Specific Yield Functions for Estimating Evapotranspiration from Diurnal Surface Water Cycles

The White method has been routinely used to estimate evapotranspiration using diurnal variations in groundwater levels. Applications to surface water systems (e.g., wetlands) are less common. For applications to surface water systems, a stage‐dependent specific yield function must be defined. This is especially important for small wetlands formed in topographic depressions with bowl shaped bathymetries. Existing formulations of the specific yield function include weighting factors that impact the relative importance of the soil and open water specific yields on the composite value. Three formulations of the specific yield function from the literature were compared and found to produce varied results. Based on a comparison with empirical estimates of specific yield based on observed ratios of net precipitation to water level rise, one of the existing formulations is generalized and recommended for general use. The recommended function is dependent on wetland bathymetry, magnitude of the diurnal fluctuation, spatial extent of the equilibration area, and soil‐specific yield. A sensitivity analysis was conducted to examine the relative importance of these variables. The specific yield function is independent of wetland size and is strongly dependent on the basin profile coefficient (p), an indication of wetland shape. For most natural wetlands, bathymetry strongly influences specific yield.     

Evaluation of Ground‐Water and Surface‐Water Exchanges Using Streamflow Difference Analyses1

Abstract: In the karstic lower Flint River Basin, limestone fracturing, jointing, and subsequent dissolution have resulted in the development of extensive secondary permeability and created a system of major conduits that facilitate the exchange of water between the Upper Floridan aquifer and Flint River. Historical streamflow data from U.S. Geological Survey gaging stations located in Albany and Newton, Georgia, were used to quantify ground‐water and surface‐water exchanges within a 55.3 km section of the Flint River. Using data from 2001, we compared estimates of ground‐water flux using a time adjustment method to a water balance equation and found that these independent approaches yielded similar results. The associated error was relatively large during high streamflow when unsteady conditions prevail, but much lower during droughts. Flow reversals were identified by negative streamflow differences and verified with in situ data from temperature sensors placed inside large spring conduits. Long‐term (13 years) analysis showed negative streamflow differentials (i.e., a losing stream condition) coincided with high river stages and indicated that streamflow intrusion into the aquifer could potentially exceed 150 m³/s. Although frequent negative flow differentials were evident, the Flint River was typically a gaining stream and showed a large net increase in flow between the two gages when examined over the period 1989‐2003. Ground‐water contributions to this stream section averaged 2‐42 m³/s with a mean of 13 m³/s. The highest rate of ground‐water discharge to the Flint River occurred during the spring when regional ground‐water levels peaked following heavy winter and spring rains and corresponding rates of evapotranspiration were low. During periods of extreme drought, ground‐water contributions to the Flint River declined.     

Riparian Ground‐Water Flow Patterns Using Flownet Analysis: Evapotranspiration‐Induced Upwelling and Implications for N Removal1

Abstract: Ground‐water flow paths constrain the extent of nitrogen (N) sinks in deep, stratified soils of riparian wetlands. We examined ground‐water flow paths at four forested riparian wetlands in deep, low gradient, stratified deposits subjected to Southern New England’s temperate, humid climate. Mid‐day piezometric heads were recorded during the high water table period in April/May and again in late November at one site. Coupling field data with a two‐dimensional steady‐state ground‐water flow model, flow paths and fluxes were derived to 3 m depths. April/May evapotranspiration (ET) dominated total outflux (44‐100%) while flux to the stream was <10% of total outflux. ET exerted upward ground‐water flux through shallow carbon‐rich soils, increasing opportunities for N transformations and diverting flow from the stream. Dormant season results showed a marked increase in flux to the stream (27% of the total flux). Riparian sites with deep water tables (naturally or because of increased urbanization or other hydrologic modifications) or shallow root zones may not generate ground‐water upwelling to meet evaporative demand, thereby increasing the risk of N movement to streams. As water managers balance issues of water quality with water quantity, they will be faced with decisions regarding riparian management. Further work towards refining our understanding of ET mediation of N and water flux at the catchment scale will serve to inform these decisions.     

