Soil and Water Conservation Flashcards
needed in estimating runoff, planning erosion control measures, planning for irrigation and drainage, and water conservation in low rainfall regions
Precipitation
needed in designing structures and channels that will handle natural flows of water
Runoff
required in planning irrigation and drainage systems, moisture conservation practices, etc.
Infiltration, Evaporation and transpiration
caused by the rising of warmer, lighter air in colder, denser surroundings.
Convective Precipitation
results from mechanical lifting of the air mass over mountain barriers.
Orographic Precipitation
air is lifted through horizontal convergence of the inflow into a low pressure area
Non-frontal
associated with the movement of air masses from high-pressure to low-pressure regions.
Cyclonic Precipitation
lifting of warm air over cold air at the contact zone between air masses having different characteristics
Frontal
this involves the averaging arithmetically all the rainfall depths measured by the rain gages within the area.
Arithmetic mean
– location of the rain gauges are plotted on the map of the area and stations are connected by straight lines. Perpendicular bisectors are constructed on each of the connecting lines thereby forming polygons enclosing each rain gage. The average rainfall over the area is then estimated as the area weighted average for all polygons
Thiessen polygon
station locations and amounts of rainfall are plotted on a suitable map, and contours of equal precipitation (isohyets) are then drawn. The equation used is similar to that of Thiessen method except that the area, Ai, is the area under one isohyet, Pi.
Isohyetal
computer aided method wherein it averages estimated precipitation at all points of a superimposed grid.
Grid-Point Method
for rolling areas and non-uniform distribution of gages, distance factor fixed by location of gages is used for analysis using this method. It operates under the principle that the value of points close together in space are more likely to be similar than with points farther apart.
Inverse Distance Ratio Method
the number of years during which one storm of a given duration and intensity is expected to occur.
frequency or recurrence interval
one in which a definite law of certainty exists
Deterministic Process
governed by chance; time series INDEPENDENT
Probabilistic Process
governed by chance; time series DEPENDENT
Stochastic Process
data is more or less normally distributed
Normal Distribution
used when there is skewness in the data distribution
Log-Normal Distribution
considers further the skewness of the logarithmically transformed hydrologic data; preferred for flood flow frequency; log-normal distribution is a special case of this method
Pearson Type III Distribution
so far gives the best fit for most stations for 1-week, 2-week, 3-week and monthly rainfall totals of the country
Gamma Density Function
that which travels over the ground surface and through the channels to reach the basin outlet
Surface Runoff
that which infiltrates into the soil surface and moves laterally through the upper soil horizons towards streams as perched groundwater above major groundwater level
Interflow
groundwater discharge into a stream due to deep percolation of the infiltrated water into groundwater aquifers
Groundwater Runoff
that part of surface runoff that flows over the surface towards the stream channel
Overland flow
surface runoff plus interflow
Direct runoff
same as interflow
Subsurface runoff
same as groundwater runoff
Baseflow
used to determine the magnitude of floods and a valuable consideration in the design of structures
Peak Flow (qp)
used for flood forecasting and water quality studies; watershed response time
Time to Peak (tp)
total water yield from a storm for a given watershed
Runoff Volume (V)
time for surface and interflow to recede; duration of flooding
Recession Time (tr)
total of time to peak and recession time; (tb) = (tp) + (tr)
Base time (tb)
low flow, dependable flow, groundwater discharge
Base flow
a graphical representation of the instantaneous runoff rate against time. The area under it gives the runoff volume.
hydrograph
– assumes that all hydrographs from all small watersheds (in the US) have similar forms
– it is plotted over 100 arbitrary units of flow and 100 arbitrary units of time
Basic Hydrograph
- approximation of the basic hydrograph
- - developed mainly to simplify flood routing procedures in US
Triangular Hydrograph
- a hydrograph with a unit volume (e.g. 1cm) of direct runoff for a given storm duration
- represent the response of the basin on a given storm duration and characteristics
- - enables one to synthesize hydrographs for complex storms by superimposing the hydrographs resulting from the individual components of the storm
- represent the response of the basin on a given storm duration and characteristics
Unit Hydrograph
- prepared using data from a number of watershed to develop dimensionless unit hydrographs which are applicable to ungaged watersheds
- a dimensionless hydrograph is made from natural or unit hydrographs in which the time to peak (tp) and the peak runoff rate (qp) are considered an t/tp is plotted against q/qp
Synthetic Hydrograph
provide record of the highest stage observed at a stream
Crest Stage Gage
record the pressure required to maintain a small flow of gas from an orifice submerged in the stream
Bubbler Gage
motion of a float is recorded on a graph
Float-type Water-Stage Recorder
measures the velocity of a floating object preferably in a straight section of a stream
Float method
utilizes the equation or relation:
V = a + bN
Where: V – water velocity
a – starting velocity or velocity required to overcome mechanical friction
b – constant of proportionality
N – revolution per seconds of the meter cups / propeller
Propeller-type Meter (Current Meters)
Type of water erosion, primarily caused by raindrop
Splash / Raindrop Erosion
Type of water erosion, a thin film of soil layer detached and transported by water flowing on the land surface.
