Featured Collection Introduction: Agricultural Hydrology and Water Quality

Understanding the interrelationship between modern agriculture and water resources represents a critical research need in agricultural hydrology and water quality. Agriculture occupies over 38% of Earth's land surface (FAOSTAT, Food and Agriculture Organization of the United Nations Statistics Division. Accessed April 2015, http://faostat3.fao.org/home/E) and is essential to maintaining adequate food security for human society (Godfray et al., 2010). Nevertheless, decades of research have demonstrated how agriculture's expansive global footprint influences the flow, distribution, and quality of water. For instance, agriculture exerts appreciable influence on the hydrologic cycle, consuming about 70% of the world's freshwater for irrigation (Rosegrant et al., 2009) and greatly altering evapotranspiration patterns through deforestation and changes in vegetation (Gordon et al., 2005). Globally, 93% of cropland is under some form of tillage (Huggins and Reganold, 2008) and 14% is artificially drained (Smedema et al., 2004), each of which greatly transforms the hydrology and geomorphology of terrestrial and aquatic systems. Likewise, the intensification of crop and livestock production in many areas of the world has led to excessive nutrient losses in watershed runoff, exacerbating the eutrophication of inland and coastal waters (Withers et al., 2014). In addition to the widely recognized effects of agriculture on water resources, new and emerging challenges will likely rise to the fore this century. First and foremost is the projection that global population may reach 9.6 billion by 2050, an increase of 33% from 2015 levels. This raises the dual challenge of increasing food production on existing agricultural lands while simultaneously reducing agriculture's environmental impact (Foley et al., 2011). Achieving these food and water security goals will be further tested by shifting rainfall patterns, extreme storms, and droughts brought on by rapidly changing patterns in climate and weather (IPCC, 2014). In addition to planning for a sustainable future, agriculture is also dealing with the consequences of historical management that have left behind a legacy of soil and water pollution, including excess nitrogen stored in groundwater (Howden et al., 2011), accumulated phosphorus in soils and riverine sediments (Sharpley et al., 2013), as well as other chemical and microbial contaminants of emerging environmental concern (Boxall, 2012). These legacy contaminants w