Evaluation of Rio Grande Management Alternatives Using a Surface‐Water/Ground‐Water Model1

Abstract: Previous investigations observed significant seepage losses from the Rio Grande to the shallow aquifer between Socorro and San Antonio, New Mexico. High‐resolution telescopic modeling was used along a 10‐km reach of the Rio Grande and associated drains and canals to evaluate several management alternatives aimed at improving river conveyance efficiency. Observed data consisted of ground‐water and surface‐water elevations, seepage rates along the Rio Grande and associated canals and drains, and borehole geology. Model calibration was achieved by adjusting hydraulic conductivity and specific storage until the output matched observed data. Sensitivity analyses indicated that the system was responsive to changes in hydrogeologic properties, especially when such alterations increased vertical connectivity between layers. The calibrated model predicted that removal of the low flow conveyance channel, a major channel draining the valley, would not only decrease river seepage by 67%, but also decrease total flow through the reach by 75%. The decreased flow through the reach would result in increased water logging and an average increase in ground‐water elevations of 1.21 meter. Simulations of the system with reduced riparian evapotranspiration rates or a relocated river channel also predicted decreased river seepage, but to a much lesser degree.     

Variations in Base‐Flow Nitrate Flux in a First‐Order Stream and Riparian Zone1

Abstract: Nonpoint source pollution, which contributes to contamination of surface waters, is difficult to control. Some pollutants, particularly nitrate (), are predominantly transmitted through ground water. Riparian buffer zones have the potential to remove contaminants from ground water and reduce the amount of that enters surface water. This is a justification for setting aside vegetated buffer strips along waterways. Many riparian zone hydrologic models assume uniform ground‐water flow through organic‐rich soil under reducing conditions, leading to effective removal of ground‐water prior to discharge into a stream. However, in a small first‐order stream in the mid‐Atlantic coastal plain, base‐flow generation was highly variable (spatially and temporally). Average base‐flow loads were greater in winter than summer, and higher during a wetter year than in dryer years. Specific sections of the stream consistently received greater amounts of high ground water than others. Areas within the riparian zone responsible for most of the exported from the watershed are termed “critical areas.” Over this 5‐year study, most of the exported during base flow originated from a critical area comprising less than 10% of the total riparian zone land area. Allocation of resources to address and improve mitigation function in critical areas should be a priority for continued riparian zone research.     

Surface Water Seepage Effects on Shallow Ground‐Water Quality Along the Rio Grande in Northern New Mexico1

Abstract: Interactions between surface irrigation water, shallow ground water, and river water may have effects on water quality that are important for both drinking water supplies and the ecological function of rivers and floodplains. We investigated water quality in surface water and ground water, and how water quality is influenced by surface water inputs from an unlined irrigation system in the Alcalde Valley of the Rio Grande in northern New Mexico. From August 2005 to July 2006, we sampled ground water and surface water monthly and analyzed for concentrations of major cations and anions, specific conductance, pH, dissolved oxygen, and water levels. Results indicate that irrigation ditch seepage caused an increase in ground water levels and that the Rio Grande is a gaining stream in this region. Temporal and spatial differences were found in ion concentrations in shallow ground water as it flowed from under the ditch toward the river. Ground‐water ion concentrations were higher when the ditch was not flowing compared with periods during peak irrigation season when the ditch was flowing. Ditch inputs diluted ion concentrations in shallow ground water at well positions near the ditch. Specifically, lower ion concentrations were detected in ground water at well positions located near the ditch and river compared with well positions located in the middle of an agricultural field. Results from this project showed that ditch inputs influenced ion concentrations and were associated with ground‐water recharge. In arid region river valleys, careful consideration should be given to management scenarios that change seepage from irrigation systems, because in some situations reduced seepage could negatively affect ground‐water recharge and water quality.     

