# 1:2.1 Precipitation

The precipitation reaching the earth's surface on a given day, $$R\_{day}$$, may be read from an input file or generated by the model. Users are strongly recommended to incorporate measured precipitation into their simulations any time the data is available. The ability of SWAT+ to reproduce observed stream hydrographs is greatly improved by the use of measured precipitation data. 

Unfortunately, even with the use of measured precipitation the model user can expect some error due to inaccuracy in precipitation data. Measurement of precipitation at individual gages is subject to error from a number of causes and additional error is introduced when regional precipitation is estimated from point values. Typically, total or average areal precipitation estimates for periods of a year or longer have relative uncertainties of 10% (Winter, 1981). 

Point measurements of precipitation generally capture only a fraction of the true precipitation. The inability of a gage to capture a true reading is primarily caused by wind eddies created by the gage. These wind eddies reduce the catch of the smaller raindrops and snowflakes. Larson and Peck (1974) found that deficiencies of 10% for rain and 30% for snow are common for gages projecting above the ground surface that are not designed to shield wind effects. Even when the gage is designed to shield for wind effects, this source of error will not be eliminated. For an in-depth discussion of this and other sources of error as well as methods for dealing with the error, please refer to Dingman (1994). 

The variable pcp in the master weather file ([weather-sta.cli](/io-docs/introduction-1/climate/weather-sta.cli.md)) identifies the method used to obtain precipitation data. To read in precipitation data, the variable is set to the names of the precipitation data file(s). To generate precipitation values, pcp is set to "sim". The equations used to generate precipitation data in SWAT+ are reviewed in Chapter 1:3. SWAT+ input variables that pertain to precipitation are summarized in Table 1:2-1.

Table 1:2-1: SWAT+ input variables used in precipitation calculations.

| Definition                                                                     | Source Name | Input Name | Input File                                                                      |
| ------------------------------------------------------------------------------ | ----------- | ---------- | ------------------------------------------------------------------------------- |
| Name of measured precipitation input file (.pcp) Set to "sim" to simulate data | pgage       | pcp        | [weather-sta.cli](/io-docs/introduction-1/climate/weather-sta.cli.md)           |
| Observed precipitation                                                         | pcpmm       | pcp        | [.pcp](/io-docs/introduction-1/climate/pcp.cli-and-precipitation-data-files.md) |

See description of .[pcp](/io-docs/introduction-1/climate/pcp.cli-and-precipitation-data-files.md) file in the User’s Manual for input and format requirements if measured daily precipitation data is being used.


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