Ground water sustenance of Nebraska's unique Sand Hills peatland fen ecosystems

Many ecosystems have evolved under conditions where ground water is critical to the survival of the unique species contained therein. One example is Nebraska's Sand Hills fens. The Sand Hills region comprises lush interdunal valleys amid grass-stabilized dunes. One valley type, fens, are ground water-fed wetlands that have several meters of saturated, organic soils comprising peat formed under anaerobic conditions where plant growth exceeds decomposition. These fens support unique flora and fauna such as rush aster and northern redbelly dace that are typically found in more boreal regions of North America. This study characterized the hydrogeology of a representative fen in the Jumbo Valley to explore how these relict species survive in a hot, arid climate and to understand present and historical relations between ground water and these unique ecosystems. Results indicate that ground water's present role in sustaining the fens is twofold: (1) wetland plant transpiration of shallow ground water across the valley creates a cooler, humid microclimate that sustains the fens in the region's hot, arid climate and (2) the large volume of ground water pumped through the fens by plant transpiration transports necessary nutrients to the root zone. Results suggest that, in the past, in addition to its sustaining role, ground water also played a vital role in fen development and in fen preservation through nearly 15,000 years of existence. Thus, as with like ecosystems, the role of ground water must be considered in future Sand Hills fen protection, restoration, and management.     

MODIS‐Aided Statewide Net Groundwater‐Recharge Estimation in Nebraska

Monthly evapotranspiration (ET) rates (2000 to 2009) across Nebraska at about 1‐km resolution were obtained by linear transformations of the MODIS (MODerate resolution Imaging Spectroradiometer) daytime surface temperature values with the help of the Priestley–Taylor equation and the complementary relationship of evaporation. For positive values of the mean annual precipitation and ET differences, the mean annual net recharge was found by an additional multiplication of the power‐function‐transformed groundwater vulnerability DRASTIC‐code values. Statewide mean annual net recharge became about 29 mm (i.e., 5% of mean annual precipitation) with the largest recharge rates (in excess of 100 mm/year) found in the eastern Sand Hills and eastern Nebraska. Areas with the largest negative net recharge rates caused by declining groundwater levels due to large‐scale irrigation are found in the south‐western region of the state. Error bounds of the estimated values are within 10% to 15% of the corresponding precipitation rates and the estimated net recharge rates are sensitive to errors in the precipitation and ET values. This study largely confirms earlier base‐flow analysis‐based statewide groundwater recharge estimates when considerations are made for differences in the recharge definitions. The current approach not only provides better spatial resolution than available earlier studies for the region but also quantifies negative net recharge rates that become especially important in numerical modeling of shallow groundwater systems.     

Regional estimation of total recharge to ground water in Nebraska

Naturally occurring long-term mean annual recharge to ground water in Nebraska was estimated by a novel water-balance approach. This approach uses geographic information systems (GIS) layers of land cover, elevation of land and ground water surfaces, base recharge, and the recharge potential in combination with monthly climatic data. Long-term mean recharge > 140 mm per year was estimated in eastern Nebraska, having the highest annual precipitation rates within the state, along the Elkhorn, Platte, Missouri, and Big Nemaha River valleys where ground water is very close to the surface. Similarly high recharge values were obtained for the Sand Hills sections of the North and Middle Loup, as well as Cedar River and Beaver Creek valleys due to high infiltration rates of the sandy soil in the area. The westernmost and southwesternmost parts of the state were estimated to typically receive < 30 mm of recharge a year.     

Recognition of Regional Water Table Patterns for Estimating Recharge Rates in Shallow Aquifers

We propose a new method for groundwater recharge rate estimation in regions with stream-aquifer interactions, at a linear scale on the order of 10 km and more. The method is based on visual identification and quantification of classically recognized water table contour patterns. Simple quantitative analysis of these patterns can be done manually from measurements on a map, or from more complex GIS data extraction and curve fitting. Recharge rate is then estimated from the groundwater table contour parameters, streambed gradients, and aquifer transmissivity using an analytical model for groundwater flow between parallel perennial streams. Recharge estimates were obtained in three regions (areas of 1500, 2200, and 3300 km²) using available water table maps produced by different methods at different times in the area of High Plains Aquifer in Nebraska. One region is located in the largely undeveloped Nebraska Sand Hills area, while the other two regions are located at a transition zone from Sand Hills to loess-covered area and include areas where groundwater is used for irrigation. Obtained recharge rates are consistent with other independent estimates. The approach is useful and robust diagnostic tool for preliminary estimates of recharge rates, evaluation of the quality of groundwater table maps, identification of priority areas for further aquifer characterization and expansion of groundwater monitoring networks prior to using more detailed methods.     

