Interception and Depression Storage 2011

INTERCEPTION ANDINTERCEPTION ANDDEPRESSION STORAGEDEPRESSION STORAGE

• Water BudgetWater Budget:: Recall a water budget is an Recall a water budget is an accounting of the volume of flow rate of accounting of the volume of flow rate of water in all possible locations.water in all possible locations.

• You need to focus interest on a region and You need to focus interest on a region and determine how the quantity of water in the determine how the quantity of water in the region can be changed.region can be changed.

• The regional boundaries have to be The regional boundaries have to be determined across which water may move determined across which water may move or be confined. You also need to specify a or be confined. You also need to specify a time period.time period.

Depression StorageDepression Storage

• So what is So what is storage depressionstorage depression??

• It is the water retained in puddles, It is the water retained in puddles, ditches, and other depressions in the ditches, and other depressions in the surface of the ground.surface of the ground.

InterceptionInterception

• As water reaches the surface in various As water reaches the surface in various forms of precipitation, it is intercepted forms of precipitation, it is intercepted by plants or falls directly to the surface.by plants or falls directly to the surface.

• Precipitation that collects on the leaves Precipitation that collects on the leaves or stems of plants is known as or stems of plants is known as interception.interception.

• The amount of water intercepted by a The amount of water intercepted by a plant largely depends on the plant form plant largely depends on the plant form (type).(type).

Interception (cont’d)Interception (cont’d)

• Interception (or canopy interception) typically Interception (or canopy interception) typically refers to precipitation that does not reach the refers to precipitation that does not reach the soil, but is instead intercepted by the leaves soil, but is instead intercepted by the leaves and branches of plants.and branches of plants.

• It occurs in the canopy and in the forest litter.It occurs in the canopy and in the forest litter.

• Because of evaporation, interception of liquid Because of evaporation, interception of liquid water generally tends to loss of that water generally tends to loss of that precipitation for the drainage basin, except precipitation for the drainage basin, except for cases such as fog interception.for cases such as fog interception.


• Intercepted snowfall does not result in any Intercepted snowfall does not result in any notable amount of evaporation, and most of notable amount of evaporation, and most of the snow falls off the tree by wind or melt.the snow falls off the tree by wind or melt.

• However, intercepted snow can more easily However, intercepted snow can more easily drift with the wind, out of the watershed.drift with the wind, out of the watershed.

• Conifers have greater interception than Conifers have greater interception than hardwoods. Their needs gives them more hardwoods. Their needs gives them more surface area for droplets to adhere to, and they surface area for droplets to adhere to, and they have foliage in spring and fall; therefore, have foliage in spring and fall; therefore, interception also depends on the type of interception also depends on the type of vegetation in the wooded area.vegetation in the wooded area.


• Water is held on the leaf surface until it Water is held on the leaf surface until it either drips off as through fall or trickles either drips off as through fall or trickles down the leaf finally reaching the down the leaf finally reaching the ground as stem flow.ground as stem flow.

• Interception of falling rain buffers the Interception of falling rain buffers the surface against erosion.surface against erosion.

• Coniferous trees tend to intercept more Coniferous trees tend to intercept more water than deciduous trees on an water than deciduous trees on an annual basis because deciduous trees annual basis because deciduous trees drop their leaves for a period of time.drop their leaves for a period of time.


• Upon reaching the ground, some water Upon reaching the ground, some water infiltrates into the soil, possible infiltrates into the soil, possible percolating down to the groundwater percolating down to the groundwater table, or it may run across the surface as table, or it may run across the surface as runoffrunoff..

• InfiltrationInfiltration refers to water that refers to water that penetrates into the surface of the soil.penetrates into the surface of the soil.

• Infiltration is controlled by soil texture, Infiltration is controlled by soil texture, soil structure, vegetation, and soil soil structure, vegetation, and soil moisture content.moisture content.


• High infiltration rates occur in dry soils, with High infiltration rates occur in dry soils, with infiltration slowing as the soil becomes wet.infiltration slowing as the soil becomes wet.

• Coarse textured soils with large well-Coarse textured soils with large well-connected pore spaces tend to have higher connected pore spaces tend to have higher infiltration rates than fine textured soils.infiltration rates than fine textured soils.

