Generally, a Hydrograph is a graphical representation of:
Understanding the Hydrograph Concept in Hydrology
A hydrograph is a fundamental graphical representation used in hydrology to study the behavior of rivers and streams over time. It provides crucial information about how water flow changes at a specific point in a stream or river.
Defining the Hydrograph
Generally, a hydrograph is defined by the relationship between two key variables:
- Water flow rate or discharge
- Time
Specifically, a hydrograph plots the discharge of a stream or river on the vertical axis (y-axis) against time on the horizontal axis (x-axis). This graphical representation shows how the volume of water flowing past a point per unit of time changes over a specific period.
For example, a typical storm hydrograph shows a rise in discharge following rainfall, reaching a peak flow, and then receding back to baseflow conditions over time.
Analyzing the Options
Let's examine the given options based on the definition of a hydrograph:
- Option 1: Evaporation of a stream as a function of time
Evaporation is the process of water changing from liquid to gas. While evaporation rates can change over time, a standard hydrograph does not plot evaporation rates. It focuses on the water flowing within the stream channel. This option is incorrect. - Option 2: Discharge of a stream as a function of temperature
Temperature is a meteorological variable that can influence stream discharge (e.g., through snowmelt or affecting evaporation rates), but a standard hydrograph plots discharge directly against time, not temperature. While temperature data might be analyzed alongside a hydrograph, it is not the defining graphical representation itself. This option is incorrect. - Option 3: Evaporation of a stream as a function of temperature
This option relates evaporation and temperature. Both are relevant in hydrology, but neither variable (evaporation or temperature) is plotted as the primary component of a standard hydrograph, which focuses on stream discharge. This option is incorrect. - Option 4: Discharge of a stream as a function of time
This aligns precisely with the definition of a hydrograph, which graphically represents how the discharge (flow rate) of a stream changes over time. This is the correct representation.
Conclusion on Hydrograph Representation
Based on hydrological principles, a hydrograph is universally understood as the plot of stream discharge versus time. This graph is essential for analyzing runoff events, understanding watershed response, and managing water resources.
| Axis | Variable Represented | Units (Common Examples) |
|---|---|---|
| Vertical (y-axis) | Discharge (Flow Rate) | Cubic meters per second ($\text{m}^3/\text{s}$) or Cubic feet per second ($\text{ft}^3/\text{s}$) |
| Horizontal (x-axis) | Time | Hours, Days, Months, Years |
Revision Table: Key Hydrology Terms
| Term | Definition |
|---|---|
| Hydrograph | A graph showing the rate of flow (discharge) of water past a specific point in a river, channel, or conduit plotted against time. |
| Discharge | The volume of water flowing through a channel per unit time. |
| Runoff | The flow of water over land surfaces or through channels; the part of precipitation that appears in surface streams. |
| Baseflow | The portion of streamflow that is not due to direct runoff from precipitation; it is sustained by groundwater discharge. |
Additional Information on Hydrograph Analysis
Hydrographs are powerful tools in hydrological analysis. By studying the shape and characteristics of a hydrograph, hydrologists can infer important information about the watershed:
- Peak Discharge: The maximum flow rate during a runoff event.
- Time to Peak: The time elapsed from the start of the rainfall excess to the peak discharge.
- Rising Limb: The initial portion of the hydrograph where discharge is increasing due to storm runoff.
- Falling Limb (Recession Curve): The portion of the hydrograph after the peak where discharge is decreasing.
- Hydrograph Shape: Reflects watershed characteristics like size, shape, slope, soil type, and land cover. A flashy watershed (small, steep, impervious) typically produces a hydrograph with a rapid rise and fall and a high peak.
Understanding hydrographs is crucial for flood forecasting, designing hydraulic structures, and assessing the impact of land-use changes on water resources.
Important Questions from Hydrograph
The use of unit hydrographs for estimating floods is generally limited to catchments of size less than?
What are the lower and upper limits of catchment area for applicability of the use of unit-hydrograph?
Select the correct option with regard to the following two statements (H1 and H2) pertaining to the hydrograph of a storm in a catchment.
H1: The rising limb of the hydrograph depends on the catchment characteristics only.
H2: The recession limb of the hydrograph depends on the storm characteristics and catchment characteristics.
Basin lag in hydrology is the time difference between the ______.
Isolated storm is represented in a hydrograph with
