{"id":53,"date":"2020-09-29T20:29:27","date_gmt":"2020-09-29T20:29:27","guid":{"rendered":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/?post_type=chapter&p=53"},"modified":"2020-12-19T05:17:24","modified_gmt":"2020-12-19T05:17:24","slug":"the-scale-of-exchange","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/chapter\/the-scale-of-exchange\/","title":{"raw":"1.8 The Scale of Exchange","rendered":"1.8 The Scale of Exchange"},"content":{"raw":"Cross sections can be used to represent the exchange process at various dimensions depending on the site context. For example, at the landscape scale, hydrogeologists generally visualize surface-water features as either receiving groundwater discharge or acting as sources of groundwater recharge (e.g., Figure 11). Toth (1963) developed a regional conceptual model that shows nested groundwater flow systems, and multiple groundwater recharge and discharge locations (exchange sites). In the context of groundwater-surface water exchange, surface-water features corresponding with discharge areas are effluent and features located in recharge areas are influent. When a topographically varying landscape receives sufficient groundwater recharge so that the water table is higher and mirrors topographic highs, then local, intermediate and\/or regional flow systems develop (Figure 11a). In settings where recharge rates and geologic conditions do not create a water table that mirrors the topography, some surface-water features may occur where water collects in topographic lows. These features can act as a recharge sources under these conditions (Figure 11b). Regional groundwater exchange is also influenced by the distribution of the hydrogeologic properties of the underlying geologic framework. Hinton (2014) schematically shows variations in flow system exchange locations as influenced by the hydraulic conductivity distribution (Figure 12).<\/a>\r\n\r\n[caption id=\"attachment_362\" align=\"alignnone\" width=\"657\"]\"Schematic Figure 11 - <\/strong>Schematic cross sections showing the exchange of local, intermediate and regional groundwater flow systems with streams, lakes and wetlands. Groundwater flow lines are blue, shaded areas represent local and intermediate groundwater flow systems. R is placed over a zone receiving recharge and D is over a zone where groundwater discharge is occurring. a) Conditions where recharge is enough to create a water table that mirrors the topography (Carter, 1996). b) Setting in which recharge rates and geologic conditions do not result in a topographically dominated water table configuration. (Woessner, 2020).[\/caption]\r\n\r\n
<\/div>\r\n\r\n[caption id=\"attachment_363\" align=\"alignnone\" width=\"572\"]\"Figures Figure 12 - <\/strong>Examples showing how variations of hydraulic conductivity can impact local, intermediate and regional groundwater flow systems. It is assumed recharge rates are the same in all three settings and the changes in flow systems shown are caused by the hydraulic conductivity of underlying earth materials. a) Flow paths under isotropic and homogenous conditions with the water table mirroring the topography. b) Flow paths when the first layer of earth materials is underlain by a saturated layer with a higher hydraulic conductivity. c) Flow paths when the first layer of earth material is underlain by a saturated layer with a lower hydraulic conductivity (Hinton, 2014).[\/caption]\r\n\r\nExchange occurring at the scale of tens of meters to meters, and the sub-meter scale add complexity as slight variations in bed and bank configuration and elevation, hydraulic properties and hydraulic gradients drive local exchanges (Figure 13). Though often requiring extensive instrumentation, exchange at these scales is not only of interest to hydrogeologists, but also to those linking ecological systems to surface-water features and those focusing on exchange of contaminants (e.g., Hauer and Lambert, 2017; Conant, 2004; Conant et al., 2019).\r\n\r\n[caption id=\"attachment_364\" align=\"alignnone\" width=\"869\"]\"Schematic Figure 13 \u2013 <\/strong>Schematic of a river systems with groundwater exchange (hyporheic flow) at the 10\u2019s of meters to sub-meter scale (modified from USGS, 2015)[\/caption]\r\n