• However, coarse textured soils fill more However, coarse textured soils fill more quickly than fine textured soils due to a quickly than fine textured soils due to a smaller amount of total pore space in a unit smaller amount of total pore space in a unit volume of soil.volume of soil.

• Runoff is generated quicker than one might Runoff is generated quicker than one might have with a finer textured soil.have with a finer textured soil.


• Vegetation also affects infiltration.Vegetation also affects infiltration.

• For instance, infiltration is higher for soils For instance, infiltration is higher for soils under forest vegetation than bare soils.under forest vegetation than bare soils.

• Tree roots loosen and provide conduits Tree roots loosen and provide conduits through which water can enter the soil.through which water can enter the soil.

• Foliage and surface litter reduce the Foliage and surface litter reduce the impact of falling rain keeping soil impact of falling rain keeping soil passages from becoming sealed.passages from becoming sealed.


• An article has been provided to you as an An article has been provided to you as an e-mail attachment addressing the impact e-mail attachment addressing the impact of interception losses on the water of interception losses on the water balances in forested mountain ranges.balances in forested mountain ranges.

Significance of Interception Significance of Interception and Interception Lossand Interception Loss

• By how much does interception reduce inputs By how much does interception reduce inputs into the watershed hydrological cycle?into the watershed hydrological cycle?

• Not much, because interception is offset by Not much, because interception is offset by decreased transpiration.decreased transpiration.

• The amount of solar energy for evaporation The amount of solar energy for evaporation and transpiration is constant for any time and and transpiration is constant for any time and place, so evaporation of intercepted moisture place, so evaporation of intercepted moisture simply replaces the evapotranspiration that simply replaces the evapotranspiration that would have occurred in the absence of would have occurred in the absence of precipitation.precipitation.

Significance of Interception and Significance of Interception and Interception Loss (cont’d)Interception Loss (cont’d)

• Experiments have shown decreased Experiments have shown decreased transpira-tion from wet foliage. Therefore:transpira-tion from wet foliage. Therefore:

• Interception loss represents a net los of Interception loss represents a net los of water.water.

– The rate of evaporation of intercepted water exceeds The rate of evaporation of intercepted water exceeds rates of transpiration, because transpiration is limited rates of transpiration, because transpiration is limited by soil moisture conditions and the rate at which by soil moisture conditions and the rate at which water is transferred to leaves and interception loss water is transferred to leaves and interception loss can occur from dead (nontranspiring) vegetation.can occur from dead (nontranspiring) vegetation.

– Evaporation of intercepted water cools the plant, Evaporation of intercepted water cools the plant, suppress-ing transpiration and causes a heat flux suppress-ing transpiration and causes a heat flux from the air which contributes to further evaporation.from the air which contributes to further evaporation.

Depression Storage (cont’d)Depression Storage (cont’d)

• Consider an example of a water budget Consider an example of a water budget from a parking lot.from a parking lot.

• First, you must determine the surface First, you must determine the surface boundaries of the parking lot that boundaries of the parking lot that contribute water to a collection point.contribute water to a collection point.

• The boundary may be defined on the The boundary may be defined on the surface as an imaginary line bordering surface as an imaginary line bordering the surface area from which the surface area from which precipitation may be accumulated and precipitation may be accumulated and routed to some control point.routed to some control point.


• At the control point, a decision can be At the control point, a decision can be made on the volume and rate of dis-made on the volume and rate of dis-charge. As an example, the control charge. As an example, the control point can be an inlet grate at the point can be an inlet grate at the lowest elevation of the parking lot. If lowest elevation of the parking lot. If the parking lot is constructed with the parking lot is constructed with curbs to contain the water on site, curbs to contain the water on site, the boundary is easily determined. the boundary is easily determined.


• A water budget (balance) for a parking A water budget (balance) for a parking lot is useful; to show our mass balance lot is useful; to show our mass balance principles.principles.

• Let’s first assume that all the Let’s first assume that all the precipitation remains on the surface of precipitation remains on the surface of the parking lot, and is routed to a the parking lot, and is routed to a control point. This water volume is given control point. This water volume is given the term “the term “rainfall excessrainfall excess ”. The rate at ”. The rate at which rainfall excess appears over time which rainfall excess appears over time at a discharge (control) point is called at a discharge (control) point is called runoffrunoff. .


