The hydrograph

Streamflow is a valuable source of hydroclimatic information. It conveys the complex details of how the climate system and the hydrologic cycle are interacting. The runoff process can be viewed directly for relatively small areas, but streamflow for a large watershed results from distant events and relationships not readily apparent. It is necessary to understand how water arrives at the stream channel in order to assess the coupling of climate and the hydrologic response.

Streamflow is portrayed graphically using a hydrograph that is a plot of discharge against time (Fig. 6.2). A hydrograph relates stream discharge to the water supplied by rainfall or snowmelt. It is a graphical representation of the sequence of relationships between precipitation and the various basin environmental factors important to the runoff process and their occurrence over time. Hydrographs provide information about the runoff process and the behavior of streams during drought, floods, or under normal weather conditions. In general, a long-term stream discharge hydrograph is a series of irregular increases and decreases superimposed on a relatively consistent flow. Precipitation over the watershed produces a discharge increase and dry periods produce gradually declining flows. The magnitude of the peaks and troughs is an expression of the size of the drainage area, the intensity of the storm producing runoff, antecedent watershed conditions, and the distance and path the water travels before reaching the stream gauge. Information supporting detailed analysis of stream hydrographs is found in the hydrology literature (e.g. Dingman, 1994; Ward and Robinson, 2000).

The nature of the storages, delays, and time of travel is different for each of the surface and subsurface paths water follows in becoming runoff as presented in Section 6.2. Discharge in the channel results from the integration of flow from all runoff sources. Many of the sources exert distinctive influences on the quantity and timing of streamflow, which elucidate how the runoff process works in a watershed and how the processes are different from one watershed to another.

6.9.1 Hydrograph components

Transforming precipitation into runoff by hydroclimatic processes is evident in the hydrograph form. Hydrographs also portray information on the change in runoff rates with time, the peak runoff rate, and the volume of runoff. The hydroclimatic role in the runoff process can be characterized by the path followed by water in arriving at the stream channel after it is delivered to the surface by precipitation. Tracing a rainfall event serves as the framework for assessing other combinations of precipitation.

A common convention to facilitate description of the discharge hydrograph is to recognize two streamflow components designated as event flow and base flow. Event flow, also called direct flow, surface flow, storm flow, or quick flow, is water that enters the stream channel promptly in response to individual water-input events. Event flow is dominantly water that moves over the surface to the stream or travels as throughflow.

Base flow is water that enters the stream from persistent, slowly varying sources, and it maintains streamflow between water-input events. It is usually assumed that most, if not all, base flow is supplied by groundwater. However, streamflow between water-input events can also derive from drainage of lakes or wetlands or from the slow drainage of relatively thin soils on upland hillslopes. Some surface stream baseflow comes from throughflow in the soil (Ward and Robinson, 2000). Streams that receive large proportions of flow as groundwater tend to have relatively low temporal flow variability.

Hydrograph separation is a convenient method for gaining insight into the array of runoff components represented by streamflow. A concentrated rainfall event produces a typical hydrograph with a single peak and a skewed distribution curve (see Fig. 6.2). Multiple peaks can result from variations in rainfall intensity, a succession of storms, or other causes. Therefore, the shape of the hydrograph provides an integration of the climatic and watershed characteristics responsible for runoff.

The customary method for examining hydrographs is to recognize commonly recurring features of the curves. This is a reasonable approximation approach, and it provides a perspective that is useful for developing a general understanding of the watershed response. Complete details of methods for analyzing storm hydrographs are found elsewhere (e.g. Dingman, 1994; Ward and Robinson, 2000).

For descriptive purposes, the hydrograph is composed of a rising limb, a crest, and a recession limb. Some time after the beginning of rainfall, the flow rate begins to increase relatively quickly from a preexisting level. This period of rapid discharge increase is the rising limb of the hydrograph. The slope of the rising limb is largely determined by the storm intensity influencing the proportion of rainfall allocated to surface runoff. Rainfall following surface routes causes the rising limb to be very steep because water is delivered quickly to the channel.

The peak discharge defines the hydrograph crest. This is approximately the time when surface inflow related to the rainfall event ceases. Large contributions by surface flow and throughflow contribute to high peak flows.

Declining flows following the crest form the recession limb of the hydrograph. In general, surface inflow ceases and water is provided by basin storages. The groundwater contribution strongly influences the character of the recession flow, which is described mathematically by exponential, regression, and wave transform equations that result in decreasing flow to near the pre-event value (Sujono et al., 2004). The exponential function is the most commonly used form. Base flow is conventionally identified on the hydrograph by a line extending from the foot of the rising limb to the point of intersection on the recession limb (Mosley and McKerchar, 1993).

6.9.2 Hydrograph insights

Hydroclimatic relationships are evident when the time and space scales of hydrographs are expanded. Discharge time-series for a number of years

Wetland Hydrograph

Year

Fig. 6.3. Annual mean streamflow for the Mississippi River at St. Louis, Missouri (39° N), for 1934-2004. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

emphasize information on the year-to-year variations in both peak discharge and low flows. Figure 6.3 displays annual mean streamflow for the Mississippi River at St. Louis, Missouri (39° N). The drainage area above this gauge is 1812 200 km2, and the average annual discharge is 5386 m3s-1. The lowest annual mean streamflow is 2233 m3 s-1 in 1934 and the highest is 12 435 m3 s-1 in 1993. Other prominent wet years are 1951 and 1973, and dry years are 1934, 1940, and 1956. Eight of the nine years when streamflow is less than one standard deviation below the mean occur prior to 1967. In contrast, eight of the nine years when streamflow is greater than one standard deviation above the mean occur after 1967. Overall, the 82-year streamflow time-series indicates a slight increasing trend.

Seasonal cycles imbedded in annual streamflow data are evident in the greater variability of monthly mean streamflow shown in Figure 6.4. However, the increasing streamflow trend suggested in Figure 6.3 is more difficult to identify in the mean monthly data. Extreme wet events in 1951, 1973, and 1993 are amplified in Figure 6.4 by the magnitude of individual high stream-flow months. The maximum monthly value is 22 905 m3 s-1 for July 1993, but six additional wet years are evident when monthly streamflow exceeds 15 000 m3 s-1. Dry events are seen in the monthly data in terms of the duration of low flows and the absolute streamflow values. The 1934 dry event results from persistently low monthly streamflow and not from a single month of extremely low flow. The lowest monthly flow (888 m3 s-1) in the record occurs in January 1940 (see Fig. 6.4) which is the second driest year (see Fig. 6.3). The second lowest monthly value (1360 m3 s-1) occurs in December 1989, but 1989 is only a moderately dry year as shown by the annual mean streamflow. A final

25 000

3 20 000 T

E 15 000

3 20 000 T

E 15 000

1933 1943 1953 1963 1973 1983 1993 2003 Year

Fig. 6.4. Monthly mean streamflow for the Mississippi River at St. Louis, Missouri (39° N), for April 1933 to June 2005. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

1933 1943 1953 1963 1973 1983 1993 2003 Year

Fig. 6.4. Monthly mean streamflow for the Mississippi River at St. Louis, Missouri (39° N), for April 1933 to June 2005. (Data courtesy of the U.S. Geological Survey from their website at http://waterdata.usgs.gov/nwis/.)

feature of Figure 6.4 is that the slightly undulating pattern of minimum monthly discharge delineates a base flow component not evident in the annual mean streamflow.

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