hydrology

What drives the hydrologic cycle?

 

Solar Energy

Let’s say three different regions, a city, a farmland and a dessert has same acreage (and each of them are a watershed of their own). Now, if a rainfall event with P mm precipitation occurs in these three locations, which one would have the maximum runoff?

The City

The Farmland

The Desert

The City

Let’s assume that in a certain watershed, 1/3^rd of annual rainfall contributes to surface runoff, evapotranspiration, and groundwater recharge each. What type of climatic condition you would say the watershed is in?

 Humid

Arid

Semi Humid

Semi Arid

Humid

The fraction of precipitation that leaves a watershed as streamflow is called:

Runoff Ratio

What is the approximate change in runoff when an area changes from natural ground cover to 35%-50% impervious surface? (refer to the approximate percentages based on the lecture slides)

+20%

What is the approximate change in shallow infiltration when an area changes from natural ground cover to 75%-100% impervious surface? (refer to the approximate percentages based on the lecture slides)

-15%

What is the approximate change in runoff when a town returns an abandoned industrial area back to a park with native vegetation, changing the neighborhood from 75%-100% impervious surface back to 35%-50% impervious surface? (refer to the approximate percentages based on the lecture slides)

-25%

What is the order of difference in residence time between water in a river channel and in swamps?

Years

Mark if the statement is true or false:

The actual evapotranspiration is always greater than the potential evapotranspiration.

False

What could be the most likely cause that a site has a higher actual evapotranspiration compared to the precipitation:

Irrigation

Groundwater recharge is possible during the winter months, even-though there is not much rainfall because the snowmelt infiltrates the soil in pervious regions. (T/F)

True

If Hurricane Harvey occurred during 1996, how less would have been the depth of resulting runoff (flood depth) compared to 2017?

0.75 inches

0.25 inches

1.50 inches

2.75 inches

2.75 inches

Estimate the relative change in runoff for the New River (runoff ratio w = 0.10), if climate change results in a 15% increase in precipitation and a 15% decrease in evaporation.

3.85

True or False: The number of floods, and associated human and economic losses in the US are decreasing due to climate change and urbanization.

False

Increasing the elevation to the pour point for grid cells completely surrounded by higher terrain is called

pit filling

Which of the following can distinguish a drainage basin?

Wind patterns

Groundwater flows

Topography

Vegetation

Topography

How many HUC-8 regions is Blacksburg a part of?

2

Which of the following statements is True?

Higher flow accumulation values are ridge tops

Lower flow accumulation values are valleys & stream channels

Flow direction is only calculated for the cells with the steepest descent

Flow direction is calculated as the direction of steepest downward descent

Flow direction is calculated as the direction of steepest downward descent

Which of the following statements is false?

Water flows along the steepest descent

Streamlines flow towards drainage divides and intersect them

Water flows perpendicular to elevation contour lines

Topographic maps show lines of equal elevation

Streamlines flow towards drainage divides and intersect them

According to the following eight direction pour point model, what cell does the water flow out of? Identify the cell as (row, column) with the bottom left corner being (1,1) and the top right corner being (5,5).

(1,4)

Use the following DEM elevation map for this question. Elevations are given in meters (m) and the map has a 30m resolution (each square is 30m by 30m). What is the slope of cell (5,2) to the cell that it drains to? Identify the cells as (row, column) with the bottom left corner being (1,1) and the top right corner being (8,8). Hint: Cell (5,2) has an elevation of 10m. (If the image does not show up, try clicking the link next to it).

0.071

According to the Strahler Stream Order, what order stream is created when a 2nd order and 3rd order stream meet?

3rd

A ________________ based on flow accumulation or grid order is needed to define a stream network.

threshold

Why is it a fundamental rule in topographic mapping that contour lines do not intersect?

It ensures that one point doesn’t have multiple elevations at the same time.

In the Strahler stream order system, when two streams of the same order come together, what is the order of the resulting stream?

It increases by 1

Which of the following methods is NOT commonly used to determine the boundary of a watershed?

Topographic maps

Flow accumulation

Groundwater Mapping

Aerial photography

Groundwater Mapping

Flow distance from each cell of the watershed to the outlet is defined as,

flow length

Which of these spatial methods would you expect to produce more accurate estimates for rainfall interpolated from point gages?