Sensitivity and Uncertainty of Ground‐Water Discharge Estimates for Semiarid Shrublands1

Abstract: We proposed a step‐by‐step approach to quantify the sensitivity of ground‐water discharge by evapotranspiration (ET) to three categories of independent input variables. To illustrate the approach, we adopt a basic ground‐water discharge estimation model, in which the volume of ground water lost to ET was computed as the product of the ground‐water discharge rate and the associated area. The ground‐water discharge rate was assumed to equal the ET rate minus local precipitation. The objective of this study is to outline a step‐by‐step procedure to quantify the contributions from individual independent variable uncertainties to the uncertainty of total ground‐water discharge estimates; the independent variables include ET rates of individual ET units, areas associated with the ET units, and precipitation in each subbasin. The specific goal is to guide future characterization efforts by better targeting data collection for those variables most responsible for uncertainty in ground‐water discharge estimates. The influential independent variables to be included in the sensitivity analysis are first selected based on the physical characteristics and model structure. Both regression coefficients and standardized regression coefficients for the selected independent variables are calculated using the results from sampling‐based Monte Carlo simulations. Results illustrate that, while as many as 630 independent variables potentially contribute to the calculation of the total annual ground‐water discharge for the case study area, a selection of seven independent variables could be used to develop an accurate regression model, accounting for more than 96% of the total variance in ground‐water discharge. Results indicate that the variability of ET rate for moderately dense desert shrubland contributes to about 75% of the variance in the total ground‐water discharge estimates. These results point to a need to better quantify ET rates for moderately dense shrubland to reduce overall uncertainty in estimates of ground‐water discharge. While the approach proposed here uses a basic ground‐water discharge model taken from an earlier study, the procedure of quantifying uncertainty and sensitivity can be generalized to handle other types of environmental models involving large numbers of independent variables.     

Estimating Water Budgets and Vertical Leakages for Karst Lakes in North‐Central Florida (United States) Via Hydrological Modeling1

Lin, Zhulu, 2011. Estimating Water Budgets and Vertical Leakages for Karst Lakes in North‐Central Florida (United States) Via Hydrological Modeling. Journal of the American Water Resources Association (JAWRA) 1‐16. DOI: 10.1111/j.1752‐1688.2010.00513.x Abstract: Newnans, Lochloosa, and Orange Lakes are closely hydrologically connected karst lakes located in north‐central Florida, United States. The complex karst hydrology in this region poses a great challenge to the hydrological modeling that is essential to the development of Total Maximum Daily Loads for these lakes. We used a Hydrological Simulation Program – Fortran model coupled with the parallel Parameter ESTimation model calibration and uncertainty analysis software to estimate effectively the hydrological interactions between the lakes and the underlying upper Floridan aquifer and the water budgets for these three lakes. The net results of the lake‐groundwater interactions in Newnans and Orange Lakes are that both lakes recharge the underlying upper Floridan aquifer, with the recharge rate of the latter one magnitude greater than that of the former. However, for Lochloosa Lake, the net lake‐groundwater interaction is that the lake gains water from groundwater in a significant amount, approximately 40% of its total terrestrial water input. The annual average vertical leakages estimated for Newnans, Lochloosa, and Orange Lakes are 6.0 × 10⁶, ?8.9 × 10⁶, and 44.4 × 10⁶ m³, respectively. The average vertical hydraulic conductance (K_v/b) of the units between a lake bottom and the underlying upper Floridan aquifer in this region are also estimated to be from 1.26 × 10^?4 to 1.01 × 10^?3 day^?1.     

TEMPORAL RESPONSES OF SURFACE‐WATER AND GROUND‐WATER TO PRECIPITATION IN ILLINOIS1

ABSTRACT: Illinois data from 168 months (1986–1999) were investigated to determine the responses of surface‐water and ground‐water resources to precipitation. Such responses were generally within the month of occurrence or one to two months later, with recovery being reached another one to three months into the future, depending on season of the year. Although the drought of 1988 immediately impacted surface‐water and ground‐water resources, the time of recovery was substantially longer compared to those of individual dry months, generally continuing for several months. The extremely wet summer of 1993 resulted in elevated responses in water resources almost immediately, but in this instance continued through the following fall and winter, into the spring of 1994.     

