LAKE EVAPORATION STUDIES USING SATELLITE THERMAL INFRARED DATA1

Thermal infrared radiation data were acquired by the Heat Capacity Mapping Mission (HCMM) satellite over the surface area (385 km²) of Utah Lake during periodic overpasses in 1978 and 1979. The thermal infrared data were converted to lake surface temperatures which were subsequently used in correlations with lake evaporation. Correlations between HCMM surface temperature and pan-derived evaporation exceeded r = 0.90 when HCMM night and day/night average temperatures and two-day average evaporation values were tested. Similar regression studies were done using monthly data from a conceptual evaporation model and the evaporation pan versus monthly HCMM temperature data. In this test both the HCMM day and night monthly temperature versus the monthly model or pan evaporation had correlations exceeding r = 0.95. Empirical estimates of both short and long term lake evaporation using satellite thermal infrared data seem feasible. Attempts to use the HCMM thermal information as direct input to a theoretical approach to calculating evaporation were inconclusive; however, a definite potential seems to exist.     

EVAPOTRANSPIRATION MEASUREMENT AND ESTIMATION OF THREE WETLAND ENVIRONMENTS IN THE UPPER ST. JOHNS RIVER BASIN,FLORIDA1

ABSTRACT: Accurate estimates of evapotranspiration from areas dominated by wetland vegetation are needed in the water budget of the Upper St. Johns River Basin. However, local data on evapotranspiration rates, especially in wetland environments, were lacking in the project area. In response to this need, the St. Johns River Water Management District collected evapotranspiration field data in Fort Drum Marsh Conservation Area over the period 1996 through 1999. Three large lysimeters were installed to measure the evapotranspiration from different wetland environments: sawgrass (Cladium jamaicense), cattail (Typha domingensis), and open water. In addition, pan evaporation was measured with a standard class “A” pan. Concurrently, meteorological data including rainfall, solar radiation, wind speed, relative humidity, air temperature, and atmospheric pressure were collected. By comparing computed evapotranspiration rates with those measured in the lysimeters, parameters in the Penman‐Monteith, the Priestley‐Taylor, and Reference‐ET methods, and evaporation pan coefficients were estimated for monthly and seasonal cycles. The results from the data collected in this study show that mean monthly evapotranspiration rates, computed by the different methods, are relatively close. From a practical point of view, results indicate that the evaporation pan can be used equally well as the more complex and data‐intensive methods. This paper presents the measured evapotranspiration rates, evaporation pan coefficients, and the estimated parameter values for three different methods to compute evapotranspiration in the project area. Since local data on evaporation are often scarce or lacking, this information may be useful to watershed hydrologists for practical application in other project regions.     

UNCERTAINTIES IN ESTIMATING THE WATER BALANCE OF LAKES1

ABSTRACT: Evaluation of hydrologic methodology used in a number of water balance studies of lakes in the United States shows that most of these studies calculate one or more terms of the budget as the residual. A literature review was made of studies in which the primary purpose was error analysis of hydrologic measurement and interpretation. Estimates of precipitation can have a wide range of error, depending on the gage placement, gage spacing, and areal averaging technique. Errors in measurement of individual storms can be as high as 75 percent. Errors in short term averages are commonly in the 15-30 percent range, but decrease to about 5 percent or less for annual estimates. Errors in estimates of evaporation can also vary widely depending on instrumentation and methodology. The energy budget is the most accurate method of calculating evaporation; errors are in the 10–15 percent range. If pans are used that are located a distance from the lake of interest, errors can be considerable. Annual pan-to-lake coefficients should not be used for monthly estimates of evaporation because they differ from the commonly used coefficient of 0.7 by more than 100 percent. Errors in estimates of stream discharge are often considered to be within 5 percent. If the measuring section, type of flow profile, and other considerations, such as stage discharge relationship, are less than ideal errors in estimates of stream discharge can be considerably greater than 5 percent. Errors in estimating overland (nonchannelized) flow have not been evaluated, and in most lake studies this component is not mentioned. Comparison of several lake water balances in which the risdual consists solely of errors in measurement, shows that such a residual, if interpreted as ground water, can differ from an independent estimate of ground water by more than 100 percent.     