Transient Changes in the Exchange Process<\/h1>\r\nIn some settings, exchange conditions are influenced by temporal long- or short-term hydrologic conditions such as seasonal variations in water availability, stage responses to individual storm and flood events, and changes in surface-water temperatures. Such conditions result in temporal changes in rates and locations of groundwater exchanges with streams, lakes and wetlands (e.g., LaBaugh and Rosenberry, 2008) (Figure 14). For example, when the water table rises in response to a wet period a surface-water feature may become a gaining feature that was previously influent. However, when groundwater recharge becomes limited during drying periods and\/or drought, a gaining feature could become flow-through and\/or losing later in the year (Figure 14). When rapid changes in surface-water stage occur in response to a short-term precipitation event or resulting flooding, exchange conditions may change from gaining to losing and then, as surface-water stages decline, gaining conditions are re-established (e.g., bank storage, Freeze and Cherry, 1979, p. 225-226). Influent seepage rates can also be impacted by surface-water temperature changes (e.g., Constantz et al., 1994). This occurs because vertical bed hydraulic conductivities increase and decrease slightly as surface-water temperatures vary (changes in the specific weight and viscosity of water) (e.g., Freeze and Cherry, 1979; Fetter, 2001). Zamora (2007) suggests temperature impacts on seepage rates should be evaluated to determine if seasonal surface-water temperatures impact local seepage rates. Activation of production wells impacting the shallow groundwater system near surface-water features may locally reverse the surface-water exchange (e.g., Barlow and Leake, 2012). Studies of exchange using only a snapshot analysis (single moment or period) are often insufficient to characterize seasonal and longer-term trends in the movement of water between surface-water features and groundwater. Thus, in most settings, researchers assessing exchange processes should design studies that account for changing conditions.\r\n\r\n[caption id=\"attachment_365\" align=\"alignnone\" width=\"807\"]\"Figures Figure 14 - <\/strong>Seasonal variation (periods of wet and dry) of groundwater exchange with streams, lakes and wetlands in a hypothetical setting. a) The wet period corresponds to a setting where groundwater recharge drives the system and effluent conditions dominate (dashed line is the water table and blue arrows represent groundwater flow). b) Drying conditions represent times with a lower surface-water level and reductions in groundwater inflow resulting in a shift to flow-through conditions. c) Drought conditions may cause influent conditions to dominate (Woessner, 2020).[\/caption]\r\n\r\nThe conceptual models presented in Figures 4<\/a> through 14 represent the baseline conditions underlying similarities and differences of exchange process in rivers, lakes, and wetlands. The next three sections use these conceptual models to describe exchanges with streams, lakes and wetlands.","rendered":"

Cross sections can be used to represent the exchange process at various dimensions depending on the site context. For example, at the landscape scale, hydrogeologists generally visualize surface-water features as either receiving groundwater discharge or acting as sources of groundwater recharge (e.g., Figure 11). Toth (1963) developed a regional conceptual model that shows nested groundwater flow systems, and multiple groundwater recharge and discharge locations (exchange sites). In the context of groundwater-surface water exchange, surface-water features corresponding with discharge areas are effluent and features located in recharge areas are influent. When a topographically varying landscape receives sufficient groundwater recharge so that the water table is higher and mirrors topographic highs, then local, intermediate and\/or regional flow systems develop (Figure 11a). In settings where recharge rates and geologic conditions do not create a water table that mirrors the topography, some surface-water features may occur where water collects in topographic lows. These features can act as a recharge sources under these conditions (Figure 11b). Regional groundwater exchange is also influenced by the distribution of the hydrogeologic properties of the underlying geologic framework. Hinton (2014) schematically shows variations in flow system exchange locations as influenced by the hydraulic conductivity distribution (Figure 12).<\/a><\/p>\n

\"Schematic
Figure 11 – <\/strong>Schematic cross sections showing the exchange of local, intermediate and regional groundwater flow systems with streams, lakes and wetlands. Groundwater flow lines are blue, shaded areas represent local and intermediate groundwater flow systems. R is placed over a zone receiving recharge and D is over a zone where groundwater discharge is occurring. a) Conditions where recharge is enough to create a water table that mirrors the topography (Carter, 1996). b) Setting in which recharge rates and geologic conditions do not result in a topographically dominated water table configuration. (Woessner, 2020).<\/figcaption><\/figure>\n
<\/div>\n
\"Figures
Figure 12 – <\/strong>Examples showing how variations of hydraulic conductivity can impact local, intermediate and regional groundwater flow systems. It is assumed recharge rates are the same in all three settings and the changes in flow systems shown are caused by the hydraulic conductivity of underlying earth materials. a) Flow paths under isotropic and homogenous conditions with the water table mirroring the topography. b) Flow paths when the first layer of earth materials is underlain by a saturated layer with a higher hydraulic conductivity. c) Flow paths when the first layer of earth material is underlain by a saturated layer with a lower hydraulic conductivity (Hinton, 2014).<\/figcaption><\/figure>\n

