Threshold discharge area " means an on-site area draining to a single natural discharge location or multiple natural discharge locations that combine within. The relationship between discharge and drainage area also . time work against the latter, resulting in a natural limit to the values of c. S:\CSC\Document_Warehouse\FORMS\Current Forms\NPDES Updates\ Threshold Discharge caztuning.info EFF: Figure TB Threshold.
The stream power law is used to model aggradation and incision within a fluvial system and is generally written in the form: Validations of this model have been reported from several drainages in temperate climatic regimes, including the California Coast Range Snyder et al.
Field-tests in arid and semiarid environments are less common, although data reported from five drainages in New Mexico suggest that the stream power law does not always agree with observed rates of fluvial incision Mitchell, The substitution of contributing area A for discharge Q is a common assumption in many fluvial models, including the stream power laws.
When contributing area A is substituted for discharge Q in the stream power law, it results in the new equation: It is commonly held that a power function defines the empirical relation between discharge and area in the form: Appropriate values of c generally range from 0.
For example, a plot of mean annual discharge versus contributing drainage area for several basins in western Washington indicates an appropriate c value of 0.
Plot showing drainage area vs. The validity of assuming a linear or nearly linear relation between discharge and drainage area may not be valid in all environments, especially in semiarid landscapes where there is not a large influx of water from local catchments.
This may be a common source of error in fluvial models attempting to extend hydrologic relations beyond the bounds in which they were previously defined.
This study attempts to quantify the relation between discharge and area in the semiarid landscape of the Colorado Plateau using a GIS to compare contributing drainage area to observed discharge from several basins. Objectives The goal of this study is to test whether the discharge-area relation in the semiarid environment of the Colorado Plateau is linear as shown to be true in the temperate climatic zones of the Pacific Northwest.
This study will attempt to answer 2 questions: A quantitative analysis of this nature will provide a basis upon which further testing of the discharge-area relation in semiarid environments can completed and will ultimately provide us with a better understanding of the fluvial systems within the Colorado Plateau. Study Area Six drainage basins were selected from the Colorado Plateau for analysis: Digital elevation models were compiled from the U.
The models were compiled into a geographic information system Arc v. The relief of the watershed is m, and the bedrock lithology is primarily extrusive igneous rocks. There are five gauging stations for both mean and peak annual discharges, with three stations the minimum number for the linear regressions operating since for peak and for mean discharges.
Although there are not large dams or reservoirs on the John Day River, there are some small diversions, mostly for irrigation Herrett et al. The km-long Salmon River, Idaho Fig.
The watershed has m of relief and contains mostly intrusive and extrusive igneous bedrock lithologies. Five gauging stations were used for linear regression; the records begin in for both mean and peak annual discharges.
The Salmon River is the least human-affected watershed in this study, with few dams and little consumptive water withdrawals Fig. The Wabash River watershed covers 85, km2 and is almost entirely within Indiana Fig. The watershed averages cm of annual precipitation, contains mostly siliciclastic and carbonate bedrock lithologies, and has m of relief Fig.
The main channel is km long and has 14 USGS gauges that are used in this study; records date to for peak annual and for mean annual discharges. The watershed averages cm of annual precipitation, has limestone and shale bedrock lithologies, and has m of relief. There are four gauging stations on the Greenbrier, the earliest from The Yellowstone River Fig. The watershed averages 42 cm of precipitation annually, iskm2, contains mostly sedimentary and extrusive and intrusive igneous lithologies, and has relief of m.
Eight gauging stations recorded peak annual and five stations recorded mean annual discharges on the Yellowstone. The Yellowstone watershed has some unique characteristics that may influence the discharge-drainage area relationship.
First, the tributary of the Wind-Bighorn River actually has more upstream drainage area 58, km2 than the main-stem Yellowstone 30, km2 when the two join. Second, there are more reservoirs on the tributaries than in the other five watersheds, which most likely influence the discharge characteristics of the main stem.
RESULTS The physical characteristics of each watershed include area, length of the main trunk channel, watershed relief, precipitation, number of USGS gauging stations used in this study, and the length of discharge records Figs.
The watersheds vary from small and in a humid-temperate setting Greenbrier, Fig. The scaling factor c exhibits both secular and nonsecular trends over the length of record. The rivers can be categorized based on the long-term averages and trends of their c values. For the Yellowstone the peak c approaches maximum values of 0. As a river gains drainage area with additional downstream tributaries, those tributaries also contribute discharges that are proportional to the added drainage area.
The Yellowstone River, where c is closer to 0. Discharges Scaling from 0. Nevertheless, our study and others show that some watersheds as large and complex as the John Day River in Oregon and the Susquehanna River in Pennsylvania Slingerland et al.
The John Day River is interesting from the perspective that it has both higher precipitation and a greater concentration of dams in the upstream areas of the watershed.
In terms of the effect on the c value, the precipitation gradient and the upstream distribution of dams may counteract each other, the former acting to increase peak and mean annual discharges and the latter decreasing discharges. A river with a c value of 0. There are several ways to interpret these results. One would be to argue for a downstream influent trunk channel, but this is unlikely for all of these rivers except the Greenbrier, which is underlain by carbonate.