Evaluation of the soil water balance in an alluvial flood plain with a shallow groundwater table

Abstract

Many of the hydrological and ecological functions of alluvial flood plains within watersheds depend on the water flow exchanges between the vadoze soil zone and the shallow groundwater. The water balance of the soil in the flood plain is investigated, in order to evaluate the main hydrological processes that underlie the temporal dynamics of soil moisture and groundwater levels. The soil moisture and the groundwater level in the flood plain were monitored continuously for a three-year period. These data were integrated with the results derived from applying a physically-based numerical model which simulated the variably-saturated vertical water flow in the soil. The analysis indicated that the simultaneous processes of lateral groundwater flow and the vertical recharge from the unsaturated zone caused the observed water table fluctuations. The importance of these flows in determining the rises in the water table varied, depending on soil moisture and groundwater depth before precipitation. The monitoring period included two hydrological years (September 2009–September 2011). About 13% of the precipitation vertically recharged the groundwater in the first year and about 50% in the second. The difference in the two recharge coefficients was in part due to the lower groundwater levels in the recharge season of the first hydrological year, compared to those observed in the second. In the latter year, the shallow groundwater increased the soil moisture in the unsaturated zone due to capillary rise, and so the mean hydraulic conductivity of the unsaturated soil was high. This moisture state of soil favoured a more efficient conversion of infiltrated precipitation into vertical groundwater recharge. The results show that groundwater dynamics in the flood plain are an important source of temporal variability in soil moisture and vertical recharge processes, and this variability must be properly taken into account when the water balance is investigated in shallow groundwater environments.

Citation Pirastru, M. and Niedda, M., 2013. Evaluation of the soil water balance in an alluvial flood plain with a shallow groundwater table. Hydrological Sciences Journal, 58 (4), 898–911.     

Field data and flow system response in clay (vertisol) shale terrain,north central Texas,USA

The water budget in clay shale terrain is controlled by a complex interaction between the vertisol soil layer, the underlying fractured rock, land use, topography, and seasonal trends in rainfall and evapotranspiration. Rainfall, runoff, lateral flow, soil moisture, and groundwater levels were monitored over an annual recharge cycle. Four phases of soil–aquifer response were noted over the study period: (1) dry‐season cracking of soils; (2) runoff initiation, lateral flow and aquifer recharge; (3) crack closure and down‐slope movement of subsurface water, with surface seepage; (4) a drying phase. Surface flow predominated within the watershed (25% of rainfall), but lateral flow through the soil zone continued for most of the year and contributed 11% of stream flow through surface seepage. Actual flow through the fractured shale makes up a small fraction of the water budget but does appear to influence surface seepage by its effect on valley‐bottom storage. When the valley soil storage is full, lateral flow exits onto the valley‐bottom surface as seasonal seeps. Well response varied with depth and hillslope position. FLOWTUBE model results and regional recharge estimates are consistent with an aquifer recharge of 1·6% of annual precipitation calculated from well heights and specific yield of the shale aquifer. Copyright © 2005 John Wiley & Sons, Ltd.     

