WRM web version: Aquifers
A WRM scenario can include any number of aquifers
representing underground bodies of ground water,
that are linked to specific node types and reaches.
Conceptually, and aquifer is very similar to a RESERVOIR but
with a very simple rectangular (and terrain following) geometry.
An aquifer is represented as a fully mixed reservoir
of a given volume and substrate-to-water ratio with an
initial state, that simply keeps track of all inputs/outputs over time,
also reporting its head (distance of the water table from the lower bottom,
i.e., head is relative (to the bottom of the aquifer) always positive and can vary from
0 (aquifer completely empty) to a maximum value of aquifer depth of thickness).
The spatial distribution for the interaction of the (averaged) auifer with the
individual nodes and reaches is based on an initial distance to the groundwater level
specified for each of the relevant objects as initial condition, updated dynamically
(daily) as the groundwater volume and thus head changes:
everything is relative to the initial conditios.
Aquifers are supposed to be unconstrained at the surface (which means they can
"flood" ! and consist of one layer only; a aggregate loss term (deep percolation)
as a function of head can be specified (mm/m and day, equivalent to seepage from reservoirs).
Water movement (flow) within the aquifer is assumed to be instantaneous
(at the daily time step used !) in the initial lumped parameter implementation,
i.e., the groundwater reservoir is assumed to be "fully mixed", its dynamic surface
maintains slope (parallel to the soil surface).
Several node types interact directly with an aquifer;
nodes and reaches that can/do interact with the groundwater have to specify the
respective aquifer by name from a drop down menu of all aquifers
defined for the scenario. They include:
- Start, GWater (representing pumped well fields
- Start, spring (natural flow from the groundwater system)
- Recharge (artificial recharge into the groundwater system
- Demand nodes (conveyance and returnflow or drainage losses,
with a multiplier for percolation versus evaporation fractions);
- Reservoir nodes (seepage, as a function of storage level)
- Reaches can receive lateral inflow;
- lateral flow defined by a pre-compuyted time series;
- lateral inflow defined a local precipitation and a runoff coefficient;
- RRM coup,ing: specify for each reach a direct catchment and run in a standard (oe simplified)
RRM scenario, export the resulting time series with a complete water budget
for option 1), pre-defined inflow time series.
Data and attributes
- Name and ID (internal); reference from other linked OBJECTS is by name;
- METADescription (free text, 1024 characters), author, last modification date;
- OBJECT link (optional, can be used to inherit default data);
- Area in km² (for recharge and volume/depth calculations; the aquifer does NOT have an
explicit location, but is again only topologically liniked to the network);
- Avg. Depth in m (soil surface to the bottom layer) - together with the area,
this defines the aquifer volume.
- Porosity (in %)
- Hydraulic conductivity (K) of the aquifer (m/d, only used for aquifer coupling)
- DegDayCoeff: degree day coefficient for evaporation (like reservoirs):
simple linear representation: EL = DegDay Coeff * Temperature * MAX(0,M-D)
where M is the (deepest) groundwater depth where evaporation approaches 0 (e.g., 20m),
D the actual depth (distance of groundwater head to soil surface),
and Temperature daily average temperature.
- Recharge coefficient (in %) defines the % of daily rainfall that becomes available as
natural recharge - such a linear percentage os overly simplisitic, but pragmatic in terms of
its minimal data requirements - a full (vertical) water budget like calculated in RMM
should be used eventually.
- Percolation loss coefficient (deep percolation, like reservoir seepage):
calculated dynamically from the head:
PERC = AREA * COEFF * head where COEFF is in mm/day per meter of groundwater level,
i.e., LEVEL = Avg.depth - head
- Initial volume (defines also initial head !) in %.
Obviously, this is always positive and must be between 0 and 100.
- Input Time Series:
- Air temperature
Dynamic output time series:
An AQUIFER generates the following time series:
- Mass budget INPUTS:
- Natural recharge (natural recharge from precipitation);
- Pumped recharge (input from specific recharge nodes);
- Seeping (sum of all losses - demand nodes, reservoirs);
- (exfiltration from river reaches as a function of the difference of flow/level and head
at each reach connected, not yet implemented).
- Mass budget OUTPUTS:
Extraction, as the sum of all GW start node pumping rates;
- Evapotranspiration losses (function of temperature)
- Deep percolation (loss to lower layer of groundwater, function of head/height).
- (Infiltration to river reaches, or base flow contribution
(all inputs to reaches as lateral inflow that are not derived from precipitation input,
not yet implemented)
- STATE VARIABLES:
- Head (a POSITIVE number that signifies the distance from
the soil surface to the groundwater leveL, varies between 0 (start of water logging)
and the maximum depth (groundwater reservoir empty) spatial average over the area;
This is to be understood as the current state of RELATIVE CHANGE
from the initial conditions rather than an absolute value
(which would represent a basin wide average !).
TABLE of TS summaries: min, max, average, totals, of input, output, states;
- Water BUDGET per aquifer: total input, total extraction, losses, annual balance.
- Sustainable Yield: Sum of inputs - Sum of outputs.
- Water Logging: N of days when the distance from surface becomes NEGATIVE
(concept and interpretation to be discussed).
- CONSISTENCY CHECK: the initial depth (distance from surface must be positive
when calculated from Aquifer area, depth, porosity, and initial volume)
- also needs an extension for all nodes that can interact with the Aquifer !
- The simple lumped representation should be replaced by a spatially distributed model
eventually (like MOFAT).
- Aquifer NODE can be linked to an Aquifer OBJECT that in turn links to
groundwater model scenarios.
Coupling of Multiple Aquifers
For large or steep basins, an aquifer can be divided into a number of cells that can be coupled
to allow for groundwater flow, using a very simple model based on Darcy's flow.
Please note that the cells are of arbitrary shape, other than the basic vertical aquifer geometry
(volume, depth) data only the are (W, D) of the interface and the downstream distances
(in flow direction) between centroids of coupled cells are relevant.
To COUPLE AQUIFERS, a concept of INTERFACES (analog to reaches) must be defined:
An INTERFACE is defined by two Aquifers and an Area as well as L (average flow path),
the distance of the two Aquifer centriods.
Q = discharge (m3/d)
K = hydraulic conductivity of aquifer (m/d)
h2, h1 = hydraulic heads measured along flow path (m)
L = distance between centroids of the aquifer cells (m)
W = width of cross-sectional flow (m)
D = height of cross-sectional flow (m)