Inverse Distance Weighting Method

Both have the same level of accuracy

Artimetic Mean Method

Inverse Distance Weighting Method

The IDF curve for 10-year storms in Raintown, Waterstate is given by

where i is the average intensity in inches/hr and td is the duration in minutes. Assuming that the maximum rainfall intensity occurs at 50% of the rainfall duration, estimate the triangular hyetograph for a 70-min storm. At what time (t ; measured in minutes from beginning of storm) does the peak rainfall intensity occur?

35

The IDF curve for 10-year storms in Raintown, Waterstate is given by

where i is the average intensity in inches/hr and t is the duration in minutes. Assuming that the maximum rainfall intensity occurs at 50% of the rainfall duration, estimate the triangular hyetograph for a 70-min storm. What is the peak intensity (i ) in inches/hr of the triangular hyetograph? Give answer rounded to one decimal

place.

4.1

Calculate the spatially averaged rainfall for a watershed if there are five gages located inside the watershed using the Thiessen Polygon method. The area-weighted Thiessen polygon representative areas corresponding to each of the five rain gages and recorded rainfall are as follows:

58

Assuming all other factors stay the same, when the wind speed increases, the pan coefficient of evaporation ________________

stays the same

is negative

decreases

increases

decreases

Select the correct statement:

• Rainfall is uniform over across space and time in a watershed

• On average, snow/snow melt events deliver more volume to soils compared to rain only events

• Accurate ET modeling is more important for event based modeling than continuous modeling

• NRCS Type II distributions are applied over a minority percent coverage over the US

On average, snow/snow melt events deliver more volume to soils compared to rain only event

Answer true or false:

A non-exceedance probability of 90% means that there is a 10% chance that a rainfall event of a magnitude would occur in any given year and this corresponds to an event of 10-year return period

True

One of the disadvantages of using the water balance method to estimate evaporation is that it _____________________.

combines measurement uncertainties

Calculate the spatially interpolated rainfall for a point P located at (1, 1) using the inverse distance weighting method using five gages. The coordinates of the five rain gages and recorded rainfall are as follows:

35

Which of the following does NOT impact interception loss.

Precipitation Depth

Forest Cover

Soil type

Canopy density

Soil type

When a soil is saturated then the water content is equal to the ___________

Porosity

Interception losses are ultimately _____________________.

evaporated

As the water content in the soil increases the absolute value of the pressure head ___________

decreases

If the infiltration capacity is higher than rainfall intensity, then the rate of infiltration will be equal to ___________________.

the rainfall intensity

By what percentage would the effective saturation increase if the moisture content of the soil changes from 0.3 to 0.46? Assume that the porosity is equal to 0.75 and the residual moisture content is equal to 0.07. Report the value in nearest integers.

70

Which of the following statements is false? 

Horton model assumes infinite depth of vadose zone

Depression storage can reduce the onset time of runoff in a watershed

Green-Ampt method assumes a sharp wetting front

The pressure head in the saturated zone is hydrostatic

Depression storage can reduce the onset time of runoff in a watershed

Which of the following is not a parameter in Green-Ampt method calculations?

suction head

Datum head

Interception storage

porosity

Interception storage

Which of the following is false about the flow through soils?

• The infiltration rate is affected by vegetation

• The flow rate through porous media is inversely proportional to the area of medium

• The difference between tension heads between wetting and drying is called hysteresis

• Horton model assumes that infiltration tends to decrease in an exponential manner

The flow rate through porous media is inversely proportional to the area of medium

According to Darcy's Law, how will the flow change if the length and the hydraulic conductivity both increase?

depends

As the moisture content in a soil increases, the hydraulic conductivity, K:

increases

Which of the following is true:

• The Horton's method assumes that the soil has a fixed depth up to the water table.

• The infiltration rate in Hortons method increases exponentially with time.

• The Green Ampt method assumes a sharp wetting front dividing the saturated and unsaturated soil.

• Sorptivity is the rate at which water is drawn out of a soil

The Green Ampt method assumes a sharp wetting front dividing the saturated and unsaturated soil.

Of the infiltration models, which is a fully empirical model? 

• Green-Ampt Model

• NRCS Curve Number Method

• Horton's Equation

• Philip's Equation

NRCS Curve Number Method

If the antecedent runoff condition for a watershed has been Dry recently, would you expect the runoff, Q, to increase or decrease compared to moderate conditions?

decrease

According to the NRCS Curve Number Method, if a dirt road is soil group C and is in antecedent runoff condition III (wet), what would be the initial abstraction with a rainfall depth of 0.3 in? Enter you answer in three decimal places.