A variable source area for groundwater evapotranspiration: impacts on modeling stream flow

Evapotranspiration (ET) plays a crucial role in catchment water budgets, typically accounting for more than 50% of annual precipitation falling within temperate deciduous forests. Groundwater ET is a portion of total ET that occurs where plant roots extend to the capillary fringe above the phreatic surface or induce upward movement of water from the water table by hydraulic redistribution. Groundwater ET is spatially restricted to riparian zones or other areas where the groundwater is accessible to plants. Due to the difficulty in measuring groundwater ET, it is rarely incorporated explicitly into hydrological models. In this study, we calibrated Topographic Model (TOPMODEL) using a 14‐year hydrograph record and added a groundwater ET pathway to derive a new model, Groundwater Evapotranspiration TOPMODEL (GETTOP). We inspected groundwater elevations and stream flow hydrographs for evidence of groundwater ET, examined the relationship between groundwater ET and topography, and delineated the area where groundwater ET is likely to take place. The total groundwater ET flux was estimated using a hydrological model. Groundwater ET was larger where the topography was flat and the groundwater table was shallow, occurring within about 10% of the area in a headwater catchment and accounting for 6 to 18% of total annual ET. The addition of groundwater ET to GETTOP improved the simulation of stream discharge and more closely balanced the watershed water budget. Copyright © 2013 John Wiley & Sons, Ltd.     

Net Recharge vs. Depth to Groundwater Relationship in the Platte River Valley of Nebraska,United States

One‐km resolution MODIS‐based mean annual evapotranspiration (ET) estimates in combination with PRISM precipitation rates were correlated with depth to groundwater (d) values in the wide alluvial valley of the Platte River in Nebraska for obtaining a net recharge (Rn) vs. d relationship. MODIS cells with irrigation were excluded, yielding a mixture of predominantly range, pasture, grass, and riparian forest covers on sandy soils with a shallow groundwater table. The transition depth (d_t) between negative and positive values of the net groundwater recharge was found to be at about 2 (±1) m. Within 1 (±1) m of the surface and at a depth larger than about 7 to 8 (±1) m, the mean annual net recharge became independent of d at a level of about ?4 (±12)% and 13 (±10)%, respectively, of the mean annual precipitation rate. The obtained Rn(d) relationship is based on a calibration‐free ET estimation method and may help in obtaining the net recharge in shallow groundwater areas of negligible surface runoff where sufficient groundwater‐depth data exist.     

Importance of river water recharge to the San Joaquin Valley groundwater system

Groundwater is not a sustainable resource, unless abstraction is balanced by recharge. Identifying the sources of recharge in a groundwater basin is critical for sustainable groundwater management. We studied the importance of river water recharge to groundwater in the south‐eastern San Joaquin Valley (24,000 km², population 4 million). We combined dissolved noble gas concentrations, stable isotopes, tritium, and carbon‐14 analyses to analyse the sources, mechanisms, and timescales of groundwater recharge. Area‐representative groundwater sampling and numerical model input data enabled a stable isotope mass balance and quantitative estimates of river and local recharge. River recharge, identified by a lighter stable isotope signature, represents 47 ± 4% of modern groundwater in the San Joaquin Valley (recharged after 1950) but only 26 ± 4% of premodern groundwater (recharged before 1950). This implies that the importance of river water recharge in the San Joaquin valley has nearly doubled and is likely the result of a 40% increase in total recharge, caused by river water irrigation return flows and increased stream depletion and river recharge due to groundwater pumping. Compared with the large and long‐duration capacity for water storage in the subsurface, storage of water in rivers is limited in time and volume, as evidenced by cold river recharge temperatures resulting from fast infiltration and recharge. Groundwater banking of seasonal surface water flows and expansion of managed aquifer recharge practices therefore appear to be a natural and promising method for increasing the resilience of the San Joaquin Valley water supply system.     

Groundwater flow and implications for groundwater contamination north of Prewitt,New Mexico,U.S.A.

In the southern San Juan Basin, New Mexico, strata of Permian and younger age dip gently toward the center of the basin. Most previous investigators believed that recharge to these strata occurred by precipitation on the outcrops and groundwater flowed downdip to the north and northeast. Recent water-level measurements in an undeveloped part of the basin near Prewitt, New Mexico, show that groundwater at shallow depths in alluvium and bedrock flows southward, opposite to the dip direction, and toward a major ephemeral drainage in a strike valley. North of this area, groundwater in deep bedrock aquifers does appear to flow northward. This information suggests that there are two groundwater circulation patterns; a shallow one controlled by topography and a deeper one controlled by geologic structure.Significant amounts of recharge to sandstone aquifers by infiltration through outcrops is unlikely due to the near-vertical exposures on cliffs, the gentle dip of the strata, and small annual precipitation. Numerical model results suggest that recharge to bedrock aquifers may be from downward leakage via aquitards over large areas and leakage from narrow alluvial aquifers in the subcrop area. The recharge mechanism is controlled by the hydraulic conductivity of the strata.As the flow path is controlled by hydraulic conductivity contrasts, geologic structure, and topography, contamination movement from surface impoundments is likely to be difficult to predict without a thorough hydrogeological site investigation.     

