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- Hydrograph Separation
- Comparing groundwater recharge and base flow in the Bukmoongol small-forested watershed, Korea
- Baseflow Separation Using Straight Line Method
A hydrograph depicts the response of a stream to rainfall or snowmelt. What can hydrographs tell us about watershed processes? The goal of this project is to partition a storm hydrograph into event flow and baseflow components, and old and new water components. A storm hydrograph depicts a streams response to a water input event rainfall or snowmelt in graphical form Figure 1.
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The essence of hydrograph separation is traditionally considered to render a deconstructive rationale of streamflow as a two-component or multi-component process. The most commonly used scheme is the two-component scenario that considers streamflow consisting of direct flow i. Direct flow is in general formed by surface precipitation, overland flow i. There exists a wide variety of methods for baseflow separation, which are categorized into different types based on selected crite-ria, such as tracer-based method, graphical method, filtering method, digital filter and recession analysis see Table 1 for a summary.
Among the various methods, the tracer-based method yields the most realistic results Buttle, ;Sklash and Farvolden, ; however, it is laborious and expensive; thus its application is restricted to small number of events, which prohibits statistical analysis. Graphical is the most intuitive method Chow et al.
Another technique is the filtering method which is typically designated for long-term, daily time scale, data records. Digital filter is a commonly used baseflow separation method nowadays, and it can be sorted as one-, two-or multi-parameter filter depending on the number of parameters used Lyne and Hollick, ;Jakeman and Hornberger, ;Chapman, ;Eckhardt, Common parameters for these filters are the recession coefficient and the maximum baseflow index the longterm ratio of baseflow to total streamflow Eckhardt, ; the one-parameter filter has a predefined maximum baseflow index.
The recession coefficient rate of change of discharge depletion during periods of little or no precipitation can be determined reasonably well from the recession limb of the hydrograph Nathan and McMahon, , while to get a value for the maximum baseflow index would require running in-situ measurements, or acquiring it from the literature.
A three-parameter filter is rarely used since it was concluded to produce baseflow hydrographs with sharp peaks compared to observations Chapman, The most physically based method without running field experiments could be the analytical solution of recession equation.
This group of methods generally starts from assuming either a linear Blume et al. To ensure a reasonable recession coefficient for a digital filter, or the analytical solution of recession, fine temporal resolution flow data is required. Use of baseflow separation methods to study properties of hydrologic events requires determining the start and end times of these events in the streamflow record. Typically this is carried out manually by visual inspection of the time series data.
However, visually inspecting the timing of events is cumbersome when it comes to long-term data records. Few studies on this topic have attempted to develop automatic event identification methods that can apply to large data records Dhakal et al. Merz et al. Norbiato et al. In short, the Merz et al. Khanal , on the other hand, proposed a semi-automatic approach based on the unit hydrograph method, which he applied on 90 mid-slope watersheds in Texas to extract single peak events over a year period.
The method required manual identification and extraction of the multi-peak events as well as events with un-wanted shapes. The event database was then used in several subsequent studies. Cleveland et al. The essence of SARR is to associate each rainfall event i.
In their study, Koskelo et al. In this study, we investigate an automatic hydrograph separation method that is based on long time series and hourly time scale data on basin rainfall and runoff.
The new aspects of this technique are its physical basis, which requires less data to constrain the even separation algorithm. Specifically, a set of parameters recession coefficient, maximum baseflow index, time lag between rainfall and runoff mass centers are derived by mining the basin areal rainfall and runoff time series and used to drive the event separation algorithm. An important point to note is that the algorithm is designated for basins with clear recession period this limitation is detailed in the conclusion part.
In Section 2 below we describe the study area, rainfall and runoff data used to demonstrate the efficiency of the proposed technique. The technique's procedures used for baseflow separation and event identification are described in Sections 3.
The results are discussed in Section 4, while conclusions are summarized in Section 5. Information about the basin can be found in Mei et al. Drainage areas for the eight sub-basins range from to km 2 refer to Table 2 for details about the basins. There are three separate drainage systems Fig. These three rivers join as the Tar River nears the basin outlet we name the entire basin as TSF using the first letter of each river's name.
The basin is characterized by low elevations ranging between 4 and m above sea level and mild slopes i. Eight years of data were analyzed for this work. Mean annual precipitation for all sub-basins was over mm; among the study years, was the wettest with more than mm of annual cumulative precipitation for every sub- basin, while and were comparatively drier years receiving less than mm of annual cumulative precipitation Table 3.
The streamflow datasets used in this study are hourly streamguage measurements from USGS that span the same eight-year period as the precipitation data information regarding the streamflow datasets are listed in Table 3. The mean annual runoff volumes are around mm for all sub-basins. Similarly to precipitation, was hydrologically the wettest with annual cumulative runoff volume over mm mm for the different sub-basins, while and had relatively low mm annual cumulative runoff volumes.
Besides, values of the runoff ratio mean annual runoff over mean annual precipitation for the different sub-basins are nearly the same at 0. MethodologyThis section describes the details on the proposed methodology including inputs, outputs, implemented steps and variables involved. Flow charts showing the connection of the various steps and flow of data and information are shown in Fig.
As we can see from the flow charts, the required inputs are merely rainfall and flow time series. Outputs from the baseflow separation part are the baseflow time series constructed under the revised constant k method, and its filtered ver-sion denoted below as b RCK and b FRCK.
Timings of the analyzed events i. Every process step and variable computed from this steps are labeled on the flow chart and the corresponding descriptions are documented in Sections 3. Baseflow separationWe considered the two-component streamflow scenario baseflow and direct flow in formulating this technique. In short, we revised the constant k method developed in Blume et al.
