Water Table Decline Research Articles

Evaporative loss from irrigated interrows in a highly advective semi-arid agricultural area

Agricultural productivity has increased in the Texas High Plains at the cost of declining water tables, putting at risk the sustainability of the Ogallala Aquifer as a principal source of water for irrigated agriculture. This has led area producers to seek alternative practices that can increase water use efficiency (WUE) through more careful management of water. One potential way of improving WUE is by reducing soil evaporation (E), thus reducing overall evapotranspiration (ET). Before searching for ways to reduce E, it is first important to quantify E and understand the factors that determine its magnitude. The objectives of this study were (1) to quantify E throughout part of the growing season for irrigated cotton in a strongly advective semi-arid region; (2) to study the effects of LAI, days after irrigation, and measurement location within the row on the E/ET fraction; and (3) to study the ability of microlysimeter (ML) measures of E combined with sap flow gage measures of transpiration (T) to accurately estimate ET when compared with weighing lysimeter ET data and to assess the E/T ratio. The research was conducted in an irrigated cotton field at the Conservation & Production Research Laboratoryof the USDA-ARS, Bushland, TX. ET was measured by a large weighing lysimeter, and E was measured by 10 microlysimeters that were deployed in two sets of 5 across the interrow. In addition, 10 heat balance sap flow gages were used to determine T. A moderately good agreement was found between the sum E+T and ET (SE=1mm or ∼10% of ET). It was found that E may account for >50% of ET during early stages of the growing season (LAI<0.2), significantly decreasing with increase in LAI to values near 20% at peak LAI of three. Measurement location within the north-south interrows had a distinct effect on the diurnal pattern of E, with a shift in time of peak E from west to east, a pattern that was governed by the solar radiation reaching the soil surface. However, total daily E was unaffected by position in the interrow. Under wet soil conditions, wind speed and direction affected soil evaporation. Row orientation interacted with wind direction in this study such that aerodynamic resistance to E usually increased when wind direction was perpendicular to row direction; but this interaction needs further study because it appeared to be lessened under higher wind speeds.

Controls on in situ oxygen and dissolved inorganic carbon dynamics in peats of a temperate fen

Changes in hydrological conditions are expected and may alter carbon cycling in peatlands. Peat aeration with water table change has not commonly been investigated, and the water table is often assumed to constitute the oxic‐anoxic boundary in peat. We analyzed temperature, moisture, oxygen (O2), and carbon dioxide (CO2) concentrations in profiles of a temperate fen during two seasons. A drying‐rewetting cycle and flooding were induced and compared to controls. The response of moisture and water table position varied greatly and was related to gradients of peat compaction and ash content. Background drought raised air‐filled porosity (AFP) to a maximum of 15%–38% in shallow peat and experimental drought up to 50%. Decline in water table and soil moisture broadly led to O2 penetration and CO2 degassing, and rewetting and flooding led to anoxic conditions and CO2 accumulation in peat pore water. In dense peat with ≥20% ash content the unsaturated zone remained partly low in oxygen, however, and up to 5% AFP and 20 cm above water table O2 concentrations frequently remained below 50 μmol L−1. Moderately intense and short drying did not induce substantial oxygen penetration in the compacted soil profiles. The likelihood of the presence of oxygen in the peat was predicted by logistic regression using water table and ash content or bulk density as predictors (p &lt; 0.0005). The model is potentially useful for predicting the position of the redoxcline in peat deposits and may assist in improving statistical models of trace gas emission from peatlands.

