CADDIS Volume 2: Sources, Stressors & Responses
Ways to Measure Flow Alteration
In the context of causal assessment, water depth, volume, velocity, and discharge (each of which varies with time and space) are considered part of the flow regime. Discharge is often a primary focus of hydrologic studies (especially large scale studies), but the existence, quality, and accessibility of hydrologic data may dictate which measures of flow alteration are used in any given causal assessment.
A good place to start your search for hydrologic data might be the U.S. Geological Survey's (USGS's) StreamStats. This on-line tool provides watershed characteristics and flow statistics for both gauged and ungauged streams. Gauged site data come from data collection stations, while ungauged site information is estimated (assuming natural unaltered or natural flow) using Geographical Information System (GIS) technology.
Continuous discharge rarely is measured directly. Instead, flow gauges typically measure instantaneous water depth (i.e., stage) at intervals (e.g., every 5 to 60 minutes); a rating curve (plot of discharge versus stage height) then can be used to convert stage to discharge. To construct a rating curve, measurements of discharge are made for a channel cross-section under a range of flow conditions (e.g., baseflow, small storms, and large storms). Discharge can be estimated by measuring flow velocity and water depth at several points across the width of a channel [for more information on field hydraulic measures, see Rantz (1982)]. The USGS is the primary U.S. agency that collects flow data. In addition, local government agencies, state biologists, consulting firms, or other entities may provide causal assessors with discharge data over time or rating curves and measured depth of flow; causal assessors should consider differences between raw and calculated data in terms of accuracy, collection techniques, and underlying calculation assumptions.
Hydrographs, plotted as discharge versus time (Figure 13), can be used to characterize flow at a given stream cross-section. The shape of a hydrograph following a precipitation event reveals information about the contributing watershed's flow regime, and is characterized by several parameters, including: volume of flow, or the area under the hydrograph; magnitude, or peak flow; duration of the event above a certain flow; rate of change from, say, low to peak discharge and back again (i.e., a flashy system); and lag time, or time between the rainfall center of mass and flow volume center of mass.
For causal assessment, hydrographs of reference versus impaired streams or hydrographs before and after a disturbance might be compared to determine, for example, the impacts of urbanization (Figure 14), dams, or channelization.
Figures 13 and 14 show discharges resulting from single rain events. Sometimes, it is advantageous to analyze a hydrograph for one or more water years (typically, October 1st of year X, to September 30th of year X+1) to better understand frequency of small events, seasonal variation of flow, and baseflow levels or groundwater inputs between precipitation events.
Long-term flow data may be used to develop statistical descriptors of flow regime. A common statistical measure of flow is event frequency (sometimes expressed as recurrence interval or return period). A 10-year flow event is an event with a magnitude predicted to recur once every ten years; such an event has a 10% chance of happening in any given year. USGS provides guidance for estimating flow frequency (U.S. Interagency Advisory Committee on Water Data 1982). Flood management often focuses on large events with low frequency (e.g., 10- to 100-year events), although smaller events with greater frequency play an equally important role in geomorphological and ecological processes. Cunnane (1978) provides an equation for estimating sub-annual return periods—that is, recurrence of small events within a year.
Duration was mentioned above in the context of a single event (i.e., length of time flow is above a certain magnitude). Flow duration also can be calculated for longer time periods. For example, flow duration can be defined as the length of time (generally, the number of days) per year in which a stream's discharge is greater than a particular value.
Software has been developed to calculate hydrologic statistics. PEAKFQ is an application based on USGS's guidelines for determining flood flow frequency (U.S. Interagency Advisory Committee on Water Data 1982), and is publicly available at U.S. Geological Survey-PEAKFQ. USGS recommends having ten years of flow data before conducting a basic determination of event frequency. Such data might be in the form of ten consecutive annual peak flow values—that is, the highest estimated discharge for each of ten years, input as a list into the application. The Nature Conservancy also has developed software for estimating statistical indicators describing flow, and the Indicators of Hydrologic Alteration (IHA) software application and supporting information can be downloaded from The Nature Conservancy. IHA requires daily discharge data for input (refer to the IHA user's manual for additional information including data requirements; The Nature Conservancy 2006).
The location of flow gauges along a stream or river may not coincide with biological sampling sites. Furthermore, data from an impaired site may not be matched with comparable reference site data. If a flow gauge is downstream of a biological sampling site, it may be appropriate to scale down flow estimates for the biological sampling site, using a ratio of watershed area between the two locations. Alternatively, a computer model characterizing a watershed's hydrologic behavior may assist causal assessment by providing data to compare with observed data, and allowing further understanding of flow regimes. Model inputs might include precipitation, watershed shape and size, infiltration rates, and lengths and roughness of flow paths. These data also may allow for simple analyses. For example, an investigator could compare the percent of total precipitation reaching stream channels among watersheds with varying amounts of impervious surface area. Hydrologic data also may allow investigators to develop more complicated simulations that depict a range of flow characteristics. Hydrologic computer models can be used to compare pre- and post-development flow regimes, to compare reference versus impaired watershed flow characteristics, and to consider potential impacts of restoration designs. Modeling software continues to evolve as computational technology improves. For example, the U.S. EPA and the U.S Army Corps of Engineers Hydrologic Engineering Center (HEC) provide modeling applications and supporting materials (BASINS and HEC, respectively).
Conducting a hydrologic study often involves developing a map of the watershed, as hydrologic modeling frequently entails using Geographic Information Systems (GIS) to characterize watershed features. Examples of GIS components in hydrologic modeling include digital elevation models (DEMs or topography) and soil-type layers, which are sometimes made available by local government agencies. Causal assessors may search for existing hydrologic studies and models of impaired watersheds, perhaps conducted or contracted by local government agencies.