The discovery of surface runoff in the megadunes of Badain Jaran Desert,China, and its significance

The Badain Jaran Desert exhibits the greatest difference in altitude of all of the world’s deserts. On the slopes of megadunes in the desert, there are physical and chemical deposits produced by surface runoff. In addition, we have observed rarely-seen infiltration-excess surface runoff in the megadune depressions as well as spring streams at the base of megadunes. We used electron microscopy, energy spectrum analysis, infiltration experiments, moisture content determinations and grain-size analysis to study the mineral and chemical composition of the runoff precipitates, and grain-size of the deposits associated with the runoff, together with the hydrological balance in the megadune area, and the atmospheric precipitation mechanism responsible for groundwater recharge and for supplying water to lakes. The observations of shallow runoff and infiltration-excess surface runoff indicate the occurrence of strong and effective precipitation in summer, which would provide an important source for groundwater recharge. Several lines of evidence, such as the physical and chemical deposits resulting from shallow subsurface runoff, spring streams, infiltration-excess runoff, and gravity capillary water with a moisture content of 3–6%, demonstrate that precipitation reaches the base of the megadunes through infiltration and subsequently becomes groundwater. The chemical deposits, such as newly-formed calcite and gypsum, and gray-black physical deposits, as well as different stages in the development of fan-shaped landforms resulting from shallow subsurface runoff, indicate that groundwater recharge in the area is the result of long-term precipitation, rather than intermittent individual major rainfall events. Fine sand layers with a low infiltration capacity lead to subsurface runoff emerging at the ground surface. Five factors play an important role in maintaining a positive water balance and in replenishing groundwater via rainfall: effective rainfall as a water source, the high infiltration capacity of the sands enabling rainfall to rapidly become capillary water in the dunes, low evapotranspiration rates due to the sparse vegetation, the fact that the depth of the sand layer influenced by evaporation is shallow enough to maximize the deep infiltration of rainfall, and rapidly-moving gravity capillary water in the sandy dunes. These five factors together constitute a mechanism for groundwater recharge from rainfall, and explain the origin of the groundwater and lakes in the area. Our findings represent a significant advance in research on the hydrological cycle, including groundwater recharge conditions and recharge mechanisms, in this desert region.     

The groundwater recharge regime of some slightly metamorphosed neoproterozoic sedimentary rocks: an application of natural environmental tracers

Isotope data of precipitation and groundwater in parts of the Voltaian Basin in Northern Ghana were used to explain the groundwater recharge regime in the area. Groundwater recharge is an important parameter in the development of a decision support system for the management and efficient utilization of groundwater resources in the area. It is therefore important to establish the processes and sources of groundwater recharge. δ¹⁸O and δ²H data for local precipitation suggest enrichment relative to the Global Meteoric Water Line (GMWL) and indicate that precipitation takes place at a relative humidity less than 100%. The groundwater data plot on an evaporation line with a slope of 5, suggesting a high degree of evaporative enrichment of the precipitation in the process of vertical infiltration and percolation through the unsaturated zone into the saturated zone. This finding is consistent with the observation of high evapotranspiration rates in the area and ties in with the fact that significant clay fraction in the unsaturated zone limits vertical percolation and thus exposes the percolating rainwater to the effects of high temperatures and low humidities resulting in high evapotranspiration rates. Groundwater recharge estimates from the chloride mass balance, CMB, method suggest recharge in the range of 1.8–32% of the annual average precipitation in the form of rainfall. The highest rates are associated with areas where open wells encourage significant amount of groundwater recharge from precipitation in the area. In the northern parts of the study area, groundwater recharge is lower than 12%. The recharge so computed through the application of the CMB methodology takes on a spatial distribution akin to the converse of the spatial pattern of both δ¹⁸O and δ²H in the area. As such, the locations of the highest recharge are associated with the most depleted values of the two isotopes. This observation is consistent with the assertion that low vertical hydraulic conductivities slow down vertical percolation of precipitation down to the groundwater water. The percolating precipitation water thus gets enriched in the heavier isotopes through high evapotranspiration rates. At the same time, the amount of water that finally reaches the water table is considerably reduced. Copyright © 2013 John Wiley & Sons, Ltd.     