Ground Water Use Across Command Area of Pabnawa Minor of Bhakra Irrigation System

The northwest Indo-Gangetic plain of India endowed with fertile soil, defined seasons and fair climate produces about 52 % of the national food requirement. Currently, this plain is facing a number of second generation problems, declining water table being the major one. The paper presents the status of ground water table, and looks at the scope of conjunctive use of water at field scale in the command area of the Pabnawa minor of the Bhakra irrigation system. The study showed that there is a steep decline in water table after 1998 in middle and tail reaches. The study revealed that ground water contribution to irrigation and productivity in the canal command is very high. While canal irrigated area is shrinking, ground water irrigated area is increasing every year. The contribution of shallow and medium depth tube wells is negligible /limited, and the area covered by deep tube wells is large in all the reaches of the command area. This system of intensive irrigated agriculture is unsustainable in the long run because of declining water tables and high use of energy.

Groundwater markets under the water scarcity and declining watertable conditions: The upland Balochistan Region of Pakistan

The study documents comprehensive analysis on informal groundwater marketing in upland Balochistan, Pakistan. Informal groundwater markets are emerging as a feasible option to manage increasing water scarcity and declining water tables resulting from poor groundwater policies. Firstly, we evaluated the groundwater trading mechanism; we then examined factors affecting the groundwater trading using empirical data and logit econometric models. We did not observe any permanent groundwater transactions; only temporary groundwater exchange takes place. Two common transactions methods were noted – water in exchange for a given crop share and cash payment at a flat rate per hour. In all, 60% of respondents reported selling water in exchange for a crop share. This form of transaction intensifies as we move from high altitude areas to low altitude areas where water and land are relatively abundant. In general, crop sharing rates were 33% of crop output. Water trading in the form of cash transactions was practiced by 40% of respondents. The charge per hour also varied with altitude, with average prices of Rs. 100,1USD=Pakistan Rupees (Rs) 85 as of June 2011.1 Rs. 112, and Rs. 205 per hour reported at low, medium and high altitudes, respectively. The increase in this form of water marketing as we move from low to high altitude areas is mainly in response to relatively scarce water and land in high altitude areas.The results of empirical models indicates that among others, cropping intensity, area under high value fruits, decline in watertable, tubewell reliability, alternate source of irrigation, and soil quality are important variables influencing water buying and selling decisions. Importantly, personal attributes such as kinships, age, and education were also found to be important factors affecting water buying and selling decisions. In upland Balochistan, the water markets appears to provide a cushion against increasing water scarcity by averting risk to high value horticultural crops and enhancing water use efficiency, as it helps overcome the problem of over irrigation or misuse of water by facilitating the sale of surplus or extra water and more sparing and efficient use of purchased water. It is envisaged that groundwater markets will continue to plays a key role in sustaining high value crops and will continue to make a significant contribution in upland Balochistan.

Climate Change Induced Land Degradation and Socio-Economic Deterioration: A Remote Sensing and GIS Based Case Study from Rajasthan, India

The present study attempts to identify and quantify climate change induced land degradation at watershed and village level in Jaggar Watershed of Eastern Rajasthan using remote sensing and GIS technique. The study utilizes Standard Geocoded FCC LISS II data of 1989, and LISS III data of 1998 and 2009 on 1:50,000 scale for Land use/land cover mapping. Maps were digitized, edited and analyzed in GIS to ascertain land use/land cover changes. Comparative analysis of the Land use/land cover statistics and village level household survey reveals that climate change has severely affected land use/land cover especially agriculture land. Agricultural land in the watershed has decreased from 12,026 ha (34%) to 10,400 ha (29.65%) from 1998 to 2009. The area occupied by surface water resources of the major water body has decreased by 207 ha owing to decline in rainfall over the years. Climate data analysis suggests that average maximum and minimum temperatures during the period 1977-2007 have increased by 1.2?C and 0.4?C respectively. Increasing trends of temperature suggests warming up of the area. Decline of ground water table by 1 - 2 m on annual basis coupled with significant drawdown has led to water scarcity in many parts of the watershed. The water table has gone down to a depth of 240 feet, which was reported at 60 - 70 feet 20 years back. The area has shown sharp decline of rainfall by 269 mm from 1977 to 2007. The survey results show that there has been shift in the cropping pattern during the last 20 years due to change in climate as well as decline in availability of water for irrigation. Climate change seems to have played a key role in Jaggar watershed resulting in land degradation and making rainfed agriculture more vulnerable.

