Cropping systems for groundwater security in India: Groundwater responses to agricultural land management

The total annual groundwater withdrawals in India (251 billion km 3) are the highest of any nation. Depletion of groundwater resources is increasingly common in much of India, and farmers bear significant costs and greater vulnerability resulting from the loss or reduction of a reliable irrigation source. Three hypotheses were tested: (1) current rice cropland extent and management practices are depleting groundwater supplies, (2) tillage for water harvesting can significantly increase groundwater recharge in rainfed croplands, and (3) there are combinations of tillage, crop selection, and irrigation that are likely to increase groundwater recharge and reduce groundwater withdrawals. In order to test these hypotheses, there was the objective to evaluate improvements to the Green-Ampt infiltration routines of a hydrologic model, the Soil and Water Assessment Tool (SWAT), through the addition of a dynamic surface storage depth used for tillage parameterization. Also, the final objective was to assess the social and economic impacts of alternative agricultural land management. SWAT was used for simulating the groundwater balance (recharge -- irrigation pumping) of a 512 ha watershed to examine a variety of possible agricultural management options for groundwater sustainability. The best options for groundwater sustainability were evaluated based on predictions of groundwater recharge and withdrawals, evapotranspiration, and estimated household incomes. Reductions in rice cropland areas significantly improved the groundwater balance of the study area; water harvesting tillage simulated in all rainfed areas increased groundwater recharge by about 30 mm/year. Surface storage depth was shown to be the most important parameter for infiltration prediction in agricultural systems having 1.5 to 5.0 cm of surface storage capacity; surface storage depth was still important for infiltration prediction in systems having 0 to 1.0 cm of surface storage capacity. The vast extent of rice cropland areas and their highly negative groundwater balance suggest that irrigation from groundwater resources has caused much of the observed groundwater decline in India. Sensitivity analyses suggest that the addition of a variable surface storage depth head to the Green-Ampt infiltration routine can reduce uncertainty in infiltration simulations. Evidence of rainfall characterized by storms of greater intensity suggests that surface storage of runoff will become increasingly important for maintaining or improving current levels of groundwater recharge. Estimates of the economic impacts of selected management scenarios show promise that moderate management changes to improve the groundwater balance can still maintain or increase total watershed-scale income.