From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, December 14, 2007
2007-08 Birdsall-Dreiss Lecture:
Bureau of Economic Geology
The most widespread changes in land use have occurred because of agricultural expansion. In the past 300 years, cultivated cropland and pastureland have increased globally by 560% and 660%, respectively. Irrigated agriculture has expanded by 580% since 1900 and is projected to increase by 20% by 2030 in developing countries Agricultural food production accounts for ~85% of global fresh water consumption, led by irrigated agriculture. What impacts have these land-use changes had on water resources?
Measurements of pressure head, soil pore water chemistry, groundwater levels, and groundwater quality provide an archive of system response to past land-use changes. The presentation will focus on the Texas Southern High Plains, which is one of the largest agricultural areas in the United States. Cultivation of natural grasslands has changed the system from discharging through evapotranspiration since Pleistocene times (~10,000 to 15,000 yr) to recharging during the past 50 to 100 yr. Recharge under rain-fed agriculture is shown by large groundwater-level rises (average 7 m over 3,400 km2 area of rain-fed agriculture) during the last few decades, resulting in a median recharge rate of 21 mm/yr (5% of precipitation). Changes from discharge to recharge conditions reflect long fallow periods (~7 months/yr) associated with cultivation. Recharge under irrigated agriculture is shown by downward hydraulic head gradients. Large groundwater-level declines (as much as 75 m) under irrigated areas indicate that irrigated agriculture is not sustainable. Results from land-use changes in this region will be compared with those from other regions globally. Although past land-use changes had unintended impacts on the water cycle, a comprehensive understanding of these impacts could be used to alter land-use practices for better management of water resources.