Other watershed variables such as slope, elevation, and evapotranspiration may account for scaling values of 0. Higher slopes in headwater regions increase the amount of discharge generated by a unit area of drainage area. The headwaters of a watershed are commonly steeper with many exceptionsthe steep slopes generating larger amounts of overland flow and runoff with less water infiltrating into the ground than more gentle slopes Knighton, Higher elevations also create orographic effects that tend to increase precipitation amounts in the headwater of a watershed, creating more runoff Dunne and Leopold, ; Smith, More runoff will consequently generate higher discharges, especially peak annual discharge.
In addition, the mean annual discharge will also be increased because it compiles all flows, including both higher and lower, into its value.
This unequal distribution of precipitation influences a river's discharge as well as the river's long profile Roe et al. Higher elevations also create orographic effects that tend to increase precipitation amounts in the headwater of a watershed Smith, The orographic effect on weather systems and subsequent increased precipitation can spatially skew the total annual precipitation toward the headwaters, decreasing the value of c Dunne and Leopold, Higher elevations in a watershed also tend to shift precipitation toward snowfall rather than rainfall.
As a snowpack grows over a winter season it stores water that, when quickly melted in spring, releases months of precipitation over days or weeks, thereby increasing the peak annual discharge. While the melting of winter snow does not dominate the hydrology of all of the studied watersheds, it is a critical factor for generating peak discharges in some watersheds, as discussed here for the Yellowstone watershed.
Conversely, these same mechanisms that act to relatively increase the upstream discharge decrease the downstream discharge. Downstream areas of a watershed are generally less steep and have less relief, causing the above-mentioned processes to work in reverse and lessen the amount of runoff delivered to the river channel.
Furthermore, the gentler slopes produce lower flow velocities and higher transit times for the runoff to the main channel that would tend to spread a peak discharge out over time, decreasing the maximum value of the peak discharge.
Gentler slopes and lower elevations in the downstream part of a watershed would also drive higher evapotranspiration ET rates. ET is inversely correlated with elevation in both arid Shevenell, and humid environments Kovnee, ; Swift et al. These higher rates of ET would transfer larger amounts of precipitation back to the atmosphere, decreasing the amount of runoff generated. Lower-elevation areas are generally warmer, which would also mean a smaller or nonexistent winter snowpack, resulting in a smaller available reservoir to melt and produce runoff and peak discharges.
Collectively, slope, elevation, and evapotranspiration work to relatively increase the discharge generated per unit area of watershed in the drainage headwaters while concurrently acting to decrease discharge per unit area down basin. The c values of the Wabash River are the lowest of these four rivers. The long-term average for both the peak 0. The Wabash River watershed is unique in this set of four rivers: However, it remains unclear why the glaciation or glacial drift cover would cause the Wabash to systematically have lower c values.
There may be other factors than the previously invoked relief and precipitation gradient to lower the c value here, including the travel time of water in a watershed with a glacially caused poorly integrated channel network Gupta and Waymire, The Yellowstone watershed would also have the same processes slope, elevation, ET operating within it that serve to reduce the value of c by proportionally increasing the discharges upstream and decreasing them downstream. However, there are temporal and spatial changes in the hydrologic characteristics of the Yellowstone watershed that uniquely affect the scaling of the discharge and act to decrease c below 0.
Discharge-Area relations from Selected Drainages on the Colorado Plateau:
The spatial component is the variation in precipitation across the watershed, whereas the temporal aspects are the changes in precipitation, fire frequency, and land use over the 90 year length of the discharge record.
Both of these influences change the scaling in a distinct way when compared to the other rivers in this study. The precipitation gradient in the Yellowstone watershed is strongly oriented from southwest to northeast, roughly parallel to the major axis of the watershed Fig. The higher precipitation in the headwaters produces more runoff per unit drainage area and consequently higher discharges in the headwaters; the opposite occurs in the downstream, drier climate section of the river Zelt et al.
This trend has also been observed for other rivers in semiarid conditions Gupta and Waymire, The relationship between discharge and drainage area also becomes more nonlinear for the Yellowstone peak annual discharges with increasing drainage area Fig. Such a trend suggests that a power law relationship equation 1 may not be appropriate for semiarid watersheds. At this point in the season the lower elevations downstream have lost all or a majority of their winter snowpack, creating a gradient in the amount of runoff generated by snowmelt at the time of the annual peak discharge Animation 1 1.
The snowier, higher-elevation headwaters generate more runoff than in the lower, warmer, and drier downstream sections of the watershed, creating a peak discharge that slowly increases in size moving downstream, resulting in the low c values for the discharges Arora and Boer, The annual mean and peak discharges are also affected by human influences within the Yellowstone watershed.
The construction of several large reservoirs has also affected the discharge characteristics of the Yellowstone watershed. The three largest reservoirs in the watershed Boysen, builtcapacity 0.
These reservoirs, through consumptive use and storage capacity, would decrease the mean and peak annual discharges that these downstream tributaries contribute to the Yellowstone River. The concentration of these human influences on the downstream tributaries decreases the downstream peak and mean discharges for the Yellowstone River, which subsequently decreases the c values for both the peak and mean annual discharges.
There is also a secular decreasing trend in the c values for the annual discharges for Yellowstone River over the past 75 years, especially for the peak discharges.
This secular decrease in c values corresponds with climatic and land use changes in the Yellowstone watershed. Climatic data that overlap with the discharge record during the twentieth century indicate that the headwaters became progressively warmer during the summer months and that the January—June precipitation there decreased Balling et al. The average peak annual discharge for the Yellowstone River has decreased at the largest drainage area Zelt et al.