ENH: optionally use "w"-flag in r.watershed to enable disk swap for l…

…arge grids

- updated docu
closes #41

R/subbasin.R

@@ -34,6 +34,7 @@
 #'      If \code{drainage_dir} or \code{flowaccum} is set to \code{NULL} (default value), both will be calculated by GRASS function \emph{r.watershed}.
 #' @param drainage_dir (optional) Existing raster map of drainage direction in GRASS location. Must correspond to map generated by \code{r.watershed}.
 #'      If \code{flowaccum} or \code{flowaccum} is set to \code{NULL} (default value), both will be calculated by GRASS function \emph{r.watershed}.
+#' @param disk_swap (optional) Only needed if memory requirements exceed available RAM (large DEMs): If set to \code{TRUE}, \code{r.watershed} uses the "-m"-flag (slow)
 #' @param basin_out Output: Name of subbasins raster map exported into GRASS location.
 #' @param stream Output: Prefix of calculated stream segments vector (<stream>_vect) and
 #'      raster (<stream>_rast) maps exported into GRASS location. Only generated if
@@ -108,6 +109,7 @@ calc_subbas <- function(
   river=NULL,
   flowaccum=NULL,
   drainage_dir=NULL,
+  disk_swap=FALSE,
 
   ### OUTPUT ###
   basin_out=NULL,
@@ -204,8 +206,9 @@ calc_subbas <- function(
     if(is.null(river)) {
       message("\nCalculate drainage and river network...\n")
       # GRASS watershed calculation #
+      ws_flags = ifelse(disk_swap,c("overwrite","w"),"overwrite") #flags to use for r.watershed
       if(is.null(flowaccum) )
-        execGRASS("r.watershed", elevation=dem, accumulation="accum_t", drainage="drain_t", flags = "overwrite")
+        execGRASS("r.watershed", elevation=dem, accumulation="accum_t", drainage="drain_t", flags = ws_flags)
       # check thresh_stream parameter
       cmd_out <- execGRASS("r.univar", map="accum_t", fs="comma", flags=c("t"), intern=TRUE, ignore.stderr = TRUE)
       cmd_out <- strsplit(cmd_out, ",")
@@ -226,7 +229,7 @@ calc_subbas <- function(
       if(is.null(flowaccum) )
       {
         message("\nCalculate drainage...\n")
-        execGRASS("r.watershed", elevation=dem, drainage="drain_t")
+        execGRASS("r.watershed", elevation=dem, drainage="drain_t", flags = ws_flags)
       }
     }
 
@@ -237,7 +240,7 @@ calc_subbas <- function(
       message("\nCalculate subbasins based on given area threshold...\n")
 
       # calculate subbasins
-      execGRASS("r.watershed", elevation=dem, basin="basin_calc_t", threshold=thresh_sub)
+      execGRASS("r.watershed", elevation=dem, basin="basin_calc_t", threshold=thresh_sub, flags = ws_flags)
     }