5_rainfall_runoff

Percolation.....
Or, it may move as unsaturated flow mostly due to capillary forces.
Percolation….the point
The vertical percolation of the water into various levels or zones allows for storage in the subsurface – these zones will be very important in the SAC-SMA model. SACThis stored subsurface water is held and released as either evaporation, transpiration, or as streamflow eventually reaching the watershed outlet.
Evapotranspiration....
The terms transpiration and evaporation are often combined in the form :
EVAPOTRANSPIRATION
Storage....
• Storage occurs at several “locations” in the hydrologic cycle and varies in both space and time - spatially and temporally. • Water can be stored in: - The unsaturated portion of the soil - The saturated portion (below the water table) - On the soil or surface - snow/snowpack, puddles, ponds, lakes, wetlands. - Rivers and stream channels - even though they are generally in motion!
Interception...........
First, the falling precipitation may be intercepted by the vegetation in an area. It is typically either distributed as runoff or evaporated back to the atmosphere. The leafy matter may also be a form of interception.
Canopy…(or lack of)
Leafy Matter also intercepts...
Very thick ground litter layers can hold as much as 0.5 inches!
Interception…the point
The point of the interception is that the precipitation is temporarily stored before the next process begins. The intercepted/stored precipitation may not reach the ground to contribute to runoff. Interception may be referred to as an abstraction and is accounted for as initial abstraction in some models. This is also true for snowfall which may sublimate and leave the watershed!
Infiltration, cont...........
Infiltration is nearly impossible to measure directly - as we would disturb the sample in doing so. We can infer infiltration in a variety of ways (to be discussed at a later point). The exact point at which the atmosphere ends and the soil beings is very difficult to define and generally we are not concerned with this fine detail! In other words, we mostly want to know how much of the precipitation actually enters the soil.
Transpiration....
The process of water moving from the soil via the plants internal moisture supply system. This is a type of evaporative process. The water moves through the stomates, tiny openings in the leaves (mostly on the underside), that allow the passage of oxygen, carbon dioxide, water vapor, and other gases.
Storage....
Water in storage can still be involved in a process. i.e. : Water in a puddle may be evaporating.....
The hydrologic cycle represented as a series of storage units & processes....
Surface runoff
no
Storage....
The thought process....... RO
=
Surface runoff yes
P
Depresstorm Flow
Is I > f?
Base Flow
no Channel runoff
Is retention full?
BEGIN
RainfallRainfall-Runoff Models
Excess Precipitation or Runoff Volume Models
May be:
– – – – – – – Physically Based Empirical Descriptive Conceptual Generally Lumped Etc…… May not only estimate excess precipitation – hence, we will refer to them as rainfall-runoff rainfallmodels….. models…..
The Basic Process….
Focus on Excess Precipitation Excess Precip. Model
Excess Precip.
Necessary for a single basin
Excess Precip.
Basin “Routing” UHG Methods
Channel Storage
yes
Detention Storage Ground Water Storage Retention Storage Vegetation Storage
Surface runoff
no
Storage....
Percolation.....
Once the water infiltrates into the ground, the downward movement of water through the soil profile may begin.
Percolation.....
The percolating water may move as a saturated front - under the influence of gravity…
Runoff Hydrograph
Runoff Hydrograph
Stream and/or Reservoir “Routing”
Downstream Hydrograph
Goal of Rainfall-Runoff RainfallModels
The fate of the falling precipitation is: …modeled in order to account for the destiny of the precipitation that falls and the potential of the precipitation to affect the the runoff hydrograph. … losses include interception, evapotranspiration, storage, infiltration, percolation, and finally - runoff. Let’s look at the fate of the precipitation…..
Infiltration...........
Precipitation reaching the ground may infiltrate. This is the process of moving from the atmosphere into the soil. Infiltration may be regarded as either a rate or a total. For example: the soil can infiltrate 1.2 inches/hour. Alternatively, we could say the soil has a total infiltration capacity of 3 inches. Note that in both cases the units are Length or length per time!
Evaporation....
Is the movement of water from the liquid state to the vapor state - allowing transport to the atmosphere. Occurs from any wet surface or open body of water. Soil can have water evaporate from within, as can leafy matter, living leaves and plants, etc.. The water evaporates from a storage location....
=
Surface runoff yes
RO
P
Depression Storage
E
T
Storm Flow
Is I > f?
Base Flow
no Channel runoff
Is retention full?
Channel Storage
yes
Detention Storage Ground Water Storage Retention Storage Vegetation Storage