[R] Finding center of mass in a hydrologic time series

[Bert Gunter]

... and for the record:

The apply() family of functions -- here sapply() -- are *not* vectorized.
This means that one should not expect them to be necessarily more efficient
than expicit for() loops. Their advantage for many is clarity of code.

Cheers,
Bert



Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )

On Mon, Dec 18, 2017 at 7:42 AM, Eric Berger <ericjberger at gmail.com> wrote:

> Hi Eric,
> the following works for me.
>
> HTH,
> Eric
>
> library(EGRET)
>
> StartDate <- "1990-10-01"
> EndDate <- "2017-09-30"
> siteNumber <- "10310000"
> QParameterCd <- "00060"
>
> Daily <- readNWISDaily(siteNumber, QParameterCd, StartDate, EndDate)
>
> # Define 'center of mass' function
> com <- function(x) {
>   match(TRUE, cumsum(x/sum(x)) > 0.5) - 1
> }
> wyrs <- unique(Daily$waterYear)
> x <- as.Date(sapply( wyrs, function(yr) { Df <-
> Daily[Daily$waterYear==yr,];  Df$Date[com(Df$Q)] } ), "1970-01-01")
>
>
>
> On Mon, Dec 18, 2017 at 4:47 PM, Morway, Eric <emorway at usgs.gov> wrote:
>
> > Eric B's response provided just the kind of quick & simple solution I was
> > hoping for (appears as the function com below).  However, I once again
> > failed to take advantage of the power of R and have reverted back to
> using
> > a for loop for the next step of the processing.  The example below (which
> > requires the library EGRET for pulling an example dataset) works, but
> > probably can be replaced with some version of the apply functionality?
> So
> > far, I've been unable to figure out how to enter the arguments to the
> apply
> > function.  The idea is this: for each unique water year (variable 'wyrs'
> in
> > example below) in a 27 year continuous time series of daily values, find
> > the date of the 'center of mass', and build a vector of those dates.
> > Thanks, -Eric M
> >
> > library(EGRET)
> >
> > StartDate <- "1990-10-01"
> > EndDate <- "2017-09-30"
> > siteNumber <- "10310000"
> > QParameterCd <- "00060"
> >
> > Daily <- readNWISDaily(siteNumber, QParameterCd, StartDate, EndDate)
> >
> > # Define 'center of mass' function
> > com <- function(x) {
> >     match(TRUE, cumsum(x/sum(x)) > 0.5) - 1
> > }
> >
> >
> > wyrs <- unique(Daily$waterYear)
> > for(i in (1:length(wyrs))){
> >     OneYr <- Daily[Daily$waterYear==wyrs[i], ]
> >     mid <- com(OneYr$Q)
> >     if(i==1){
> >         midPts <- as.Date(OneYr$Date[mid])
> >     } else {
> >         midPts <- c(midPts, as.Date(OneYr$Date[mid]))
> >     }
> > }
> >
> >
> >
> > Eric Morway
> > Research Hydrologist
> > Nevada Water Science Center
> > U.S. Geological Survey
> > 2730 N. Deer Run Rd.
> > <https://maps.google.com/?q=2730+N.+Deer+Run+Rd.Carson+
> City,+NV+89701+(775&entry=gmail&source=g>
> > Carson City, NV 89701
> > <https://maps.google.com/?q=2730+N.+Deer+Run+Rd.Carson+
> City,+NV+89701+(775&entry=gmail&source=g>
> > (775
> > <https://maps.google.com/?q=2730+N.+Deer+Run+Rd.Carson+
> City,+NV+89701+(775&entry=gmail&source=g>)
> > 887-7668
> > *orcid*:  0000-0002-8553-6140 <http://orcid.org/0000-0002-8553-6140>
> >
> >
> >
> > On Sat, Dec 16, 2017 at 5:32 AM, Eric Berger <ericjberger at gmail.com>
> > wrote:
> >
> >> Hi Eric,
> >> How about
> >>
> >> match( TRUE, cumsum(hyd/sum(hyd)) > .5 ) - 1
> >>
> >> HTH,
> >> Eric
> >>
> >>
> >> On Sat, Dec 16, 2017 at 3:18 PM, Morway, Eric <emorway at usgs.gov> wrote:
> >>
> >>> The small bit of script below is an example of what I'm attempting to
> do
> >>> -
> >>> find the day on which the 'center of mass' occurs.  In case that is the
> >>> wrong term, I'd like to know the day that essentially cuts the area
> under
> >>> the curve in to two equal parts:
> >>>
> >>> set.seed(4004)
> >>> Date <- seq(as.Date('2000-09-01'), as.Date('2000-09-30'), by='day')
> >>> hyd <- ((100*(sin(seq(0.5,4.5,length.out=30))+10) +
> >>> seq(45,1,length.out=30)) + rnorm(30)*8) - 800
> >>>
> >>> # View the example curve
> >>> plot(Date, hyd, las=1)
> >>>
> >>> # By trial-and-error, the day on which the center of mass occurs is the
> >>> 11th day:
> >>> # Add up the area under the curve for the first 11 days and compare
> >>> # with the last 19 days:
> >>>
> >>> sum(hyd[1:11])
> >>> # 3546.364
> >>> sum(hyd[12:30])
> >>> # 3947.553
> >>>
> >>> # Add up the area under the curve for the first 12 days and compare
> >>> # with the last 18 days:
> >>>
> >>> sum(hyd[1:12])
> >>> # 3875.753
> >>> sum(hyd[13:30])
> >>> # 3618.164
> >>>
> >>> By day 12, the halfway point has already been passed, so the answer
> that
> >>> would be returned would be:
> >>>
> >>> Date[11]
> >>> # "2000-09-11"
> >>>
> >>> For the larger problem, it'd be handy if the proposed function could
> >>> process a multi-year time series (a runoff hydrograph) and return the
> day
> >>> of the center of mass for each year in the time series.
> >>>
> >>> I appreciate any pointers...Eric
> >>>
> >>>         [[alternative HTML version deleted]]
> >>>
> >>> ______________________________________________
> >>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> >>> https://stat.ethz.ch/mailman/listinfo/r-help
> >>> PLEASE do read the posting guide http://www.R-project.org/posti
> >>> ng-guide.html
> >>> and provide commented, minimal, self-contained, reproducible code.
> >>>
> >>
> >>
> >
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/
> posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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