1:3.2.2 Daily Maximum Half-Hour Rain Value
The user has the option of using the monthly maximum half-hour rainfall for all days in the month or generating a daily value. The variable sed_det in the basin input file (codes.bsn) defines which option the user prefers. The randomness of the triangular distribution used to generated daily values can cause the maximum half-hour rainfall value to jump around. For small plots or microwatersheds in particular, the variability of the triangular distribution is unrealistic.
The triangular distribution used to generate the maximum half-hour rainfall fraction requires four inputs: average monthly half-hour rainfall fraction, maximum value for half-hour rainfall fraction allowed in month, minimum value for half-hour rainfall fraction allowed in month, and a random number between 0.0 and 1.0.
The maximum half-hour rainfall fraction, or upper limit of the triangular distribution, is calculated from the daily amount of rainfall with the equation:
1:3.2.3
where is the largest half-hour fraction that can be generated on a given day, and is the precipitation on a given day (mm HO). The minimum half-hour fraction, or lower limit of the triangular distribution, , is set at 0.02083.
The triangular distribution uses one of two sets of equations to generate a maximum half-hour rainfall fraction for the day. If then
1:3.2.4
If then
1:3.2.5
where is the maximum half-hour rainfall fraction for the day, is the average maximum half-hour rainfall fraction for the month, is a random number generated by the model each day, is the smallest half-hour rainfall fraction that can be generated, is the largest half-hour fraction that can be generated, and is the average of , , and .
Table 1:3-2: SWAT+ input variables that pertain to generation of maximum half-hour rainfall.
Definition | Source Name | Input Name | Input File |
---|---|---|---|
Code governing calculation of daily maximum half-hour rainfall: 0-generate daily value 1-use monthly maximum half-hour rainfall value | sed_det (not used) | sed_det (not used) | |
: extreme half-hour rainfall for month (mm HO) | rainhmx | pcp_hhr | |
: peak rate adjustment factor | adj_pkr | adj_pkrt | |
average amount of precipitation falling in month (mm HO) | pcpmm | pcp_ave | |
: average number of days of precipitation in month (= PCPMM / PCPD) | pcpd | pcp_days | |
: number of years of data used to obtain values for RAINHHMX pcp_hhr | rain_yrs | yrs | |
: amount of rain falling on a given day (mm HO) | pcp | *.pcp |
Last updated