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Showing posts from April, 2016

### Surface water hydrology modeling: scaling issue in Precipitation Runoff Modeling System(PRMS)

Grid-based HRU network for stream routing in PRMS could be convenient for several reasons.
Apparently the ideal data structure as matrix could be one of them.But it is not always the case if the scaling issue is not well handled.
Below is an example how the scale issue could be a potential problem for the simulation framework.
First, I would like to introduce the CRT. Cascade Routing Tool (CRT) is a computer application for watershed models that include the coupled Groundwater and Surface-water FLOW model GSFLOW and the Precipitation-Runoff Modeling System (PRMS). More information could be found at (http://water.usgs.gov/ogw/CRT/)
The example run result is listed as:
Read the instruction of CRT and you will easily understand the legend/label of the above result.The stream line datasets used here is vector and the grid resolutions in horizontal are both 100 meters.
The next step, the subbasin was delineated using stream line and DEM datasets. Note that in these operations, the resoluti…

### Groundwater hydrology modeling: relation between steady state and transient simulation in MODFLOW simulation

Standard groundwater modeling usually uses the three-dimensional Richard's equation to describe the groundwater flow. Solution to the Richard equation usually requires numerical methods such as finite-difference equation used in USGS MODFLOW.
To solute the array of equations within MODFLOW, an initial head is required regardless of steady state (SS) or transient (TR) simulation.
As the MODFLOW manual states that in a steady state simulation, the storage terms are ignored since storage of each grid cell is constant with time.
One of the common problems to run the MODFLOW is  that we don't have the initial head data for the simulation. Therefore it is usually suggested that a SS simulation could be run at first to provide the head for the TR simulation. This is because SS is independent with initial head, but TR isn't.
However, whether placing the SS simulation ahead of TR simulations is doubtful in many practices! Here are the explanations:
First, the SS flow equation shoul…

### Surface water hydrology modeling: deal with different types of precipitation

In surface water hydrology, precipitation is one of the most important components.
However, withing most hydrology models, it is unclear of how precipitation is actually represented.
For example, in a typical water cycle illustration from Wiki, precipitation is described as
Here is the question, what form does precipitation actually take when it falls to land surface? Water can be in either liquid (water, rain), solid(ice, snow) or gas(water vapor) forms. How about precipitation? Surely most of time precipitation is either rain of snow. In some cases, it takes form in hail, etc.
Since the physical proprieties of water and snow are significantly different, it is necessary to distinguish them within surface water hydrology models. In some scenarios, rain and snow may co-exist in a mixed precipitation event. In this case, surface water hydrology models need to deal with both of them. The difficulty is how to manage the two-phase mass and energy balance. A complete comparison of how differ…