Impact of Storm Water Recharge Practices on Boston Groundwater Elevations

Over the past century, the City of Boston has periodically experienced a decline in groundwater elevations and the associated deterioration of untreated wood piles, which support building foundations. To combat declining water tables, Boston enacted a groundwater conservation overlay district enforced by city zoning boards to require storm water recharge practices for any activity that triggers the zoning bylaw. In Boston, recharge to the water table results from the infiltration of rainfall and snowmelt, leakage from water mains, and recharge from artificial systems. Increased mitigation activities to reduce unaccounted-for water have reduced leakage from water mains in the city. Given the high percentage of impervious cover in Boston, the remaining sources of recharge are primarily artificial systems, including pump and infiltrate systems and storm water recharge best management practices (BMPs). The primary objective of this research was to exploit existing information on groundwater elevations and recharge practices to quantify the effect of the required recharge BMPs on the behavior of groundwater elevations in the Back Bay region of Boston. Regional multivariate regression models were developed to determine the potential effects of recharge BMPs on observed groundwater elevations. The literature review revealed several analogous multivariate linear regression studies, none which focused on behavior of stormwater BMPs. The model reveals that the installation of recharge BMPs has a small but highly statistically significant positive effect on groundwater elevations in the Back Bay with the effect being proportional to their capacity and inversely proportional to their distance from the location of interest. The resulting model can be used to predict the effect on average groundwater elevations at a particular location resulting from the installation of a recharge BMP or a set of such BMPs of a particular capacity at a particular distance from the location of interest.

Modeling the effects of hydrology on gross primary productivity and net ecosystem productivity at Mer Bleue bog

[1] The ecosys model was applied to investigate the effects of water table and subsurface hydrology changes on carbon dioxide exchange at the ombrotrophic Mer Bleue peatland, Ontario, Canada. It was hypothesized that (1) water table drawdown would not affect vascular canopy water potential, hence vascular productivity, because roots would penetrate deeper to compensate for near‐surface dryness, (2) moss canopy water potential and productivity would be severely reduced because rhizoids occupy the uppermost peat that is subject to desiccation with water table decline, and (3) given that in a previous study of Mer Bleue, ecosystem respiration showed little sensitivity to water table drawdown, gross primary productivity would mainly determine the net ecosystem productivity through these vegetation–subsurface hydrology linkages. Model output was compared with literature reports and hourly eddy‐covariance measurements during 2000–2004. Our findings suggest that late‐summer water table drawdown in 2001 had only a minor impact on vascular canopy water potential but greatly impacted hummock moss water potential, where midday values declined to −250 MPa on average in the model. As a result, simulated moss productivity was reduced by half, which largely explained a reduction of 2–3 mmol CO2 m −2 s −1 in midday simulated and measurement‐derived gross primary productivity and an equivalent reduction in simulated and measured net ecosystem productivity. The water content of the near‐surface peat (top 5–10 cm) was found to be the most important driver of interannual variability of annual net ecosystem productivity through its effects on hummock moss productivity and on ecosystem respiration.

SIMULATION OF A GROUNDWATER ARTIFICIAL RECHARGE IN A SEMI‐ARID REGION OF IRAN

ABSTRACTThe Bahar aquifer is one of the most economically important aquifers in the western part of Iran, covering the water demands for irrigation supplies. Intensive pumping for irrigation has caused a water table decline. The local water organisation has constructed a water storage structure to augment the groundwater resources. Groundwater recharge using water from the river via 13 dug wells and a related structure has been estimated as 2.7 million m3, for the hydrological year 2002–2003. A three‐dimensional model was developed to predict the aquifer system response. The effect of radial flow was found to be 4 km, and the area covering this recharge more than 40 km2. The results of the numerical model could be used by local authorities and decision makers for groundwater resources management and planning. Finally, it is concluded that aquifer recharge is one environmental solution, as a part of integral water resources management. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Over‐extraction from shallow bedrock versus deep alluvial aquifers: Reliability versus sustainability considerations for India's groundwater irrigation

