In previous chapter we discussed about Data Processing, Files formats etc. In This chapter I will discuss about models:
The basic fundamental models are:
i) Hydrological Model
ii) Hydrodynamic Model
iii) Reservoir Model
i) Hydrological Model: About this I have discussed in Step 2. After that It requires the characteristics of catchment e.g. Canopy, Routing method, Baseflow, loss method etc. You have to take a look at your catchment to describe these parameters. After that you can do some modifications(touch-ups) to achieve the desired result/accuracy. I won't go to detail of these parameters as different model name them differently but the concept is same. Also they are different for different areas. You can have satellite data or survey data to get the parameters. In this method the storage coefficient and time of concentration you have to set . The HMS (Hydrological Modelling System) model of BBMB looks like this :
In the above figure we have junctions, reaches, subbasins and reservoirs. Subbasin have rainfall which results into flow, that flow go to junction. that junction passes that flow to another junction via a reach. Finally it reaches to reservoir. Different models may have different structures.
Note: i) Junction can take flow from reach/subbasin or both(multiples).
ii) At every subbasin outlet there will be a junction(or a reservoir).
iii) Subbasin parameters change the volume, time(delay or advance) in flow.
iv) Reaches are for routing ( usually have a time delay may contain a loss/gain method as well).
After setting parameters in hydrological Model, you have to give it a Metrological model. The metrological model defines the metrological data type such as rainfall, snowfall, temperature etc.
If you choose for no snowfall all you have to provide is a precipitation data whether gaged or gridded. You can also provide other data depending upon the method you choose such as Solar Radiation, Humidity, wind-speed etc.
For precipitation we can use gage data as well as gridded data. Gridded data gives better distribution as gage weights method is not that accurate. Further it is not possible to install gages at some site (which are inaccessible due to various reasons such as transboundary conditions or due to very high altitude). For gage precipitation to work the density of the network should be very high, it's something difficult to achieve.
If your catchment contains snowfall then you have to provide the temperature data required for snowmelt. There are two methods mainly used for this:
a) Temperature Index.
b) Gridded Temperature Index.
a) Temperature Index: In this method you have to provide temperature gages data, elevation data(above Mean sea level) of gages, lapse rate. Lapse rate is how much increase or decrease in temperature with the change in the elevation. We all know that temperature decreases with increase in elevation.
b) Gridded Temperature Index: In this method satellite does the work for you. In this you need not to have elevation data of gages as the gridded data will distribute the temperature in pixel by itself. This is mostly used for snowmelt due to better accuracy.
NOTE: Nowadays gage data is mostly used for validation purpose.
When you have run the simulation some results will come. For validation of results you have to insert observed discharge data. Then you have to fine tune your data so that it matches with the observed discharge. The more closely it matches more accurate will be your result. One may have different preferences in choosing the accuracy. You can go for peaks or you can go for the volume.
The HEC-HMS has an automated approach to do so you can choose graphical edit or automatic computation of the parameters so that it matches the observed discharge. You can choose either or both to serve your purpose.
In HEC-HMS you can also insert your Reservoir data so that it can tell you the level of the reservoir after the simulation run.
In next chapter I will discuss about Hydrodynamic model.
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