Use of Streamflow Data to Estimate Base Flow/Ground‐Water Recharge For Wisconsin1

Abstract: The average annual base flow/recharge was determined for streamflow‐gaging stations throughout Wisconsin by base‐flow separation. A map of the State was prepared that shows the average annual base flow for the period 1970‐99 for watersheds at 118 gaging stations. Trend analysis was performed on 22 of the 118 streamflow‐gaging stations that had long‐term records, unregulated flow, and provided aerial coverage of the State. The analysis found that a statistically significant increasing trend was occurring for watersheds where the primary land use was agriculture. Most gaging stations where the land cover was forest had no significant trend. A method to estimate the average annual base flow at ungaged sites was developed by multiple‐regression analysis using basin characteristics. The equation with the lowest standard error of estimate, 9.5%, has drainage area, soil infiltration and base flow factor as independent variables. To determine the average annual base flow for smaller watersheds, estimates were made at low‐flow partial‐record stations in 3 of the 12 major river basins in Wisconsin. Regression equations were developed for each of the three major river basins using basin characteristics. Drainage area, soil infiltration, basin storage and base‐flow factor were the independent variables in the regression equations with the lowest standard error of estimate. The standard error of estimate ranged from 17% to 52% for the three river basins.     

Error Autocorrelation and Linear Regression for Temperature‐Based Evapotranspiration Estimates Improvement1

Medeiros, Patrick Valverde, Francisco Fernando Noronha Marcuzzo, Cristián Youlton, and Edson Wendland, 2012. Error Autocorrelation and Linear Regression for Temperature‐Based Evapotranspiration Estimates Improvement. Journal of the American Water Resources Association (JAWRA) 48(2): 297‐305. DOI: 10.1111/j.1752‐1688.2011.00614.x Abstract: Estimates of evapotranspiration on a local scale is important information for agricultural and hydrological practices. However, equations to estimate potential evapotranspiration based only on temperature data, which are simple to use, are usually less trustworthy than the Food and Agriculture Organization (FAO)‐Penman‐Monteith standard method. The present work describes two correction procedures for potential evapotranspiration estimates by temperature, making the results more reliable. Initially, the standard FAO‐Penman‐Monteith method was evaluated with a complete climatologic data set for the period between 2002 and 2006. Then temperature‐based estimates by Camargo and Jensen‐Haise methods have been adjusted by error autocorrelation evaluated in biweekly and monthly periods. In a second adjustment, simple linear regression was applied. The adjusted equations have been validated with climatic data available for the Year 2001. Both proposed methodologies showed good agreement with the standard method indicating that the methodology can be used for local potential evapotranspiration estimates.     

Drought Resilience of the California Central Valley Surface‐Ground‐Water‐Conveyance System1

Abstract: A series of drought simulations were performed for the California Central Valley using computer applications developed by the California Department of Water Resources and historical datasets representing a range of droughts from mild to severe for time periods lasting up to 60 years. Land use, agricultural cropping patterns, and water demand were held fixed at the 2003 level and water supply was decreased by amounts ranging between 25 and 50%, representing light to severe drought types. Impacts were examined for four hydrologic subbasins, the Sacramento Basin, the San Joaquin Basin, the Tulare Basin, and the Eastside Drainage. Results suggest the greatest impacts are in the San Joaquin and Tulare Basins, regions that are heavily irrigated and are presently overdrafted in most years. Regional surface water diversions decrease by as much as 70%. Stream‐to‐aquifer flows and aquifer storage declines were proportional to drought severity. Most significant was the decline in ground water head for the severe drought cases, where results suggest that under these scenarios the water table is unlikely to recover within the 30‐year model‐simulated future. However, the overall response to such droughts is not as severe as anticipated and the Sacramento Basin may act as ground‐water insurance to sustain California during extended dry periods.     

