2:2.2.4 Reading Measured or Estimated PET

Users are now able to read in measured or estimated potential evapotranspiration (PET) values from an external data source for each climate station. This process is explained below.

First, each hydrological response unit (HRU) needs to be associated with a specific climate station. The variable (iob), which is initialized with hru(j)%obj_no is the index corresponding to the object number for the current HRU and links it to the climate data.

iob = hru(j)%obj_no

where iob: This variable holds the object number of the HRU, which is crucial for linking the HRU to its corresponding weather data.

Next, the iwst variable is used to obtain the weather station index associated with the current HRU by accessing ob(iob)%wst. This effectively retrieves the identifier for the climate station that provides the PET data.

iwst = ob(iob)%wst

where iwst: This retrieves the specific weather station identifier associated with the HRU from the array of objects.

Finally, the measured or estimated PET values for the specified weather station are then accessed using wst(iwst)%weat%pet. Here, weat refers to a weather data structure that holds various weather-related parameters, including the PET. The specific PET value is retrieved and assigned to the variable $PET_{day}$.

$PET_{day}$ = wst(iwst)%weat%pet

where $PET_{day}$: This variable is then assigned the PET value from the weather data structure, allowing the subroutine to utilize pre-existing climate data instead of calculating PET through other methods.

This method relies on the availability of PET data for the specific climate stations associated with each HRU. The PET values must be formatted and stored correctly in the data structures used by the model to ensure proper retrieval. The PET data should be organized in a hierarchical structure that reflects the relationship between HRUs, climate stations, and the specific weather data. The weather data structure (weat) must include a field for PET. Each weather station entry in the main weather data array should be indexed in a way that relates it to the HRUs. For instance, if you have an array of weather stations (wst), each HRU might reference its corresponding weather station via an index (iwst).

The PET data could be read from various file formats, such as:

• CSV/TSV Files: Easy to parse and read using Fortran I/O operations.

• Binary Files: More efficient for large datasets.

• Database Formats: Can be accessed via specific libraries or APIs.

This approach allows the model to use real observed data, which can improve accuracy and reliability. By integrating measured data, the model can account for local variations in climatic conditions that might not be captured by theoretical methods. By incorporating this functionality, the model enhances its capability to simulate real-world conditions, making it a valuable tool for hydrological studies and water resource management.

Table 2:2-2: SWAT+ input variables used in potential evapotranspiration calculations summarized in this section.