• Photographs shown on the following Photographs shown on the following slide depict storage depression slide depict storage depression during a storm (in the faculty parking during a storm (in the faculty parking lot near Hoehn Engineering Building).lot near Hoehn Engineering Building).

Storage Depression in Parking Storage Depression in Parking Lot near Hoehn Engineering Lot near Hoehn Engineering

BuildingBuilding


• A water budget (water balance) can then A water budget (water balance) can then be written in volume terms:be written in volume terms:

Inputs Inputs Outputs Outputs Accumulation = 0 Accumulation = 0 {1}{1}

• If water is not stored on the parking lot If water is not stored on the parking lot surface, the accumulation term is zero surface, the accumulation term is zero with input equal to precipitation, and with input equal to precipitation, and output equal to rainfall excess, or:output equal to rainfall excess, or:Rainfall excess = Volume of precipitationRainfall excess = Volume of precipitation {2}{2}


• If precipitation is abstracted by If precipitation is abstracted by depression storage in the parking lot, depression storage in the parking lot, then the water balance in volume then the water balance in volume terms is altered as shown below:terms is altered as shown below:

Rainfall excess =Rainfall excess =Precipitation Precipitation Depression storage Depression storage {3}{3}


• The complexity of the water budget The complexity of the water budget (water balance) depends on the (water balance) depends on the physical system and the ultimate use physical system and the ultimate use of the balance.of the balance.

• Water budgets for large areas are Water budgets for large areas are very complex with many parameters. very complex with many parameters.


• All of the rain that falls on the impervious All of the rain that falls on the impervious portion of the basin is considered rainfall excess.portion of the basin is considered rainfall excess.

• The equation for impervious area runoff can be The equation for impervious area runoff can be written to account for depression storage or written to account for depression storage or evaporation (if applicable), using the following evaporation (if applicable), using the following equation:equation:

RR ((II ) = ) = dd' [' [PP ((t) – t) – DD ((t)] t)] EVPEVP {4} {4}

where:where: RR ((II ) = impervious area runoff, (depth)) = impervious area runoff, (depth)dd‘ ‘ == impervious portion of the impervious portion of the

drainage basin, drainage basin, (fraction) (fraction)PP ((t) = rainfall depth during time increment t) = rainfall depth during time increment t, t,

(depth) (depth)DD ((t) = depression storage during time t) = depression storage during time

increment increment t, t, (depth) (depth)EVP EVP == portion of rainfall excess which is portion of rainfall excess which is

evaporated evaporated before runoff (fraction) before runoff (fraction)


• The nature of the depression, as well as their The nature of the depression, as well as their size, is largely dependent on the original land size, is largely dependent on the original land form and local land-use practices.form and local land-use practices.

• Ultimately, all the water stored in depressions Ultimately, all the water stored in depressions will either evaporate or seep into the will either evaporate or seep into the complex. Thus, depressions vary widely in complex. Thus, depressions vary widely in size, degree of interconnection, and size, degree of interconnection, and contributing drainage area.contributing drainage area.

• The amount of precipitation going into The amount of precipitation going into depression storage will approach zero, given depression storage will approach zero, given that there is a large enough volume of that there is a large enough volume of precipitation to exceed other losses to surface precipitation to exceed other losses to surface storage, such as infiltration and evaporation. storage, such as infiltration and evaporation.


• The volume of water stored by surface The volume of water stored by surface depressions at any given time can be depressions at any given time can be approximated using the following relationship:approximated using the following relationship:

V V = = SS dd (1 – e (1 – e kk P P ee)) {5} {5}

where:where: V V = volume actually in storage at the = volume actually in storage at the time of time of interest interest

SS d d = maximum storage capacity of the = maximum storage capacity of the depression depression

PP e e = the rainfall excess (gross rainfall = the rainfall excess (gross rainfall evaporation, interception, and evaporation, interception, and

infiltration)infiltration)

k k = constant equivalent to 1/= constant equivalent to 1/SS dd


• The value of The value of kk can be determined by can be determined by considering the case where considering the case where PP ee ~ 0, so that all ~ 0, so that all the water fills the depressions and dthe water fills the depressions and dVV /d/dPP ee will will equal 1.0. With this case, it requires that equal 1.0. With this case, it requires that kk = = 1/1/SS dd..