Also, we coupled the revised constant k method with the recursive digital filter RDF constructed by Eckhardt to compute the baseflow hydrograph, namely FRCK method. The two methods are described next.
Blume et al. Based on the form of Eq. The specific steps of the RCK method are shown in the flow chart of Fig. This is consistent with the behavior of k. The reason of this restriction is visualized in Fig. This implies that quantifying recession limb with Eq. The remaining rising and recessing points are named as turning points.
A quality control procedure used in the other studies is also implemented here; namely, baseflow is set as total flow when it is greater than total flow. Envelope baseflow hydrograph As shown in the procedure 3 of RCK method, a null-change ratio is determined for every potential turning point from recession limbs. This leads to an issue; this ratio cannot satisfy every single recession limb due to the heterogeneity of temporal flow properties.
That is although most of the turning points appear at the visually acceptable locations, a portion of them would show up at inappropriate locations, e. Therefore, envelope baseflow time series denoted by b en are constructed to limit the occurrences of turning points at the high flow rate locations.
The envelope baseflow time series are simply the product between the flow rate maxima for every block envelope flow time series, Q en and the baseflow index determined for the entire study period denoted by BFI. Therefore, Q en are constructed as following also shown in the flowchart of Fig. This searching period should be able to capture most of the flow maxima associated with an event. We apply one of the classical empirical methods to determine the number of hours from the peak hour to the event ending hour Hornberger, ;Chow et al.
To limit the value range of b en , upper and lower limit of envelope baseflow rate, b u and b l are set. Values of b u and b l and mean flow quantiles are determined from the CDF of the flow time series step vi and are basin scale dependent refer to Table 5 for values.
As noted in the figure, turning points above the envelope baseflow rate are excluded. BFI is the baseflow index defined as the ratio of baseflow volume in a given time interval e.
The parameterization process of RDF see Fig. UKIH was designed for daily flow data Gustard et al. The initial BFIs are calculated based on the entire 8-year period see Table 4 for values. Procedures 1 through 12 document the entire process of baseflow separation. Among the involved parameters, K is derived based on recession analysis, L SP is determined empirically based on a classical method, R NC is reported in previous studies and the other parameters are arbitrarily selected.
Thus, only the choices of BFI could be ambiguous to some extent. Event identificationIn this section we describe a method to identify single flow events from the stream flow record together with the information from the FRCK method.
The flowchart of this method is presented in Fig. In short, the essence of this method is to synthesize flow events with three characteristic points start, peak s and end of the event and then associate the flow event to a corresponding rainfall event from the rainfall time series; the method for rainfall event identification used here is based on a procedure used in Mei et al.
This method is named as characteristic point method CPM. Flow eventsA flow event is defined with three characteristic points, the start, peak s and end. If these points are ensured, an event is established. Hence, this section presents a method to locate the flow peak and then associate the peak flow points with the closest rising and recessing points.
Groundwater recharge and base flow using different investigated methods are simulated in the ha Bukmoongol small-forested watershed located at the southern part of Korea. Results show that about 15—31 per cent of annual rainfall might be contributed for base flow. The watershed groundwater recharge proportions are computed to about 10—21 per cent during the wet period and 23—32 per cent for the remainder periods. Mean annual base flow indices vary from 0. However, the study found out that all methods were significantly correlated with each other. The similarity of various methods is expressed as a weighted relationship provided by the matrix product from the principal component analysis. The case study recommends the application of different models to other watersheds as well as in low-lying areas where most observation groundwater wells are located with available streamflow data.
Comparing groundwater recharge and base flow in the Bukmoongol small-forested watershed, Korea
Baseflow also called drought flow , groundwater recession flow , low flow , low-water flow , low-water discharge and sustained or fair-weather runoff is the portion of the streamflow that is sustained between precipitation events, fed to streams by delayed pathways. It should not be confused with groundwater flow. Fair weather flow is also called base flow. Baseflow is important for sustaining human centers of population and ecosystems.
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This report is a user guide for the streamflow-hydrograph analysis methods provided with version 1. The Groundwater Toolbox is a customized interface built on the nonproprietary, open source MapWindow geographic information system software. The program provides graphing, mapping, and analysis capabilities in a Microsoft Windows computing environment. In addition to the four hydrograph-analysis methods, the Groundwater Toolbox allows for the retrieval of hydrologic time-series data streamflow, groundwater levels, and precipitation from the USGS National Water Information System, downloading of a suite of preprocessed geographic information system coverages and meteorological data from the National Oceanic and Atmospheric Administration National Climatic Data Center, and analysis of data with several preprocessing and postprocessing utilities. With its data retrieval and analysis tools, the Groundwater Toolbox provides methods to estimate many of the components of the water budget for a hydrologic basin, including precipitation; streamflow; base flow; runoff; groundwater recharge; and total, groundwater, and near-surface evapotranspiration.
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Baseflow Separation Using Straight Line Method
This step demonstrates the use of straight line method to separate baseflow from a streamflow hydrograph by using data for a single storm event. This step is developed for a storm event at Hall Creek in Indiana. Baseflow is a portion of streamflow that is not directly generated from the excess rainfall during a storm event. In other words, this is the flow that would exist in the stream without the contribution of direct runoff from the rainfall.
Download page Selecting a Baseflow Method. While a subbasin element conceptually represents infiltration, surface runoff, and subsurface processes interacting together, the actual subsurface calculations are performed by a baseflow method contained within the subbasin. A total of six different baseflow methods are provided. Some of the methods are designed primarily for simulating events while others are intended for continuous simulation. You can select a baseflow method from the list of six available choices. Use the selection list to choose the method you wish to use.
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