0.129

According to the NRCS Curve Number Method, if a dirt road is soil group C and is in antecedent runoff condition III (wet), what would be the depth of the runoff (Q) with a rainfall depth of 0.3 in and an initial abstraction of 0.128? Enter you answer in three decimal places.

0.036

Would the runoff calculated in question 3 and 4 be considered accurate (using the NRCS Curve Number Method)?

depends on several factors

What is the lag time of a hydrograph?

excess precipitation centroid to peak discharge

How is time of concentration measured on a hydrograph?

end of excess precipitation event to the inflection point of the recession limb

Using the constant slop method for a given storm hydrograph, does the baseflow increase or decrease during the storm?

Increases

If a given area's land cover increases the impervious area and all other factors remain the same, how would the lag time change for the same storm?

decrease

What are the main components of a water budget?

Precipitation, evaporation, transpiration, streamflow, groundwater inflow/outflow, soil storage, and human uses

What does residence time represent?

The average time water spends in a subsystem of the hydrologic cycle.

What is the residence time of atmospheric water?

Approximately 8.2 days

Which water reservoir has the longest residence time?

Ice caps, glaciers, and permafrosts (1,000 to 10,000 years)

How does rapid development affect flood risk

Increases runoff and decreases subsurface storage, raising flood risk

What assumptions are made in long-term water balance models?

Groundwater discharge and change in storage are negligible

What does the steady-state assumption imply?

No change in storage over time

Why is the runoff ratio limited?

It varies with soil moisture, precipitation rate, and interception.

What is a watershed?

A region draining into a river, river system, or body of water; defined by a drainage point and its upstream area.

catchment

The surface water portion of a watershed that drains to a single outlet

drainage divide

A topographic barrier (e.g., ridge or hill) that separates adjacent drainage basins

drainage area

horizontal projection of the area contributing flow to a point in a stream

What does HUC stand for?

Hydrologic Unit Code – a system for classifying watershed scales

List the HUC levels from largest to smallest.

HUC2 (region) → HUC4 (subregion) → HUC6 (basin) → HUC8 (subbasin) → HUC10 (watershed) → HUC12 (sub-watershed)

Are watersheds nested?

Yes, smaller watersheds are nested within larger ones

Digital Elevation Model (DEM)

A raster representation of the earth’s surface elevation

DEM resolution improvement

From 90 m in the 1980s to ~1 m with modern LIDAR

National Elevation Dataset (NED)

USGS dataset offering DEMs at 30 m, 10 m, and 3 m resolutions

LIDAR

Remote sensing technology that uses laser pulses to measure elevation and create DEMs and DSMs

Digital Surface Model (DSM)

Elevation model that includes canopy and structures

Pit

A cell or group of cells in a DEM with no downslope neighbor

Pit removal (Fill routine)

Increases elevation of pits to ensure drainage, creating a depressionless DEM

Pour point

The lowest cell adjacent to a pit where water can drain

Eight

direction pour point model

Hydrologic slope

Direction of steepest descent from a cell

Flow direction

Calculated for each cell based on steepest descent, stored as coded values

Flow accumulation

Number of upstream cells draining into a given cell

Low flow accumulation values

Indicate ridges or high points

High flow accumulation values

Indicate valleys or stream channels

Strahler stream order

Classification where first-order streams have no tributaries; stream order increases at confluence

Grid order

Application of Strahler stream order to DEM grid cells

Stream network definition

Based on flow accumulation threshold or grid order

Stream segmentation

Divides stream network into uniquely identified links

Catchment

Area that drains into a specific stream segment

Catchment delineation

Based on stream network and flow direction from DEM

Watershed

Total area draining to a pour point or outlet

Watershed delineation

Uses pour points and flow direction to define boundaries

Pour point placement

Should be in high-flow pathways and converted to grid layer

Drainage line

Line through center of stream cells connecting pour points

Flow length

Distance from each cell to the outlet

Assumption in DEM-based delineation

Water flows downhill and each cell has a defined flow path

Limitation of DEMs

Artificial pits and internal drainage areas must be corrected before analysis

Limitation of automated delineation

Accuracy depends on DEM resolution and quality