The excessive exploitation of groundwater aquifers is emerging as a worldwide problem, but it is nowhere as dramatic and consequential as it is in India, the world's largest consumer, where hundreds of millions of people depend on it. Usually the problem is framed in terms of a long‐term decline in water tables and its consequence for extraction costs, resource depletion, and the sustainability of irrigated agriculture. Here a comparative analysis is provided of coupled groundwater, energy, and irrigation dynamics in two groundwater intensive regions in India that differ in their underlying hydrogeology—the Indian Punjab with its deep alluvial aquifers and the Telangana region in south‐central India with its shallow hard rock aquifers. Using a simple modeling framework and piezometric and agricultural time series, we show that in shallow aquifers the sense in which extraction is excessive is different, and is related to the short‐term reliability of water supply rather than long‐term sustainability. This has important repercussions for irrigated agricultural economies.

Water-Use Dynamics of an Invasive Reed, Arundo donax, from Leaf to Stand

We investigated water use of an invasive riparian reed species, Arundo donax (L.), along moisture gradients to determine how extensively this plant might affect water resources. On an approximately 250 m stretch of the Lower Rio Grande in South Texas, we measured the gas exchange of water vapor at the leaf scale and structural characteristics, such as leaf area and shoot density, at the stand scale. To assess the effect of water availability, we used transects perpendicular to the edge of the river along a potential moisture gradient. Stands of A. donax used approximately 8.8 ± 0.9 mm of water per day during the peak of the 2008 growing season; this rate of water use is at the high end of the spectrum for plants. Transpiration and leaf area index varied with water availability, which suggests this plant is sensitive to drought and declining water tables. This provides a baseline for future studies comparing water use between A. donax and other plant species, especially native species considered in riparian restoration efforts.

Surface shading effects on soil C loss in a temperate muck soil

Histosols are a huge reservoir for C, covering < 1% of the world's land surface but storing up to 12% of total soil C. Thorough comprehension of factors controlling the rate of soil C loss from Histosols is critical for proper management of these C sinks. Two experiments evaluated how formerly cultivated, warm-climate Histosols undergoing wetland restoration respond to decreases in soil temperatures via vegetative shading, under different water table conditions. We compared temperature and soil CO 2 efflux differences from intact soil cores under three levels of light reduction in a greenhouse: 0%, 70%, and 90%. Soil in full sun was consistently warmer and showed higher efflux rates than 70% and 90% shade treatments: 4.132, 3.438, and 2.054 μmol CO 2 m −2 s −1, respectively. Shade treatments reached peak efflux rates at similar water potential, −2 to − 4 kPa. A field experiment subjected in-situ soil to full sun, 70% light reduction, and light reduction from naturally occurring herbaceous vegetation. Shade treatment effects on soil temperature and C mineralization were evident throughout the growing season. Vegetative shade effects on soil temperature were greatest in August and September when soil under vegetation was 5–11 °C cooler than unshaded soil. Soil CO 2 efflux was correlated strongly with soil temperature; daily efflux rates were consistently highest from unshaded soil. Efflux across treatments showed a strong seasonal correlation to soil moisture, increasing as soil dried in response to water table decline. Soil water potential was unaffected by shade treatment, suggesting temperature effects were solely responsible for efflux differences between treatments. All results confirm that surface shading has a strong influence on soil temperatures and C mineralization rates. Management to enhance vegetative shading in wetland restoration projects may be an effective strategy for slowing soil C losses and promoting soil C sequestration when O 2 is not limiting.