• Estimates of Estimates of SS dd can be obtained by making can be obtained by making sample field measurements for the area under sample field measurements for the area under consideration. Combining that data with esti-consideration. Combining that data with esti-mates of mates of PP ee allows you to estimate the volume allows you to estimate the volume VV..

• The manner in which The manner in which VV varies with time must varies with time must either be determined or assumed if depression either be determined or assumed if depression storage losses are to be abstracted from the storage losses are to be abstracted from the gross rainfall input.gross rainfall input.


• A common assumption invoked for dA common assumption invoked for dVV/d/dtt is is that all depressions must be full before that all depressions must be full before overland flow supply begins.overland flow supply begins.

• This usually does not agree with reality, This usually does not agree with reality, unless the largest depressions are graded unless the largest depressions are graded with the largest ones occurring with the largest ones occurring downstream. If abstrac-tion storage were downstream. If abstrac-tion storage were abstracted in this way, the total volume abstracted in this way, the total volume would be deducted from the initial storm would be deducted from the initial storm such as depicted in Figure 5.2 of your text.such as depicted in Figure 5.2 of your text.

Figure 5.2. Simple depression storage abstraction scheme.


• Note that the overland flow supply rate Note that the overland flow supply rate plus the depression storage intensity plus the depression storage intensity equals equals ii – – ff ::where:where: i i = rainfall intensity reaching the = rainfall intensity reaching the ground ground surface surface

f f = infiltration rate= infiltration rate

• Then the ratio of the overland flow Then the ratio of the overland flow supply to the overland flow plus supply to the overland flow plus depression storage supply is given by depression storage supply is given by the following relationship:the following relationship:

= 1 – e= 1 – e k P k P ee {6}{6}

ii – – ff


• This can be rewritten as:This can be rewritten as: = = ii – – ff – – vv

{7}{7}ii – – ff i i – – ff

where where vv is the derivative of Equation {5} is the derivative of Equation {5} with respect to time.with respect to time.

• Thus:Thus: v v = = d d [ [SS dd (1 – e (1 – e – –k P k P ee)])] ddtt

== ((SS dd kk e e – –k P k P ee) ) ddPPee {8} {8} ddtt

and:and: k k == 1_ 1_ SS dd


• Thus:Thus: v v == ee – –k P k P ee ddPP ee {9} {9}

ddtt

• Because the excess precipitation Because the excess precipitation PP ee equals equals the gross rainfall minus the infiltrated water, the gross rainfall minus the infiltrated water, and noting that the derivative with respect to and noting that the derivative with respect to time can be approximated by the equivalent time can be approximated by the equivalent intensity (intensity (ii – – ff ), the intensity of depression ), the intensity of depression storage then becomes:storage then becomes:

v v == ee – –k P k P ee ( (ii – – ff )){10}{10}


• Then, inserting this into Equation Then, inserting this into Equation {7}, we obtain:{7}, we obtain:

= = ii – – ff – – vvii – – ff i i – – ff

= = ((ii – – ff ) – (i – ) – (i – ff ) e) e – –k P k P ee

ii – – ff

= = ((ii – – ff ) (1 – e) (1 – e – –k P ek P e)) = 1 – e = 1 – e – – k P k P ee {11}{11} ii – – ff


• Figure 5.3 in your text shows a plot of this Figure 5.3 in your text shows a plot of this function versus the mass overland flow function versus the mass overland flow and the depression storage supply (and the depression storage supply (PP – – FF ), ), where where FF is the accumulated mass is the accumulated mass infiltration and infiltration and PP represents the gross represents the gross precipitation.precipitation.

• This figures indicates that as rainfall This figures indicates that as rainfall accumulation increases, the ratio of accumulation increases, the ratio of /(/(ii – –ff ) ) in its limit approaches a value of 1.0. in its limit approaches a value of 1.0.

Figure 5.3. Depth distribution curve

of depression storage. Enter

graph from top, read down to

selected curve, and project right or left as desired.


