WRM web version: Aquifers

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:

1. Start, GWater (representing pumped well fields
2. Start, spring (natural flow from the groundwater system)
3. Recharge (artificial recharge into the groundwater system
4. Demand nodes (conveyance and returnflow or drainage losses, with a multiplier for percolation versus evaporation fractions);
5. Reservoir nodes (seepage, as a function of storage level)
6. Reaches (NOT YET IMPLEMENTED) receive lateral flow; initially, this is only lateral inflow associated with or taken from an aquifer selected for each reach). However, this can be generalised to a generic river-groundwater interaction:
   LATERAL INFLOW = (GWhead - RVhead) * COEFF
   
   which will move water from and to the river at REACH rn depending on the relative levels (head) of river and groundwater.
   Alternatively, we can specify for each reach a direct catchment and run a simplified version of RRM for it with a complete water budget.

Data and attributes

1. Name and ID (internal); reference from other linked OBJECTS is by name;
2. METADescription (free text, 1024 characters), author, last modification date;
3. OBJECT link (optional, can be used to inherit default data);
4. 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);
5. Avg. Depth in m (soil surface to the bottom layer) - together with the area, this defines the aquifer volume.
6. Porosity (in %)
7. Hydraulic conductivity (K) of the aquifer (m/d, only used for aquifer coupling)
8. 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.
9. 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.
10. 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
11. Initial volume (defines also initial head !) in %. Obviously, this is always positive and must be between 0 and 100.

12. Input Time Series:
    • Precipitation
    • Air temperature

Dynamic output time series:

1. 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).
2. 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)
3. STATE VARIABLES:
   • Volume
   • 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 !).
4. SUMMARIES:
   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).

Development Notes:

• 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

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)