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A modern way of automate calibration of a hydrologic model

Calibration of hydrologic model can be tedious, that is why we spent great efforts to automate this process. And sometimes we need some tool that is universal, reusable, so that we don't have to re-invent the wheel again and again.

Today I want to introduce a very effective framework to conduct a hydrologic model calibration. I call it framework because you can apply this method to any model and use any of your preferred language in some steps.

Here is the framework:
Let me explain what is going on:
PEST generate new parameter file based on a simple template;PEST call Python interface to start model simulation;Python interface translates parameter file to model input files;Python interface launches SWAT simulation;Python interface extracts results; andPEST analyzes result and updates parameters.
A few highlights here:
This is an example for a SWAT model, and you can change it to any model you are calibrating;I used Python, but you can also use any other language such as C/C++ or eve…

The problem with the HRU definition in the current SWAT model

Model calibration is a critical step in most numerical simulations, including hydrology simulations.

While I am conducting the model calibration for a SWAT model in one of my current projects, I found there are several issues in current methods.

In SWAT, a watershed is represented by a list of Hydrologic Response Unit (HRU), which is the smallest unit that has the same hydrologic property.

Besides, a watershed is also divided into a list of subbasin using watershed delineation process.
In the end, a watershed is actually represented using the following structure.

Figure 1. The structure of a watershed in SWAT model.

The HRU is commonly defined using land cover, soil type and slope.

On the other side, data and parameters are usually assigned using different levels. For example, an unique ID is assigned to each HRU and some parameters are assigned to subbasin.

In reality, when two HRUs share the same land cover, soil and slope type, they are supposed to have the same hydrologic characte…