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This function performs a Mann-Kendall trend test with associated p-values on a time seris of gridded satellite data
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| ############################################################################################# | |
| # title : Perform a Mann-Kendall trend test of satellite image time series | |
| # purpose : Mann-Kendall test of monotonic trend and associated statistical significance | |
| # author : Abdulhakim Abdi (@HakimAbdi) | |
| # input : Raster stack/brick comprising a time series of observations | |
| # output : Raster object of monotoic trend (Kendall's tau), significance or both (i.e. brick) | |
| # data : Test data: https://1drv.ms/u/s!AsHsKb_qtbkwg4ti8pJCxvUer9rMqg?e=BGcH1K | |
| # notes : Depending on the time series, it is might be useful prewhiten the data to remove serial correlation | |
| # : prior to extracting the trend. See blog post on www.hakimabdi.com for more information. | |
| ############################################################################################# | |
| MKraster <- function(rasterstack, type=c("trend","pval","both")){ | |
| # Values for (layers, ncell, ncol, nrow, method, crs, extent) come straight from the input raster stack | |
| # e.g. nlayers(rasterstack), ncell(rasterstack)... etc. | |
| print(paste("Start MKraster:",Sys.time())) | |
| print("Loading parameters") | |
| layers=nlayers(rasterstack);ncell=ncell(rasterstack); | |
| ncol=ncol(rasterstack);nrow=nrow(rasterstack);crs=crs(rasterstack); | |
| extent=extent(rasterstack);pb = txtProgressBar(min = 0, max = ncell, initial = 0) | |
| print("Done loading parameters") | |
| mtrx <- as.matrix(rasterstack,ncol=layers) | |
| empt <- matrix(nrow=ncell, ncol=2) | |
| print("Initiating loop operation") | |
| if (type == "trend"){ | |
| for (i in 1:length(mtrx[,1])){ | |
| if (all(is.na(mtrx[i,]))){ | |
| empt[i,1] <- NA | |
| } else | |
| if (sum(!is.na(mtrx[i,])) < 4){ | |
| empt[i,1] <- NA | |
| } else | |
| empt[i,1] <- as.numeric(MannKendall(as.numeric(mtrx[i,]))$tau) | |
| } | |
| print("Creating empty raster") | |
| trend <- raster(nrows=nrow,ncols=ncol,crs=crs) | |
| extent(trend) <- extent | |
| print("Populating trend raster") | |
| values(trend) <- empt[,1] | |
| print(paste("Ending MKraster on",Sys.time())) | |
| trend | |
| } | |
| else | |
| if (type == "pval"){ | |
| for (i in 1:length(mtrx[,1])){ | |
| if (all(is.na(mtrx[i,]))){ | |
| empt[i,1] <- NA | |
| } else | |
| if (sum(!is.na(mtrx[i,])) < 4){ | |
| empt[i,1] <- NA | |
| } else | |
| empt[i,1] <- as.numeric(MannKendall(as.numeric(mtrx[i,]))$sl) | |
| } | |
| pval <- raster(nrows=nrow,ncols=ncol,crs=crs) | |
| extent(pval) <- extent | |
| print("Populating significance raster") | |
| values(pval) <- empt[,1] | |
| print(paste("Ending MKraster on",Sys.time())) | |
| pval | |
| } | |
| else | |
| if (type == "both"){ | |
| for (i in 1:length(mtrx[,1])){ | |
| if (all(is.na(mtrx[i,]))){ | |
| empt[i,1] <- NA | |
| } else | |
| if (sum(!is.na(mtrx[i,])) < 4){ | |
| empt[i,1] <- NA | |
| } else | |
| tryCatch({ | |
| empt[i,1] <- as.numeric(MannKendall(as.numeric(mtrx[i,]))$tau) | |
| empt[i,2] <- as.numeric(MannKendall(as.numeric(mtrx[i,]))$sl) | |
| }) | |
| } | |
| tr <- raster(nrows=nrow,ncols=ncol,crs=crs) | |
| pv <- raster(nrows=nrow,ncols=ncol,crs=crs) | |
| print("Populating raster brick") | |
| values(tr) <- empt[,1] | |
| values(pv) <- empt[,2] | |
| brk <- brick(tr,pv) | |
| extent(brk) <- extent | |
| names(brk) <- c("trend","p.value") | |
| print(paste("Ending MKraster on",Sys.time())) | |
| brk | |
| } | |
| } |
antobaro00
commented
Jul 2, 2023
via email
***@***.***
Thank you very much
Il dom 2 lug 2023, 10:47 Charles Bwalya Chisanga ***@***.***>
ha scritto:
… ***@***.**** commented on this gist.
