Hydrograph

• The plot of discharge against time
• Has three regions: rising limb, crest segment, and falling limb
• The nature of hydrograph depends on rainfall and watershed characters.
• Isolated storm results in single peak hydrograph and complex storm yields multiple peak hydrographs.

Rising limb

• Ascending portion representing rising discharge due to gradual increase in flow in the stream
• Slope depend on storm and basin characteristics

Crest Segment

• Inflection point on a rising limb to falling limb
• Indicate the peak flow
• Controlled by storm and watershed characteristics
• Multiple peaks –due to the occurrence of two or more storms of different intensities in a closer interval

Falling limb (recession limb)

• From point of inflection at the end of the crest segment to base flow.
• Inflection point indicates the time at which rainfall stopped
• Shape independent on storm characteristics but dependant on watershed characteristics

Factors affecting the shape of a hydrograph

Climatic factors

1.     Form of precipitation

• Rainfall and snow fall –rainfall tends to produce runoff rapidly generating hydrograph with high peak and narrow base

2.     Rainfall Intensity

• Affect volume of runoff, the occurrence of peak flow, duration of surface flow
• Higher the intensity quicker the peak flow and conical hydrograph

3.     Duration of rainfall

• Longer the duration more the volume
• Longer duration, peak flow occur after a long time and hydrograph is flatter with a broad base

4.     Distribution of rainfall

• When heavy rain occurs near an outlet
  • Peak flow occurs quickly
• When heavy rain occurs in upper areas
  • Peak flow occurs after a few hours
  • Lower peak and broad base (more time taken for flow to reach outlet)

5.     Direction of storm movement

• Affects the amount of peak flow and surface flow duration
• Upward direction –lower peak and broad base
• Downward direction –sharp peak and narrow base

Physiographic factors -characteristics of watershed

1.     Shape of basin

• Affects the shape of hydrograph affecting time of concentration
• Broad shaped –peak flow occurs soon because of less time of concentration, narrow hydrograph with high peak
• Fan-shaped –peak flow occurs at a longer time interval because of long time of concentration, broad base lower peak hydrograph

2.     Size of basin

• Small basin –flow dominated by overland flow that joins channel quickly, peak flow occurs quickly

3.     Stream slope

• More the stream slope higher the slope of recession limb, reduce base width of a hydrograph
• The small slope makes recession limb flatter, base width wider

4.     Nature of valley

• Greater valley slope higher the slope of recession limb

5.     Drainage density

• Higher the drainage density, quicker the peak flow, recession limb is steeper with the narrow hydrograph
• Lesser the drainage density, slow-moving rising limb, and wide base width

6.     Land use

• Vegetation increases the loss of water
• Higher the vegetation density, lesser the peak flow

7.     Surface depression

• Presence of ponds, rills, etc. delay and modify flow pattern
• Decreases peak flow and wide base width

Types of hydrographs

• Storm hydrographs
• Flood hydrographs
• Annual hydrographs a.k.a. regimes
• Direct Runoff Hydrograph
• Effective Runoff Hydrograph
• Raster Hydrograph
• Storage opportunities in the drainage network

Baseflow separation

A stream hydrograph is commonly determining the influence of different hydrologic processes on discharge from the subject catchment. Because the timing, magnitude, and duration of groundwater return flow differ so greatly from that of direct runoff, separating and understanding the influence of these distinct processes is key to analyzing and simulating the likely hydrologic effects of various land use, water use, weather, and climate conditions and changes.

However, the process of separating “baseflow” from “direct runoff” is an inexact science. In part this is because these two concepts are not, themselves, entirely distinct and unrelated. Return flow from groundwater increases along with overland flow from saturated or impermeable areas during and after a storm event; moreover, a particular water molecule can easily move through both pathways end route to the watershed outlet. Therefore, separation of a purely “baseflow component” in a hydrograph is a somewhat arbitrary exercise.