• The overland flow supply rate The overland flow supply rate represents the amount of gross represents the amount of gross precipitation that can be delivered precipitation that can be delivered overland after infiltration and overland after infiltration and depression storage losses have been depression storage losses have been accounted for (i.e., deducted from the accounted for (i.e., deducted from the water balance).water balance).

• The overland flow eventually becomes The overland flow eventually becomes the runoff that produces a stream flow the runoff that produces a stream flow hydro-graph.hydro-graph.


• Generally, depression storage Generally, depression storage deductions are estimated during the deductions are estimated during the first part of a storm (early in the first part of a storm (early in the storm); the amount subtracted is a storm); the amount subtracted is a function of the site topography, ground function of the site topography, ground cover, and the extent and type of land cover, and the extent and type of land development.development.

• During major storm events, depression During major storm events, depression storage deductions are usually storage deductions are usually considered as being negligible. considered as being negligible.

InterceptionInterception

• Part of the storm precipitation that Part of the storm precipitation that occurs is intercepted by vegetation occurs is intercepted by vegetation and other forms of cover on the and other forms of cover on the drainage area.drainage area.

• Interception can be defined as that Interception can be defined as that segment of the gross precipitation segment of the gross precipitation input which wets and adheres to the input which wets and adheres to the ground objects until it is returned to ground objects until it is returned to the atmosphere through evaporation.the atmosphere through evaporation.


• Precipitation striking vegetation may be Precipitation striking vegetation may be retained on leaves or blades of grass, retained on leaves or blades of grass, flow down the stems of plants and flow down the stems of plants and become stem flow, or fall off the leaves become stem flow, or fall off the leaves to become part of the throughfall. The to become part of the throughfall. The modifying effect that a forest canopy modifying effect that a forest canopy can have on rainfall intensity at the can have on rainfall intensity at the ground (the throughfall) can be put to ground (the throughfall) can be put to practical use in watershed practical use in watershed management schemes.management schemes.


• The amount of water intercepted is a The amount of water intercepted is a function of the following:function of the following:

The storm character;The storm character;

The species, age, and density of The species, age, and density of prevailing plants and trees; andprevailing plants and trees; and

The season of the year.The season of the year.


• Typically, 10Typically, 1020% of the precipitation 20% of the precipitation that falls during a growing season is that falls during a growing season is inter-cepted and returned to the inter-cepted and returned to the hydrologic cycle via evaporation. Your hydrologic cycle via evaporation. Your text cites various percentages of text cites various percentages of interception for various plant types and interception for various plant types and seasons.seasons.

• Most interception loss develops during Most interception loss develops during the initial storm period, and the rate of the initial storm period, and the rate of interception quickly approaches zero interception quickly approaches zero thereafter.thereafter.


• Potential storm interception losses can be Potential storm interception losses can be estimated using the following equation:estimated using the following equation:

LL i i = = S S + + K EK E tt {12}{12}

where: where: LL ii = volume of water intercepted, (in)= volume of water intercepted, (in)

SS = interception storage that = interception storage that will be retained on will be retained on the the foliage against the forces of wind and gravity foliage against the forces of wind and gravity [the [the value typically varies between 0.01 and 0.05 value typically varies between 0.01 and 0.05 in.)in.)

KK = ratio of surface area of = ratio of surface area of intercepting intercepting leaves leaves to to horizontal projection of this area; horizontal projection of this area;

EE = amount of water evaporated per hour during = amount of water evaporated per hour during the the precipitation period, (in) precipitation period, (in)

tt = time, (hr)= time, (hr)


• Total interception by an individual plant is Total interception by an individual plant is directly related to the amount of foliage directly related to the amount of foliage and its character and orientation.and its character and orientation.

• Table 5.2 illustrates the mean Table 5.2 illustrates the mean interception for various crops and interception for various crops and grasses.grasses.

Table 5.2. Observed percentages of interception by various crops and

grasses.


• The above equation can be used to estimate The above equation can be used to estimate total interception losses.total interception losses.

• For analyses of individual storms, it is also For analyses of individual storms, it is also necessary to know or assume the necessary to know or assume the distribution of this abstraction.distribution of this abstraction.

• A general equation for estimating such A general equation for estimating such losses is not available, since most studies losses is not available, since most studies have been related to particular species or have been related to particular species or experimental plots strongly associated with experimental plots strongly associated with a given locality. a given locality.