------------------------------
Send me you email so that I send you the complete script that calculates
the significance and p-value.
This is the script below:
# Perform a Mann-Kendall trend test on satellite image time series in RThe
Mann-Kendall trend test has become popular in the remote sensing community
to test whether a time series of satellite observations is consistently
increasing or decreasing. This consistency is referred to as monotonicity,
and the function that describes it is called a monotonic function. On the
other hand, a non-monotonic trend increases and decreases at different
intervals in the time series.A number of reasons are behind the
popularity of the Mann-Kendall test in remote sensing: (1) it is a
non-parametric test, which means it makes no assumptions on the
distribution of the data. (2) It also does not assume the data to be
homoscedastic, which means the variance around the regression line does not
have to be the same for all values of the predictor variable. (3) It is
resistant to outliers, so these do not have to be removed prior to trend
detection.The Mann-Kendall test does require that the data be independent
and is sensitive to presence of serial correlation (i.e. data points are
correlated with one another). This makes the null hypothesis of no trend to
be rejected too frequently, making the test very liberal. In this case, it
is might be useful to remove the serial correlation from the data through a
process called prewhitening (case for prewhitening). On the other hand, if
the sample size of the data and the magnitude of the trend are large,
serial correlation does not affect the Mann-Kendall test statistics and
prewhitening the data may remove part of the trend itself (case against
prewhitening). So, the short answer on whether to prewhiten or not is that
it depends on your data.The Mann-Kendall test is usually performed on
univariate time series such as hydrometeorological data derived from rain
or streamflow gauges. However, satellite image time series has a different
data structure that presents a unique challenge, i.e. each pixel is its own
time series. While conducting analysis for the last research paper in my
PhD, I developed a function in R that can take in raster stacks or bricks
to perform the Mann-Kendall trend test and calculate its statistical
significance (p values). The advantage of this function is that it only
requires two R packages and only has two parameters, which makes it
straightforward to run.# TUTORIAL: HOW TO PERFORM THE M-K TEST ON RASTER
DATA IN RHere I’ve provided sample data to try out the function. The
sample dataset comprises biweekly observations of normalized difference
vegetation index (NDVI) over South America between 2000 and 2009. The
spatial resolution is 0.5 degrees (~50km), so the dataset is not large
(about 5.2 MB).The function depends on the kendall and raster packages in
R, so you will need to install them. Once you’ve loaded them as well as
custom function I’ve developed then you’re good to go.require(raster)
require(kendall)
library(Kendall)
library(raster)
load NDVI time series stack from file.ndvi.stack =
stack("ndvi_SouthAmerica_2000-2009.tif")
setwd("D:/2023_modeled_climate_data/CHIRPS/chirps_geotif/Output_TIF/input_geotif/")
all_NDVI_files <- Sys.glob('zambia_chirps-v2*.tif')
all_NDVI_files
tiffiles <- all_NDVI_files
chirps_stack <- stack(tiffiles)
chirps_stack
source("MKraster.R") # this script should in folder where geotiff files
are located
get the Mann-Kendall trend and p-values for the time series
MKraster(chirps_stack, type = "both")
mk_precip <- MKraster(chirps_stack, type = "both")
system.time({
mk_precip = MKraster(chirps_stack, type = "both")
})
plot the trend and p-value
plot(mk_precip$trend)
plot(mk_precip$p.value)
plot(mk_precip[[1]])
plot(mk_precip[[2]])
write the result to file
writeRaster(mk_precip,
"D:/2023_modeled_climate_data/kendall_analysis/outputs/mk.Annual_evi_Trend.2000.2022.tif",
overwrite=TRUE)
getwd()
calculate the trend, p-value and significanceset the path
path <- "D:/MOOC/kanona_forest/correlation_r/"
set the working directory
