Base Flow Values Research Articles

Hydrologic response to climatic variability in a Great Lakes Watershed: A case study with the SWAT model

Summary Process-based hydrologic models are usually calibrated prior to application to ensure that they closely match reality. However, different hydrologic response to varied climatic conditions might affect model calibration and validation. A case study was conducted for a 901 km 2 watershed of northern Michigan to compare the effects of calibrating the Soil and Water Assessment Tool (SWAT) watershed model with different climatic datasets representing drought (1948–1949) versus average (1969–1970) conditions. The effects of the different climatic conditions on parameter response and sensitivity were evaluated, and performance of the two calibration versions was compared using a common validation period, 1950–1965. For the drought- and average-calibration periods, models were well calibrated, as indicated by high Nash-Sutcliffe efficiency coefficients ( E = 0.8 and 0.9), and low deviation of discharge values ( D = 2.9% and 3.4%). Evapotranspiration parameters differed under the two sets of climatic conditions. The plant water uptake compensation factor (EPCO), appropriately reflected plant water uptake patterns in varied climatic conditions. Snow melting parameters differed between the two scenarios. A comparison of baseflow values simulated by SWAT versus those computed by a hydrograph separation method showed that the SWAT method treated most snowmelt as surface runoff, whereas the latter method treated much of it as baseflow. The drought-calibrated version of the model performed much better during the validation period (1950–1965) ( E = 0.8, D = 2.6%) than did the average-calibrated version ( E = 0.4, D = 41.4%).

Modelling of base flow of the basin of Kolubara river in Serbia

Summary This study presents a physically founded hydrologic–hydraulical model for determining the base flow of the catchment by analysing the water balance of the basin and vertical exchange of water between the atmosphere, unsaturated media and ground water, basing on the known physical characteristics of the basin (climatic, paedological, geological and vegetation ones) on the example of the 340 km 2 Kolubara River basin up to “Valjevo” water level monitoring station in Serbia in three characteristic years: an extremely rainy year (1970), an average year (1985) and an extremely dry year (1990). Taking into consideration that the amount of base flow of a basin is impossible to measure at the catchment outlet, the hydrograph of the base flow values obtained by separating from the streamflow hydrograph registered at the outlet of a drainage basin by application of the graphical local minimum method was used for checking the model’s functioning. It was finally determined that the total base flow values at the outlet point of the catchment, obtained by application of the graphical local minimum method, equal approximately the total groundwater recharge from the whole surface area of the basin, representing the summary of groundwater recharge originating from different soil types covered with different types of vegetation, under the influence of atmospheric conditions of the soil surface. Thus it was confirmed that when characteristics of a basin are known, it is possible to obtain information on the base flow values.

Redox Chemistry in Minnesota Streams during Episodes of Increased Methylmercury Discharge

Mercury (Hg) and methylmercury (MeHg) are flushed from watersheds during hydrological events, contaminating downstream surface waters and resident fish populations. We monitored total mercury (THg), MeHg, and ancillary water chemistry parameters in two streams (Cedar Creek and Trott Brook) in east-central Minnesota on a weekly or semiweekly basis from April through October 2003. Heavy precipitation in late June resulted in discrete episodes of high concentrations (>1.2 ng/L) of MeHg in both streams in early July. The MeHg/THg ratio increased from 0.15 to 0.36 in Cedar Creek and from 0.13 to 0.46 in Trott Brook during the event. The high MeHg concentrations were accompanied by low dissolved oxygen concentrations and increased concentrations of dissolved organic carbon, Mn, Fe, and orthophosphate. A prolonged absence of precipitation during August and early September brought stream levels back to baseflow values, and MeHg concentrations decreased to less than 0.1 ng/L. These results suggest that warm-weather, high-discharge events are the primary route of export of MeHg from these watersheds, and baseflow contributes much less MeHg to downstream waters. The redox water chemistry during the,events sampled here suggests that MeHg in these streams is discharged from wetland areas where anoxic/anaerobic conditions prevail.