• In addition, the loss function varies with the In addition, the loss function varies with the storm’s character.storm’s character.

• If adequate experimental data are available, If adequate experimental data are available, the nature of the variance of interception the nature of the variance of interception versus time may be inferred.versus time may be inferred.

• Otherwise, common practice is to deduct the Otherwise, common practice is to deduct the estimated volume entirely from the initial estimated volume entirely from the initial period of the storm (“period of the storm (“initial abstractioninitial abstraction”). ”).


• The above equation was developed with The above equation was developed with the assumption that rainfall is sufficient the assumption that rainfall is sufficient to fully satisfy the storage term to fully satisfy the storage term SS..

• The next equation then accounts for the The next equation then accounts for the rainfall amount:rainfall amount:

L L I I == SS (1 – e (1 – e – P/S – P/S) + ) + K EK E t t {13}{13}

where where PP is the precipitation (rainfall), is the precipitation (rainfall), and the other terms are as described in and the other terms are as described in Equation {12}.Equation {12}.


• Precipitation type, rainfall intensity and Precipitation type, rainfall intensity and duration, wind, and atmospheric duration, wind, and atmospheric conditions affecting evaporation all are conditions affecting evaporation all are factors that affect interception losses.factors that affect interception losses.

• Estimates of loss to gross precipitation Estimates of loss to gross precipitation may be significant in annual or log-term may be significant in annual or log-term modeling.modeling.

• For heavy rainfalls during individual For heavy rainfalls during individual storm events, accounting for interception storm events, accounting for interception may have minimal (negligible) may have minimal (negligible) significance in the overall water balance.significance in the overall water balance.


• You should also note that there can You should also note that there can be considerable variation in be considerable variation in interception losses areally.interception losses areally.

• Common practice is to subtract the Common practice is to subtract the estimated volume from the initial estimated volume from the initial period of the storm (commonly period of the storm (commonly termed termed initial abstractioninitial abstraction).).

Interception ExampleInterception Example

• ExampleExample: : The following equations derived The following equations derived by Horton were derived for interception by by Horton were derived for interception by ash and oak trees:ash and oak trees:

For ash trees: For ash trees: LL ii = 0.015 + 0.23 = 0.015 + 0.23 PP For oak trees: For oak trees: LL i i = 0.03 + 0.22 = 0.03 + 0.22 PP

where where LL ii is the volume of water intercepted is the volume of water intercepted (in inches) and (in inches) and PP is the precipitation (in is the precipitation (in inches). Estimate the interception loss inches). Estimate the interception loss beneath trees during a storm having a total beneath trees during a storm having a total precipitation of 1.5 inches.precipitation of 1.5 inches.

Interception Example (cont’d)Interception Example (cont’d)

• SolutionSolution::

1.1. For ash trees:For ash trees:

LL i i = 0.015 + 0.23 = 0.015 + 0.23 PP

= 0.015 + 0.23 (1.5 in) = (0.015 + 0.345) in= 0.015 + 0.23 (1.5 in) = (0.015 + 0.345) in

= 0.360 in= 0.360 in

2.2. For oak trees:For oak trees:

LL i i = 0.03 += 0.03 + 0.22 0.22 PP

= 0.03 += 0.03 + 0.22 (1.5 in) 0.22 (1.5 in) = (0.03 + = (0.03 + 0.33) in0.33) in

= 0.36 in= 0.36 in

Interception ExampleInterception Example

3.3. Using the curve for mean percentage Using the curve for mean percentage interception by trees, the percentage interception by trees, the percentage interception ~24%. Thus:interception ~24%. Thus:

0.24 x 1.5 in = 0.36 in0.24 x 1.5 in = 0.36 in

• For this example, the storm loss For this example, the storm loss beneath the trees resulting from beneath the trees resulting from interception would be ~0.36 inches.interception would be ~0.36 inches.

