diff --git a/NAMESPACE b/NAMESPACE
index ca80cd7..6e6c3bd 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -90,6 +90,7 @@ export(celsius.to.fahrenheit)
 export(celsius.to.kelvin)
 export(convert_temperature)
 export(cv_coef)
+export(ddays)
 export(deg2rad)
 export(detect_flood_events)
 export(detect_groups)
diff --git a/R/flood_divide.R b/R/flood_divide.R
index c97d893..feac058 100644
--- a/R/flood_divide.R
+++ b/R/flood_divide.R
@@ -12,9 +12,9 @@ get_event_info <- function(date, Q, extend = ddays(5)) {
 }
 
 #' @param gap_max distance smaller than gap_max is considered as a same group
-#' 
+#'
 #' @rdname flood_divide
-#' @export 
+#' @export
 detect_groups <- function(df, inds, gap_max = 5, extend = ddays(5)) {
   grps <- cumsum(c(0, diff(inds) > gap_max)) # 分组
   info <- cbind(group = grps, df[inds, ]) # 分组
@@ -22,16 +22,21 @@ detect_groups <- function(df, inds, gap_max = 5, extend = ddays(5)) {
 }
 
 #' detect_flood_events
+#'
 #' @param Q_min minimum discharge to detect flood events
 #' @param Q_peak peak discharge to detect flood events
-#' 
+#'
+#' @param gap_max Default `5`. if `index gap > gap_max`, events will be regarded as two.
+#' If Q is hourly, gap_max can be set `5*24`.
+#' @param extend Default `ddays(5)`. Extend `nday` in the left and right of a event
+#'
 #' @rdname flood_divide
 #' @export
-detect_flood_events <- function(date, Q, Q_min = 2, Q_peak = 10, gap_max = 5) {
+detect_flood_events <- function(date, Q, Q_min = 2, Q_peak = 10, gap_max = 5, extend = ddays(5)) {
   df <- data.table(date, Q)
   inds <- df[, which(Q > Q_min)] #
 
-  info_group <- detect_groups(df, inds, gap_max) %>%
+  info_group <- detect_groups(df, inds, gap_max, extend) %>%
     .[Q_max > Q_peak, ]
 
   ## 数据压缩
@@ -41,7 +46,7 @@ detect_flood_events <- function(date, Q, Q_min = 2, Q_peak = 10, gap_max = 5) {
     lgl[date >= info$date_beg & date <= info$date_end] <- TRUE
   }
 
-  # 由于扩展了5天，这里对洪水事件重新进行编号
+  # 由于已经扩展了5天，这里对洪水事件重新进行编号，不需要二次扩展
   inds <- which(lgl)
   info_group <- detect_groups(df, inds, gap_max, extend = 0)
   info_group
@@ -51,12 +56,12 @@ detect_flood_events <- function(date, Q, Q_min = 2, Q_peak = 10, gap_max = 5) {
 #' flood_divide
 #' @param df A data.table with date and Q columns
 #' @param ... parameters passed to [detect_flood_events()]
-#' @export 
+#' @export
 flood_divide <- function(df, ...) {
   date <- df$date
   if ("time" %in% names(df)) date <- df$time
   df <- df[order(date)]
-  info_group <- detect_flood_events(date, df$Q, ...)
+  detect_flood_events(date, df$Q, ...) # info_group
 }
 
 #' @rdname flood_divide
diff --git a/R/hydroTools-package.R b/R/hydroTools-package.R
index 28d3592..b6b5bff 100644
--- a/R/hydroTools-package.R
+++ b/R/hydroTools-package.R
@@ -13,6 +13,10 @@
 ## usethis namespace: end
 NULL
 
+#' @importFrom lubridate ddays
+#' @export
+lubridate::ddays
+
 .onLoad <- function(libname, pkgname) {
   options(rmarkdown.html_vignette.check_title = FALSE)
   
diff --git a/man/flood_divide.Rd b/man/flood_divide.Rd
index 7139922..87857e4 100644
--- a/man/flood_divide.Rd
+++ b/man/flood_divide.Rd
@@ -9,7 +9,14 @@
 \usage{
 detect_groups(df, inds, gap_max = 5, extend = ddays(5))
 
-detect_flood_events(date, Q, Q_min = 2, Q_peak = 10, gap_max = 5)
+detect_flood_events(
+  date,
+  Q,
+  Q_min = 2,
+  Q_peak = 10,
+  gap_max = 5,
+  extend = ddays(5)
+)
 
 flood_divide(df, ...)
 
@@ -18,7 +25,10 @@ merge_flood(df, info_flood, format = "\%Y.\%m")
 \arguments{
 \item{df}{A data.table with date and Q columns}
 
-\item{gap_max}{distance smaller than gap_max is considered as a same group}
+\item{gap_max}{Default \code{5}. if \verb{index gap > gap_max}, events will be regarded as two.
+If Q is hourly, gap_max can be set \code{5*24}.}
+
+\item{extend}{Default \code{ddays(5)}. Extend \code{nday} in the left and right of a event}
 
 \item{Q_min}{minimum discharge to detect flood events}
 
diff --git a/man/reexports.Rd b/man/reexports.Rd
new file mode 100644
index 0000000..2432ec1
--- /dev/null
+++ b/man/reexports.Rd
@@ -0,0 +1,16 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/hydroTools-package.R
+\docType{import}
+\name{reexports}
+\alias{reexports}
+\alias{ddays}
+\title{Objects exported from other packages}
+\keyword{internal}
+\description{
+These objects are imported from other packages. Follow the links
+below to see their documentation.
+
+\describe{
+  \item{lubridate}{\code{\link[lubridate:duration]{ddays}}}
+}}
+