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Pat J-F. Yeh and J. S. Famiglietti

1. Introduction In shallow water table areas, summer drying of root-zone soil moisture often results in an upward capillary flux from the underlying aquifer to sustain the strong evapotranspiration demand; this is referred to as groundwater evapotranspiration (hereafter groundwater evaporation). In wetland and playa areas, or in semiarid riparian environments, groundwater evaporation represents the major source of water for plants, particularly during drought periods ( Schmidhalter et al. 1994

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Jeffrey Cardille, Michael T. Coe, and Julie A. Vano

1. Introduction Lakes are a major geologic feature in humid regions throughout the world. In groundwater-dominated systems, multiple lake hydrologic types exist with varying physical and chemical characteristics, connections among lakes, and relationships to the landscape. Hydraulically mounded lakes, those highest in the landscape, are connected to the groundwater system with a flow gradient sloping away from the lake in all directions ( Kratz et al. 1997 ) and typically have only a small

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Pennan Chinnasamy, Jason A. Hubbart, and Govindasamy Agoramoorthy

1. Introduction Groundwater is the primary source of freshwater in many regions globally ( Rodell et al. 2009 ). However, in many locations groundwater is consumed unsustainably to the detriment of quantity and quality of the resource. Regulations governing the use of groundwater are not well developed, even in many developed nations ( Livingston and Garrido 2004 ). Lack of regulations is largely attributable to insufficient knowledge of groundwater supplies and rates of recharge. In many

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M. Sekhar, M. Shindekar, Sat K. Tomer, and P. Goswami

1. Introduction Groundwater acts as a decentralized source to provide “safe drinking water” for millions in rural and urban areas. It accounts for nearly 85% of rural domestic water needs and 50% of urban water needs ( Kumar and Shah 2006 ). Accelerated population growth, increasing per capita water use, and poor reliability of imported surface water from distant sources are resulting in increasing dependence on groundwater for water supply in urban towns and cities ( Foster et al. 2010

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David M. Bjerklie, Thomas J. Trombley, and Roland J. Viger

1. Introduction Knowledge of historical and future hydrologic trends is needed to develop and evaluate regional water-management strategies. In New England, the amount of groundwater discharge to streams and rivers (base flow), the amount of snow, and the timing of snowmelt are especially important factors for evaluating, managing, and planning for the health of in-stream habitat and recreation. This knowledge needs to include patterns of hydrologic change in both time and space. Regional

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Y. Govender, E. Cuevas, L. D. S. Sternberg, and M. R. Jury

spite of this, it is generally accepted that, once water enters the subsurface and is away from evaporative effects, δ 18 O and δD are conservative in their mixing relationships ( Craig 1961 ). By analyzing the natural variations of the oxygen and hydrogen isotope ratios in rainfall (δ 18 O and δD, respectively), researchers have been able to trace the origins of rainfall and groundwater recharge ( Rodriguez-Martínez 1997 ; Alyamani 2001 ; Bowen and Wilkinson 2002 ; Jones and Banner 2003 ; Rosen

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M. Decker, A. J. Pitman, and J. P. Evans

1. Introduction Southeastern (SE) Australia has recently recovered from a long and intense drought that caused observable reductions in both groundwater ( van Dijk et al. 2011 ) and surface water stores ( Cai et al. 2009 ; Leblanc et al. 2009 ). While long-term droughts in Australia, caused in part by variability in the Indian Ocean sea surface temperatures ( Ummenhofer et al. 2009 ), can last a decade or longer, forests dominated by the genus Eucalyptus flourish over significant parts of SE

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Akarsh Asoka and Vimal Mishra

1. Introduction Groundwater storage and its relationship with socioenvironmental demands is a complex issue ( Hora et al. 2019 ); yet, groundwater remains the lifeline for food production in India. Unsustainable groundwater pumping for irrigation is the primary cause of groundwater depletion in many regions of the world ( Döll et al. 2012 ; Rodell et al. 2009 ; Taylor et al. 2013 ). India is the largest user of groundwater, where more than 60% of irrigated agriculture is dependent on

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Augusto Getirana, Matthew Rodell, Sujay Kumar, Hiroko Kato Beaudoing, Kristi Arsenault, Benjamin Zaitchik, Himanshu Save, and Srinivas Bettadpur

extreme events, GRACE data assimilation (DA) has been shown to improve identification of droughts in parts of North America ( Houborg et al. 2012 ; Kumar et al. 2016 ), Europe ( Li et al. 2012 ), Asia ( Girotto et al. 2017 ) and globally ( Li et al. 2019 ). Such studies demonstrated that GRACE-DA improves the simulations of water storage variability, in particular, groundwater. A series of major drought events in the United States in the early 2000s made the development of an integrated drought early

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Guoyong Leng, Maoyi Huang, Qiuhong Tang, Huilin Gao, and L. Ruby Leung

assumed unlimited irrigation water supply from local runoff or other unidentified sources (e.g., Sacks et al. 2009 ; Ozdogan et al. 2010 ; Sorooshian et al. 2012 ). This can have important implications to estimating irrigation effects, especially for regions dominated by groundwater pumping for irrigation. In the United States, about 60% of irrigation relies on groundwater. Irrigation in California’s Central Valley and the High Plains accounts for about 50% of groundwater use nationwide ( Scanlon

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