Efficient and Practical Approaches to Ground‐Water Right Transfers Under the Prior Appropriation Doctrine and the Snake River Example1

Abstract: Water right transfers are one of the basic means of implementing changes in water use in the highly appropriated water resource systems of the western United States. Many of these systems are governed by the Prior Appropriation Doctrine, which was not originally intended for application to ground‐water pumping and the conjunctive management of ground water and surface water, and thus creates an administrative challenge. That challenge results from the fact that ground‐water pumping can affect all interconnected surface‐water bodies and the effects may be immeasurably small relative to surface water discharge and greatly attenuated in time. Although we may have the ability to calculate the effects of ground‐water pumping and transfers of pumping location on surface‐water bodies, mitigating for all the impacts of each individual transfer is sufficiently inefficient that it impedes the transfer process, frustrates water users, and consequently inhibits economic development. A more holistic approach to ground‐water right transfers, such as a ground‐water accounting or banking scheme, may adequately control transfer third‐party effects while reducing mitigation requirements on individual transfers. Acceptance of an accounting scheme can accelerate the transfer process, and possibly reduce the administrative burden.     

Hydrologic and Water Quality Functions of a Disturbed Wetland in an Agricultural Setting1

Abstract: In 2006, we collected flow, sediment, and phosphorus (P) data at stream locations upstream and downstream of a small degraded wetland in south‐central Wisconsin traversed by a stream draining a predominantly agricultural watershed. The amount of sediment that left the wetland in the two largest storms, which accounted for 96% of the exported sediment during the observation period, was twice the amount that entered the wetland, even though only 50% of the wetland had been inundated. This apparently anomalous result is due to erosion of sediment that had accumulated in the low‐gradient channel and to the role of drainage ditches, which trapped sediment during the wetland‐filling phase. In the case of total P, the inflow to the wetland approximately equaled the outflow, although the wetland sequestered 30% of the incoming dissolved reactive P. The discrepancy is almost certainly due to net export of sediment. Many wetlands in the glaciated midwestern United States are ditched and traversed by low‐gradient channels draining predominantly agricultural areas, so the processes observed in this wetland are likely to be common in that region. Knowledge of this behavior presents opportunities to improve water quality in this and similar regions.     

Estimating Forest Ecosystem Evapotranspiration at Multiple Temporal Scales With a Dimension Analysis Approach1

Abstract:  It is critical that evapotranspiration (ET) be quantified accurately so that scientists can evaluate the effects of land management and global change on water availability, streamflow, nutrient and sediment loading, and ecosystem productivity in watersheds. The objective of this study was to derive a new semi‐empirical ET modeled using a dimension analysis method that could be used to estimate forest ET effectively at multiple temporal scales. The model developed describes ET as a function of water availability for evaporation and transpiration, potential ET demand, air humidity, and land surface characteristics. The model was tested with long‐term hydrometeorological data from five research sites with distinct forest hydrology in the United States and China. Averaged simulation error for daily ET was within 0.5 mm/day. The annual ET at each of the five study sites were within 7% of measured values. Results suggest that the model can accurately capture the temporal dynamics of ET in forest ecosystems at daily, monthly, and annual scales. The model is climate‐driven and is sensitive to topography and vegetation characteristics and thus has potential to be used to examine the compounding hydrologic responses to land cover and climate changes at multiple temporal scales.     

Air‐Water Temperature Relationships in the Trout Streams of Southeastern Minnesota's Carbonate‐Sandstone Landscape

Carbonate‐sandstone geology in southeastern Minnesota creates a heterogeneous landscape of springs, seeps, and sinkholes that supply groundwater into streams. Air temperatures are effective predictors of water temperature in surface‐water dominated streams. However, no published work investigates the relationship between air and water temperatures in groundwater‐fed streams (GWFS) across watersheds. We used simple linear regressions to examine weekly air‐water temperature relationships for 40 GWFS in southeastern Minnesota. A 40‐stream, composite linear regression model has a slope of 0.38, an intercept of 6.63, and R² of 0.83. The regression models for GWFS have lower slopes and higher intercepts in comparison to surface‐water dominated streams. Regression models for streams with high R² values offer promise for use as predictive tools for future climate conditions. Climate change is expected to alter the thermal regime of groundwater‐fed systems, but will do so at a slower rate than surface‐water dominated systems. A regression model of intercept vs. slope can be used to identify streams for which water temperatures are more meteorologically than groundwater controlled, and thus more vulnerable to climate change. Such relationships can be used to guide restoration vs. management strategies to protect trout streams.     

