$title Simple Farm Level Model (DEMO1,SEQ=91)

$onText
This is the first in a series of single farm models. This simplest
version has only 7 principal crops and 2 basic inputs, land and labor,
which are specified on a monthly basis.

Kutcher, G P, Meeraus, A, and O'Mara, G T, Agriculture Sector and Policy
Models. The World Bank, 1988.

Keywords: linear programming, agricultural economics, farming, crop yields
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$sTitle Crop Data
Set
   c 'crops'  / wheat, clover, beans, onions, cotton, maize, tomato        /
   t 'period' / jan, feb, mar, apr, may, jun, jul, aug, sep, oct, nov, dec /;

Table landreq(t,c) 'months of land occupation by crop (hectares)'
         wheat  clover  beans  onions  cotton  maize  tomato
   jan     1.     1.     1.      1.
   feb     1.     1.     1.      1.
   mar     1.      .5    1.      1.       .5
   apr     1.            1.      1.      1.
   may     1.                     .25    1.      .25
   jun                                   1.     1.
   jul                                   1.     1.       .75
   aug                                   1.     1.      1.
   sep                                   1.     1.      1.
   oct                                   1.      .5     1.
   nov      .5     .25    .25     .5      .75            .75
   dec     1.     1.     1.      1.                         ;

Table laborreq(t,c) 'crop labor requirements (man-days per hectare)'
         wheat  clover  beans  onions  cotton  maize  tomato
   jan    1.72     4.5    .75    5.16
   feb     .5      1.     .75    5.
   mar    1.       8.     .75    5.       5.
   apr    1.            16.     19.58     5.
   may   17.16                   2.42     9.    4.3
   jun    2.34                            2.    5.04
   jul                                    1.5   7.16     17.
   aug                                    2.    7.97     15.
   sep                                    1.    4.41     12.
   oct                                   26.    1.12      7.
   nov    2.43     2.5   7.5    11.16    12.              6.
   dec    1.35     7.5    .75    4.68                       ;

Parameters
   yield(c)   'crop yield         (tons per hectare)'
              / wheat  1.5, clover 6.5,  beans  1, onions 3
                cotton 1.5, maize  2  ,  tomato 3           /
   price(c)   'crop prices         (dollars per ton)'
              / wheat  100, beans 200, onions 125
                cotton 350, maize  70, tomato 120 /
   miscost(c) 'misc cash costs (dollars per hectare)'
              / wheat  10, beans 5, onions 50
                cotton 80, maize 5, tomato 50 /;

* arm data, size labor availability etc.
Scalar
   land   'farm size                    (hectares)' /  4. /
   famlab 'family labor available (days per month)' / 25  /
   owage  'hire-out  wage rate   (dollars per day)' /  3. /
   twage  'temporary labor wage  (dollars per day)' /  4  /
   dpm    'number of working days per month'        / 25  /;

$sTitle Endogenous Variables and Equations
Variable
   xcrop(c) 'cropping activity  (hectares)'
   yfarm    'farm income         (dollars)'
   revenue  'value of production (dollars)'
   mcost    'misc cash cost      (dollars)'
   labcost  'labor cost          (dollars)'
   labearn  'labor income        (dollars)'
   flab(t)  'family labor use       (days)'
   fout(t)  'hiring out             (days)'
   tlab(t)  'temporary labor        (days)';

Positive Variable xcrop, flab ,fout, tlab;

Equation
   landbal(t)  'land balance           (hectares)'
   laborbal(t) 'labor balance              (days)'
   flabor(t)   'family labor balance       (days)'
   arev        'revenue accounting      (dollars)'
   acost       'cash cost accounting    (dollars)'
   alab        'labor cost accounting   (dollars)'
   aout        'labor income accounting (dollars)'
   income      'income definition       (dollars)';

landbal(t)..   sum(c, xcrop(c)*landreq(t,c))  =l= land;

laborbal(t)..  sum(c, xcrop(c)*laborreq(t,c)) =l= flab(t) + tlab(t);

flabor(t)..    famlab  =e= flab(t) + fout(t);

arev..         revenue =e= sum(c, xcrop(c)*yield(c)*price(c));

acost..        mcost   =e= sum(c, xcrop(c)*miscost(c));

alab..         labcost =e= sum(t, tlab(t)*twage);

aout..         labearn =e= sum(t, fout(t)*owage);

income..       yfarm   =e= revenue + labearn - labcost - mcost;

Model demo1 'farm labor model' / all /;

solve demo1 using lp maximizing yfarm;

$sTitle Report on Solution
Set
   crep / landuse, output, revenue                     /
   lrep / demand,  family, temporary, unused, hire-out /;

Parameter
   croprep 'crop report summary'
   labrep  'labor report summary(days)';

croprep("landuse",c)  = xcrop.l(c);
croprep("output",c)   = xcrop.l(c)*yield(c);
croprep("revenue",c)  = croprep("output",c)*price(c);
croprep(crep,"total") = sum(c, croprep(crep,c));

labrep(t,"demand")    = sum(c, xcrop.l(c)*laborreq(t,c));
labrep(t,"family")    = flab.l(t);
labrep(t,"temporary") = tlab.l(t);
labrep(t,"unused")    = -laborbal.l(t);
labrep(t,"hire-out")  = fout.l(t);
labrep("total",lrep)  = sum(t, labrep(t,lrep));

display "landuse -- hectares "
        "output  -- tons     "
        "revenue -- dollars  ", croprep, labrep;