Assessing hydrogeologic controls on dynamic groundwater storage using long‐term instrumental records of water table levels

This study analyzes a long‐term regional compilation of water table response to climate variability based on 124 long‐term groundwater wells distributed across New England, USA, screened in a variety of geologic materials. The New England region of the USA is located in a humid‐temperature climate underlain by low‐storage‐fractured metamorphic and crystalline bedrock dissected by north–south trending valleys filled with glacial and post‐glacial valley fill sediments. Uplands are covered by thin glacial till that comprises more than 60% of the total area. Annual and multi‐annual responses of the water table to climate variability are assessed to understand how local hydraulic properties and hydrogeologic setting (located in recharge/discharge region) of the aquifer influence the hydrologic sensitivity of the aquifer system to climate variability. This study documents that upland aquifer systems dominated by thin deposits of surface till comprise ~70% of the active and dynamic storage of the region. Total aquifer storage changes of +5 to ?7 km³ occur over the region during the study interval. The storage response is dominated by thin and low permeability surficial till aquifer that fills and drains on a multi‐annual basis and serves as the main mechanism to deliver water to valley fill aquifers and underlying bedrock aquifers. Whereas the till aquifer system is traditionally neglected as an important storage reservoir, this study highlights the importance of a process‐based understanding of how different landscape hydrogeologic units contribute to the overall hydrologic response of a region.     

Hydrological cycle and water balance estimates for the megadune–lake region of the Badain Jaran Desert,China

The hydrology and water balance of megadunes and lakes have been investigated in the Badain Jaran Desert of China. Field observations and analyses of sand layer water content, field capacity, secondary salt content, and grain size reveal 3 types of important natural phenomenon: (a) vegetation bands on the leeward slope of the megadunes reflect the hydrological regime within the sandy vadose zone; (b) seepage, wet sand deposits, and secondary salt deposits indicate the pattern of water movement within the sandy vadose zone; (c) zones of groundwater seeps and descending springs around the lakes reflect the influence of the local topography on the hydrological regime of the megadunes. The seepage exposed on the sloping surface of the megadunes and gravity water contained within the sand layer confirm the occurrence of preferential flow within the vadose zone of the megadunes. Alternating layers of coarse and fine sand create the conditions for the formation of preferential flows. The preferential flows promote movement of water within the sand layer water that leads to deep penetration of water within the megadunes and ultimately to the recharging of groundwater and lake water. Our results indicate that a positive water balance promotes recharge of the megadunes, which depends on the high permeability of the megadune material, the shallow depth of the surface sand layer affected by evaporation, the occurrence of rainfall events exceeding 15 mm, and the sparse vegetation cover. Water balance estimates indicate that the annual water storage of the megadunes is about 7.5 mm, accounting for only 8% of annual precipitation; however, the shallow groundwater per unit area under the megadunes receives only 3.6% of annual precipitation, but it is still able to maintain a dynamic balance of the lake water. From a water budget perspective, the annual water storage in the megadunes is sufficient to serve as a recharge source for lake water, thereby enabling the long‐term persistence of the lakes. Overall, our findings demonstrate that precipitation is a significant component of the hydrological cycle in arid deserts.     

