Changes from the original module are written in bold text
This module determines whether measured precipitation is rain or snow and distributes it to the HRUs. The daily precipitation values for the weather sites are interpolated to each HRU by an inverse distance weighting scheme and then averaged.
Monthly factor to adjust rain proportion in a mixed rain/snow event.
The square of the distance between an HRU and each weather station. Does not need to be input because it is calculated. Identified as parameter because not a state variable. Units are the square of the input unit for hru_xlong, hru_ylat, psta_xlong, and psta_ylat parameters (see below).
The x or longitude of the centroid of an HRU that is input in projection coordinates such as State Plane, Lambert, or UTM. Used to calculate distance between HRU and a weather site. Units can be feet, miles, meters, or kilometers but must be consistent with hru_ylat, psta_xlong, and psta_ylat.
The y or latitude of the centroid of an HRU that is input in projection coordinates such as State Plane, Lambert, or UTM. Used to calculate distance between HRU and a weather site. Units can be feet, miles, meters, or kilometers but must be consistent with hru_xlong, psta_xlong, and psta_ylat.
Mean monthly precipitation for each weather station, in inches. Mean monthly values should be consistent with period of record used for rain_mon and snow_mon described below.
The x or longitude of each weather station that is input in projection coordinates such as State Plane, Lambert, or UTM. Used to calculate distance between HRU and a weather site. Units can be feet, miles, meters, or kilometers but must be consistent with hru_ylat, hru_xlong, and psta_ylat.
The y or latitude of each weather station that is input in projection coordinates such as State Plane, Lambert, or UTM. Used to calculate distance between HRU and a weather site. Units can be feet, miles, meters, or kilometers but must be consistent with hru_xlong, psta_xlong, and hru_ylat.
Maximum daily precipitation that can occur in each month for each weather station. If precipitation at a station is greater than this value for the month, it is assumed to be in error. Value is used to constrain bad or missing values, in inches.
Mean monthly HRU rain to account for differences in elevation, spatial variation, and topography, in inches. Can be obtained from the National Weather Service's spatial distribution of mean annual precipitation for the 1961-90 climate normal period.
Mean monthly HRU snow to account for differences in elevation, spatial variation, and topography, in inches. Can be obtained from the National Weather Service's spatial distribution of mean annual precipitation for the 1961-90 climate normal period.
If HRU maximum temperature is greater than or equal to this value precipitation is assumed to be rain, by month, in ° C or ° F, depending on units of data.
If HRU maximum temperature is less than or equal to this value, precipitation is assumed to be snow, by month, in ° C or ° F, depending on units of data.
Monthly indicator for prevalent storm type, 0 indicates frontal storms prevalent, 1 indicates convective storms prevalent during month.
Mean elevation for each HRU, in feet. [basin]
Area for each HRU, in acres. [basin]
Indicator for units for temperature data, 0= ° F and 1= ° C. [basin]
Average basin precipitation, in inches.
Precipitation on HRU, rain and snow, in inches.
Indicator for new snow during time step.
Observed precipitation at each measurement station, in inches. [obs]
Indicator to override computed determination of the precipitation form, rain or snow. [obs]
Observed maximum temperature at each temperature measurement station, ° F or ° C, depending on units of data. [obs]
Maximum HRU temperature, ° F or ° C, depending on units of data. [temp]
Minimum HRU temperature, ° F or ° C, depending on units of data. [temp]
Total daily precipitation depth (hru_ppt) received on an HRU is computed by
pcor = rain_mon/psta_mon if precipitation is rain, or
pcor = snow_mon/pstat_mon if precipitation is snow,
precip = observed precipitation at a measurement station corresponding to the HRU, and
dist2 = the square of the distance between the station and the centroid of the HRU.
The above method of interpolating precipitation from a weather site to a HRU using the inverse of the square of the distance between the site and an HRU is from Dean and Snyder (1977). The correction factor (pcor) is based on the method of Bauer and Vaccaro (1987), which used mean annual values, and has been modified to use mean monthly values.
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.
Bauer, H.H., and Vaccaro, J.J., 1987, Documentation of a deep percolation model for estimating ground-water recharge: U. S. Geological Survey Open-File Report 86-536, 180 p.
Dean, J.D., and Snyder, W.M., 1977, Temporally and areally distributed rainfall: Journal of Irrigation Division, American Society of Civil Engineers, TA 103, No. IR2, p. 221-224.
Leavesley, G.H., Lichty, R.W., Troutman, B.M., and Saindon, L.G., 1983, Precipitation-runoff modeling system--User's manual: U. S. Geological Survey Water-Resources Investigations Report 83-4238, 207 p.
Willen, D.W., Shumway, C.A., and Reid, J.E., 1971, Simulation of daily snow water equivalent and melt, in Western Snow Conference, Billings, Mont., 1971, Proceedings: v. 39, p. 1-8.
URL for this page is http://pubsdata.usgs.gov/pubs/of/2002/ofr02362/htdocs/precip/precip_prms_dist2.htm
Page contact: Mark Mastin (mcmastin@usgs.gov),
253-428-3600, ext. 2609
Last modified: Friday, 11-Jan-2013 03:19:50 EST
![prmx = [(tmax minus tmax_allsnow) divided by (tmax minus tmin)] multiplied by adjmix_rain](../../images/precip_prms_dist2-2.gif)