Relationships between groundwater use, water table, and recovery of willow on Yellowstone's northern range

Excessive levels of herbivory, incision of stream channels, and climate warming are believed to be responsible for the decline of woody deciduous plants in riparian zones in western North America, declines that are likely to be associated with diminished biological diversity. In the northern elk wintering range of Yellowstone National Park, USA, overbrowsing by elk (Cervus elaphus), lowered water tables resulting from stream incision, and loss of activity by beaver (Castor canadensis) have been implicated in the decline of willow (Salix sp.) communities. Reducing elk browsing appears sufficient for willow recovery in some areas, but where water table changes have been dramatic, recovery may be slow or absent. The importance of water table changes is disputed because experimental results demonstrate water table limitations, but water table depth has failed to explain variation in willow height at landscape scales. One explanation for this apparent discrepancy is that willows that have survived intensive browsing by elk have maintained access to groundwater despite declining water tables. Using stable isotopes of water, we examined the relationships between groundwater use, water table depth, and height of heavily browsed Salix geyeriana. Salix geyeriana groundwater use varied from 30% to 80%, and was higher later in the growing season, when soil water was less available and shoot water potentials were lower. Late season groundwater use explained 26% of the variation in total height of willows (P = 0.002), with taller plants using more groundwater. Water table depth explained only 8% of the variability in total height (P = 0.051), with shorter willows having deeper water table depths. Groundwater use and water table depth were uncorrelated. Height recovery following a winter of heavy browsing was related to groundwater use, but not groundwater depth. We suggest that access to deeper water sources alleviates late season water stress, allowing for more rapid height recovery and higher total plant height. Variability in groundwater access may account for variability in height recovery at landscape scales.

Resilience of phreatophytic vegetation to groundwater drawdown: is recovery possible under a drying climate?

AbstractBanksia (Proteaceae) woodlands are one of a number of groundwater‐dependent ecosystems in southwestern Western Australia that are threatened by groundwater abstraction. In addition to this threat is an ongoing decline in regional water tables due to a drying climate. We used ecological resilience theory to analyse and interpret a long‐term vegetation monitoring dataset from a site that has experienced an abstraction‐induced acute groundwater drawdown in the late 1980s and early 1990s. Despite reduced plant abundance, all dominant over‐ and understorey species were still found on all transect plots in which they were recorded pre‐drawdown. This suggests a notional resilience and a strong likelihood of recovery, in the event that pre‐drawdown ecohydrological habitat conditions were to return. However, since the drawdown event, the regional water table continued to decline, with the vegetation responding through progressive and uni‐directional change in abundance and composition. The change in composition was primarily manifested as a shift towards non‐woody, shallow‐rooted species not dependent on specific hydrological conditions. This slow, progressive change in hydrology associated with reduced rainfall and land use changes has continued to force a transition in the floristics towards an alternative ecohydrological state. Despite the absence of an acute drawdown event, the same progressive floristic response was also observed at two reference sites that were not under the immediate influence of production bores. The challenge for adaptive water resource management will be to enhance the capacity for resilience in these groundwater‐dependent ecosystems in a drying environment through appropriate regulation of groundwater abstraction. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Evaluation of groundwater quantity and quality in the Kashan Basin, Central Iran

This paper investigates groundwater changes in the Kashan Basin quantitatively and qualitatively. According to the data obtained from 53 observation wells, it is found that the mean water table is decreased about 7.93 m between 1990 and 2006, indicating a mean water table decline of 0.496 m/year. To study groundwater quality, 21 samples are analyzed for their physicochemical characteristics such as pH, hardness, chloride (Cl), electrical conductivity (EC) and total dissolved solid (TDS) values. Comparing the results with drinking water quality standards issued by World Health Organization (WHO), it is found that most of the water samples are not potable. Hydrochemical facies using Piper diagram indicate that in most part of this basin, the chemical character of water is dominated by NaCl. However, Na% values indicate that just 53% of samples are permissible for irrigation. The chloride–bicarbonate ratios reveal that salt lake intrusion to this basin is the main source of salinity. It is also found that functional relationships between EC and Cl are logarithmic.

