basic aspects of hydrology. The aim of hydrology. Determine how much water will be in a given location and condition. The hydrological cycle. A continuum, broken by the observator into storages water bodies
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Etienne LebloisCemagref Lyon
Determine how much water will be in a given location and condition
A continuum, broken by the observator into
with possible internal evolutionary laws
inside or between water bodies
associated to hydrological processes
Ranked here by increasing time constant
soil moisture (non saturated area)
lakes ; reservoirs
groundwater (saturated area)
Infiltration and seepage (= ex filtration)
Runoff (on slopes)
Discharge (in rivers)
according to Horton
runoff occurs where and when rain rate exceeds infiltration capacity
according to Capus, Hewlett, Beven, ...
runoff occurs where and when rain falls on saturated areas
importance of the soil structure
The continuous model
unsatured zone : the Richards equation
satured zone : the Darcy equation
integrated form for alluvial aquifers
integrated form for constrained aquifer
The problem of parameters estimation
importance of K(, x, y, z) (a tensor)
The river network upstream
both side of the rivers
up to the water divides
surface and subsurface storagesin relation to the river
one input (rain, other atmospheric conditions)
one output (discharge at the outlet)
the best possible unit for effective management
what I do here is my problemWhy study catchments ?
the fractal nature of the river network
the fractal nature of the topography
the partially unreachable description of the under ground
the unsteady character of the topography and soil properties at detailed scaleThe catchment : limits
Two gauging stations define either nested or non-nested catchments
Data out of many catchments are part of a data hierarchy that must sometimes be considered explicitly (discharge mapping).The catchmentlimits (continued…)
A balanced description of a catchment (hydrological monography) can be very interesting.
It will not solve all possible and unexpected questions.
« Production »
relates the gross precipitation over the catchment to the net precipitation that is to flow through the outlet.
non-flowing water is only considered as a soil moisture controling factor, influencing the soil behaviour under further rains.
« Transfert »
relates the produced « net precipitation » to the discharge.
It is common choice to
upload the production function with all the non-linearity of the rain-discharge transformation.
consider the transfert function as linear.
This approximation may be valid for heavy rains
Unit Hydrograph (Sherman, 1932)
the transfert function is assumed linear.
the structure of the non-linear production function remains author-dependant.
parameters for both parts are identified from a joint pair of long rainfall/discharge time series.
an improvement from the previous approach
the shape of the unit hydrograph is related to distances and slopes along the runoff pathways from the catchment to the outlet
this gives clearer constraints to what the production function can beConceptual approaches to the transfert aspects
Isotopes evaluations show that most of the water of the flood has been in the soil long before the begin of the rain.
potential energy + kinetic energy
along the stream (energy loss in turbulence, interactions between the water and the reach)
localised (in hydraulic jumps from torrential to fluvial conditions
3D equations (Navier Stokes)
small scale studies like geomorphology, flow around a bridge
2D equations (Barré Saint-Venant)
where overland flow is most relevant : dam breakes, flooding of broader areas with non negligible speed in the flooded partVarious levels of description for hydraulic transfer
where the flooded area is broken in independent storages, where speed is negligible
1D (Barré Saint-Venant) :
where streamflow is concentrated in the minor riverbed (no flooding).
including dam breaks, working spillways, moving hydraulic jumps, ...Various levels of description for hydraulic transfer (continued)
Diffusive wave approximation :
flood diffusion in gentle, sub-horizontal rivers
Cinematic wave approximation :
flood propagation in steep rivers or lateral slopesVarious levels of description for hydraulic transfer (continued)
if time variations are negligible. Mostly broad, gentle rivers,
a important step for text-books in hydraulics (clear, intuitive relation of results to energetic consideration and limits)
1D, uniform approximation :
to be considered only in regular, chenalized reachesGoverning equations for hydraulic transfer (continued)
To predict floods, or to assess flood hazard?
Given a current stage of water and observed or predicted rain, guess the shape, time of arrival and water stage to occur in the next future at the interest point.
Given a observed discharge time-serie, give probability of a given flood characteristic (peak flow, duration, volume,…) to be over-seeded
civil servants ; river authorities ; majors ; meteorologists ; hydrologists
real time data collection
quick data processing, mostly empirical models or analogues
365 days, 24 H communication system to people
some rivers have long time constants
gentle rain, so progressive saturation ; broad basins, so long hydraulic transferts
some rivers have short time constants
steep, small catchments ; convective storms.
enable different kind of human measures
induced an “hydrology of flash floods” to exist
but hydrology is one !
is a natural event
can be characterised as an random event
can yield damages
this depends on the sensitivity of land use
considers vulnerability as the cost of damages
to minimise by
protective measures (levees),
storing or evacuating waters via various works,
as far as monetary evaluation proves efficient.
the probabilistic nature of events,
the short memory of human beings,
teleconnections of local actions and basin-wide effects,
… spontaneous local management exhibits a drift towards heavy works that appears to be unsustainable at the basin scale (spiral of corrective measures).The dammage approach : drawback
some areas, like marshes, may have a positive demand for flooding.The alea / vulnerability approach
the one are in a lack of protection (red)
the other one are “underflooded” (green).
