Elevation Bands

Orographic precipitation is a significant phenomenon in certain areas of the world. To account for orographic effects on both precipitation and temperature, SWAT+ allows up to 10 elevation bands to be defined in each subbasin. Precipitation and maximum and minimum temperatures are calculated for each band as a function of the respective lapse rate and the difference between the gage elevation and the average elevation specified for the band. For precipitation,

$R_{band}=R_{day}+(EL_{band}-EL_{gage})*\frac{plaps}{days_{pcp,yr}*1000}$ when $R_{day}>0.01$

1:4.1.1

where $R_{band}$ is the precipitation falling in the elevation band (mm H$_2$O), $R_{day}$ is the precipitation recorded at the gage or generated from gage data (mm H$_2$O), $EL_{band}$ is the mean elevation in the elevation band (m), $EL_{gage}$ is the elevation at the recording gage (m), $plaps$ is the precipitation lapse rate (mm H$_2$O/km), $days_{pcp,yr}$ is the average number of days of precipitation in the subbasin in a year, and 1000 is a factor needed to convert meters to kilometers. For temperature,

$T_{mx,band}=T_{mx}+(EL_{band}-EL_{gage})*\frac{tlaps}{1000}$ 1:4.1.2

$T_{mn,band}=T_{mn}+(EL_{band}-EL_{gage})*\frac{tlaps}{1000}$ 1:4.1.3

$\overline T_{av,band} =\overline T_{av} +(EL_{band}-EL_{gage})*\frac{tlaps}{1000}$ 1:4.1.4

where $T_{mx,band}$ is the maximum daily temperature in the elevation band (°C), $T_{mn,band}$ is the minimum daily temperature in the elevation band (°C), is the mean daily temperature in the elevation band (°C), $T_{mx}$ is the maximum daily temperature recorded at the gage or generated from gage data (°C), $T_{mn}$ is the minimum daily temperature recorded at the gage or generated from gage data (°C), is the mean daily temperature recorded at the gage or generated from gage data (°C), $EL_{band}$ is the mean elevation in the elevation band (m), $EL_{gage}$ is the elevation at the recording gage (m), tlaps is the temperature lapse rate (°C/km), and 1000 is a factor needed to convert meters to kilometers.

Once the precipitation and temperature values have been calculated for each elevation band in the subbasin, new average subbasin precipitation and temperature values are calculated:

$R_{day}= \sum_{bnd=1}^{b}R_{band}*fr_{bnd}$ 1:4.1.5

$T_{mx}=\sum_{bnd=1}^b T_{mx,band}*fr_{bnd}$ 1:4.1.6

$T_{mn}=\sum_{bnd=1}^b T_{mn,band} *fr_{bnd}$ 1:4.1.7

$\overline T_{av}=\sum_{bnd=1}^b \overline T_{av,band}*fr_{bnd}$ 1:4.1.8

where $R_{day}$ is the daily average precipitation adjusted for orographic effects (mm H$_2$O), $T_{mx}$ is the daily maximum temperature adjusted for orographic effects (°C), $T_{mn}$ is the daily minimum temperature adjusted for orographic effects (°C), is the daily mean temperature adjusted for orographic effects (°C), $R_{band}$ is the precipitation falling in elevation band $bnd$ (mm H2O), $T_{mx,band}$ is the maximum daily temperature in elevation band $bnd$ (°C), $T_{mn,band}$ is the minimum daily temperature in elevation band $bnd$ (°C), is the mean daily temperature in elevation band $bnd$ (°C), $fr_{bnd}$ is the fraction of subbasin area within the elevation band, and $b$ is the total number of elevation bands in the subbasin.

The only processes modeled separately for each individual elevation band are the accumulation, sublimation and melting of snow. As with the initial precipitation and temperature data, after amounts of sublimation and snow melt are determined for each elevation band, subbasin average values are calculated. These average values are the values that are used in the remainder of the simulation and reported in the output files.

Table 1:4-1: SWAT+ input variables that pertain to orographic effects.