Alternative linearization of water table kinematic condition for unconfined aquifer pumping test modeling and its implications for specific yield estimates

Summary An alternative linearization of the nonlinear kinematic condition at the water table used in unconfined aquifer flow theory is proposed. It simulates the downward propagation of the water table, during water extraction via a pumping well, as a diffuse process controlled by a dimensionless linearization parameter, β D . The limiting value of β D = 0 corresponds to the solution of Neuman (1972) whereas that of β D → ∞ corresponds to the early-time Theis solution. Using the solution obtained with this linearization, data collected during a pumping test conducted at the Boise Hydrogeophysical Research Site in Idaho, US, were analyzed. The estimates of hydraulic conductivity and specific storage obtained in this study compare well with those obtained with the other models. The most significant improvement is in the estimated value of specific yield, with the proposed model yielding values that are close to those expected from known porosity values ( n ∼ 0.25) at the site. The results show that meaningful estimates of specific yield can be obtained by treating the water table as a diffuse moving material boundary, without recourse to more complex models for water table decline or inclusion of unsaturated zone flow.

Riparian Seedling Mortality from Simulated Water Table Recession, and the Design of Sustainable Flow Regimes on Regulated Rivers

Seasonal water limitation exerts a strong ecological filter on stream communities in semiarid regions. For first‐year riparian willow and poplar tree seedlings, desiccation from rapidly declining river flows can limit reproduction, especially on rivers in which flow regulation and land conversion have limited the amount of area available for recruitment. We investigated survivorship of first‐year riparian seedlings to simulated river stage declines, focusing on the three dominant species in California's heavily regulated San Joaquin Basin: Fremont cottonwood, Goodding's black willow, and sandbar willow. Seedlings grown in mesocosms were subjected to water table decline rates typical in spring on unregulated and regulated rivers. We compared species' differences in survival time and fit the empirical data to accelerated failure time models that predicted time until death as a function of drawdown rate, initial seedling size, and maternal line. We used Akaike information criteria to select the best model for each species. Water table decline rates ≥ 6 cm/day were lethal to all species. At an intermediate rate (3 cm/day) survival varied most among species (12–38%) and was highest for Goodding's black willow. Failure time models indicated no maternal effects on survival but that initial seedling size was important for cottonwood. Using these models, we simulated survivable flow scenarios on the Tuolumne River (CA) and assessed the survivability of actual flow releases in two representative years. This modeling approach shows promise for optimizing flow releases to restore pioneer riparian habitat on regulated rivers in some of the world's most water‐limited regions.

Natural and induced sinkholes of the Obruk Plateau and Karapınar-Hotamış Plain, Turkey

The number of sinkholes (locally known as obruks) has increased rapidly in recent years near Karapınar, located in the semi-arid Konya Closed Basin in Central Anatolia. Nineteen sinkholes have formed in the last 33 years (1977–2009) as a result of the collapse of cavity roofs in the Neogene lacustrine limestone in the Obruk Plateau and beneath Quaternary lake sediments in the Karapınar-Hotamış Plain. Of these, 13 have formed within the past 4 years (2006–2009). The Obruk Plateau takes its name from the presence of several hundred paleo-sinkholes which formed as a result of natural processes during the Quaternary period. More recently, human activity has induced the formation of new sinkholes, which presents a hazard to life and property. Changing agricultural patterns have led to the opening of thousands of deep wells in recent years, and increased water pumping currently exceeds the sustainable yield of the aquifer. Thus the formation of sinkholes has been triggered by a combination of natural and human causes. The groundwater level has dropped almost 24 m in the vicinity of Karapınar during the last 26 years (1983–2008). Approximately 8 m of this drop occurred within the 4 years prior to the study (2005–2008). Legally-binding precautions must be taken to prevent further water table decline, in order to decrease sinkhole formation within the basin in the years to come.