setwd(path)
list.files()
gimms <- brick("annual.evi.2000.2022.nc", varname= "_250m_16_days_EVI")
gimms[] <- NAgimms[gimms == -999] <- NAplot(gimms)Remove NAsgimms[is.na(gimms)]
<- -9999gimms <- na.omit(gimms)first, let’s create annual sums, as
autocorrelation might be present with monthly values. To keep NDVI as the
unit, the results are devided by 24:The MOD11A2 Version 6 product
provides an average 8-day per-pixel Land Surface Temperature and Emissivity
(LST&E) with a 1 kilometer (km) spatial resolution
fun <- function(x) {
gimms.ts = ts(x, start=c(2000,2), end=c(2022,1), frequency=1) # annual
timestep
x <- aggregate(gimms.ts)
}
gimms.sum <- calc(gimms, fun)
gimms.sum=gimms.sum/1
plot(gimms.sum)
nlayers(gimms.sum)
#then the slope is calculated to get the direction and magnitude of
trends, multiplied by the number of years to get the change in NDVI units:
time <- 1:nlayers(gimms.sum)
fun=function(x) { if (is.na(x[1])){ NA } else { m = lm(x ~ time);
summary(m)$coefficients[2] }}
gimms.slope=calc(gimms.sum, fun)
gimms.slope=gimms.slope*25
plot(gimms.slope)
now we need to see which trends are significant. Thus we first extract the
p-value:
fun=function(x) { if (is.na(x[1])){ NA } else { m = lm(x ~ time);
summary(m)$coefficients[8] }}
p <- calc(gimms.sum, fun=fun)
plot(p, main="p-Value")
then mask all values >0.05 to get a confidence level of 95%:
m = c(0, 0.05, 1, 0.05, 1, 0)
rclmat = matrix(m, ncol=3, byrow=TRUE)
p.mask = reclassify(p, rclmat)
fun=function(x) { x[x<1] <- NA; return(x)}
p.mask.NA = calc(p.mask, fun)
and finaly mask all insignificant values in the trend map, so we only get
NDVI change significant at the 95% level:
trend.sig = mask(gimms.slope, p.mask.NA)
plot(trend.sig, main="significant EVI change")plot(shp, add=T)
evi.trend.sig <- trend.sig
plot(evi.trend.sig, main="significant EVI change")
plot(shp, add=T)
output<-mask(trend.sig, shp) # mask netcdf data using shp
output
The result could look like that:The duration will vary depending on your
data and computing configuration. If you have a large dataset and a
multicore computer, you can use the clusterR function from within the
raster package to speed up the process.Using the sample NDVI data, this
Mann-Kendall trend and its significance took 2 seconds on my laptop.Using
the sample NDVI data, this Mann-Kendall trend and its significance took 2
seconds on my laptop.The function was developed for the paper below, so
please cite it if you use any part of the code in a publication or if you
build upon it.# Citation:Abdi, A. M., Boke-Olén, N., Jin, H., Eklundh,
L., Tagesson, T., Lehsten, V., & Ardö, J. (2019). First assessment of the
plant phenology index (PPI) for estimating gross primary productivity in
African semi-arid ecosystems. International Journal of Applied Earth
Observation and Geoinformation, 78, 249-260.The function is available as
a Gist on Github. The sample NDVI data I used to produce the figure above
along with a tutorial script can be downloaded here. These sample data can
conceivably be used in a study, if anyone is interested.
—
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--
@cbchisanga
Unfortunately, my e-mail address is censored.
In any case, I would need the procedure that can be used via R but in QGIS.
Could you explain how to do it?
Thank you very much
Antonio
You can use QGIS 2.18.24. It has plugin for SPI Utility for Raster & SPI Utility for Vector. Read the manual on input data.
R scripts for computing drought indices are plenty.
For python install the climate-indices module in Miniconda or Anaconda or in QGIS. Using the python console.
@cbchisanga Unfortunately, my e-mail address is censored. In any case, I would need the procedure that can be used via R but in QGIS. Could you explain how to do it?
Thank you very much Antonio
You can use QGIS 2.18.24. It has plugin for SPI Utility for Raster & SPI Utility for Vector. Read the manual on input data.
R scripts for computing drought indices are plenty.
For python install the climate-indices module in Miniconda or Anaconda or in QGIS. Using the python console.
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