Estimating Annual Groundwater Evapotranspiration from Phreatophytes in the Great Basin Using Landsat and Flux Tower Measurements

Escalating concerns about water supplies in the Great Basin have prompted numerous water budget studies focused on groundwater recharge and discharge. For many hydrographic areas (HAs) in the Great Basin, most of the recharge is discharged by bare soil evaporation and evapotranspiration (ET) from phreatophyte vegetation. Estimating recharge from precipitation in a given HA is difficult and often has significant uncertainty, therefore it is often quantified by estimating the natural discharge. As such, remote sensing applications for spatially distributing flux tower estimates of ET and groundwater ET (ET_g) across phreatophyte areas are becoming more common. We build on previous studies and develop a transferable empirical relationship with uncertainty bounds between flux tower estimates of ET and a remotely sensed vegetation index, Enhanced Vegetation Index (EVI). Energy balance‐corrected ET measured from 40 flux tower site‐year combinations in the Great Basin was statistically correlated with EVI derived from Landsat imagery (r² = 0.97). Application of the relationship to estimate mean‐annual ET_g from four HAs in western and eastern Nevada is highlighted and results are compared with previous estimates. Uncertainty bounds about the estimated mean ET_g allow investigators to evaluate if independent groundwater discharge estimates are “believable” and will ultimately assist local, state, and federal agencies to evaluate expert witness reports of ET_g, along with providing new first‐order estimates of ET_g.     

ESTIMATING REFERENCE EVAPORATION IN THE CENTRAL VALLEY OF CALIFORNIA USING THE LINACRE MODEL1

ABSTRACT: The Linacre (1988) model for calculating evaporation from open water or well-watered surfaces only requires inputs of air temperature, latitude and elevation, and windspeed if it is available. The model was developed using data collected at a large number of sites in different climatic regions of the world, while independent tests of the model have shown it to be suitable for estimating evaporation in a variety of locations. This study was intended to contribute to the broad goal of evaluating temperature-based evaporation models for use in California by testing the Linacre model in the agriculturally intensive Central Valley. Observed monthly mean reference evaporation (E_o) and meteorological data for periods ranging up to 72 months were obtained from 25 California Irrigation and Management Information System (CIMIS) stations distributed throughout the Central Valley. Uncalibrated and calibrated Linacre models were used to estimate monthly mean reference evaporation, and the performance of each model was evaluated using indices that quantified the random and systematic errors and overall model performance. The accuracy of the radiation and ventilation components of the model were evaluated separately. The uncalibrated model was found to systematically overestimate E_o with most of the model error being attributed to the ventilation component. Calibration of the radiation and ventilation components removed most of the systematic model errors, and the root mean square error for monthly mean E_o was 0.676 mm day^?1 (16.8 percent of the mean observed value). (KEY TERMS: reference evaporation; Linacre model; irrigation scheduling.)     

ESTIMATING EVAPORATION FROM UTAH'S GREAT SALT LAKE USING THERMAL INFRARED SATELLITE IMAGERY1

ABSTRACT: Water surface temperatures can be obtained from satellite thermal remote sensing. Landsat and other satellites sense emitted thermal infrared radiation on a regular basis over much of the earth's surface. Evaporation is accomplished by the net transport of mass from the water surface to the atmosphere. The evaporative transfer predominantly determines the water surface temperature. Hence, there should be good correlations between evaporation and surface temperatures. Previous investigations on Utah Lake with satellite-derived temperatures and pan- and model-derived evaporation values have produced good correlations. However, more study was required with additional satellite data and evaporation measurements for saltwater conditions. The applicability of this method for estimating evaporation on Utah's Great Salt Lake was of particular interest at this time because of the unprecedented rise of this terminal lake. Satellite thermal data and evaporation data from four different years were obtained for the Great Salt Lake and the surrounding region. More than 350 correlation and linear regression analyses were performed on the temperature and evaporation data. The lake salt concentrations were also factored into the analyses in several different ways. The correlation results were generally very good and a methodology for using satellite-derived water surface temperatures along with salt concentrations was developed to estimate evaporation.     

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.     

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.     

DEFINING WATERSHED-SCALE EVAPORATION USING A NORMALIZED DIFFERENCE VEGETATION INDEX1

Monthly composites of the Normalized Difference Vegetation Indices (NDVI), derived from the National Oceanic and Atmospheric Administration's (NOAA) Advanced Very High Resolution Radiometer (AVILRR), were transformed linearly into monthly evaporation rates and compared with detailed hydrologic-model simulation results for five watersheds across the United States. Model-simulated monthly evaporation values showed high correlations (mean R²= .77) with NDVI-derived evaporation estimates. These latter estimates, used in a classical water balance model, resulted in equally accurate simulations of monthly runoff than when the model was run to estimate monthly evaporation via soil moisture accounting. Comparison of NDVI-derived evaporation estimates with pan data showed promise for transforming NDVI values into evaporation estimates under both wet and water-limiting conditions without resorting to the application of any kind of calibrated hydrologic models.     

