Approximately 70% of overall freshwater withdrawals are for agriculture.
Finding an estimate for the percent of farmland worldwide that is dependent on groundwater/aquifer withdrawals is more limited by available data... but the most comprehensive global inventory, also posted on the FAO-AQUASTAT site, says that 43% of that water use comes from groundwater sources... and 38% of irrigated farmland is dependent on groundwater.
"Globally, area equipped for irrigation is currently about 301 million ha of which 38% are equipped for irrigation with groundwater."
"Total consumptive groundwater use for irrigation is estimated as 545 km3 yr−1, or 43% of the total consumptive irrigation water use of1277 km3 yr−1. The countries with the largest extent of areas equipped for irrigation with groundwater, in absolute terms, are India (39 million ha), China (19 million ha) and the USA (17 million ha)."
"Globally, irrigation accounts for more than 70% of total water withdrawals and for more than 90% of total consumptive water use (Doll, 2009; FAO, 2010; Shiklomanov et al., 2000)"
"The inventory is mainly based on statistics published in national census reports available on-line or made available from the FAO-AQUASTAT library. In addition, by linking these statistics to the Global Crop Water Model GCWM (Siebert and Doll, 2010), consumptive irrigation water use from groundwater and surface water is estimated."
S. Siebert, J Burke, JM Faures, K Frenken, J Hoogeveen, P Doll, and FT Portma. “Groundwater use for irrigation – a global inventory.” Hydrology and Earth Systems Sciences 14. (2010) doi:10.5194/hess-14-1863-2010
Accesed April 25, 2014. http://www.fao.org/docrep/013/al816e/al816e00.pdf