Depression Storage ExampleDepression Storage Example

• Example (Problem 5.1 of your text)Example (Problem 5.1 of your text):: Using the precipitation input of Figure Using the precipitation input of Figure 5.2 of your text, estimate the volume 5.2 of your text, estimate the volume of depression storage for a 3.0-acre of depression storage for a 3.0-acre paved drainage area. State the paved drainage area. State the volume in ftvolume in ft 3 3 and m and m 3 3. Convert it to an . Convert it to an equivalent depth over the area in equivalent depth over the area in inches and centimeters.inches and centimeters.

Figure 5.2. Simple depression Figure 5.2. Simple depression storage abstraction scheme.storage abstraction scheme.


SolutionSolution::

• Use a basis of 1.0 minute (based on the Use a basis of 1.0 minute (based on the representation of Figure 5.2).representation of Figure 5.2).

• Using Figure 5.2, estimate the initial Using Figure 5.2, estimate the initial depression storage abstraction:depression storage abstraction:

V V ~ [0.5 + 1.75] ft~ [0.5 + 1.75] ft 3 3/sec-acre/sec-acre

== 2.25 ft2.25 ft 3 3/sec-acre/sec-acre

• The time scale on Figure 5.2 is in minutes; The time scale on Figure 5.2 is in minutes; thus:thus:

V V ~~ 2.25 ft 2.25 ft 3 3 60 sec60 sec = = 135.0 ft 135.0 ft 3 3 sec-acresec-acre min min min-acre min-acre

Depression Storage Example Depression Storage Example (cont’d)(cont’d)

• We are given the paved drainage area is 3.0 We are given the paved drainage area is 3.0 acres, so:acres, so:

V V == 135.0 ft135.0 ft 3 3 (3.0 acres) (3.0 acres) = 405.0 ft= 405.0 ft 3 3

acreacre

= (405.0 ft= (405.0 ft 3 3) ) 12 in12 in 3 3 2.54 cm2.54 cm 3 3 1.0 L 1.0 L 1.0 m1.0 m 3 3 ft ft in in 1000 cm1000 cm 3 3 1000 1000

== (405.0) (1728) (16.3871) (10(405.0) (1728) (16.3871) (1066) m) m 3 3 = 11.468 m= 11.468 m

33


• Determine the volume – use a basis of 1.0 acres:Determine the volume – use a basis of 1.0 acres:

V V = = 2.25 ft2.25 ft 3 3 60 sec60 sec (1 min) (1.0 acre) = 135.0 ft (1 min) (1.0 acre) = 135.0 ft 3 3

sec-acresec-acre min min

• Recall:Recall: 1.0 acres = 43,560 ft1.0 acres = 43,560 ft 2 2

• Thus:Thus: d d = = VV = = 135.0 ft 135.0 ft 3 3 = 3.099 x 10 = 3.099 x 1033 ft ft 12 in12 in A A 43,560 ft 43,560 ft 2 2 ft ft

= 0.0372 in x = 0.0372 in x 2.54 cm2.54 cm = 0.0945 cm = 0.0945 cm in in

Surface Flow PhenomenaSurface Flow Phenomenawithin a Watershedwithin a Watershed

• During a given rainfall, water is continually being During a given rainfall, water is continually being abstracted to saturate the upper levels of the abstracted to saturate the upper levels of the soil surface; however, this saturation or soil surface; however, this saturation or infiltration is only one of many continuous infiltration is only one of many continuous abstractions.abstractions.

• Rainfall is intercepted by trees, plants, and roof Rainfall is intercepted by trees, plants, and roof surfaces, and at the same time is evaporated.surfaces, and at the same time is evaporated.

• Once rain falls and fulfills initial requirements of Once rain falls and fulfills initial requirements of infiltration, natural depressions collect falling infiltration, natural depressions collect falling rain to form small puddles, creating rain to form small puddles, creating depression depression storagestorage. In addition, minute depths of water . In addition, minute depths of water forming forming detention storagedetention storage build up on permeable build up on permeable and impermeable surfaces within the watershed. and impermeable surfaces within the watershed.

Surface Flow PhenomenaSurface Flow Phenomenawithin a Watershed (cont’d)within a Watershed (cont’d)

• The stored water gathers in small rivulets The stored water gathers in small rivulets which carry the water originating as overland which carry the water originating as overland flow into small channels, then into larger flow into small channels, then into larger channels, and finally as channels, and finally as channel flowchannel flow to the to the watershed outlet.watershed outlet.