Temporal pattern of toxicity in runoff from the Tijuana River Watershed

Samples were collected from the Tijuana River under both dry weather (baseflow) conditions and during wet weather, and tested for toxicity using Ceriodaphnia dubia tests. Toxicity of waters in the Tijuana River was generally low under baseflow conditions, but increased markedly during high flow runoff events. In order to determine the temporal pattern of toxicity during individual rain events, sequential grab samples were collected using an autosampler at 5–7 h intervals after the start of the rain event, and tested for acute toxicity. In all cases, peak toxicity values (ranging from 2.8 to 5.8 TU) for each storm occurred within the first 1–2 h of initiation of the rain event, and were statistically higher (using the 95% CL) for each of the pre-storm base flow values. However, there was no statistically significant correlation ( p<0.05) between flow rate and toxicity when all storm data was pooled. Additionally, we used toxicity identification evaluation (TIE) procedures to attempt to identify the classes of chemicals that account for this early storm toxicity. Solid phase extraction was the only treatment that showed consistent and significant ( P<0.05) removal of toxicity. These TIEs, conducted on the most toxic sample of the river's flow during runoff events, suggest that non-polar organics may be responsible for such toxicity. The temporal pattern of toxicity, both during a given storm event and seasonally, indicates that wash-off from the watershed by rainfall may deplete the supply of toxicity available for wash-off in subsequent events, so that a clearly consistent relationship between flow and toxicity was not evident.

Role of organic nitrogen in the nitrogen cycle of a high‐elevation catchment, Colorado Front Range

Here we report on 3 years (1996–1998) of measurements of organic and inorganic nitrogen (N) fluxes to and from Green Lakes Valley, a high‐elevation ecosystem in the Colorado Front Range of the Rocky Mountains. Nitrate‐N (NO3−N) was the dominant form of N in both precipitation and stream water. Annual precipitation contained 52% NO3−N, 32% ammonium‐N (NH4−N), 9% dissolved organic N (DON), and 7% particulate organic N (PON). Annual export of N in streamflow was composed of 70% NO3−N, 4% NH4−N, 14% DON, and 12% PON. Thus the percentage of organic N increased from 16% of total N in precipitation to 26% of total N in streamflow. Subtracting inputs from outputs, Green Lakes Valley always shows net retention of inorganic N. The only form of N that showed net export was DON. DON export was low (0.18 to −0.13 kg ha−1 yr−1), with net export recorded in 2 years and basin retention recorded in 1 year. There was a seasonal pattern in the concentrations of inorganic N (NO3−N+NH4−N) and organic N (DON+PON). Concentrations of inorganic N were ∼15–25 μmol L−11 during base flow, increased to ∼30 μmol L−1 on the rising limb of the hydrograph during snowmelt runoff, then decreased to ∼5 μ L−1 on the recession limb of late summer, with a return to base flow values in the autumn. In contrast, organic N was 7–15 μ L−1 during base flow and decreased to near or below detection limits on the rising limb of the hydrograph, with a gradual but consistent increase on the recession limb and on into the autumn. The amount of N in dissolved organic matter changed over time, with the dissolved organic carbon (DOC):DON ratio decreasing from ∼45 on the rising limb of the hydrograph to &lt;20 in the autumn. Spatially, there was a striking difference in the ratios of NO3−N and DON between talus and tundra areas. Nitrate concentrations in surface water draining talus areas were always greater than DON. In contrast, DON concentrations in surface water draining tundra areas were always greater than NO3−N. Concentrations of DON were not significantly correlated with DOC (R2 = 0.04, p &gt; 0.05), indicating that controls on DON export may be different than controls on DOC export. Our results suggest that the ratio of the annual mass flux of inorganic N to organic N in stream waters may provide a novel index to evaluate the N status of terrestrial ecosystems from various biomes.

Groundwater residence times in Shenandoah National Park, Blue Ridge Mountains, Virginia, USA: a multi-tracer approach

