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Table of Contents
MODULE PROCESS (TYPE) DEFINITION KEYWORDS CREATION DATE PARAMETERS DECLARED VARIABLES DECLARED EXTERNAL VARIABLES USED DESCRIPTION REFERENCES DEVELOPER NAME AND ADDRESS NAMEprecip_prms.f
MODULE PROCESS (TYPE)Precipitation form and distribution
DEFINITIONThis module determines whether measured precipitation is rain or snow and distributes it to the HRU's.
KEYWORDSCREATION DATEAugust 1992
PARAMETERS DECLAREDadjmix_rainMonthly factor to adjust rain proportion in a mixed rain/snow event. hru_psta Index of precipitation station used to compute rain and snow on HRU. rain_adj Monthly factor to adjust measured precipitation to each hru to account for differences in elevation, spatial variation, topography, and gage catch efficiency. snow_adj Monthly factor to adjust measured precipitation to each hru to account for differences in elevation, spatial variation, topography, and gage catch efficiency. tmax_allrain If HRU maximum temperature is greater than or equal to this value precipitation is assumed to be rain, by month, in \xb0 C or ë\xb0 F, depending on units of data. tmax_allsnow If HRU maximum temperature is less than or equal to this value, precipitation is assumed to be snow, by month, in \xb0 C or \xb0 F, depending on units of data. tstorm_mo Monthly indicator for prevalent storm type, 0 indicates frontal storms prevalent, 1 indicates convective storms prevalent during month. basin_area Mean elevation for each HRU, in feet. [basin] hru_area Area for each HRU, in acres. [basin] temp_units Indicator for units for temperature data, 0=ëûëëëëû ëëëëë\xb0 F and 1=\xb0 C. [basin]
VARIABLES DECLAREDbasin_pptAverage basin precipitation, in inches. hru_ppt Precipitation on HRU, rain and snow, in inches. hru_rain Rain on HRU, in inches. hru_snow Snow on HRU, in inches. newsnow Indicator for new snow during time step. pptmix Indicator for mixed rain and snow during time step. prmx The proportion of rain in a mixture of rain and snow.
EXTERNAL VARIABLES USEDprecipObserved precipitation at each measurement station, in inches. [obs] form_data Indicator to override computed determination of the precipitation form, rain or snow. [obs] tmax Observed maximum temperature at each temperature measurement station, \xb0 F or \xb0 C, depending on units of data. [obs] tmaxf or tmaxc Maximum HRU temperature, \xb0 F or \xb0 C, depending on units of data. [temp] tminf or tminc Minimum HRU temperature, \xb0 F or \xb0 C, depending on units of data. [temp]
DESCRIPTIONTotal daily precipitation depth (hru_ppt) received on an HRU is computed by:(1)  where pcor = rain_adj if precipitation is rain, or pcor = snow_adj if precipitation is snow, and precip = observed precipitation at the measurement station corresponding to the HRU. Precipitation form (rain, snow, or a mixture of both) on each HRU is estimated from the HRU maximum and minimum daily air temperatures and their relationship to a base temperature (tmax_allsnow). Precipitation is all snow if the maximum temperature is less than or equal to the tmax_allsnow and all rain if the minimum temperature is greater than or equal to tmax_allsnow. If the maximum temperature is greater than tmax_allsnow and the minimum temperature is below tmax_allsnow, then the precipitation is considered a mixture, and the rain is assumed to occur first. The portion of the total precipitation occurring as rain (prmx) is computed by: tmax is the maximum HRU temperature, tmin is the minimum HRU temperature, and adjmix_rain is a monthly factor to adjust the rain proportion in a mixed rain/snow event. This mixture algorithm can be overridden in two ways. One is the use of the parameter tmax_allrain, which is an air temperature value that when exceeded by tmax, forces the precipitation to be considered all rain. This parameter is useful for periods, such as in the spring, when the minimum daily temperatures may be below tmax_allsnow but precipitation is predominantly convective afternoon storms. The form of the precipitation may also be explicitly specified by including the variable form_data in the observed data file.
REFERENCES
DEVELOPER NAME AND ADDRESSGeorge H. Leavelsey
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