In this section, five examples of Web retrievals document how to get raw data. This data includes site information (\ref{sec:usgsSite}), measured parameter information (\ref{sec:usgsParams}), historical daily values(\ref{sec:usgsDaily}), unit values (which include real-time data but can also include other sensor data stored at regular time intervals) (\ref{sec:usgsRT}), and water quality data (\ref{sec:usgsWQP}). We will use the Choptank River near Greensboro, MD as an example. Daily discharge measurements are available as far back as 1948. Additionally, nitrate has been measured since 1964.
In this section, examples of Web retrievals document how to get raw data. This data includes site information (\ref{sec:usgsSite}), measured parameter information (\ref{sec:usgsParams}), historical daily values(\ref{sec:usgsDaily}), unit values (which include real-time data but can also include other sensor data stored at regular time intervals) (\ref{sec:usgsRT}), water quality data (\ref{sec:usgsWQP}), groundwater level data (\ref{sec:gwl}), peak flow data (\ref{sec:peak}), rating curve data (\ref{sec:rating}, and surface-water measurement data (\ref{sec:meas}). We will mainly use the Choptank River near Greensboro, MD as an example. Daily discharge measurements are available as far back as 1948. Additionally, nitrate has been measured since 1964.
The USGS organizes hydrologic data in a standard structure. Streamgages are located throughout the United States, and each streamgage has a unique ID (referred in this document and throughout the dataRetrievaldemo package as \enquote{siteNumber}). Often (but not always), these ID's are 8 digits. The first step to finding data is discovering this siteNumber. There are many ways to do this, one is the National Water Information System: Mapper \url{http://maps.waterdata.usgs.gov/mapper/index.html}.
Once the siteNumber is known, the next required input for USGS data retrievals is the \enquote{parameter code}. This is a 5-digit code that specifies the measured parameter being requested. For example, parameter code 00631 represents \enquote{Nitrate plus nitrite, water, filtered, milligrams per liter as nitrogen}, with units of \enquote{mg/l as N}. A complete list of possible USGS parameter codes can be found at \url{http://nwis.waterdata.usgs.gov/usa/nwis/pmcodes?help}.
Any data collected at regular time intervals (such as 15-minute or hourly) are known as \enquote{unit values.} Many of these are delivered on a real time basis and very recent data (even less than an hour old in many cases) are available through the function \texttt{readNWISunit}. Some of these unit values are available for many years, and some are only available for a recent time period such as 120 days. Here is an example of a retrieval of such data.
Any data collected at regular time intervals (such as 15-minute or hourly) are known as \enquote{unit values.} Many of these are delivered on a real time basis and very recent data (even less than an hour old in many cases) are available through the function \texttt{readNWISuv}. Some of these unit values are available for many years, and some are only available for a recent time period such as 120 days. Here is an example of a retrieval of such data.
@@ -531,7 +529,7 @@ The retrieval produces the following dataframe:
head(dischargeToday)
@
Note that time now becomes important, so the variable datetime is a POSIXct, and the reported time zone is included in a separate column. The datetime column is converted automatically to \enquote{UTC} (Coordinated Universal Time). To override the timezone
Note that time now becomes important, so the variable datetime is a POSIXct, and the reported time zone is included in a separate column. The datetime column is converted automatically to \enquote{UTC} (Coordinated Universal Time). To override the timezone,
Data are retrieved from \url{http://waterservices.usgs.gov/rest/IV-Test-Tool.html}. There are occasions where NWIS values are not reported as numbers, instead a common example is \enquote{Ice.} Any value that cannot be converted to a number will be reported as NA in this package.