Seasonal ice and hydrologic controls on dissolved organic carbon and nitrogen concentrations in a boreal‐rich fen

Boreal wetland carbon cycling is vulnerable to climate change in part because hydrology and the extent of frozen ground have strong influences on plant and microbial functions. We examined the response of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) across an experimental manipulation of water table position (both raised and lowered water table treatments) in a boreal‐rich fen in interior Alaska. DOC and TDN responses to water table manipulation exhibited an interaction with seasonal ice dynamics. We observed consistently higher DOC and TDN concentrations in the lowered water table treatment (71.7 ± 6.5 and 3.0 ± 0.3 mg−L) than in both the control (55.6 ± 5.1 and 2.3 ± 0.2 mg−L) and raised (49.1 ± 4.3 and 1.9 ± 0.1 mg L−1, respectively) water table treatments. Across all plots, pore water DOC concentrations at 20 cm increased as the depth to water table increased (R2 = 0.43, p &lt; 0.001). DOC concentrations also increased as the seasonal thaw depth increased, with solutes increasing most rapidly in the drained plot (R2 = 0.62, p &lt; 0.001). About half of the TDN pool was composed of dissolved organic N (DON). Inorganic N and DON were both highly correlated with changes in DOC, and their respective constraints to mineralization are discussed. These results demonstrate that a declining water table position and dryer conditions affect thaw depth and peat temperatures, and interactions among these ecosystem properties will likely increase DOC and TDN loading and potential for export in these systems.

Groundwater‐dependent vegetation: Quantifying the groundwater subsidy

The typical stratigraphy of riparian ecosystems consists of fine‐grained overbank deposits overlying coarser‐grained materials. Plants within these regions rely on soil moisture in the fine‐grained sediments as well as supplemental groundwater for root water uptake. The additional water available as a result of shallow water table conditions is defined here as groundwater subsidy and is found to be a significant contribution to root water uptake. Work presented here quantifies the effect of groundwater subsidy on root water uptake as a result of variations in the soil thickness of the upper fine‐grained sediments, rate of water table decline, and maximum water table depth. Variations in soil thickness and water table decline regimes produce a complex response with respect to both the rate of groundwater subsidy and the cumulative groundwater subsidy. These simulated regimes are analogs to environmental scenarios in riparian ecosystems that result from stream incision, soil erosion, and climate change. These results have implications for identifying ecosystems most susceptible to future change as well as those most amenable to restoration.

Spatial variability of specific yield and vertical hydraulic conductivity in a highly permeable alluvial aquifer

Summary Lower-than-expected specific yield values of an unconfined aquifer estimated from pumping test data have been a concern in the groundwater community over the last several decades. Delayed yield above the declining water table during pumping has been considered the primary cause of this uncertainty in the specific yield. The drainage process above the declining water table is a dominantly downward flow. Thus, the vertical hydraulic conductivity ( K v ) must have an effect on the drainage process. However, careful examination of K v variation has not been done. The spatial variation of S y has not been well evaluated either. This paper examines the heterogeneity of the specific yield and the vertical hydraulic conductivity of an alluvial aquifer. A drainage experiment and permeameter tests using more than 60 sediment cores were conducted to determine specific yield and the vertical hydraulic conductivity. Drainage experiments indicated that more than 50% of drainable water was released from the permeable sediment cores in less than 2 h, and nearly 100% of drainable water was released from most of the cores within 24 h. Thus, the appreciable effect of delayed yield in a permeable aquifer is unlikely to last longer than 24 h. Both S y and K v indicated a strong tendency of variation between one and another sediment core, and the two variables have some correlation. Because K v is heterogeneous and the two parameters are correlated, holding a constant K v value for different observation wells in the analysis of pumping test data can introduce errors to the derived S y values.