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temperature = graph(time(1)) +
time(1)/20
total harvest = sum([patch_harvest])
rain = rand_var(0,2500)
Temperature follows an annual cycle, expressed in
a sketched graph, plus a rise of 1 degree C every
20 years. (Simile provides a tool to enable users to
sketch the graph, and uses linear interpolation
between the points to evaluate the sketched
function.)
We sum the harvests from all the patches to give
the total for the whole system.
Rainfall varies randomly on each time step, with an
annual average of 1250.
Submodel Patch
land use = floor(rand_const(0,2))
x = floor((index(1)-1)/40)+1
y = fmod(index(1)-1,40)+1
soil type = (if x+y>rand_const(45,55)
then floor(rand_const(1,1.3)) else
floor(rand_const(1.8,3)))
patch harvest = sum({harvest})*dt(1)
This determines randomly the land use (i.e.
whether a crop is grown) for each patch.
These two expressions give each patch unique x,y
coordinates on 1 range of 1 to 40.
As a demonstration, the soil type is determined
partly randomly, partly in terms of the x and y
coordinates of each patch.
This works out the amount harvested at the start of
each year for each patch.
Submodel Patch/Crop
cond1 = (land_use==1)
biomass = 0
growth = (if soil_type==1 and
water>100 then 20*0.04*temperature
elseif water>150 then
18*0.04*temperature else 0)
growth coef = 17.0
harvest = (if fmod(time(1),1)==0 then
biomass/dt(1)else 0)
This condition determines which patch is grown,
determined by the patch's land use.
Initial crop biomass is set to zero.
Crop growth is worked out from soil type, soil
water content and temperature.
The growth coefficient (the growth rate for optimal
conditions of temperature and soil water) is
different for each crop, being 17,20 and 24 tonnes
per year for crop types 1, 2 and 3 respectively.
Current crop biomass becomes the harvest flow at
the start of each year.
Submodel Patch/Soil water
water = 200
rain = rain
transpiration = (if water>50 then
20*sum({biomass})else 0)
drainage = 7*water
Initial soil water is set to 200 mm
The rain flow for each patch is set equal to the
overall rainfall.
Transpiration depends on crop biomass, if there is
sufficient water in the soil.
Soil water drainage is proportional to current soil
water content.
Table 8.1 Equations for the spatial crop model.