Temporal variation of stable isotopes in a precipitation–groundwater system: implications for determining the mechanism of groundwater recharge in high mountain–hills of the Loess Plateau,China

The groundwater in shallow loess aquifers in high mountain–hills in the western Loess Plateau in China is almost the sole water resource for local residents. However, the question about how the loess groundwater naturally circulates in these high mountain–hills, characterized by low precipitation and high potential evaporation, remains unclear. The objectives of this study are to evaluate the application of hydrogen and oxygen isotopes to (1) examine temporal variations of the isotopic composition of precipitation and shallow groundwater and (2) uncover the mechanism of groundwater recharge in high mountain–hills. Results from 2 years of monitoring data show a difference in the stable isotopes for groundwater and local precipitation between the winter and summer periods. Similar to precipitation, stable isotopes in groundwater are observed to be depleted in winter and enriched in summer, particularly in oxygen isotope. A prominent characteristic is that H and O isotopes of groundwater show a very clear response to strong precipitation in the rainy season in 2013. The results highlight that local precipitation is the likely recharge source for groundwater in shallow loess aquifers. Annual recharge from local precipitation maintains the groundwater resource in the shallower loess aquifer. The mechanisms governing shallow loess groundwater recharge in high mountain–hills were evaluated. In addition to possible vertical slow percolation of soil water through the unsaturated zone, rapid groundwater recharge mechanisms have been identified as temporal preferential infiltration through sinkholes, slip surface or landslide surface and through the interface of loess layer and palaeo‐soils. Most groundwater can be recharged after a heavy rainy season. Copyright © 2015 John Wiley & Sons, Ltd.     

Seasonal expansion and contraction of stream networks in shallow groundwater systems

Surface water and groundwater are normally closely connected in areas with shallow aquifer systems. Stream systems can thus be considered as the outcrops of associated groundwater flows in areas with a shallow groundwater table and a previous subsurface. This situation prevails in sandy lowland areas where almost all rainfall percolates into the subsurface so that the surplus over evapotranspiration becomes part of a groundwater drainage system before it reappears at the surface in a stream. The stream network, being the interface with the groundwater system, must have the capacity to release the seasonally dependent precipitation surplus through the continuum of ground and surface waters. A river network therefore consists of a hierarchical system of different order and incision depth, of which the discharge-contributing component contracts and expands with the seasonal fluctuation in recharge and water table depth.

Coupling the mathematical expressions for groundwater drainage and stream flow enables development of a conjunctive model which relates the properties of a seasonally contracting and expanding stream network and related groundwater level fluctuation to the seasonal rainfall character for given geological and geomorphological conditions. This model further allows for assessment of drainage network response to a changing environment.     

Modelling the potential impacts of groundwater hydrology on long‐term drainage basin evolution

Fluvial erosion processes are driven by water discharge on the land surface, which is produced by surface runoff and groundwater discharge. Although groundwater is often neglected in long‐term landscape evolution problems, water table levels control patterns of Dunne runoff production, and groundwater discharge can contribute significantly to storm flows. In this analysis, we investigate the role that groundwater movement plays in long‐term drainage basin evolution by modifying a widely used landscape evolution model to include a more detailed representation of basin hydrology. Precipitation is generated by a stochastic process, and the precipitation is partitioned between surface runoff and groundwater recharge using a specified infiltration capacity. Groundwater flow is simulated by a dynamic two‐dimensional Dupuit equation for an unconfined aquifer with an irregular underlying impervious layer. The model is applied to the WE‐38 basin, an experimental catchment in Pennsylvania, because 60–80 per cent of the discharge is derived from groundwater and substantial hydrologic and geomorphic information is available. The hydrologic model is first calibrated to match the observed streamflows, and then the combined hydrologic/geomorphic model is used to simulate scenarios with different infiltration capacities. The results of this modelling exercise indicate that the basin can be divided into three zones with distinct streamflow‐generating characteristics, and different parts of the basin can have different geomorphic effective events. Over long periods of time, scenarios in which groundwater discharge is large tend to modify the topography in a way that promotes groundwater discharge and inhibits Dunne runoff. Copyright © 2006 John Wiley & Sons, Ltd.     

