The current crises in food and water scarcity and related biofuel production highlight linkages between water, energy, and food production. These issues are critical in semiarid regions where water resources are extremely limited. The objectives of our studies are to address the following questions:

• How does agriculture impact water resources in semiarid regions?
• How can we measure these impacts?
• How can we use our understanding of impacts and feedbacks to develop sustainable water resources?

Much of our original work has focused on the High Plains in Texas (94,000 km2 area). Our recent work has involved reconnaissance studies in China (North China Plain, Loess Plateau), India (Rajasthan), and collaborations in Africa (Niger), Mexico, and Australia.

Impacts
Rainfed agriculture increased groundwater recharge from 0 under natural ecosystems to 24 mm/yr under rainfed cropland in the Texas High Plains. Similar increases were recorded in Australia (0 to ~10 mm/yr) and Niger (1- 5 to 10-50 mm/yr). There is a tradeoff between water and salts as increased recharge mobilizes natural salts and degrades groundwater quality slightly in the Texas High Plains and Niger but much more in Australia. Irrigated agriculture depletes groundwater resources in the Texas High Plains and North China Plain, decreasing groundwater levels by up to 1 m/yr. Reducing irrigation applications, as proposed for most irrigated regions, is increasing salt buildup in soils in the Texas High Plains where groundwater quality is poor.

Methods
High correlations between Terrestrial Water Storage (TWS) monitored by the GRACE satellite and measured groundwater (1000 sites) and soil moisture storage (90 stations) in the US High Plains (450,000 km2 area) (r= 0.9) indicates that the GRACE satellite can be used to track changes in groundwater storage in semiarid regions subjected to intensive irrigation with deep water tables, e.g. North China Plain, western India. Unsaturated zone water pressure and chemistry (chloride) provide an archive of subsurface flow response to cultivation and irrigation over decades to centuries. These data are readily collected in remote regions and provide information on water and salt balances, as shown by profiles in Texas, Australia, China, India, and Africa. Groundwater level monitoring and groundwater chemistry record impacts of cultivation on groundwater resources in semiarid regions globally.

Sustainability
Irrigated agriculture as currently practiced is not sustainable. Irrigation with too much groundwater in the Texas High Plains and North China Plain depletes aquifer storage. Irrigation with too little water in the Texas High Plains salinizes soils. Rainfed agriculture is sustainable from a water resource standpoint in the Texas High Plains. Increased recharge under cropland could sustain limited irrigation in this region (~1 yr out of 10 yr) and other regions. Water resources will recover if irrigated agriculture is converted to rainfed agriculture in Texas High Plains. Policies and economic incentives need to be developed that encourage sustainable water resources management through agriculture in these semiarid regions.