Chemical and isotopic properties of water discharging from springs and wells in Shenandoah National Park (SNP), near the crest of the Blue Ridge Mountains, VA, USA were monitored to obtain information on groundwater residence times. Investigated time scales included seasonal (wet season, April, 1996; dry season, August–September, 1997), monthly (March through September, 1999) and hourly (30-min interval recording of specific conductance and temperature, March, 1999 through February, 2000). Multiple environmental tracers, including tritium/helium-3 ( 3H/ 3He), chlorofluorocarbons (CFCs), sulfur hexafluoride (SF 6), sulfur-35 ( 35S), and stable isotopes ( δ 18O and δ 2H) of water, were used to estimate the residence times of shallow groundwater discharging from 34 springs and 15 wells. The most reliable ages of water from springs appear to be based on SF 6 and 3H/ 3He, with most ages in the range of 0–3 years. This range is consistent with apparent ages estimated from concentrations of CFCs; however, CFC-based ages have large uncertainties owing to the post-1995 leveling-off of the CFC atmospheric growth curves. Somewhat higher apparent ages are indicated by 35S (>1.5 years) and seasonal variation of δ 18O (mean residence time of 5 years) for spring discharge. The higher ages indicated by the 35S and δ 18O data reflect travel times through the unsaturated zone and, in the case of 35S, possible sorption and exchange of S with soils or biomass. In springs sampled in April, 1996, apparent ages derived from the 3H/ 3He data (median age of 0.2 years) are lower than those obtained from SF 6 (median age of 4.3 years), and in contrast to median ages from 3H/ 3He (0.3 years) and SF 6 (0.7 years) obtained during the late summer dry season of 1997. Monthly samples from 1999 at four springs in SNP had SF 6 apparent ages of only 1.2 to 2.5±0.8 years, and were consistent with the 1997 SF 6 data. Water from springs has low excess air (0–1 cm 3 kg −1) and N 2–Ar temperatures that vary seasonally. Concentrations of He and Ne in excess of solubility equilibrium indicate that the dissolved gases are not fractionated. The seasonal variations in N 2–Ar temperatures suggest shallow, seasonal recharge, and the excess He and Ne data suggest waters mostly confined to gas exchange in the shallow, mountain-slope, water-table spring systems. Water from wells in the fractured rock contains up to 8 cm 3 kg −1 of excess air with ages in the range of 0–25 years. Transient responses in specific conductance and temperature were observed in spring discharge within several hours of large precipitation events in September, 1999; both parameters increased initially, then decreased to values below pre-storm base-flow values. The groundwater residence times indicate that flushing rates of mobile atmospheric constituents through groundwater to streams draining the higher elevations in SNP average less than 3 years in base-flow conditions.

High DON bioavailability in boreal streams during a spring flood

Riverine inputs of nitrogen is an important factor that controls productivity of coastal marine waters. Dissolved organic nitrogen (DON) comprises most of the N in boreal rivers. During spring floods, DON flux may exceed the baseflow flux by several orders of magnitude. However, little is known about the biological availability of spring flood DON and, thus, its potential effect on coastal productivity.We have investigated the dynamics of DON bioavailability and chemical composition during a spring flood in two streams in northern Sweden. Potential bioavailability was determined by employing bacterial regrowth bioassays with brackish medium and a coastal bacterial inoculum. In addition, concentrations of urea and amino acids and the proportion of D‐isomers in total dissolved amino acids were analyzed because a high proportion of D‐isomers is suggested to indicate old and refractory organic material.During the flood, potential DON bioavailability increased from 19–28% at baseflow to 55–45% during peak flow in the two streams, while DON concentration remained relatively constant. At the end of the flood, bioavailability returned to the baseflow values. Potential DON bioavailability was positively correlated with the concentration of dissolved combined amino acids and the proportion of L‐enantiomers of amino acids. However, only 5–18% of DON was identified as urea and free and combined amino acids, suggesting that bacteria also utilized other DON compounds. Our results imply that a major portion of the annual export of labile nitrogen occurred during a few weeks of spring flood.

DETECTING BASE FLOW IMPACTS IN COASTAL PLAIN STREAMS1

ABSTRACT: An established trend analysis methodology was applied to the problem of identifying and quantifying stream base flow impacts from water withdrawals and water loss through interbasin transfers. Impacts were simulated using base flow values selected from two U.S. Geological Survey (USGS) continuous record streamflow sites located within the Pinelands of southern New Jersey. Study site base flows were regressed against index site base flows with monotonic and step trend tests applied to the residuals from the regression model. The smallest, significantly detectable (α= 0.10) percentage reduction within a given simulation was used as an estimate of the sensitivity of a trend test. Evaluation of the trend analysis methodology led to the following practical considerations regarding trend test sensitivity. The proportion of study site base flow variability explained by index site base flows should be maximized, while at the same time minimizing positive, first‐order autocorrelation in the regression residuals. Given the importance of detecting autocorrelation, missing values should be avoided or minimized. The quarterly (three‐month) interval reduced the magnitude of autocorrelation relative to a shorter two‐month sampling interval. Sensitivity appeared to improve when equalizing the number of values before and after a base flow impact(s) while seasonally biased sampling appeared to reduce sensitivity. Based primarily on past trend detection studies, nonparametric tests were deemed a better choice over their parametric counterparts, due to the lack of stringent data distributional requirements coupled with little or no loss of power even when applied to normally distributed data.

