Basic and Full Modules

Scales

We assume that we are looking at an area in the range of 200 x 200 m2 up to 1 km2, located in a relatively flat terrain that is not too much affected by horizontal fluxes of groundwater. The area modelled could be larger provided that it is spatially homogeneous, however it would be hard to think of very large areas with the same geomorphological parameters. The time scale assumed is 1 day. It is such that the groundwater table is rather stable and tends to be at equilibrium at the initial conditions. The climatic data are also assumed at a daily time step. Over smaller time steps we would have to consider ponding of surface water during rainfall events and diel variations in evapotranspiration due to changes in solar irradiation. Also the climatic conditions would need to be more detailed. The module may be used for larger time scales, however it may be somewhat redundant in that case.

Structure

A simplified conceptual model of hydrologic processes may be presented in the following way:

Surface water - Unsaturated layer - Ground water
Conceptual model of unit hydrology

This diagram is only the top of an iceberg, with a lot of fairly complex processes that may be further described in much more detail and complexity. You may click on the diagram to see some more details about the variables and processes involved. At this point it is important to decide what are the most important features of the system that need be considered.

For the LHEM modules we chose the following 4 variables:

  1. SURFACE WATER - water on the surface of the land (in most cases it is in rivers, creeks, ponds, depressions)
  2. SNOW/ICE - at freezing temperatures surface water becomes ice, which then melts as temperature grows above 0o(C)
  3. UNSATURATED WATER - is actually the amount of water in the unsaturated layer of ground. We think of the ground as a sponge. If we pour water on a sponge it will hold a certain amount of water before water will start dripping from the sponge. While water can be still added and held by the sponge, it is in the unsaturated condition.
  4. SATURATED WATER - is the amount of water in the saturated ground. Once the sponge can no longer hold additional water, it becomes saturated. As with surface water, if we add water to the saturated zone, it's level increases.

The major processes and assumptions we make to create the module:

  • Precipitation comes with rainfall and snowfall. If temperature is below 0o(C) (32 F) the precipitation is channeled into the Snow/Ice variable. Otherwise part of it infiltrates into the Unsat Water and the rest goes into the Surface Water.
  • We assume that, rainfall infiltrates immediately to the unsaturated layer and only accumulates as surface water if the unsaturated layer becomes saturated or if the infiltration rate is exceeded.
  • Surface water may be present as rivers, creeks and ponds. Surface water is removed by overland flows and evaporation .
  • Surface water flow rates are a function of dynamically varying plant biomass, density, and morphology in addition to surface and water elevation.
  • Water from the Unsaturated layer is forced by gravity to percolate down towards the saturated layer. As it accumulates the level of the saturated water goes up, while the amount of water in the unsaturated layer decreases.
  • Transpiration is the process of water removal from soil by the sucking action of roots. Transpiration fluxes depend on plant growth, vegetation type and relative humidity.
  • Saturated ground water can reach the surface and feed into the flow of surface water. This process is what feeds the streams and rivers between the rainfall events - the so called baseflow.
    E-mail to Alexey Voinov