Hydraulic subsurface measurements and hydrodynamic modelling as indicators for groundwater flow systems in the Rotondo granite,Central Alps (Switzerland)

Regional groundwater flow in high mountainous terrain is governed by a multitude of factors such as geology, topography, recharge conditions, structural elements such as fracturation and regional fault zones as well as man‐made underground structures. By means of a numerical groundwater flow model, we consider the impact of deep underground tunnels and of an idealized major fault zone on the groundwater flow systems within the fractured Rotondo granite. The position of the free groundwater table as response to the above subsurface structures and, in particular, with regard to the influence of spatial distributed groundwater recharge rates is addressed. The model results show significant unsaturated zones below the mountain ridges in the study area with a thickness of up to several hundred metres. The subsurface galleries are shown to have a strong effect on the head distribution in the model domain, causing locally a reversal of natural head gradients. With respect to the position of the catchment areas to the tunnel and the corresponding type of recharge source for the tunnel inflows (i.e. glaciers or recent precipitation), as well as water table elevation, the influence of spatial distributed recharge rates is compared to uniform recharge rates. Water table elevations below the well exposed high‐relief mountain ridges are observed to be more sensitive to changes in groundwater recharge rates and permeability than below ridges with less topographic relief. In the conceptual framework of the numerical simulations, the model fault zone has less influence on the groundwater table position, but more importantly acts as fast flow path for recharge from glaciated areas towards the subsurface galleries. This is in agreement with a previous study, where the imprint of glacial recharge was observed in the environmental isotope composition of groundwater sampled in the subsurface galleries. Copyright © 2012 John Wiley & Sons, Ltd.     

A digital procedure for ground water recharge and discharge pattern recognition and rate estimation

A digital procedure to estimate recharge/discharge rates that requires relatively short preparation time and uses readily available data was applied to a setting in central Wisconsin. The method requires only measurements of the water table, fluxes such as stream baseflows, bottom of the system, and hydraulic conductivity to delineate approximate recharge/discharge zones and to estimate rates. The method uses interpolation of the water table surface, recharge/discharge mapping, pattern recognition, and a parameter estimation model. The surface interpolator used is based on the theory of radial basis functions with thin-plate splines. The recharge/discharge mapping is based on a mass-balance calculation performed using MODFLOW. The results of the recharge/discharge mapping are critically dependent on the accuracy of the water table interpolation and the accuracy and number of water table measurements. The recharge pattern recognition is performed with the help of a graphical user interface (GUI) program based on several algorithms used in image processing. Pattern recognition is needed to identify the recharge/discharge zonations and zone the results of the mapping method. The parameter estimation program UCODE calculates the parameter values that provide a best fit between simulated heads and flows and calibration head-and-flow targets. A model of the Buena Vista Ground Water Basin in the Central Sand Plains of Wisconsin is used to demonstrate the procedure.     

Evaluating actual evapotranspiration and impacts of groundwater storage change in the North China Plain

As a critical water discharge term in basin‐scale water balance, accurate estimation of evapotranspiration (ET) is therefore important for sustainable water resources management. The understanding of the relationship between ET and groundwater storage change can improve our knowledge on the hydrological cycle in such regions with intensive agricultural land usage. Since the 1960s, the North China Plain (NCP) has experienced groundwater depletion because of overexploitation of groundwater for agriculture and urban development. Using meteorological data from 23 stations, the complementary relationship areal evapotranspiration model is evaluated against estimates of ET derived from regional water balance in the NCP during the period 1993–2008. The discrepancies between calculated ET and that derived by basin water balance indicate seasonal and interannual variations in model parameters. The monthly actual ET variations during the period from 1960 to 2008 are investigated by the calibrated model and then are used to derive groundwater storage change. The estimated actual ET is positively correlated with precipitation, and the general higher ET than precipitation indicates the contributions of groundwater irrigation to the total water supply. The long term decreasing trend in the actual ET can be explained by declining in precipitation, sunshine duration and wind speed. Over the past ~50 years, the calculated average annual water storage change, represented by the difference between actual ET and precipitation, was approximately 36 mm, or 4.8 km³; and the cumulative groundwater storage depletion was approximately 1700 mm, or 220 km³ in the NCP. The significantly groundwater storage depletion conversely affects the seasonal and interannual variations of ET. Irrigation especially during spring cause a marked increase in seasonal ET, whereas the rapid increasing of agricultural coverage over the NCP reduces the annual ET and is the primary control factor of the strong linear relationship between actual and potential ET. Copyright © 2013 John Wiley & Sons, Ltd.