Hydrology and Water Budget for a Forested Atlantic Coastal Plain Watershed,South Carolina1

Abstract: Increases in timber demand and urban development in the Atlantic Coastal Plain over the past decade have motivated studies on the hydrology, water quality, and sustainable management of coastal plain watersheds. However, studies on baseline water budgets are limited for the low‐lying, forested watersheds of the Atlantic Coastal Plain. The purpose of this study was to document the hydrology and a method to quantify the water budget of a first‐order forested watershed, WS80, located within the USDA Forest Service Santee Experimental Forest northeast of Charleston, South Carolina. Annual Rainfall for the 2003 and 2004 periods were 1,671 mm (300 mm above normal) and 962 mm (over 400 mm below normal), respectively. Runoff coefficients (outflow as a fraction of total rainfall) for the 2003 and 2004 periods were 0.47 and 0.08, respectively, indicating a wide variability of outflows as affected by antecedent conditions. A spreadsheet‐based Thornthwaite monthly water balance model was tested on WS80 using three different potential evapotranspiration estimators [Hamon, Thornthwaite, and Penman‐Monteith (P‐M)]. The Hamon and P‐M‐based methods performed reasonably well with average absolute monthly deviations of 12.6 and 13.9 mm, respectively, between the measured and predicted outflows. Estimated closure errors were all within 9% for the 2003, 2004, and seasonal water budgets. These results may have implications on forest management practices and provide necessary baseline or reference information for Atlantic Coastal Plain watersheds.     

Modeling of Vertical Solute Dispersion in a Sediment Bed Enhanced by Wave‐Induced Interstitial Flow1

Abstract: Mass (solute) transport in a stream or lake sediment bed has a significant effect on chemical mass balances and microbial activities in the sediment. A “1D vertical dispersion model” is a useful tool to analyze or model solute transfer between river or lake water and a sediment bed. Under a motionless water column, solute transfer into and within the sediment bed is by molecular diffusion. However, surface waves or bed forms create periodic pressure waves along the sediment/water interface, which in turn induce flows in the pores of the sediment bed. The enhancement of solute transport by these interstitial periodic flows in the pores has been incorporated in a 1D depth‐dependent “enhanced dispersion coefficient (D_E).” Typically, D_E diminishes exponentially with depth in the sediment bed. Relationships have been developed to estimate D_E as a function of the characteristics of sediment (particle size, hydraulic conductivity, and porosity) and pressure waves (wave length and height). In this paper, we outline and illustrate the calculation of D_E as well as the penetration depth (d_p) of the flow effect. Sample applications to illustrate the computational procedure are provided for dissolved oxygen transfer into a stream gravel bed and release of phosphorus from a lake bed. The sensitivity of the results to input parameter values is illustrated, and compared with the errors obtained when interstitial flow is ignored. Maximum values of D_E near the sediment surface can be on the order of 1 cm²/s in a stream gravel bed with standing waves, and 0.001 cm²/s in a fine sand lake bed under progressive surface waves, much larger than molecular diffusion coefficients.     

SPATIAL VARIABILITY IN WATER‐BALANCE MODEL PERFORMANCE IN THE CONTERMINOUS UNITED STATES1

ABSTRACT: A monthly water‐balance (WB) model was tested in 44 river basins from diverse physiographic and climatic regions across the conterminous United States (U.S.). The WB model includes the concepts of climatic water supply and climatic water demand, seasonality in climatic water supply and demand, and soil‐moisture storage. Exhaustive search techniques were employed to determine the optimal set of precipitation and temperature stations, and the optimal set of WB model parameters to use for each basin. It was found that the WB model worked best for basins with: (1) a mean elevation less than 450 meters or greater than 2000 meters, and/or (2) monthly runoff that is greater than 5 millimeters (mm) more than 80 percent of the time. In a separate analysis, a multiple linear regression (MLR) was computed using the adjusted R‐square values obtained by comparing measured and estimated monthly runoff of the original 44 river basins as the dependent variable, and combinations of various independent variables [streamflow gauge latitude, longitude, and elevation; basin area, the long‐term mean and standard deviation of annual precipitation; temperature and runoff; and low‐flow statistics (i.e., the percentage of months with monthly runoff that is less than 5 mm)]. Results from the MLR study showed that the reliability of a WB model for application in a specific region can be estimated from mean basin elevation and the percentage of months with gauged runoff less than 5 mm. The MLR equations were subsequently used to estimate adjusted R‐square values for 1,646 gauging stations across the conterminous U.S. Results of this study indicate that WB models can be used reliably to estimate monthly runoff in the eastern U.S., mountainous areas of the western U.S., and the Pacific Northwest. Applications of monthly WB models in the central U.S. can lead to uncertain estimates of runoff.     

