BEGIN. Rainfall-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 models …. The Basic Process….
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Necessary for a single basin
Focus on Excess Precipitation
Excess Precip. Model
Basin “Routing” UHG Methods
Stream and/or Reservoir “Routing”
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.
Very thick ground litter layers can hold as much as 0.5 inches!
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!
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.
Once the water infiltrates into the ground, the downward movement of water through the soil profile may begin.
The percolating water may move as a saturated front - under the influence of gravity…
Or, it may move as unsaturated flow mostly due to capillary forces.
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....
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.
The terms transpiration and evaporation are often combined in the form :
Water in storage can still be involved in a process.
Water in a puddle may be evaporating.....
Ground Water Storage
The hydrologic cycle represented as a series of storage units & processes....
Is I > f?
Is retention full?
Initial Abstraction - It is generally assumed that the initial abstractions must be satisfied before any direct storm runoff may begin. The initial abstraction is often thought of as a lumped sum (depth). Viessman (1968) found that 0.1 inches was reasonable for small urban watersheds.
Would forested & rural watersheds be more or less?
The Soil Conservation Service (SCS) now the NRCS uses a percentage of the ultimate infiltration holding capacity of the soil - i.e. 20% of the maximum soil retention capacity.
A constant infiltration rate is the most simple of the methods. It is often referred to as a phi-index or f-index.
In some modeling situations it is used in a conservative mode.
The saturated soil conductivity may be used for the infiltration rate.
The obvious weakness is the inability to model changes in infiltration rate.
The phi-index may also be estimated from individual storm events by looking at the runoff hydrograph.
Continuous process represented with discrete time steps
1.65 inches of excess precipitation
Uniform loss rate of
0.2 inches per hour.
This is purely a mathematical function - of the following form:
fi = infiltration capacity at time, t
fc = final infiltration capacity
fo = initial infiltration capacity
Effect of fo or fc
Effect of K
Assumes that precipitation supply is greater than infiltration rate.
There are now 2 parameters to estimate or calibrate for a watershed!!
fo & k
Stopped raining for a short period – how does the soil recover?
Soil Conservation Service is an empirical method of estimating EXCESS PRECIPITATION
We can imply that precipitation minus excess precipitation = infiltration/retention :
P - Pe = F
Pe = P - Ia
Effective precipitation equals total precipitation minus initial abstraction…
We will use effective precipitation in place of precipitation…
CN ranges from 1 to 100 (not really!)
Land use and treatment Hydrologic soil group
practice condition A B C D
Straight row ---- 77 86 91 94
Straight row Poor 72 81 88 91
Straight row Good 67 78 85 89
Contoured Poor 70 79 84 88
5-day antecedent rainfall, inches
Dormant Season Growing Season
I Less than 0.5 Less than 1.4
II 0.5 to 1.1 1.4 to 2.1
III Over 1.1 Over 2.1
The curve number is assumed to be 70.
The cumulative runoff (c) is calculated from the cumulative precipitation (b), using equation (4).
The potential maximum storage, S, is calculated to be S = (1000/70) - 10 = 4.286 inches.
Using 20% as the initial abstraction percentage yields 0.2 x 4.286 = 0.8572 inches and will require that at least 0.8572 inches of precipitation must accrue before runoff may begin.
These rainfall-runoff models have varied in complexity – but would have difficulty in modeling a continuous event, as they all lack the ability to allow the soil zones to “recover” when the precipitation stops….. This leads us to model systems that are intended for continuous simulation with “updating” abilities.
UZTW Free Water
Total Channel Inflow
Tension Water P S
Subsurface DischargeSacramento Model Structure
LZSK - Supplemental baseflow recession (always > LZPK)
Flow that typically persists anywhere from 15 days to 3 or 4 months