Estimating Ground‐Water Recharge from Streamflow Hydrographs for a Small Mountain Watershed in a Temperate Humid Climate, New Hampshire, USA

AbstractHydrographs of stream discharge were analyzed to determine ground‐water recharge for two small basins draining into Mirror Lake, New Hampshire. Two methods of hydrograph analysis developed for determining ground‐water recharge were evaluated, the instantaneous recharge method and the constant recharge method. For the instantaneous recharge method, recharge is assumed to be instantaneous and uniform over the basin. For the constant recharge method, recharge is assumed to be constant and uniform over the basin for a period of weeks to months. Both methods require that a ground‐water recession slope be determined. The recession slope is used directly in the calculation for the instantaneous recharge method, and it is used as a base of reference for fitting a type curve in the constant recharge method. Results of the study indicated that the estimates of ground‐water recharge for both methods agree to within about 10 percent. Two approaches to the instantaneous recharge method, manual and automated, were also evaluated, and the results were statistically similar.The baseflow component of streamflow commonly is assumed to be equivalent to ground‐water recharge; therefore, two methods developed for determining the baseflow component of streamflow, graphical partitioning and digital filtering, were evaluated also. Baseflow values determined by graphical partitioning of hydrographs were about 25 percent less than the ground‐water recharge values. Baseflow values determined by two different approaches to the mathematical digital filtering method were generally less than baseflow determined by graphical partitioning. However, one of the approaches to digital filtering agreed reasonably well with graphical partitioning if an appropriate filter constant was used. The other approach to digital filtering resulted in baseflow values that were much less than the other baseflow values and was therefore deemed inappropriate for use on these small mountain watersheds.

ANALYSIS AND EXTRACTION OF LOW FLOW RECESSION CHARACTERISTICS1

ABSTRACT: It is important to extract and assess low flow recession characteristics for water resources planning in the upper reaches of streams. However, it is very difficult to express synthetically the low flow recession characteristics for a stream flow. In this paper, first a new method of constructing the master recession curve based on the exponential expression is proposed and applied with the restriction that there are no regulation or diversion structures in the upper reaches above the measurement station. Daily precipitation and stream flow were used for the analysis. Second, analysis for a recession constant was conducted and the relationship between the recession constant and low flow and/or geology was qualitatively examined. In conclusion, the application of the proposed method indicated that it is objective and useful for constructing the master recession curve. It became apparent that the recession constant of a master recession curve may be defined as the total index of low flow characteristics. In addition, it was found that baseflow value increases in the order of Paleozoic, Mesozoic, Tertiary, and Quaternary.

Assessing the contribution of individual dissolved ions to depressions in acid neutralising capacity of streams in the adirondack and Catskill Mountains, New York

Methods for quantifying the influence of different ions on depressions in ANC, based on those used by Molot et al. (1989), have been employed on streams in the Adirondack and Catskill Mountains of New York. Streams were intensively monitored during the Episodic Response Project ofthe US EPA. Baseflow values were determined for each variable, and for each low-ANC sample the proportion of ANC depression (relative to baseflow) contributed by each ion was calculated. In the Catskill streams Ca 2+ dilution and NO 3 - increases were major causes of ANC depression ; SO 4 2- dilution and Al mobilisation increased ANC. In the Adirondack streams Ca 2+ and Na + dilution, and NO 3 - and SO 4 2- increases, all contributed to ANC depressions. Inter-stream differences in results were linked to differences in dream acidity ; in both regions Ca 2+ dilution dominated ANC depressions in circumneutral streams, whereas NO 3 - increases were dominant in acidic streams. Organic anions contributed more to ANC depressions in acidic streams. Al buffering, was negligible in circumneutral streams, but more than halved ANC depressions in the most acidic stream. Individual base cation behaviour differed widely, suggesting that caution should be used when treating them as a uniform group.

Initial contributing area of a small watershed

The initial contributing area (ICA) of a watershed at the onset of rain is defined as consisting of the stream surface and the area of surface saturation caused by the groundwater table intersecting the land surface above the elevation of the stream. Subsurface flow discharges upward within this area, so the ICA exhibits zero infiltration capacity and potential storage for rainfall. Thus, the ICA is the initial runoff-producing area in variable-source-area generation of storm runoff. The ICA of a 42-ha east-central Pennsylvania watershed is shown to be a function of prestorm baseflow; a prediction equation is developed using rainfall-runoff data from small rainstorms. This equation could be incorporated directly into a continuous streamflow model where a baseflow value is part of the simulation, or into an event-based hydrograph model where prestorm baseflow is specified as a design parameter.