THE VRANA LAKE HYDROLOGY (ISLAND OF CRES - CROATIA)1

ABSTRACT: The Vrana Lake on the island of Cres in the Adriatic Sea represents a specific phenomenon of karst hydrology. The island of Cres covers an area of 404.3 km² with an average volume of 220 × 10⁶ m³ of fresh water in the lake. The island has an average rainfall of 1,063 mm, with a Mediterranean climate. The lake has a bottom reaching a depth of 62 m below mean sea level. The average water level is 14 m above mean sea level. The most probable theories on the origin of the lake and its hycirologic-hydrogeologic functioning state that it is a flooded poije in karst. The water budget method was used to define the lake catchments area at approximately 25 km². During the last six years, there has been drastic decrease of about 3 m in the lake's water level. This phenomenon was analyzed and it was calculated that 53 percent of the water-level decline was caused by water discharges from the lake to satisfy water supply demands, and 47 percent was due to a period of low precipitation during the analyzed period.     

HYDROLOGIC BUDGET FOR A FRESHWATER MARSH IN FLORIDA1

ABSTRACT: Accurate water balance calculations are essential for water resource and environmental management decisions, but many of the terms used in the equation are difficult to measure. In this study, a method for measuring rates of evapotranspiration and net seepage from a freshwater marsh in southwest Florida is described. The results are compared to evaporation pan estimates as well as to calculations that balanced all the terms in the hydrologic budget. The measured rates of evapotranspiration showed a. distinct seasonal trend ranging from an average high of 0.24 in/d during July 1992 to a low of 0.06 in/d in January 1993. Evapotranspiration rates were higher than Class A evaporation pan measurements during July and August, indicating transpiration by plants exceeded evaporation by pans. Net ground water seepage flowed out of the marsh except during periods of high water table conditions. When all terms in the hydrologic budget were evaluated, the equation balanced on a yearly basis with an error of 2 percent, on a seasonal basis with errors less than 7 percent, but on a monthly basis errors were as great as 30 percent. Total annual rainfall on the marsh was 45 percent of the total marsh hydrologic input and was approximately equal to the loss by evapotranspiration of 41 percent.     

A SENSITIVITY AND ERROR ANALYSIS CF PROCEDURES USED FOR ESTIMATING EVAPORATION1

ABSTRACT: Sensitivity and error analyses were used to examine the following objectives: (1) analyze the structure of commonly used evaporation models; (2) provide estimates of the effect of variation in meteorological factors on observed evaporation rates; and (3) estimate the effect of error in measurements of the meteorological factors. The results indicate error in evaporation estimates resulting from measurement error in meteorological factors is probably much less than five percent of the computer evaporation rate. Variation with both time and space of the importance of the different meteorological factors is demonstrated. The sensitivity analysis indicates that the Fractional-Evaporation Equivalent method is structurally inadequate and the Weather Bureau model is more flexible than the Penman model. However, the Penman model appears to provide more realistic estimates of the importance of the various meteorological factors.     

RESPONSE OF NORTH AMERICAN FRESHWATER LAKES TO SIMULATED FUTURE CLIMATES1

ABSTRACT: We apply a physically based lake model to assess the response of North American lakes to future climate conditions as portrayed by the transient trace-gas simulations conducted with the Max Planck Institute (ECHAM4) and the Canadian Climate Center (CGCM1) atmosphere-ocean general circulation models (A/OGCMs). To quantify spatial patterns of lake responses (temperature, mixing, ice cover, evaporation) we ran the lake model for theoretical lakes of specified area, depth, and transparency over a uniformly spaced (50 km) grid. The simulations were conducted for two 10-year periods that represent present climatic conditions and those around the time of CO₂ doubling. Although the climate model output produces simulated lake responses that differ in specific regional details, there is broad agreement with regard to the direction and area of change. In particular, lake temperatures are generally warmer in the future as a result of warmer climatic conditions and a substantial loss (> 100 days/yr) of winter ice cover. Simulated summer lake temperatures are higher than 30°C over the Midwest and south, suggesting the potential for future disturbance of existing aquatic ecosystems. Overall increases in lake evaporation combine with disparate changes in A/OGCM precipitation to produce future changes in net moisture (precipitation minus evaporation) that are of less fidelity than those of lake temperature.     