Exchange occurring at the scale of tens of meters to meters, and the sub-meter scale add complexity as slight variations in bed and bank configuration and elevation, hydraulic properties and hydraulic gradients drive local exchanges (Figure 13). Though often requiring extensive instrumentation, exchange at these scales is not only of interest to hydrogeologists, but also to those linking ecological systems to surface-water features and those focusing on exchange of contaminants (e.g., Hauer and Lambert, 2017; Conant, 2004; Conant et al., 2019).<\/p>\n

\"Schematic
Figure 13 \u2013 <\/strong>Schematic of a river systems with groundwater exchange (hyporheic flow) at the 10\u2019s of meters to sub-meter scale (modified from USGS, 2015)<\/figcaption><\/figure>\n

Transient Changes in the Exchange Process<\/h1>\n

In some settings, exchange conditions are influenced by temporal long- or short-term hydrologic conditions such as seasonal variations in water availability, stage responses to individual storm and flood events, and changes in surface-water temperatures. Such conditions result in temporal changes in rates and locations of groundwater exchanges with streams, lakes and wetlands (e.g., LaBaugh and Rosenberry, 2008) (Figure 14). For example, when the water table rises in response to a wet period a surface-water feature may become a gaining feature that was previously influent. However, when groundwater recharge becomes limited during drying periods and\/or drought, a gaining feature could become flow-through and\/or losing later in the year (Figure 14). When rapid changes in surface-water stage occur in response to a short-term precipitation event or resulting flooding, exchange conditions may change from gaining to losing and then, as surface-water stages decline, gaining conditions are re-established (e.g., bank storage, Freeze and Cherry, 1979, p. 225-226). Influent seepage rates can also be impacted by surface-water temperature changes (e.g., Constantz et al., 1994). This occurs because vertical bed hydraulic conductivities increase and decrease slightly as surface-water temperatures vary (changes in the specific weight and viscosity of water) (e.g., Freeze and Cherry, 1979; Fetter, 2001). Zamora (2007) suggests temperature impacts on seepage rates should be evaluated to determine if seasonal surface-water temperatures impact local seepage rates. Activation of production wells impacting the shallow groundwater system near surface-water features may locally reverse the surface-water exchange (e.g., Barlow and Leake, 2012). Studies of exchange using only a snapshot analysis (single moment or period) are often insufficient to characterize seasonal and longer-term trends in the movement of water between surface-water features and groundwater. Thus, in most settings, researchers assessing exchange processes should design studies that account for changing conditions.<\/p>\n

\"Figures
Figure 14 – <\/strong>Seasonal variation (periods of wet and dry) of groundwater exchange with streams, lakes and wetlands in a hypothetical setting. a) The wet period corresponds to a setting where groundwater recharge drives the system and effluent conditions dominate (dashed line is the water table and blue arrows represent groundwater flow). b) Drying conditions represent times with a lower surface-water level and reductions in groundwater inflow resulting in a shift to flow-through conditions. c) Drought conditions may cause influent conditions to dominate (Woessner, 2020).<\/figcaption><\/figure>\n

The conceptual models presented in Figures 4<\/a> through 14 represent the baseline conditions underlying similarities and differences of exchange process in rivers, lakes, and wetlands. The next three sections use these conceptual models to describe exchanges with streams, lakes and wetlands.<\/p>\n","protected":false},"author":1,"menu_order":8,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-53","chapter","type-chapter","status-publish","hentry"],"part":3,"_links":{"self":[{"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/pressbooks\/v2\/chapters\/53","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/wp\/v2\/users\/1"}],"version-history":[{"count":10,"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/pressbooks\/v2\/chapters\/53\/revisions"}],"predecessor-version":[{"id":512,"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/pressbooks\/v2\/chapters\/53\/revisions\/512"}],"part":[{"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/pressbooks\/v2\/parts\/3"}],"metadata":[{"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/pressbooks\/v2\/chapters\/53\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/wp\/v2\/media?parent=53"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/pressbooks\/v2\/chapter-type?post=53"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/wp\/v2\/contributor?post=53"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/books.gw-project.org\/groundwater-surface-water-exchange\/wp-json\/wp\/v2\/license?post=53"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}