NetLogo: identification of valleys = rivers location in artificial landscape - netlogo

Question

I want to produce the artificial landscape, containing hills, slopes and valleys. So far, so good. Now, I want to place a rivers in the bottom of the valley. I thought that the easiest way will be ask turtle to move uphill/downhill by elevation as drops here: http://modelingcommons.org/browse/one_model/2352#model_tabs_browse_info
However, as my landscape is not real, my valleys are also not linears, but just the "depressions" in the terrain, and thus my river localisation is really weird looking?
Please, is there a way to create hills and valleys reflecting the real terrain in netlogo without using GIS extension? I've found great models of Erosion, Watershed and GIS gradient example, but how can I initially place my turtles to stay only in valleys?
EDIT
the GIS gradient example provide excellent problem solving of movement of all the cells over the space, and they aggregate in valleys. However, as I want to by movement of my turtle "create" the river bed on the bottom of the valley, how can I place it? maybe start at the lowest point and stop after several ticks to not to allow to get on the top of the hill? thank you !
globals [
low
high
range ]
patches-own [
altitude ]
to setup
clear-all
setup-hills
scale-patches
color-patches
end
to setup-hills
ask n-of 2 patches [
set pcolor magenta ]
ask patches [
let max_dist sqrt (world-width ^ world-width + world-height ^ world-height)
set altitude world-width - distance patch 10 10
set altitude world-width - distance min-one-of patches with [pcolor = magenta][distance myself]
]
crt 1 [ ; create a turtle, needed to identify the lowest slope
set color red
let bottom_valley min-one-of patches [altitude]
move-to bottom_valley
]
end
to scale-patches
set low [altitude] of min-one-of patches [altitude]
set high [altitude] of max-one-of patches [altitude]
set range high - low
ask patches [
set altitude altitude - low ; shift every patch down so lowest altitude is 0
set altitude altitude * 99.0 / range ; scale every patch so that the lowest is 0 and highest is 999
]
repeat 5 [
diffuse altitude 0.5 ]
end
to color-patches
ask patches [
set pcolor scale-color green altitude 0 100]
end
to create-river
ask turtles [
let p max-one-of neighbors in-radius 1 [altitude]
if [altitude] of p >= altitude [
face p
move-to p
set pcolor blue
]
]
end

Answer 1

You have the right general idea. I recommend checking out the "GIS Gradient" model on your landscape. You can find it in the Models Library under Code Examples/GIS. Then think how to filter patches that have a minimum number of water particles passing through it over time.

Answer 2

Identification of system of rivers on the landscape using GIS Gradient Example:
http://modelingcommons.org/browse/one_model/2352#model_tabs_browse_info
The whole process of identification of the river system on the complex landscape is composed by 3 subprocesses:
identify the highest points of the landscape
place here the river source (create-source)
identify patches for a downhill stream
from the highest river source, by decreasing elevetion of neighboring cells to the lowest elevation of the river valley
when turtle reaches the edge of the world, turtle dies (go-downhill)
remove top hill points (as the river does not read the summit of the hill)
the river does not start at the top of the hill, but at the highest point of the valley
the highest "river" patches from the river beds system are removed (remove-river-from-top-hill)
The number of sources depends of type of GIS data you have available. The same for the number of patches you wish to remove from the top of the hill.
;; ----------------------------------------------
;; create river
;; ----------------------------------------------
to create-source
; identify the 5 highest points of the landscape
repeat 5 [
; avoid the highest points to be close to each other
ask max-one-of patches with [ not any? turtles-here and not any? turtles in-radius 20 ] [p_elev] [
;ask patches to sporout turtles
sprout 1 [
set size 1
set color orange
]
]
]
end
to go-downhill
; stop if there is no more turtles
if not any? turtles with [color = orange]
[ stop ]
; ask turtles located on top of thehills to move downhill
ask turtles with [color = orange] [
; die when you reach the edge of the world
if ([pxcor] of patch-here = max-pxcor) or ([pycor] of patch-here = max-pycor)
or ([pxcor] of patch-here = min-pxcor) or ([pycor] of patch-here = min-pycor) [
die ]
move-to patch-here ; go to patch center
set pcolor blue ; identify the use of patch
set p min-one-of neighbors with [pcolor != blue ] [p_elev] ;; or neighbors4 with [pcolor != blue]patches in-radius 3
if p != nobody [
; move downhill if elevation of patch you are standing on is higher then one of the neighboring patches
ifelse [p_elev] of p <= p_elev
[
face p
move-to p
set pcolor blue ; identify the use of patch
]
[
;move-to min-one-of patches with [pcolor != blue] [distance myself]
move-to min-one-of patches in-radius 2 with [pcolor != blue] [p_elev]
set pcolor blue ; identify the use of patch
]
]
]
end
to remove-river-from-top-hill
; remove 5% of the blue (river) patches placed on the top of the hill
let total_blue count patches with [pcolor = blue]
repeat total_blue * 0.05 [
ask max-one-of patches with [pcolor = blue] [p_elev] [
set pcolor yellow ]
]
end
Complex river system identified:
(river bed = BLUE, removed top of the hill = YELLOW):