Estimating Hydraulic Properties of Volcanic Aquifers Using Constant‐Rate and Variable‐Rate Aquifer Tests1

Abstract: In recent years the ground‐water demand of the population of the island of Maui, Hawaii, has significantly increased. To ensure prudent management of the ground‐water resources, an improved understanding of ground‐water flow systems is needed. At present, large‐scale estimations of aquifer properties are lacking for Maui. Seven analytical methods using constant‐rate and variable‐rate withdrawals for single wells provide an estimate of hydraulic conductivity and transmissivity for 103 wells in central Maui. Methods based on constant‐rate tests, although not widely used on Maui, offer reasonable estimates. Step‐drawdown tests, which are more abundantly used than other tests, provide similar estimates as constant‐rate tests. A numerical model validates the suitability of analytical solutions for step‐drawdown tests and additionally provides an estimate of storage parameters. The results show that hydraulic conductivity is log‐normally distributed and that for dike‐free volcanic rocks it ranges over several orders of magnitude from 1 to 2,500 m/d. The arithmetic mean, geometric mean, and median values of hydraulic conductivity are respectively 520, 280, and 370 m/d for basalt and 80, 50, and 30 m/d for sediment. A geostatistical approach using ordinary kriging yields a prediction of hydraulic conductivity on a larger scale. Overall, the results are in agreement with values published for other Hawaiian islands.     

Discharge From the Edwards Aquifer Through the Leona River Floodplain,Uvalde, Texas1

Abstract: Analysis of results from an electrical resistivity survey, a magnetic survey, and an aquifer test performed on the Leona River floodplain in south‐central Texas indicates that ground‐water discharge from the Edwards Aquifer through the Leona River floodplain may be as great as 91.7 Mm³/year. When combined with an estimate of 8.8 Mm³/year for surface flow in the Leona River, as much as 100.5 Mm³/year could be discharged from the Edwards Aquifer through the Leona River floodplain. A value of 11,200 acre‐ft/year (13.82 Mm³/year) has been used as the calibration target in existing ground‐water models for total discharge from Leona Springs and the Leona River. Including ground water or underflow discharge would significantly increase the calibration target in future models. This refinement would improve the conceptualization of ground‐water flow in the western portion of the San Antonio segment of the Edwards Aquifer and would thereby allow for more accurate assessment and management of the ground‐water resources provided by the Edwards Aquifer.     

Urban Land Use,Channel Incision,and Water Table Decline Along Coastal Plain Streams,North Carolina1

Abstract: This study evaluates the effects of urban land use on stream channels and riparian ground‐water levels along low‐order Inner Coastal Plain streams in North Carolina. Six sites with stream catchments of similar size (1.19‐3.46 km²) within the Tar River Basin were selected across an urban land use gradient, as quantified by a range of catchment total impervious area (TIA; 3.8‐36.7%). Stream stage and ground‐water levels within three floodplain monitoring wells were measured manually and using pressure transducers from May 2006‐June 2007. Channel incision ratio (CIR), the ratio of bank height to bankfull height, was also measured at each monitoring site and along stream reaches within the study area (12 urban and 12 rural sites). Riparian ground‐water levels were inversely related to catchment TIA (%). As TIA (%) and stormwater runoff increased, the degree of stream channel incision increased and riparian ground‐water tables declined. In urban floodplains (>15% TIA), the median ground‐water level was 0.84 m deeper than for the rural settings (<15% TIA). This has resulted in a shift to drier conditions in the urban riparian zones, particularly during the summer months. CIR was found to be a reliable surface indicator of “riparian hydrologic drought” in these settings.