• The distribution of a prolonged uniform rainfall The distribution of a prolonged uniform rainfall is not the norm. However, the concept is useful is not the norm. However, the concept is useful to show the manner in which detention and to show the manner in which detention and depression storage would be distributed.depression storage would be distributed.


• In general, the channel of a watershed In general, the channel of a watershed possesses a certain amount of possesses a certain amount of base flowbase flow during most of the year. This flow comes from during most of the year. This flow comes from groundwater or spring contributions and may groundwater or spring contributions and may be considered as the normal day-to-day flow.be considered as the normal day-to-day flow.

• Discharge from precipitation excess – that is, Discharge from precipitation excess – that is, after abstraction deducted from the original after abstraction deducted from the original excess – constitutes the excess – constitutes the direct runoff hydro-direct runoff hydro-graphgraph (DRH). Arrival of direct runoff at the (DRH). Arrival of direct runoff at the outlet accounts for an initial rise in the DRH. outlet accounts for an initial rise in the DRH.


• As precipitation excess continues, As precipitation excess continues, enough time elapses for progressively enough time elapses for progressively distant areas to add to the outlet flow.distant areas to add to the outlet flow.

• Consequently, the duration of rainfall Consequently, the duration of rainfall dictates the proportionate area of the dictates the proportionate area of the watershed amplifying the peak, and watershed amplifying the peak, and the intensity of rainfall during this the intensity of rainfall during this period of time determines the period of time determines the resulting highest discharge.resulting highest discharge.


• If the rainfall maintains a constant If the rainfall maintains a constant intensity for a long enough period of intensity for a long enough period of time, a state of equilibrium discharge time, a state of equilibrium discharge (steady state) is reached.(steady state) is reached.

• An inflection point indicates the time An inflection point indicates the time at which the entire drainage area at which the entire drainage area contributes to the flow.contributes to the flow.

• At this time, maximum storage of the At this time, maximum storage of the watershed is only partially complete. watershed is only partially complete.


• As rainfall continues, maximum storage As rainfall continues, maximum storage capacity is attained and equilibrium capacity is attained and equilibrium (steady state) is reached where inflow (steady state) is reached where inflow (rainfall) equals outflow (runoff).(rainfall) equals outflow (runoff).

• The conditions of maximum storage and The conditions of maximum storage and equilibrium are seldom, if ever, attained in equilibrium are seldom, if ever, attained in nature.nature.

• Extended rainfall may occur, but Extended rainfall may occur, but variations in intensity throughout its variations in intensity throughout its duration negate any possibility of a DRH duration negate any possibility of a DRH of the theoretical shape for constant of the theoretical shape for constant rainfall intensity.rainfall intensity.


• A normal single-peak DRH generally possesses A normal single-peak DRH generally possesses the shape shown in curve B of Figure 3.the shape shown in curve B of Figure 3.

• The time to peak magnitude of this hydrograph The time to peak magnitude of this hydrograph depends on the intensity and duration of the depends on the intensity and duration of the rainfall, and the size, slope, shape, and storage rainfall, and the size, slope, shape, and storage capacity of the watershed.capacity of the watershed.

• Once peak flow has been reached for a given Once peak flow has been reached for a given isolated rainstorm, the DRH begins to descend, isolated rainstorm, the DRH begins to descend, its source of supply coming largely from water its source of supply coming largely from water accumulated within the watershed such as accumulated within the watershed such as detention and channel storage.detention and channel storage.


• Processes involved in forming the DRH can be Processes involved in forming the DRH can be better understood by visualizing the better understood by visualizing the precipitation excess as partially disposed of precipitation excess as partially disposed of immediately by surface runoff while a portion immediately by surface runoff while a portion remains held within the watershed remains held within the watershed boundaries and is released later from storage.boundaries and is released later from storage.

• Thus, the shape and timing of the DRH are Thus, the shape and timing of the DRH are integrated effects of the duration and integrated effects of the duration and intensity of rainfall hydrometeorological intensity of rainfall hydrometeorological factors as well as the effect of the factors as well as the effect of the physiographic factors of the watershed upon physiographic factors of the watershed upon the storage capacity.the storage capacity.

Interception and Depression Storage 2011

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