Mapping marginal agricultural lands reimagined

var mod13q1 = ee.ImageCollection("MODIS/006/MOD13Q1"),
 lulc = ee.Image("ESA/GLOBCOVER_L4_200901_200912_V2_3"),
 countries = ee.FeatureCollection("USDOS/LSIB_SIMPLE/2017");
 
var startYear = 2008;
var endYear = 2018;
var startMonth = 11;
var endMonth = 4;
var country = 'Kenya';
var scale = 2500;

var years = ee.List.sequence(startYear,endYear);
var n = endYear-startYear+1;
var ag = lulc.select('landcover').lte(30).eq(1);
var region = countries.filter(ee.Filter.eq('country_na',country));

var yearly_mod13q1 = ee.ImageCollection.fromImages(
 years.map(function (y) {
 return mod13q1.filter(ee.Filter.calendarRange(y,y,'year'))
 .filter(ee.Filter.calendarRange(startMonth,endMonth,'month'))
 .select('NDVI').mean()
 .clip(region).updateMask(ag)
 .multiply(0.0001);
 })
);

var mean_mod13q1 = yearly_mod13q1.mean().rename('mean_mod13q1');

var stddev_mod13q1 = yearly_mod13q1.map(function(img){
 return img.subtract(mean_mod13q1).pow(2);
}).sum().divide(n).sqrt().rename('stddev_mod13q1');
 
var level_mod13q1 = mean_mod13q1.reduceRegion({
 reducer: ee.Reducer.percentile([33,66]),
 geometry: region,
 scale: scale,
 maxPixels: 1e12
});

var var_mod13q1 = stddev_mod13q1.reduceRegion({
 reducer: ee.Reducer.percentile([50]),
 geometry: region,
 scale: scale,
 maxPixels: 1e12
});

var lmh_mod13q1 = mean_mod13q1.gt(ee.Number(level_mod13q1.get('mean_mod13q1_p33')))
 .add(mean_mod13q1.gt(ee.Number(level_mod13q1.get('mean_mod13q1_p66'))))
 .remap([0,1,2],[1,2,3]);

var vs_mod13q1 = stddev_mod13q1.gt(ee.Number(var_mod13q1.get('stddev_mod13q1'))).remap([0,1],[10,20]);


var productivity_mod13q1 = lmh_mod13q1.add(vs_mod13q1).remap([11,12,13,21,22,23],[1,4,5,2,3,6]).rename('productivity');
 // 11 - LS - 1 
 // 12 - MS - 3
 // 13 - HS - 5
 // 21 - LV - 2 
 // 22 - MV - 4 
 // 23 - HV - 6

var createTimeBand = function(image) {
 var num = image.get('system:index');
 return image.addBands(ee.Image(ee.Number.parse(num)).double());
};
var linearFit = yearly_mod13q1.map(createTimeBand).select(['constant', 'NDVI']).reduce(ee.Reducer.linearFit()).select('scale');
var slope = linearFit.gt(0).remap([0,1],[0,10]);
var productivity_slope = productivity_mod13q1.add(slope).remap([1,2,3,4,5,6,11,12,13,14,15,16],[3,4,7,8,11,12,1,2,5,6,9,10]);
 // 1 - 3 - LSD
 // 2 - 4 - LVD
 // 3 - 7 - MSD
 // 4 - 8 - MVD
 // 5 - 11 - HSD
 // 6 - 12 - HVD
 // 11 - 1 - LSI
 // 12 - 2 - LVI
 // 13 - 5 - MSI
 // 14 - 6 - MVI
 // 15 - 9 - HSI
 // 16 - 10 - HVI
 
Map.centerObject(region);
Map.setOptions('HYBRID');
var sColors = ['fecc5c','fd8d3c','f03b20','bd0026','a1dab4','41b6c4','2c7fb8','253494','c2e699','78c679','31a354','006837'];
Map.addLayer(ee.Image(0),{palette:'black', opacity: 0.75},'dark basemap');
Map.addLayer(productivity_slope,{min:1,max:12,palette:sColors},'productivity slope');

var categories = ['Low-Stable-Increasing', 'Low-Variable-Increasing', 
 'Low-Stable-Decreasing', 'Low-Variable-Decreasing', 
 'Medium-Stable-Increasing', 'Medium-Variable-Increasing', 
 'Medium-Stable-Decreasing', 'Medium-Variable-Decreasing', 
 'High-Stable-Increasing', 'High-Variable-Increasing', 
 'High-Stable-Decreasing', 'High-Variable-Decreasing']; 

var legend = ui.Panel({
 style: {
 position: 'bottom-left',
 backgroundColor: '202020',
 padding: '8px 15px',
 border: '5px solid #333333'
 }
});

var legendTitle = ui.Label({
 value: 'Agricultural productivity',
 style: {
 fontWeight: 'bold',
 fontSize: '18px',
 margin: '3px 0 -1px 8px',
 backgroundColor: '202020',
 color: 'dbdbdb',
 padding: '0'
 }
});
var line = ui.Label({
 value: '_________________________________________',
 style: {
 fontSize: '10px',
 margin: '0 0 10px 1px',
 backgroundColor: '202020',
 color: 'dbdbdb',
 padding: '0'
 }
});
legend.add(legendTitle);
legend.add(line);

var makeRow = function(color, name) {
 var colorBox = ui.Label({
 style: {
 backgroundColor: '#' + color,
 padding: '8px',
 margin: '0 0 4px 0'
 }
 });

 var description = ui.Label({
 value: name,
 style: {
 margin: '0 0 4px 6px',
 backgroundColor: '202020',
 color: 'dbdbdb',
 }
 });

 return ui.Panel({
 widgets: [colorBox, description],
 style: {
 backgroundColor: '202020'
 },
 layout: ui.Panel.Layout.Flow('horizontal')
 });
};

for (var i = 0; i < categories.length; i++) {
 legend.add(makeRow(sColors[i], categories[i]));
}

var site = ui.Label('cartoscience.com', {fontSize: '12px', color: 'white', stretch: 'horizontal', textAlign: 'center', margin: '0 0 0 0', backgroundColor: '202020'}, 'cartoscience.com');
legend.add(site);

Map.add(legend);