Relevant decision board can decide
to freeze some areas for them not to turn red soon, to modify land use, or to spatially modify the alea pattern with minimal river works, turning areas red to green at the “hydrological expenses” of greenThe alea / vulnerability approach
rain known via
rain gauges select 400 cm2 in 100 km2
spatial pattern, but little quantitative consistency
potential evapotranspiration known via
observed meteorological estimation of control factors (temperature, wind, …), at 100 km grid size
only via water balance estimation at the field or basin scale
known at 15 % in some gaging stations (500 working stations in France).
include non registered man-made perturbations that make the assessment of the intrinsic behaviour of the catchment very difficultDefinitively lacking data
a process can easily take precedence on an other because of
the quasi-systematic non linearity of processes
their sensitivity to the initial conditions
effect of water contents
effect of soil structure
as a behaviour that is not uniquely determine by the contents, but also by their spatial organisation
comparison with a recepie
we know the taste of each ingedient.
we can NOT predict the taste of the mealA catchment
Zebra bush in sahelian regions
Snow redistribution by the wind
Groundwater sustained rivers
Man-made linear patterns in landscape
Soil physics and plant physiology
Water quality, hydrobiology
Human and social sciences
Management and economy, law, politics
formalisation of concepts
possible formal checking
knowledge and concepts
explicitation of non-obvious structure effects
answering specialized questions
assessing impacts of land-use change
testing general management strategies
which area ?
which level of detail ?
are the details useful ?
will we be able to gather the details ?
which time scope
climate and social scenarios ?
which hydrologically related features do we need ?
floods ; water quantity ; water quality ; hydrobiology ; river geomorphology ; water uses ; land use
choice of independant and dependant features ?
Assessing the dominant processes
Is there a link to what I am interested to ?
Choosing time and space scales
Choosing a topology
Is an object oriented approach usefull ?
How to separate objects ?
How to specify the relation between objects ?
Deterministic models : deductive models
Statistical models : inductive models
directly on distributions
yielding time-series as output
boxes flowing the ones into the others through pipes...
need for calibration
useful as reference catchments in applications involving reference catchments
detection of changes
identification (which structure ?)
calibration (value of parameters)
documentation of limits
according to a regular grid
an old-fashioned, quite efficient way
according to a dominant process-based grid
slopes and contours
according to an homogeneous area concept
valid only in man-made landscape
a general tool would need the tree forms to be easily mixed !
mostly for regular grid distributed models
physical parameters unknown and spatially variable at the sub-grid resolution
effective parameters approach :
equations are kept same as in the detailed scale, but with (possibly other) numerical values that account for macroscopic scale behaviour
parameters are estimated backward from overall behaviour of the catchment
remote sensors are supposedly able to evaluate some characteristic parameters of the surface (moisture, rugosity, slope…) directly at a scale that is suitable for distributed modelingAdressing sub-grid variability
part of explicit physics quite modest.
accounted for via behavioural routines
tend to be the core of models (not just in well localized “parametrisation boxes”).
models who clame to be deterministic (for they are distributed) may be completely behavioural when one consider the scheme implemented at the cell size.
square grid, physically based
contour and slope grid, physically based
square grid, conceptual
Stanford IV, Cequeau, ModCou
CREC, GR4J, Gardenia
semi-lumped, specialised to saturation runoff : Topmodel
Privas, Ardèche dept, France
Key industry : chesnut processing (Christmas, etc.)
On the Ouvèze river, a tributary to the Rhône
Some agricultural opportunities in the valley, downstream from Privas
two tributaries of the Ouveze are used for providing water to the chesnut industry.
Water shortages in Privas
Ouveze dry off in summer in Privas
Ouveze is merely chesnut waste donwstream from Privas ; biology near to 0 down to the Rhône.
Agriculture does not really start, because lacking water
Spontaneous sectorial remediation projects
for problems in Privas
building dams on the tributaries for an enhancement of water availability in Privas ; maybe, to sustain summer discharge of the Ouveze
building a irrigation pipe from the Rhône
irrigation pipe to go up to Privas
chesnut waste to be diverted to the agricultural areas
abundant water to the industry and inhabitants
dam project can be forgotten
river will biologically recover
evaluate this and others scheme quicklyThe chesnut valley