I get about:
coffee: 80 liters per cup
cotton: 10000 liters per kg
The differences could be due to the original calculation including run off pollutants. But the numbers are actually pretty close for a back of the envelope calculation. There are some details about this in the cotton water footprint report below.
Cotton:
http://www.cottoninc.com/fiber/AgriculturalDisciplines/Engineering/Irrigation-Management/Cotton-Water-Requirements/
looking at the averages of cotton requirements for the three time periods in the table I get:
(25 * .09) + (35 * .22) + (50 * .30) = 24.95 inches of water per acre
1 acre = 6272640 sq inches
6272640 sq inches * 24.95 inches = 156502368 cubic inches per acre
156502368 cubic inches = 2564610 liters per acre
https://www.cotton.org/econ/cropinfo/costsreturns/usa.cfm
Taking the average of "Cotton Yield: pounds per planted acre": 495.56, 667, 591
584.52 lbs per acre = 265.134 kg per acre
265.134 kg per acre / 2564610 liters per acre = 9672 liters per kg
This is a pretty interesting paper on the water footprint of cotton:
http://www.waterfootprint.org/Reports/Chapagain_et_al_2006_cotton.pdf
for virtual water average in the world total cost (pg. 192)
cotton lint: 8506 cubic meters per ton = 9376.2 liters per kg
end textile: 9359 cubic meters per ton = 10316.5 liters per kg
this seems to confirm my above calculations
Coffee:
http://www.waterfootprint.org/?page=files/CoffeeTea
claims 140 liters per cup
http://www.britannica.com/EBchecked/topic/124337/coffee-production
seems to agree with:
http://www.ancoracoffee.com/class/Fast_Facts_About_Coffee.aspx
~2000 lbs of beans per acre
http://www.coffeeresearch.org/coffee/roasting.htm
~17% weight loss (can apparently be higher or lower depending on the darkness of the blend)
http://store.starbucks.com/Coffee-Preparation-FAQ/coffee-prep-faq,default,pg.html
A 1-lb bag of coffee yields approximately 64 5-oz cups* (320 oz) (53 6oz cups)
This is pretty close to most of what the individuals say that I could find ~50 cups so I'll use this number.
http://www.soap.com/p/folgers-100-columbian-ground-coffee-27-8-oz-211123
1 lb 11.8 oz (1.7375 lbs) = 240 6oz cups
But here is an up to amount from a folgers label: 240/1.7375 = 138 cups per lb
http://www.britannica.com/EBchecked/topic/124337/coffee-production
60 to 80 inches (I'll average the two and use 70)
70 inches * 1 acre = 7 195 310.72 liters per acre
2000 lbs of beans per acre
2000 lbs * .83 (less 17%) = 1660 lbs of roasted beans per acre
1660 * 53 = 87980 cups of coffee per acre
82 liters of water per cup of coffee