WATER BUDGETS FOR SMALL DIKED MARSHES1

ABSTRACT: This paper reports an analysis of the water budgets of 10 small (5–6 ha) diked areas (cells) within the Delta Marsh in southcentral Manitoba, Canada. The important terms in the water budget equation in this study were precipitation (P), water pumped in (SWI), evapotranspiration (ET), seepage in (GWI) and out (GWO), and change in storage (ΔS). P, SWI, and S were measured directly, and the sum of ET and GWO determined by difference. Estimating ET as 0.7 pan evaporation gave a seepage loss of 2.9 mm/day from the most intensively studied cell. Other methods of estimating ET produced estimates of GWO ranging from 2.4 to 3.8 mm/day. Water budgets for less intensively studied cells indicated seepage loss increased as perimeter available for seepage increased, but not proportionately. Efforts to measure seepage directly or estimate it from measured hydraulic gradients and hydraulic conductivity produced estimates much lower than the estimates from the water budget equation. Hydraulic conductivities were very heterogeneous, reflecting the sorting of water deposited sediments. Comparison of the hydraulic conductivities with seepage estimates from the water budget strongly suggests water movement downward as well as laterally from these diked areas.     

EVAPOTRANSPIRATION MEASUREMENTS AND MODELING FOR THREE WETLAND SYSTEMS IN SOUTH FLORIDA1

ABSTRACT: At the Everglades Nutrient Removal project in south Florida, three lysimeters were installed to measure daily evapotranspiration (ET) rates from cattails (Typha domingensis), mixed marsh vegetation, and an open water/algae system. The cattail lysimeter began operation in February 1993. The mixed marsh vegetation lysimeter began operation in January 1994, and the open water lysimeter with occasional algae cover began operation in December 1993. The mean measured ET rate was 3.6 mm, 3.5 mm, and 3.7 mm per day for the cattail, mixed marsh vegetation, and open water/algae system, respectively. High resolution weather data were continuously measured at the site. Six models were applied to estimate daily ET rates of the three systems. The Penman-Monteith equation best estimated ET of cattail and mixed marsh vegetation, and the Penman Combination equation was most suitable for the open water/algae system. Empirical equations based on solar radiation and maximum temperature produced estimates of daily ET from the three systems that are comparable to models that require many more parameters. In cases where limited data is available, the calibrated simple models can be used to estimate ET from wetlands in south Florida.     

WATER BALANCE OF PACIFIC ATOLLS1

ABSTRACT: This study presents an estimate of water balance components for Pacific atolls under average dimatological conditions. Figures show annual potential evapotranspiration, annual recharge for rain-fed and aquifer-fed vegetated areas, and the number of months that potential evapotranspiration exceeds actual evapotranspiration (indicating water stress) under average conditions. The method relies on the assumption that small islands have minimal influence on cloudiness and precipitation. The potential evapotranspiration is computed using the equilibrium evaporation concept, and estimates of monthly soil water storage and recharge follow Thornthwaite's bookkeeping method. Gradients in potential evapotranspiration run primarily north-south, though for the equatorial zone potential evapotranspiration declines from east to west, opposing the trend in rainfall. Recharge estimates range from 250 mm in the central Tuamotu Archipelago and zero in eastern Kiribati to over 2000 mm per year in the southern Caroline Islands (U.S. Trust Territory) and Solomon Islands. The sensitivity of the model to intra-month rainfall variability and a range of available soil moisture values is discussed.     

Lake Water Levels and Associated Hydrologic Characteristics in the Conterminous U.S.

Establishing baseline hydrologic characteristics for lakes in the United States (U.S.) is critical to evaluate changes to lake hydrology. We used the U.S. Environmental Protection Agency National Lakes Assessment 2007 and 2012 surveys to assess hydrologic characteristics of a population of ~45,000 lakes in the conterminous U.S. based on probability samples of ~1,000 lakes/yr distributed across nine ecoregions. Lake hydrologic study variables include water‐level drawdown (i.e., vertical decline and horizontal littoral exposure) and two water stable isotope‐derived parameters: evaporation‐to‐inflow (E:I) and water residence time. We present (1) national and regional distributions of the study variables for both natural and man‐made lakes and (2) differences in these characteristics between 2007 and 2012. In 2007, 59% of the population of U.S. lakes had Greater than normal or Excessive drawdown relative to water levels in ecoregional reference lakes with minimal human disturbances; whereas in 2012, only 20% of lakes were significantly drawn down beyond normal ranges. Water isotope‐derived variables did not differ significantly between survey years in contrast to drawdown. Median E:I was 20% indicating that flow‐through processes dominated lake water regimes. For 75% of U.S. lakes, water residence time was less than one year and was longer in natural vs. man‐made lakes. Our study provides baseline ranges to assess local and regional lake hydrologic status and inform management decisions in changing environmental conditions.