The first and most direct impact is the loss of baseflow. Shallow groundwaters flow nearly horizontally, in the direction of the closest depression in the Earth's crust, usually towards a stream or river draining a valley. Any shallow groundwater extracted through pumping is water that does not reach the surface waters in the vicinity. The impact depends on the intensity of pumpage and the associated loss of baseflow. If baseflow disappears and this can be traced to groundwater pumping, the effects could be far reaching. Thus, in general, it is necessary to consider the conjunctive use of surface and groundwaters, instead of treating these two resources as separate entities, subject to different management policies (National Research Council, 2000). The interaction between surface and groundwater varies from arid to humid climates. Perennial, or gaining streams, typical of subhumid and humid regions, augment their flow through baseflow. Conversely, ephemeral, or losing streams, typical of arid and semiarid regions, decrease their flow through transmission losses, which are the exact opposite of baseflow (Natural Resources Conservation Service, 2005). Therefore, when a gaining stream is transformed into a losing stream through aquifer depletion, a fraction of surface runoff is converted to shallow groundwater, increasing recharge. Effectively, the hydraulic gradient has been reversed; where originally it flowed into the stream, now it flows out of the stream. The loss of baseflow can trigger a chain reaction of negative impacts to various components of the landscape. Direct, or primary, impacts to the landscape can be expressed in terms of: * increased magnitude and frequency of floods, * loss of wetland and riparian vegetation, * loss of phreatophytes and aquatic-terrestrial transition zones, * changes in channel morphology, * accelerated erosion and gully development, * increased severity and frequency of droughts, and * loss of wildlife habitat and reduction in biodiversity. http://groundwater.sdsu.edu/groundwater_utilization_and_sustainability.html#impacts Excessive pumping can cause the ground under cities to sink, sinking the city with it. This can cause buildings to become unstable. http://www.chinadaily.com.cn/en/doc/2003-12/11/content_289290.htm * drying up of wells, * increased cost of pumping and well infrastructure, * land subsidence (Galloway et al, 2001), * salt-water intrusion, and * changes in surface albedo and related climate change (Ponce et al, 1997).