Katie: I would say that this is not an R question, so I would suggest that either
a) You ask it on a statistics help website like stats.stackexchange.com or b) You consult with someone locally who knows about nonlinear regression (possibly a statistician, but not necessarily so). -- Bert On Tue, Oct 11, 2011 at 11:34 AM, Katie Tully <katherinetu...@gmail.com>wrote: > I am trying to fit a nonlinear regression to infiltration data in order to > determine saturated hydraulic conductivity and matric pressure. The > original equation can be found in Bagarello et al. 2004 SSSAJ (green-ampt > equation for falling head including gravity). I am also VERY new to R and > to nonlinear regressions. I have searched the posts, but am still unable to > determine why my data come up with the "singular gradient" error. > > Here are the data: > time <- c(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, > 19) > #time in minutes > cumul <-c(2, 5, 7, 9.5, 11, 13, 14, 15, 16, 18.5, 21, 23, 24.5, 26.5, 28, > 29.5, 31, 31.5, 32.5) #cumulative infiltration in cm per min > df <- data.frame(time, cumul) > df$cumul.m <- df$cumul/100/60 #convert to meters per second > df$time.s <- df$time*60 #convert to seconds > b2 <- 1-(0.196/(0.06/0.01131)) #relationship between soil moisture and the > size of the ring infiltrometer (6 cm radius by 113.1 cm2 cross sectional > area) > theta <- 0.196 #difference in residual soil water and field capacity > > Here is the formula: > #Where a = K_fs and b=psi_f > nlsfit <- > > nls(time.s~(theta/a*b2)*((cumul.m/theta)-(((0.16-b)/b2)*log(1+((cumul.m*b2)/(theta*(0.16-b)))))), > data = df, start=list (a=1, b=0.5), trace=TRUE) > > ----- > I am likely "over parameterizing", but I must admit, that I am not entirely > sure what that means. Any help offered would be greatly appreciated. I am > sorry if I sound naive, but I am an ecologist, not a hydrologist. > > Kate > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > 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. > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list 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.