Sheet erosion
Type of water erosion, combination and splash and sheet erosion
Interrill erosion
Type of water erosion, advanced stage of rill erosion. Rills when neglected develop in size and become gullies.
Gully erosion
Type of water erosion, erosion of stream banks by flowing water
Stream bank erosion
Type of water erosion, erosion caused by wave action on the seashore
Coastal erosion
Type of water erosion, finger-like rills appear on the soil surface
Rill erosion
caused by high velocity winds moving over barren land surfaces.
Wind erosion
land slides and slips due to saturation of steep hills and slopes.
Slip erosion
replanting of forest trees species in the watershed area
Reforestation
a practical adaptation of reforestation whereby the species planted have economic value, such as mango, pili and so on.
Agroforestry
is the practice of growing different crops in alternate strips across the slope to serve as barriers for soil erosion.
Strip cropping
the process of covering the land surface with plant residues, plastic or other materials appropriate to arrest loss of moisture through evaporation
Mulching
consists of carrying out agricultural operations very nearly on the contour. It reduces the velocity of overland flow and retards soil erosion
Contour cultivation
modification of cropping system such as crop rotation and mixed cropping
Cropping systems
construction of earth embankment or ridge and channel across the slope at an acceptable grade to control the flow of runoff as well as soil particles.
Terracing
establishment of natural waterways or construction of canals and planting it with grasses to make it stable and arrest soil erosion
Grassed Waterways
constructed along the channel or waterway to control the velocity of flowing water and encourage deposition of sediments carried by water
Check dams or weirs
temporary detainment of water in farm pond and dams to mitigate the erosive capacity of water
Farm Ponds / Water Impounding Dams
a channel constructed around the slope and given a slight gradient to cause water to flow to a suitable and stable outlet.
Diversion Canal
concrete structure made of stones constructed along steep embankments to prevent landslide or gully erosion
Riprap
made of stones carefully and properly piled-up and arranged on steep embankments to protect from gully erosion or landslide
Stone wall
usually starts as small rills and gradually develops into deeper crevices
Gully erosion
is constructed across the slope above the gully. The grassed waterway intercepts the runoff coming from above the gully
Grassed waterways
can stabilize the slopes of the gully and hence reduce runoff velocities.
Vegetation
construction of series of platforms along the contours cut into the hill slope in a step like formation
- used for 25-30% slopes
- used for maximum moisture conservation
- used where land is at a premium
- conventional
- difficult to farm
Bench
- primary purpose is to remove excess water in such a way as to minimize erosion
- reduce slope length, conducts intercepted runoff water to outlet at non-erosive velocity
- outlet either surface or subsurface type
Broad-based Graded or channel type
- used for 9-24% slope
- easier to farm than conventional
- used for maximum soil and water conservation
Zingg or Conservation bench terrace
- no grade in channel
- primary purpose is moisture conservation, erosion control is secondary
- channel is normally closed at both ends to assure maximum detention
- adopted to deep, permeable soils
- more formable than bench types
- used where outlets are a problem
Broad-based, Level or ridge type
are open channels protected with suitable grasses constructed along the slope and act as outlet for terraces and graded bunds. They are also used to safely convey runoff from contour furrows, diversion channels and serve as emergency spillways in farm ponds.
The velocity of flow is dependent upon the ability of the vegetation to resist erosion. For design purposes, an average of 1.5 m/s to 2 m/s is used.
Grassed Waterways
Structures constructed along a channel to dissipate safely the energy of water by letting the water fall freely for a certain height.
Drop structures
A term adapted to measures utilizing both or in combination with the biological and engineering measures.
Vengineering
discharge is constant with respect to time
Steady Flow
if the depth changes abruptly over a comparatively short distance such as in a hydraulic jump
Rapidly Varied Flow
depth of flow is the same at every section of the prismatic channel
Uniform Flow
discharge is not constant with respect to time
Unsteady Flow
depth changes along the length of channel
Varied Flow
- if the depth changes smoothly over a distance
Gradually Varied Flow
Fr = 1
Critical Flow
fluid moves in parallel layers with no cross-currents; Re <= 2000
Laminar Flow
Fr < 1
Subcritical Flow
Fr > 1
Supercritical Flow
characterized by pulsatory cross-current velocities; Re => 4000
Turbulent Flow