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:

α0.5U=1exp(125Rday+5)\alpha_{0.5U}=1-exp(\frac{-125}{R_{day}+5}) 1:3.2.3

where α0.5U\alpha_{0.5U} is the largest half-hour fraction that can be generated on a given day, and RdayR_{day} is the precipitation on a given day (mm H2_2O). The minimum half-hour fraction, or lower limit of the triangular distribution, α0.5L\alpha_{0.5L}, 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 rnd1(α0.5monα0.5Lα0.5Uα0.5L)rnd_1\le(\frac{\alpha_{0.5mon}-\alpha_{0.5L}}{\alpha_{0.5U}-\alpha_{0.5L}}) then

α0.5=α0.5monα0.5L+[rnd1(α0.5Uα0.5L)(α0.5monα0.5L)]0.5α0.5mean\alpha_{0.5}=\alpha_{0.5mon}*\frac{\alpha_{0.5L}+[rnd_1*(\alpha_{0.5U}-\alpha_{0.5L})*(\alpha_{0.5mon}-\alpha_{0.5L})]^{0.5}}{\alpha_{0.5mean}} 1:3.2.4

If rnd1>(α0.5monα0.5Lα0.5Uα0.5L)rnd_1>(\frac{\alpha_{0.5mon}-\alpha_{0.5L}}{\alpha_{0.5U}-\alpha_{0.5L}}) then

α0.5=α0.5mon(α0.5U(α0.5Uα0.5mon)[α0.5U(1rnd1)α0.5L(1rnd1)α0.5Uα0.5mon]0.5α0.5mean)\alpha_{0.5}=\alpha_{0.5mon}*(\frac{\alpha_{0.5U}-(\alpha_{0.5U}-\alpha_{0.5mon})*[\frac{\alpha_{0.5U}(1-rnd_1)-\alpha_{0.5L}(1-rnd_1)}{\alpha_{0.5U}-\alpha_{0.5mon}}]^{0.5}}{\alpha_{0.5mean}}) 1:3.2.5

where α0.5\alpha_{0.5} is the maximum half-hour rainfall fraction for the day, α0.5mon\alpha_{0.5mon}is the average maximum half-hour rainfall fraction for the month, rnd1rnd_1 is a random number generated by the model each day, α0.5L\alpha_{0.5L} is the smallest half-hour rainfall fraction that can be generated, α0.5U\alpha_{0.5U} is the largest half-hour fraction that can be generated, and α0.5mean\alpha_{0.5mean}is the average of α0.5L\alpha_{0.5L}, α0.5mon\alpha_{0.5mon}, and α0.5U\alpha_{0.5U}.

Table 1:3-2: SWAT+ input variables that pertain to generation of maximum half-hour rainfall.

DefinitionSource NameInput NameInput 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)

R0.5xR_{0.5x}: extreme half-hour rainfall for month (mm H2_2O)

rainhmx

pcp_hhr

adj0.5αadj_{0.5\alpha}: peak rate adjustment factor

adj_pkr

adj_pkrt

average amount of precipitation falling in month (mm H2_2O)

pcpmm

pcp_ave

dayswetdays_{wet}: average number of days of precipitation in month (μmon\mu_{mon}= PCPMM / PCPD)

pcpd

pcp_days

yrsyrs: number of years of data used to obtain values for RAINHHMX pcp_hhr

rain_yrs

yrs

RdayR_{day}: amount of rain falling on a given day (mm H2_2O)

pcp

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