Hydrologic Cycle

* Hydrologic cycle is the processes and pathways involved in the circulation of water from land and water bodies to the atmosphere and back.

* Hydrologic balance (water budget):

Inflow - Outflow = DStorage

* Water resources: fresh water (2.6%) - distribution: polar ice caps and glaciers (77%), aquifers (11%), Circulation (12%).



Watershed hydrologic cycle


Components (More detailed study will be in Chapter 3. Watershed Hydrology)


Staring with the atmosphere, water condenses to form rain (>32oF) or ice crystals (<32oF). Based on certain atmospheric conditions, water falls to earth as precipitation. Precipitation is often intercepted by natural vegetation or impervious surfaces, which is then redistributed again as runoff (surface transfer), evaporation (transfer to the atmosphere), and infiltration (transfer to soil and groundwater systems). Surface runoff transfers water overland and enters into streams. Evaporation from water bodies and other wet surfaces. There is some loss through transpiration from vegetation. The infiltrated water percolates through soil profiles and enters the groundwater system. The groundwater system discharges into downstream water bodies and the cycle repeat. Storage is an important concept in understanding the cycle related to water budget.

Antecedent Moisture Conditions: This concept is important in hydrologic cycle. This refers to the amounts and locations of water in storage at the start of the hydrologic event or the time period under study.


Water Balance

P+w = Qs + QB + DD+ DS+EA

P-precipitation, w= water imports/exports, A=area, Qs = net surface runoff, QB = net sub-surface outflow, DD = change in surface storage, DS = change in soil water storage, E = Evaporation.


Represents stream flow changes through plotting discharge against time




Shape varies with the magnitude and the spatial and temporal distribution of precipitation.

The unit hydrograph is a discharge hydrograph resulting from one inch of direct runoff generated uniformly over a tributary area at a uniform rate during a specified period of time.


Study Questions:


1.    How does clearing vegetative cover affect the hydrologic cycle?

2.    How does a hydrograph change during an extreme flood event?

3.    How will you use water budget equation to study the effects of water withdrawals from a lake or groundwater system?




How can one use the hydrologic cycle to study watershed issues?